equipment \u0026 %

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May 2008

Disclaimer

This publication was prepared for the Canadian Association of Petroleum Producers, the Gas Processing Association Canada, the Alberta Department of Energy, the Alberta Energy Resources and Conservation Board, Small Explorers and Producers Association of Canada and Natural Resources Canada by CETAC-West. While it is believed that the information contained herein is reliable under the conditions and subject to the limitations set out, CETAC-West and the funding organizations do not guarantee its accuracy. The use of this report or any information contained will be at the user’s sole risk, regardless of any fault or negligence of CETAC-West or the sponsors. Acknowledgements

This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WEST with contributions from:

• Accurata Inc. • Clearstone Engineering Ltd. • RCL Environmental • REM Technology Inc. • Sensor Environmental Services Ltd. • Sirius Products Inc. • Sulphur Experts Inc. • Amine Experts Inc. • Tartan Engineering

CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourage advancements in environmental and economic performance in Western Canada. The corporation has formed linkages between technology producers, industry experts, and industry associates to facilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful eco-efficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry. Head Office # 420, 715 - 5th Ave SW Calgary, Alberta Canada T2P2X6 Tel: (403) 777-9595 Fax: (403) 777-9599 [email protected]

Table of Contents

1. Applicability and Objectives .......................................... 1 2. Basic Improvement Strategies....................................... 2 2.1 Technology and Equipment 2.2 Efficiency Assessment 2.3 Improving Efficiency 3. Operational Checks, Testing and Adjustments ........... 7 3.1 Establishing an Energy Reduction Program 3.2 Energy Reduction Strategies 3.3 Retrofits and New Units 3.4 Troubleshooting 3.5 Facility Engineering Input 3.6 Strategic Goals and Operating Parameters 4. Appendices.................................................................... 21

Appendix A McCabe-Thiele Method of Analysis Appendix B Using the McCabe-Thiele Diagram for Column Analysis Appendix C Additional Information Appendix D References

Tables Table 2.1 Efficiency of Fractionation Columns Figures

Figure 2.1 Schematic of a Fractionation Column Figure 3.1 Flow Plan for Reducing Energy in a

Fractionation Unit Figure 3.2 Effect of Reflux Ratio on Theoretical Trays

Background The issue of fuel gas consumption is increasingly important to the oil and gas industry. The development of this Best Management Practice (BMP) Module is sponsored by the Canadian Association of Petroleum Producers (CAPP), the Gas Processing Association Canada (GPAC), the Alberta Department of Energy, Small Explorers and Producers Association of Canada (SEPAC) Natural Resources Canada (NRC) and the Energy Resources and Conservation Board (ERCB) to promote the efficient use of fuel gas in fractionation processes used in the upstream oil and gas sector. It is part of a series of 17 modules addressing fuel gas efficiency in a range of devices.

This BMP Module:

• identifies the typical impediments to achieving high levels of operating efficiency with respect to fuel gas consumption;

• presents strategies for achieving cost effective improvements through inspection, maintenance, operating practices and the replacement of underperforming components; and

• identifies technical considerations and limitations. The aim is to provide practical guidance to operators for achieving fuel gas efficient operation while recognizing the specific requirements of fractionation processes and their service requirements.

EFFICIENT USE OF FUEL GAS IN THE UPSTREAM OIL AND GAS INDUSTRY

MODULE 12 of 17: Fractionation

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Efficient Use of Fuel Gas in Fractionation Rev Date 27/05/2008 Module 12 of 17 Page 1 of 46

1. Applicability and Objectives This module focuses on fractionation columns and units. The overwhelming majority of fractionation undertaken by CAPP members is in the area of natural gas liquid (NGL) separation. This will range from de-ethanizers, which remove sales gas from NGL components to stabilizers, which remove NGL from heavier C5

+ condensate. Plants will have either single columns, or multiple towers where the individual NGL components are successively removed, leaving a series of products. In this module the terms “fuel use” and “energy use” are used interchangeably. Many fractionation columns use a fuel-fired heater for the heat medium which provides the reboil heat. Others rely on a fuel-consuming boiler in the steam system. In addition, operation of the overhead condensers on fractionation columns will be reflected, through the energy balance, on the reboiler duty. The objectives of this module are to:

• Describe the general fundamentals of fractionation, which are applicable to all columns. This description is then extended to the most common types of columns typically found in upstream facilities. The target audience are the operators of the facilities, although input from corporate and technical functions elsewhere within the company will be necessary and of vital importance.

• Investigate energy management opportunities for the overall process and for each major piece of equipment both on the refrigerant side and on the process gas side.

• Focus on energy management opportunities through operational improvements

• Consider improvements in energy efficiency due to capital expenditures.


2. Basic Improvement Strategies

2.1 Technology and Equipment Fractionation Column Fractionation is a complex process, involving mass and heat transfer throughout a column. Improving energy efficiency in fractionation requires the understanding of certain process fundamentals such as: how the reflux rate influences the reboiler and, in turn, the energy usage. The common types of fractionation and a description of how to manage energy usage is outlined in the following sections. The components of a basic fractionation column, are shown in Figure 2.11.

Figure 2.1 Schematic of a Fractionation Column

In this column there is a total condenser, with a liquid top product and liquid is refluxed back to the top of the column.


The condensing medium in Western Canada is usually air; however, any cold fluid will work, as long as it is capable of condensing the vapour leaving the column. In some cases, it is necessary to increase the operating pressure in order to increase the condensing temperature to a level appropriate for the condensing medium. The bottom product is withdrawn from the bottom of the column, as shown in figure 2.1, or it is drawn off the shell of the reboiler. Ideally, the feed is introduced into the column at the tray where the material flowing in the column is most similar to the feed. This maximizes the performance of the column. If there is no overhead condenser, the feed is introduced into the top of the column and the liquid portion of the feed constitutes the internal reflux. For greater detail on the column components and their design, the reader is referred to Chapter 19 of the GPSA Engineering Data Book. Fractionation columns are often linked together to provide successive separation of the hydrocarbon stream into products such as propane, butane and condensate. Fractionation processes employed in the UOG are briefly discussed below. Optimization of the operations described below are outlined in section 2.3. De-ethanizers The de-ethanizer separates methane and ethane from the NGL and heavier components. The tower is often a stripper with feed entering at the top, or it could be a fractionation tower, equipped with a condenser and a reflux stream. The aim of the de-ethanizer is to produce an NGL bottom product that has a maximum ethane specification of approximately 2 mol %. The methane content of the NGL in such a circumstance will be extremely small. Steps operators can take to optimize the operation of de-ethanizers include:

• Calculating an NGL balance around the de-ethanizer. This will determine how much propane/butane is going overhead into the sales gas.

• Avoiding the tendency to over-fractionate. Ensure that bottom product consistently meets specification but not at the expense of wasted energy for reflux/reboil.

• Determining the proper feed location. Occasionally, towers are equipped with multiple feed locations, with the ability to redirect the feed. If feed composition/conditions have changed substantially from design, relocation of the feed may be warranted. This study is best done by a simulation package, although graphical analysis (as outlined in Appendix A.4- Column Configuration) will be useful.

For more background information on de-ethanizers see Appendix C.


Depropanizers For plants that fractionate their NGL, the depropanizer separates propane from the butanes. Since no fractionation is complete, there will be some ethane and very minor amounts of methane in the propane stream and there will be some propane in the butanes. The major components in depropanizers have different molal heats of vaporization (see Table B.1), but not to the same extent as seen in de-ethanizers. Nevertheless, some depropanizers at least may have non-linear operating lines2. As a result, there could be an appreciable loss of separation capability, depending upon the feed quality and the reflux ratio. Because of the similar volatility of the principle components (propane & iso butane) depropanizers required a large number of trays. Operators should be aware of the relative costs of propane and butane so that they do not over-fractionate. However, consistently making on-spec product is of paramount importance. The energy management opportunities discussed in sub-sections 3.1 and 3.2 apply. Debutanizers Debutanizers remove butanes from C5+ condensate. The molal heats of vaporization are very similar and the operating lines are linear, as outlined in conventional theory. The vapour and liquid compositional trends show great regularity and predictability, compared to those for a de-ethanizer. There are a fairly large number of theoretical trays required because of the similar volatility of the light key (normal-butane) and the heavy key (isopentane). The tray efficiency is 85-95%. Energy management opportunities are similar to those for depropanizers, except that the butane quality must be on-spec. At the same time, be aware of the relative value of butane and condensate. Butane Splitters Butane splitters are used to separate isobutane from the heavier normal-butane. These two components have very similar volatilities and as a result a very large number of theoretical trays are required – over twice the number required for a debutanizer. Moreover, a very high reflux ratio is needed.3 The tray efficiency is 90 to 100%, which means that every tray achieves essentially complete vapour-liquid equilibrium.


Condensate Stabilizers Condensate stabilizers are used to remove light end contaminants (gas, NGL components) from C5

+ condensate, removed from the feed, at the incoming flash separators and/or slug catchers. The objective is to reduce the vapour pressure of the condensate to within specification. Since the feed enters the top of the column, it is not equipped with a condenser and reflux equipment. The wide range of light ends being removed is sufficiently volatile that they readily separate from the condensate. The number of trays is generally just less than in de-ethanizers. The main energy management point that is specific to stabilizers is to minimize the carryover of condensate into the overhead stream through excessive reboil heat. Such carryover will later require the expenditure of energy to remove the heavy ends from the lighter components in the fractionation part of the plant in order to meet quality targets for those products. It should be mentioned that crude stabilizers operate in much the same way as condensate stabilizers. They should be monitored in a similar manner.

2.2 Efficiency Assessment The primary energy input to the column is the reboiler, which in turn is greatly influenced by the reflux rate. As such, the “energy health” of a column can be determined reasonably well by monitoring the reflux ratio. Table 2.1 outlines typical values for the types of fractionation columns discussed in this module.

Table 2.1 Efficiency of Fractionation Columns

2.3 Improving Efficiency Decisions to carry out adjustments and/or replace components should be made on a case- by- case basis having consideration for health, safety, environmental and economic considerations. Facility engineering and operators are responsible for preparing a strategy for maximizing overall product value and, thus, the extent to which fractionation will be employed. Operators are responsible for

Column Reflux Ratio De-ethanizer 0.9 - 2.0

Depropanizer 1.8 – 3.5

Debutanizer 1.2 – 1.5

Butane Splitter 6.0 – 14.0

Condensate Stabilizer Top Feed


implementing the strategy, ensuring that production targets are met and are the best sources of information when it is necessary to correct problems and expand performance. Section 3 provides strategies and methods to achieve this.


3. Operational Checks, Testing and Adjustments

3.1 Establishing an Energy Reduction Program The process for reducing energy involves all levels of operating staff and is determined by the identification of what constitutes good performance for each class of fractionation column. Figure 3.1 is a flow plan for evaluating a fractionation unit.

Confirm Measurement Accuracy

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Figure 3.1 Flow Plan for Reducing Energy in a Fractionation Unit

The plan is designed for active participation by all personnel – operators, facility engineering and, possibly, outside consultants. Each group has their role to play in ensuring that the energy management program is successful. The primary role of operators is to see that measurement and control equipment – pressure gauges, temperature indicators, flow meters and control valves – are operating accurately and as designed. Operators are also the group most likely to know where problems and bottlenecks occur. The other participants in the program should take advantage of this expertise. Once the identified opportunities have been implemented, operators are responsible to see that the unit is run as intended and energy consumption is as low as possible.


When conducting an energy management study of a fractionation column or unit, the main parameter to track is the absolute amount of energy consumed, such as the fuel consumption. The energy consumed needs to be expressed as a function of the product quantity. An example would be the amount of energy per volume of propane product produced on a depropanizer. It is beneficial to determine the level of fractionation, expressed in terms of the actual recovery compared with the theoretical recovery. The level of fractionation could be in terms of the mass of each component compared with the amount in the feed. An example is the mass of propane recovered as propane product as a fraction of the mass of propane entering the plant versus the amount of energy consumed in the fractionation column/unit. An alternative approach, and one which has a more direct relevance to plant operations, is to assign a value of the incoming feed to the fractionation unit – if no further processing were done, the feed would be sold as sales gas. Next, calculate the value of the actual sales gas, then the value(s) of the heavier product(s). The upgrade in product value is then compared to the value of the energy expended. Ongoing operation of the unit requires the use of current product and energy prices so that the staff can make the proper decisions in order to run the plant in the most financially-effective manner. Prices can fluctuate and it becomes necessary to normalize them in order to study long-term trends. To be truly effective, two parallel parameters, based on the essentially the same calculation procedure, are determined.

• The first uses current prices in order to maintain the economic health of the plant.

• The second uses normalized prices and is basically an energy parameter expressed in economic terms. This parameter should be trended and appropriate actions taken on the basis of the trends. It is suggested that the normalized prices either be those that existed on the base case day (such as an energy audit or production test run) or those prices that are expected to carry on into the future.

In order for these parameters, especially the economic ones, to be worthwhile, it is necessary that the operating staff is aware of the latest product values and that it is possible to determine the value of the energy expended in the fractionation unit. It also presupposes that all product specifications are met.


3.2 Energy Reduction Strategies Energy Reduction in Columns The following generic actions by operators are recommended for minimizing energy consumption on fractionation columns:

• Avoid the tendency to over-fractionate. To ensure that product consistently meets specification, it will be necessary to produce a product purer than specification but not at the expense of excessively-wasted energy for reboil heat input.

• Record operating conditions, especially when there are problems in making product specifications or if there is unusual, unexpected energy consumption. This becomes extremely valuable on three fronts:

• for ongoing day-to-day operation,

• if there are problems and outside assistance is required and,

• if there are plans to expand the fractionation capacity at the plant.

Specific steps to consider include:

• The optimization of feed and reflux rates for vapour/liquid equilibrium -reduce the feed or reflux rates. Many towers are over-refluxed in the belief that this is necessary to ensure products meet specification. Moreover, increasing reflux ratio is the usual response to an off-spec product. By running at a higher-than-required reflux flow on an existing tower, where the diameter is fixed, the liquid traffic down the column may be high enough to cause flooding. In order to maintain product quality, it is typically better to cut the feed rate, not increase the reflux rate. At the same time, there must be sufficient vapour/liquid traffic up and down the column to ensure adequate distribution across the tower.4

• Increasing column pressure. This reduces the actual volume of the vapour rising up the column and therefore also the vapour velocity. Note that this action has a smaller impact on high pressure columns since the proportional change in pressure is less.

• Do not make the pressure changes rapidly. For example, if the pressure is suddenly increased, the boiling point of the bottoms product will increase suddenly also.

• Changes to feed quality. By changing the proportion of vapour in the feed, the internal flows in the rectifying and stripping sections of the tower will be changed.

• Changes to feed location (if that option is possible). This will also help to


optimize column performance by more closely matching the feeding composition to that of the feed stream.

As explained later in this module, resolution of problems and/or optimization of fractionation systems are not always obvious and the input of the experience of the operating staff is vital for a successful outcome. When problems occur, ensure that good documentation is recorded as to how the problem is manifested, what was the situation that led up to the problem and, if available, what remedial steps seem to improve the situation, even if only temporarily. Whether the plant has a single fractionation column, or a series of columns, optimization of energy use is done in a similar manner. Energy Reduction in Condenser/Reflux In Western Canada the overhead condenser is typically an aerial cooler. As such, it consumes no fuel (unless the site generates all or a portion of its power). However, poor condensing will seriously impact the performance of the column.

• If the condenser outlet temperature rises due to a loss of cooling capability it leads to an increase in tower pressure and the relative volatility of the components decreases, leading to poorer separation. Losses in cooling capability may be the result of a number of performance issues with the fan and bundle, these are outlined in Appendix C.

• Poor separation has to be remedied by greater reflux, which means either greater amounts of reboil heat or a shift from overhead product to reflux.

By being a major contributor to the down coming liquid in a distillation tower, reflux is the primary method of influencing the quality of the products from that tower. While it is important to meet the specifications of the overhead product(s), it is also important not to over-fractionate. Generally, there is very little to no value added to the product of a fractionating column for the relatively small changes in purity. Once the decision has been made to achieve a certain product quality, the proper reflux rate must be set in order to implement that decision. In Figure 3.2 the effect of reflux ratio on a depropanizer and on a debutanizer shows how the number of trays must be significantly increased if the same product quality is to be maintained while simultaneously adjusting the reflux.


Figure 3.2

Effect of Reflux Ratio on Theoretical Trays

Note that there are three areas for each curve. In the region of higher reflux ratios, significant changes in the ratio bring about a very small change in the number of trays. At the other extreme (low reflux ratios), a small change in ratio will bring about a significant increase in the number of trays. In between, there is a “knuckle” where the trend change over occurs. Operators need to be aware of this knuckle in order to minimize reflux (in order to minimize fuel consumption) but not throw the product off-specification. For an existing column, this translates to the situation where insufficient reflux rate will mean that the desired fractionation will not occur and/or product specification will not be met. Optimization of reflux ratio is particularly recommended for fractionation towers with normally high reflux ratios. Condenser Energy Reduction Opportunities The condenser directly influences the operating pressure of the fractionation tower. As such, the condenser should be monitored frequently. The following energy management action items are recommended.

• Monitor the temperature approach. Compare the outlet temperature versus the ambient air temperature when the condenser has just been

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cleaned. A widening temperature gap is an indication that the cooling efficiency is declining.

• Ensure that the fan pitch is appropriate. If the fan motor is controlled by a VFD, set the fan blades at the optimum pitch and allow the VFD to adjust the speed and hence the air flow.

• Clean the condenser bundle. It is recommended that aerial coolers be cleaned after significant fouling events (such as poplar fluff) but prior to hot weather so that there is maximum cooling capability over the summer.

• Avoid practices that lead to fin damage. Use of high pressure water spray can lead to fin damage, which reduces the effective area of the condenser.

• Avoid the use of water spray to augment cooling. If absolutely necessary, consider spraying underneath the bundle and have the water drawn up through the bundle. Consult the cooler manufacturer prior to using water spray.

• Check to make sure that hot air from another part of the plant process is not being drawn into the condenser fan. This phenomenon can be demonstrated using smoke bombs or even sensed by positioning oneself around the perimeter of the cooler underneath the bundle. If there is air being drawn in, determine the source of the hot air and investigate ways to resolve the problem. In some plants, the outlet plenum of the offending cooler is extended approximately 6 to 8 feet.

• Check the quality of the products on a regular basis. Product specifications must be met. Aside from those restrictions, relative product prices may open opportunities to shift components from one product to another.

• Check the pressure control setting on the tower overhead. The lower the pressure the better the separation capability but it also lowers the condensing temperature. The situation could occur where the condenser is incapable of fulfilling its role, especially in hot weather. It should be noted that lower pressure also increases the vapour velocity in the tower due to the lower vapour density and the velocity increase may result in operating problems.5

• Check the reflux rate and ratio. Insufficient reflux may not just be due to a poorly performing condenser. It could also be due to a disproportionate liquid product draw from the overhead drum. The rate and the ratio are important parameters for gauging column performance.


Energy Reduction in Reboiler Unlike overhead condensers, reboilers on fractionation towers typically have no means of measuring the flow. In some cases, it would be possible to back calculate the flow from the amount of heat medium consumed. Depending upon the state of the column feed (superheated vapour, saturated vapour, vapour-liquid mixture, saturated liquid, subcooled liquid), the reboiler duty will be close to the condenser duty – sometimes slightly more, sometimes slightly less. Energy Reduction in Feed Quality Even if the composition of the feed to a distillation column remained constant, the quality of that feed could range from superheated vapour to subcooled liquid. Feed quality significantly affects the operation of a fractionation column since it influences the internal upflow of vapour and downflow of liquid. See Appendix B.2 for a more detailed discussion about feed quality. For operators, the important thing to know is that, if the column feed becomes proportionately more vapour, they will generally have to increase the reflux – and thus the reboiler duty - in order to avoid a decrease in product quality. A second option is occasionally available. Some columns, de-ethanizers for example, are equipped with two feed inlets. Therefore, it may be possible to switch the feed point to the tray lower down in the column in order to lengthen the rectifying section.

3.3 Retrofits and New Units There are many options available to the facility engineer when considering either a new fractionation column or a retrofit of an existing one. Vendors and fractionation experts should be consulted for their input on this subject. Presented here are some generic guidelines to help the engineer focus on ways and means to increase energy efficiency:

• When designing a fractionation tower consider the effect of reflux ratio upon energy costs and upon tower design. Higher reflux ratios lead to a reduction in the number of trays required but it also leads to a larger diameter column.6

• Because of the change in internal vapour and liquid flows brought about by the introduction of feed to the column, trays in the rectifying and stripping sections are designed differently. When moving feed points in an existing column, ensure that the new feed point can properly handle the changed flow profile7.


• Investigate the use of more efficient trays/packing. At the same time, ensure that there is adequate gas-liquid mixing in the feed and reflux distributors. More efficient trays/packing allow a tower to be operated at a much lower reflux ratio, which allows extra tower throughput and improved product purity.8

Fractionation involves a considerable amount of heat. One energy management technique for a fractionation train is to consider the use of the overhead condenser of one tower to serve as the reboiler or a preheat of the next column. Sometimes this will require adjustment of the operating pressures or augmentation of the heat from the condenser to bring it up to the needed intensity.9 Heat integration between columns or other process units within the plant has been practiced for many years and offers considerable energy savings. However, there is the risk of too much interdependency of the equipment, especially for non-standard conditions such as startup, reduced throughput, etc. Generally, columns greater than 3 feet in diameter utilize trays. When considering the conversion of an existing column from trays to packing first look at the quality of the existing trays. Make sure that they are suitable for the anticipated service. Perhaps only better trays – not packing – are required. Despite many advances in their design, packing options have several factors to bear in mind:

• It is important to remember that the modeling of packed columns is not as advanced as that for trayed columns.

• The efficiency of structured packing is not as good at high pressures. It is suspected that a cause of this less-than-expected performance is vapour back mixing.

• Proper distribution of the vapour and liquid streams is extremely important if considering packing. Misdistribution will result in efficiency loss. There are great benefits to be had by frequently re-distributing the flows within a packed column.10

Since an operating problem represents a departure from the norm, it is important to have an idea of what constitutes normal operation. In the event of an operating problem - or if the plant is to be debottlenecked - it is very advantageous to have a well-documented test run of each fractionation column. Obviously, the data should be as accurate as possible, with samples taken and analyzed properly and according to protocol. In addition to recording the process conditions (temperature, pressures, flows), and the heat and material balance, record the tower temperature and pressure profiles, control valve positions, controller settings, temperature and pressure performance of the condenser and reboiler.11


3.4 Troubleshooting Troubleshooting Fractionation Columns – Operators Fractionation problems mean lost throughput, downgraded product and wasted energy. In the event that there are fractionation problems at the plant, the operators are generally the first to learn of the problem and to take steps to correct it. The following notes are listed here as suggestions when troubleshooting columns. The items focus on trends that can be analyzed by operators based on the observations of the way the equipment is performing, especially if there is an unexplained change in performance and operating conditions. “Troubleshooting involves systematic elimination of possibilities.”12 “Only through a systematic problem solving effort by a team of operating, maintenance and engineering personnel can the problem be identified and a solution determined.”13 Surveys of tower problems have revealed that instrument and control issues are responsible for about 20% of fractionation problems.14 (In the experience of the author of this module, control algorithms need to be thoroughly checked in order to eliminate control “runaways”.) During tower inspections, make sure trays have been installed/re-installed correctly, no shortcuts have been taken and no internals removed or modified unless so specified. Check trays, downcomers, etc., for accumulated scale, corrosion, broken or lifted trays or packing.15 Loss of fractionation efficiency can be caused by changes in the properties of the tower streams:

• An increase in liquid viscosity will decrease efficiency.

• Low relative volatility increases tower efficiency. That is one reason why the efficiency of the de-ethanizer is less than that of the debutanizer. Relative volatility is an extremely important parameter – serious errors in tray efficiency can occur if an incorrect volatility is used.

• Pressure increases reduce efficiency.16 A major problem is flooding. Considerable information on flooding can be obtained in published literature and from the Internet. The following sub-section specifically deals with this topic. Weeping and entrainment both cause loss of efficiency. Of the two, entrainment is generally the more serious.17


Hydraulic Flooding “Hydraulic flooding occurs when liquid can no longer flow down the column and begins to accumulate excessively.”18 Unless noted otherwise, the following sub-section is taken from Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993. Some of the symptoms of flooding are the following (although flooding is not necessarily their only cause):

• excessive pressure drop,

• erratic pressure drop,

• ΔP for packing in excess of 3 inches of liquid/foot,

• ΔP for trays in excess of one half of the tray spacing,

• fluctuations in bottom level or reduced flow,

• improper temperature profile,

• surges of overhead liquid.

• rapid decrease in separation with small increases in feed or heat input,

• lack of response to control changes. Other problems include the following. Some of these may also indicate hydraulic flooding.

• low ΔP, which may indicate missing trays or packing,

• excessive carryover, caused by improperly-designed liquid inlet or wrong type of distributor or plugging on the top tray.

Suspicion of flooding should trigger a set of questions, which will prove invaluable to the investigating team by providing a sense of direction and reducing the workload.

• What specification is not being met (top, bottom, side product)? Start the investigation there first.

• Are there capacity limitations? What is the actual rate and how does it compare with the original design?

• Is this a recurring problem? If so, what was done to resolve the problem in the past?

• When was the problem first noticed? This sets the time period for the investigation. There is one potential caveat to this point.


• An important parameter is incipient-flood-point (IFP). Most new towers are designed with a margin of safety. A lack of the precise cause and location of flooding in older columns may lead to flooding problems following retrofits. Usually flooding is monitored by measuring pressure drops, temperature profiles and liquid levels. Unfortunately, in a column with a large number of trays or a large height of packing, these changes are often too imprecisely measured to know exactly where the flooding is developing and the IFP has been passed before the problem has been identified.19

• Have there been column upsets that may be a factor? Review the logs and process conditions.

• Are there any changes in lab personnel or sampling techniques? Ensure that proper protocols are being followed.

• Have there been changes to the process upstream (or downstream) equipment or operation? The composition of the feed to the column may have changed to the point that the column is no longer able to properly process it.

• Are outside resources going to be needed? Hydraulics problems are very complex and the use of specialized equipment by experts is often required in order to get a timely and effective resolution of the problem.

With the basic information garnered from the preliminary data collection and questioning, start to verify that information. Before bringing in the sophisticated analytical tools check the simple things. Make sure that instrumentation and measurement devices are working properly. Confirm the liquid levels, tower temperature and pressure profiles. (In a fully flooded column the liquid level in the bottom of the column must be below the reboiler return.)20 Pressure differential is the best indication of flooding. When doing a ΔP survey, make sure that the gauges/readings are accurate. Where possible, use the same test gauge, following calibration, for the different pressure points. This will help to minimize instrument bias. Pressure profile across the column is only possible if there are pressure gauge taps on the column. These should be designed into new columns. Once flooding is suspected, plot tower pressure drop as an aid in identifying the mechanism. Jet flooding and downcomer backup occur slowly and may be difficult to detect. Downcomer choke flood occurs very quickly and is evidenced by a sudden and distinct pressure drop increase across the tower.21 Flooding in the rectifying section can be identified by a loss of top product purity. If the loss of purity occurred at constant reflux and constant reboil heat input, flooding of the rectifying section should be suspected. At the same time, an increase in the amount of reboil heat per barrel of feed would indicate a loss of reboiler efficiency.22


Troubleshooting Fractionation Columns – Facility Engineers The following items are presented to assist facility engineers in evaluating fractionation columns and units. While the operators will supply much of the early data, based on their experience and observations, the facility engineer will generally perform, or arrange for a consultant to perform, a simulation of the tower/unit. Computer simulation is the mainstay of any evaluation of a fractionation tower, whether it be a new or an existing tower. Unfortunately, if not carefully done, the simulation can give very erroneous results leading to a poorly performing column following design and installation. This sub-section deals with potential issues that can arise when taking advantage of this tool. A review of surveys of fractionation problems and their connection to simulations indicated the three most important issues:

• Obtain good vapour-liquid equilibrium (VLE) data. This usually involves components with similar boiling points. Sometimes, small differences in relative volatility can result in large errors in tray efficiency.

• Have the simulation match the plant data. Ensure that gauges, temperature readings, flow meters and laboratory analyses are accurate and that samples have been taken properly. Another area where problems can arise is correctly modeling the feed inlet, especially if the first product drawoff is within a few trays. Does the feed enter the column in the vapour space or onto the tray or downcomer liquid?

• Use graphical techniques to troubleshoot the simulations themselves. Computer simulations do not necessarily show the presence of pinch points (where the operating line is very close to the VLE curve and there is progressively smaller change in composition on each tray as the pinch point is approached). Use of the McCabe-Thiele method can identify pinch points.23

Prior to simulating a tower, ensure the data is good. Once the tower has been simulated, review the results from many aspects. If the project is a tower revamp, ensure that there is a good match between the simulation and actual results. Internal hydraulics are important parameters to watch. Use tools such as McCabe-Thiele plots to cross-check the simulation.24 Despite extremely good precision in calculating the theoretical aspects of distillation, the conversion from theoretical to actual tower must depend upon empirical or semi-empirical correlations for describing column efficiency.25 When using empirical data, consider these sources, listed in order of preference:

• A similar process with similar tower internals. A proviso would be to ensure that the field data are accurate; that the base case tower is


modeled well; and the accuracy of the model is tested, before using the simulation for the new tower.

• Typical efficiencies found in the literature. A drawback to their use is their generic nature and the background to their derivation.

• Scale-up of experimental data for a similar process. The main problem with this method is the risk associated with any scale-up.26

The most widely used and accepted empirical formula for column efficiency is O’Connell’s correlation, which is a function of the relative volatility and feed viscosity.27 Column efficiency, as predicted by O’Connell is28 • Efficiency = 0.492 * (Relative Volatility x Feed Viscosity in centipoises)-0.245

3.5 Facility Engineering Input The foregoing review of energy management in fractionation units has illustrated that there is a wide range of opportunities for savings and/or improved product value. The ability for a plant to take advantage of them falls into three categories:

• immediate, low/no capital investment,

• minor capital investment,

• major capital investment. Experience has shown that roughly half of the savings identified in energy audits are in the first category. A well-run fractionation unit will optimize the balance between energy input and product value output. This requires operator attention therefore this module has been prepared with that goal in mind. On the other hand, capital investment – coupled with good operating practices – will greatly improve the energy efficiency of the unit. While facility engineering should take the lead role, operators can provide valuable input in view of their experiences and observations from running that specific, or a similar, unit. Facility engineering (and if, necessary, outside consulting services) also plays a significant role in monitoring the effectiveness of the fractionating equipment. Using the on-the-spot data collected by the operating staff, the facility engineer can evaluate the “health” of the columns to ensure that there are no problems that impair their effectiveness. This section discusses the role that facility engineering can play in ensuring that the fractionation unit is operated as efficiently as possible.


3.6 Strategic Goals and Operating Parameters In order for the operators to run the fractionation equipment in a cost-effective manner it is helpful for them to be aware of the values of the products. Obviously, the products must meet quality specifications and there is a need to over-fractionate in order to consistently conform to those specs. Still, there may be opportunities to not produce ultra pure streams and to take advantage of the price differentials. Facility Engineering can assist the operating staff by advising them of the relevant product values and, more importantly, of any changes in those values. The product values dictate which products will be made and the desired degree of fractionation that will be required. Especially where there will be a change in operating direction, or a new process is being integrated into the plant, Facility Engineering needs to assist Operations by providing a slate of parameters – temperatures, pressures, reflux ratios, etc. – as a guide for energy-effective operation.

.


Appendix A McCabe-Thiele Method of Analysis

When W.L. McCabe and E.W. Thiele developed their method of analyzing distillation columns in the 1920’s they had added considerably to the ability to design these columns. Since then, other graphical techniques and the tremendous increase in calculating power brought about by computers have lessened the role of the McCabe-Thiele graph in the design of fractionators. Nowadays, its strength lies in its simple, but effective, use as a tool for explaining many of the multi-faceted aspects of distillation. As such, it continues to be an important mainstay of university courses concerning fractionation and still appears in technical articles as a device to illustrate the underlying concepts being described. As found in a 1985 article,

“The point is not to reinstate antique design methods, but rather to use their visual representations to get the most out of modern computerized methods.”29

As outlined in Section 4.3, the method can be used to crosscheck computer simulation results. The preparation of a McCabe-Thiele diagram lies outside of the scope of this document. However, there is great benefit for the reader to understand the various components within the diagram. A.1 Underlying Philosophy The McCabe-Thiele graphical method was designed to determine the number of theoretical trays, or stages, in the column. It presumed a binary feed – two components. In a gas plant, for example, that presumption is obviously wrong. However, reasonably good results can be obtained if the feed to a distillation column is considered to consist of two compounds: the light key and the heavy key. The identification of the light and heavy keys depends upon the column’s service. In a butane splitter, the aim is to separate the lighter isobutane from the heavier normal-butane. To “convert” the real feed into a binary mixture, it is assumed that the light key is the isobutane and that when the isobutane rises up the column so does all of any lighter components such as propane and ethane. Similarly, normal-butane would be the heavy key and as it dropped down to the bottom of the column so would any heavier components such as the pentanes and hexane. Early designers of distillation columns were faced with the difficulty of solving simultaneous complex equations. To simplify the calculations, it was assumed


that the mols30 of vapour rising up the column remained constant31. This was known as constant molal overflow. Another way of saying it is that, if the number of mols rising up the column is constant, every time an additional mol of light material joins the upward flow an additional mol of heavy material joins the downward flow. Subsequent work in the field of distillation indicated that this assumption essentially holds reasonably true in most cases. In one demethanizer study, there was as much as 20% in variation in the ratio of liquid and vapour molal flows. This is due to the fact that, at any given theoretical tray in the column, the amount of heat released as a mol of vapour is condensed does not always equal the amount of heat required to vaporize the corresponding mol of liquid. Nevertheless, analysis of the column using the McCabe-Thiele method was not seriously affected.32 Unlike the more rigorous Ponchon-Savarit graphical method, the McCabe-Thiele analysis does not need, nor consider, the enthalpy of the streams within the tower. However, the Ponchon-Savarit method requires good enthalpy data in order to supply truly effective heat flow estimates. Unfortunately, this is often not possible in the real world, so the simplicity of the McCabe-Thiele method more than makes up for the inability to predict the thermal flows. A.2 The Diagram Components Figure A.1 shows a hypothetical McCabe-Thiele diagram. Brief descriptions of the lines on the diagram follow. Equilibrium Curve This is shown by the blue dots and heavy line. For the binary mixture (or the multi-component mixture that has been converted to a binary mixture) there will be a curve where it is possible to know the composition of the liquid and the corresponding vapour. For example, if the composition of the liquid is 50 mol% light key, the vapour that is in equilibrium with that liquid would be 88 mol%. The equilibrium line should be drawn for the entire range of compositions, ranging from 0% light key to 100% light key. The values for the curve can be determined experimentally or, more commonly with the aid of a series of computer-generated flash calculations. 45° Line and Associated Points The light pink line is the 45° line where the composition in the liquid equals the composition in the vapour. Along this diagonal there are three points, indicated by brown diamonds. At the left end of the diagonal is the bottom specification for the light key. In this case, it is 2 mol%. The large diamond in the middle of the line is the feed composition for the light key. At the right end of the diagonal is the amount of light key in the overhead product. Since the composition of the


feed is given, and the composition for the bottom product is specified as a target, the composition of the overhead product is found by a component balance. (If the overhead product had a light key specification, the composition for the bottom product would be found by a component balance.) Operating Lines The concept of constant molal overflow means that the material balance equation from tray to tray is a straight line, which is called the operating line33. There are two operating lines. The top section of the column, above the feed tray, is called the rectifying section (in some texts, the enriching section). The portion of the column below the feed tray is the stripping section. Experience has shown that most operating lines are straight but for de-ethanizers and in one study of a depropanizer, the operating line was curved. Nevertheless, the concepts discussed here still apply. The slope of the rectifying section operating line is a function of the reflux rate back into the tower. The slope of the stripping section operating line is the ratio of the mols of liquid entering the stripping section divided by the mols of vapour leaving the stripping section. Usually these internal flows are not known. Feed Point. At the feed point, the tray-to-tray material balance moves from the rectifying section operating line to the stripping section operating line. Graphically, this is where the two operating lines intersect. The feed tray can be anywhere between the points “a” and “b” on the equilibrium curve, but the closer to those points (i.e., “a” or “b”) the greater the number of trays that are required. The optimum feed point is where the minimum number of trays is achieved.34


Hypothetical McCabe-Thiele Diagram

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Figure A.1 Hypothetical McCabe-Thiele Diagram

The Feed (q) Line. The heavy brown line is the feed line. It is commonly called the q-line in most textbooks. The line passes through the overall feed composition (the large diamond on the 45° diagonal and through the intersection of the two operating lines. The slope of the q-line is important for it is an indication of the quality of the feed entering the column. Figure A.2 summarizes this information35. For the column represented in Figure A.1, the q line corresponds to a saturated vapour. It should be noted that if any two of the rectifying section operating line, the stripping section operating line and the feed (q) line are known, the third is automatically known.


q-Line Slope as Function of Feed Quality

Light Key in Liquid, mol%

Ligh

t Key

in V

apou

r, m

ol%

45° Line

Subcooled Liquid

Superheated Vapour

Saturated Vapour

Liquid and Vapour

Saturated Liquid

Equilibrium Line

Figure A.2 q-Line Slope as Function of Feed Quality

Of interest is the line where there is both liquid and vapour entering the column. The slope of the q line is q / (q-1), where q is the fraction of liquid in the feed. When there is no liquid in the feed, such as in Figure A.1, the slope of the feed line is 0/(0-1)=0 (i.e., a horizontal line). When the feed is saturated liquid, the slope is 1/(1-1)=infinity (a vertical line). The slope of the line is indicative of the molal ratio of liquid and vapour in the feed. Moreover, the q line, where it intersects the equilibrium line provides the light key composition of the liquid portion of the feed and the light key composition of the vapour portion of the feed. Number of Theoretical Trays. The step-like line connecting the equilibrium line and the operating lines indicates the number of theoretical trays or stages in the column. In this case, there are roughly 8.5 theoretical stages before the specified bottom product quality is achieved. By dividing the number of theoretical trays by the tray efficiency, the number of actual trays can be calculated. Some designers start counting trays from the tower top and some from the tower bottom. In this document, the tray count is from the top down. If the column contains packing, the number of theoretical trays can be converted into the depth of packing by the conversion factor HTU (height of a transfer unit) times the NTU (the number of transfer units), which for packing is the equivalent of a theoretical tray.


A.3 Condenser and Reboiler Aside from the heat inherent in the incoming feed, energy is supplied via the reboiler and removed via the condenser. The configuration of these column adjuncts modifies the approach to be taken when doing a McCabe-Thiele analysis, although the basic concepts remain unchanged. Condenser The condenser can be a total condenser, such as in a depropanizer or debutanizer, where only liquid is produced either as product or as reflux. Or, it can be a partial condenser, such as on a de-ethanizer, where gas is produced in addition to liquid for reflux. If the condenser is a total condenser, the step-off of the theoretical trays (the step-function in Figure A.1) is the concentration of the light key in the overhead product. The first theoretical tray in the diagram represents the top plate of the column, since the composition of the overhead vapour, the reflux back to the column and the liquid product are all the same.36 If the condenser is a partial condenser, and the condenser is in vapour-liquid equilibrium, the step-off for the theoretical trays starts at the concentration of the light key in the gas product. Theoretical tray #1 represents the partial condenser and theoretical tray #2 represents the top plate of the column. In summary, the presence of the partial condenser adds another theoretical tray to the column.37 In computer simulations, the condenser is often referred to as Tray 0 but it is actually the first theoretical tray. As an example, assume that a fractionation column has 30 actual trays with an efficiency of 60%. If the column was equipped with a total condenser, there would be 30*0.60=18 theoretical trays. If, however, the column had a partial condenser in vapour-liquid equilibrium, there will be 30*0.60+1=19 theoretical trays. (Note the term “in vapour-liquid equilibrium’ was used when referring to a partial condenser. If the cooling in the condenser is very rapid, the vapour and liquid may not reach equilibrium and there will be reduced difference in the compositions, i.e., there won’t be a complete theoretical tray. When designing a tower with a partial condenser, it is prudent to add an extra theoretical tray in the body of the column, rather than hope that the partial condenser acts as one theoretical tray.38 If there is a partial condenser, the liquid is saturated. However, the condenser is a total condenser; the possibility exists that the liquid, and the reflux, will be sub-cooled. In that case, additional vapour will be condensed on the top tray by the cold reflux and increase the internal reflux. This is equivalent to increasing the slope of the rectifying section operating line.


Reboiler For the common type of reboiler, the bottom product is drawn off the reboiler. This means that the liquid bottom product is in equilibrium with the vapour returning from the reboiler to the bottom plate in the column. In other words, the reboiler acts as an additional theoretical tray. A.4 Column Configuration Distillation columns can have quite complex configurations, such as being potentially equipped with multiple feeds, intermediate streams where heat is removed from the column or heat is added. These variations in design result in different operating lines but the same concepts apply. Figure A.3 shows how multiple streams are represented on a McCabe-Thiele diagram and Figure A.4 show an intermediate reboiler and intermediate condenser. For gas plants, these variations in tower configuration are generally not installed. An exception would be demethanizers, which are relatively uncommon in the industry. In Figure A.3, there are two feeds – on theoretical tray 3 and on theoretical tray 5. The addition of the second feed (on tray 5) results in the creation of an intermediate section operating line between the two feeds. With one feed (on tray 3) only 6 theoretical trays are required, versus 7 trays when there are two feeds. Counterbalancing the extra tray is the better fractionation that is achieved when each feed is added to its optimum tray, rather than being mixed prior to entering the column.39 As can be seen in Figure A.4, there is an increase in the number of theoretical trays when intermediate reboil and condensing facilities are installed on a column (7 theoretical trays versus 6).40 However, as discussed in Section 3.3, use of an intermediate reboiler and intermediate condenser increases the efficiency of the trays by reducing the amount of “lost work”.



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Figure A.3

Multiple Feeds to a Fractionation Column



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Figure A.4 Intermediate Reboil and Condensing on a Fractionation Column


Appendix B Using the McCabe-Thiele Diagram for Column Analysis

This appendix looks at three of the common issues facing operators of fractionation units – reflux ratio, feed quality and non-ideality. The McCabe-Thiele diagram can provide very useful guidance in these matters. While quantitative answers can be obtained using the diagrams, the discussion will focus on the qualitative aspects. It is unlikely that operating staff will be involved directly in the detailed design of fractionation units. However, because of their experience with such columns, they can provide extremely valuable input to the designers. An understanding of the fundamentals of fractionation is therefore of benefit. B.1 Reflux Ratio Figure B.1 is the same as Figure A.1 with the labels removed. In this column, the feed is a saturated vapour and there are 9 theoretical trays. The feed is on theoretical tray #5. This means that there are 4 theoretical trays in the stripping section. The primary specification is to maintain a low concentration of the light key material in the bottoms product.


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Feed at Tray 5

4 Trays in Stripping Section

Figure B.1

Original Column Conditions

Figure B.2 shows what happens when the reflux rate is reduced. That is graphed by pivoting the rectifying section operating line upward (toward the equilibrium


line) while holding the overhead composition constant. Not shown here is the fact that, with the feed still a saturated vapour, 12 theoretical trays were required. (Many of those trays were around the feed tray where very little change in composition was obtained on each tray.) Since the column is existing and has the equivalent of only 9 theoretical trays, the column will not meet the product specifications with reduced reflux. There are two options. The most obvious is to return the reflux rate to the original value. The second is to introduce more liquid into the column by changing the feed to a mixture of liquid and vapour, which appears on the graph as a line sloping up and to the left from the feed composition on the 45° line (the large diamond). This would require cooling the feed. Note that only 8 theoretical trays are required. However, since the specification is centred on the bottoms product, it is necessary to maintain at least 4 theoretical trays in the stripping section.


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Figure B.2

Column Conditions with Reduced Reflux

In the majority of plants, the first option is probably the only one available to the operators, assuming that there is no impediment to restoring the reflux rate to its original value. Consider the original column and saturated vapour as feed. This time, however, the reflux ratio is increased. Figure B.3 illustrates this. The rectifying section


operating line has been pivoted downward. Since the feed is still a saturated vapour it stays as is and the stripping section operating line shifts downward to intersect the other two lines. In this case, only 6 theoretical trays are required, with 3 trays in the rectifying section and 3 in the stripping.


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Figure B.3

Column Conditions with Increased Reflux As the reflux is increased the number of theoretical trays decreases until at infinite reflux ratio the minimum number of trays is encountered. Infinite reflux is shown graphically by drawing the operating lines along the 45° line. For the ongoing example, the minimum number of theoretical trays is 4. See the heavy stepped line in Figure B.4. Since infinite reflux ratio means that there is no product, this scenario is both impossible and infeasible. However, it does set a limit on the minimum number of trays that the column must have. At the other end of the spectrum is the minimum reflux case. In Figure B.4 this is shown by the dashed line. The minimum reflux line is found where the q line intersects the equilibrium line, which means that the two operating lines intersect on the equilibrium line. At minimum reflux, the fractionation will run into a pinch point around the feed tray and an infinite number of trays are required to get beyond one operating line and on to the other. (If the operating lines intersected above the equilibrium line it would be theoretically impossible, even with an infinite number of trays, to fractionate the feed beyond the concentration where the operating line crossed the equilibrium curve.)


While the minimum reflux case is a nonsensical situation, since it requires an infinitely tall column, the concept of minimum reflux provides an important operating parameter. The optimal reflux ratio for many towers has been found to be 1.2 to 1.3 times the minimum reflux ratio.41


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Figure B.4 Column Conditions with Infinite and Minimum Reflux

B.2 Feed Quality Figure A.2 shows how the slope of the q-line can be used to determine the feed quality – i.e., how much is liquid and how much is vapour, etc. As will be shown in this sub-section, feed quality significantly affects the number of theoretical trays. In the following examples, unless explicitly noted, the reflux ratio is constant. In Figure B.3 the feed was saturated vapour (a horizontal q-line). A total of 6 theoretical trays were required, with 3 in the rectifying section and 3 in the stripping section. Figure B.5 shows the impact of having the feed as saturated liquid. While there are still 3 trays in the stripping section, there are only 2 in the rectifying section. If the feed were a mixture of vapour and liquid, the impact would be dependent upon the vapour-liquid split. Interpolating between Figures B.3 and B.5, there


would be between 5 and 6 theoretical trays. The greater the amount of liquid, the closer to 5 theoretical trays.


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Figure B.5

Column Conditions with Saturated Liquid Feed

Figure B.6 shows what happens when the feed is further cooled and is now subcooled. Basically, it appears as if there is no change – the feed is still on Tray 2 and there are 3 trays in the stripping section. However, there has been an improvement in the fractionation. Notice that the amount of light key in the bottoms product has “decreased”. For that to happen in an actual column, it would mean that both the top and bottom products became purer – the number of theoretical trays has dropped from 5.0 to about 4.9. Obviously, if this were a column that ordinarily requires 30 or more trays, there would be a discreet change in the number of trays. Whether there is any advantage to having purer products would depend upon the relative value of the top and bottom products. Section 3.2 of the module discusses this issue.



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Figure B.6

Column Conditions with Subcooled Feed

Figure B.7 shows what happens when there is a superheated feed. In this situation, there is not only no liquid to contributed to the internal reflux in the column, but there is additional heat that reduces the internal reflux. As a result, there is an increase in the number of trays in the rectifying section. The reader will see that, if the feed were even more superheated, the q-line would slope even further down, the intersection of the q-line with the rectifying section operating line would be closer to the equilibrium line and there would a considerable increase in the number of theoretical trays. If the degree of superheat were too great, the point of intersection would be to the left of the equilibrium curve and it would be impossible to achieve the fractionation desired without a significant increase in the reflux.



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Figure B.7

Column Conditions with Superheated Feed

Assume that the feed quality is being changed for an existing column. In other words, there is a fixed number of theoretical and hence actual trays. In this example, the column as shown in Figure B.3 has 6 trays (with the feed on tray 3). In order to get back to 6 trays when there is a superheated stream as shown in Figure B.7, it will be necessary to increase the reflux ratio. This is shown in Figure B.8. In this case, it was necessary to increase the reflux by nearly 20%. (This is shown by the change in the slope of the rectifying section operating line.) In summary, for an increasing portion of vapour in the incoming feed, there will be an increasing number of theoretical trays required to maintain product quality (or, for existing columns, there will be a loss of product quality). If the feed is superheated vapour, the increase in theoretical trays becomes much more significant and it will most likely be necessary to increase the reflux, which means an increase in the reboiler duty. Note also, that the increase in the number of trays is in the rectifying section of the column.



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Figure B.8

Column Conditions with Superheated Feed – 6 Trays

B.3 Non-Ideality An underlying assumption of the McCabe-Thiele method was constant molal overflow. An important part of this assumption is that the amount of heat given off when a mol of vapour condenses on a tray equals the amount of heat required to vaporize a mol of liquid on that tray. These two assumptions combine to result in straight operating lines. As shown in Table B.1, the assumption of constant molal heat of vaporization (also known as Trouton’s Rule) is reasonably valid when dealing with propane and heavier. However, the rule is invalid when dealing with methane, ethane and propane.42 By inference, if constant molal heat of vaporization results in straight operating lines, non-constant heats would result in a curved operating line. This would change the McCabe-Thiele diagram to resemble that shown in Figure B.9. Compare this figure with Figure 5. Note how the imposition of curved operating lines results in additional trays being required, in this case an extra 4 trays.


Table B.1 Molal Heats of Vaporization of Light Hydrocarbons

Compound Molal Heat of Vaporization, BTU/lb-mol

Methane 1989

Ethane 3667

Propane 4145

isoButane 4011

n-Butane 4243

isoPentane 4041

n-Pentane 4113


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Figure B.9

Effect of Non-Ideal Operating Lines on Number of Trays


Appendix C Additional Information

Types of Column Internals Fractionation towers achieve the desired separation using either trays or packing. They are discussed briefly here. The reader is referred to Chapter 19 of the GPSA Engineering Data Book for greater detail, especially for drawings of the equipment, showing how they work and how they are installed within the column. There are three main types of trays:

• Bubble cap - This type once dominated the industry but has since been largely replaced, because of their high labour cost, their weight and their pressure drop.

• Sieve - This consists of a plate with holes cut into it.

• Valve - Somewhat similar to the sieve tray, the “valves” sit over the holes and the upcoming gas pushes the valves up so that the gas can bubble through the liquid sitting on the plate.

Valve trays offer a greater operating range. The movable valves minimize weeping of liquid down to the next tray when there are low vapour rates and deflect the entrainment of liquid in the upcoming vapour when there are high vapour rates. Packing consists of two main types: random and structured. Random packing includes: berl saddles, pall rings, raschig rings, etc. Structured packing, consisting of knitted mesh or corrugated sheets, are now common. De-ethanizer When the tower is a stripper, the only source of liquid reflux in the tower is from the liquid portion of the feed. That liquid, as it falls down the column, is then heated by a reboiler in order to strip out any downcoming ethane. Moreover, the separation of the feed into gas and liquid, when it enters the tower, leads to a considerable portion of propane and butane going overhead and exiting the fractionation process. It is important to note that any portion of the feed that enters the column as vapour is automatically lost from the NGL. The result is that the separation will typically be poor, with a great deal of propane and butane going overhead into the gas in an effort to ensure the ethane specification in the bottoms product is met. Since the propane and butane have much greater value as NGL (and even more if subsequently split into separate propane and butane streams) than as natural gas, there is an


economic incentive to maximize propane and butane retention in the NGL (bottoms) product. See Section 3.2 for specific examples. The overhead gas/bottoms NGL split would be improved by the use of a condenser and introducing the feed further down in the tower so that vapour rising from the feed point is counter-contacted by downcoming liquid reflux. Since de-ethanizers deal with the lightest components that are to be fractionated in a typical gas plant, they are often forced to run at conditions that are not optimal. Distillation is easier to undertake at lower pressures, where the relative volatility is better. However, lower pressures also mean lower condensing temperatures. Due to the nature of the components of the de-ethanizer feed, sub-zero condensing temperatures are required unless the pressure is kept in the region of 2500 kPag or higher. (A de-ethanizer that is a stripper can operate at lower pressures because a condenser for generating reflux is not required. The trade-off is reduced separation capability.) Higher column pressures change the equilibrium curve of the de-ethanizer feed and increase the number of theoretical trays that are required. Some de-ethanizers are equipped with two feed trays so that operators have some flexibility in improving fractionation. Figure 3.4 plots the vapour-liquid equilibrium curves for two de-ethanizer feeds at various pressures. In this case, the de-ethanizers receive liquid feed from low temperature separators. (De-ethanizers can also receive feed from the bottoms of a demethanizer. The latter case means that the feed has considerably less methane.) Two feeds are represented in Figure C.1. Those at 65, 200, 325 and 450 psia are the same feed. The ethane and lighter content of that stream was 51.1 mol%. The stream at 433 psia is also from a refrigeration unit. The ethane and lighter content of that stream was 43.5 mol%. The trend is that the equilibrium curve becomes flatter as the pressure increases and as the richness of the stream (propane and heavier) increases. The flatness of the curve increases the number of theoretical trays for the same degree of fractionation. Figure C.2 plots the tray-by-tray simulation results for two fractionation columns – a deethanizer and its downstream debutanizer. Note how the two columns have very different profiles. The debutanizer is shown by the two upper lines. As expected by a conventional McCabe-Thiele diagram, the concentration of the light key (ethane and lighter) declines as one moves down the column (tray 0 is the condenser and tray 27 on the debutanizer is the reboiler).


Vapour-Liquid Equilbrium Curves for De-ethanizer Feed

0

10

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40

50

60

70

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100

0 10 20 30 40 50 60 70 80 90 100

C2+Lighter in Liquid, mol%

C2+

Ligh

ter i

n Va

pour

, mol

%

65 psia200 psia325 psia450 psia45° Line433 psia

Figure C.1

Vapour-Liquid Equilibrium Curves for De-ethanizer Feed

If one were to perform a McCabe-Thiele analysis of the de-ethanizer assuming straight operating lines, the number of theoretical trays would be considerably less than the 21 trays in the simulation (again, tray 0 is the condenser and tray 21 on the de-ethanizer is the reboiler. It should be noted that this de-ethanizer was extremely lean with respect to propane and heavier. As discussed in Appendix B, the molal heats of vaporization of the major components in de-ethanizer feed show a wide difference, particularly between ethane and propane. This means that the operating lines are not straight lines but rather curved. As such, the tower will likely experience pinch points where there is very little change in the composition of the vapour as it moves up the tower and of the liquid as it moves down. This is illustrated in Figure C.2. The most significant change in the profile is at tray 2, which is the feed tray. After that point, there are 9 theoretical trays where there is very little change in composition. In fact, the liquid composition actually increases over some of these trays. It is speculated that the pinch points in the operating lines discussed in the previous paragraph, combined with the volume of cold liquid from the feed becoming part of the internal liquid traffic, are the cause of the observed profile.


Light Key Concentration on Each TrayDe-ethanizer and Debutanizer

0

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90

100

0 3 6 9 12 15 18 21 24 27

Theoretical Tray Number

Mol

% L

ight

Key

De-ethanizer

Debutanizer

Liquid

Liquid

Vapour

Vapour

Includes Condensers (Tray 0) and Reboilers (Tray 21 on

DeC2 and Tray 27 on DeC4)

Figure C.2 Light Key Concentration on Each Tray

De-ethanizer and Debutanizer In summary, de-ethanizers do not operate in a similar manner as the other fractionation towers found in the majority of gas plants because of the extremely volatile nature of the components, compounded by the uneven molal heats of vaporization. As a result, de-ethanizers require a great number of trays. The number of actual trays in a de-ethanizer is typically 1.33-1.67 times the number of theoretical trays (efficiency of 60-75%).43 Moreover, due to the curved nature of the operating lines, any reduction in the reflux ratio will bring the tower that much closer to a pinch point, with a consequent loss of separation capability.


Appendix C References

The following documents were referenced while preparing this Best Management Practice. http://www.patchingassociates.com/lan/newsletter_10.htm, Patching Associates Newsletter #10, March 1999 GPSA Engineering Data Book, 11th Edition, Chapters 19; Gas Processors Suppliers Association, Tulsa, OK; 1998 Momentum, Heat, and Mass Transfer; by C.O. Bennett and J.E. Meyers; McGraw-Hill Book Company, Inc.; 1962; pages 617-634 http://services.shell.ca/posted prices/pb_prices.jsp Online Calgary Herald http://kolmetz.com/pdf/NovaDepropanizerPaper.pdf; Properly Designed High Performance Trays Increase Column Efficiency and Capacity; by D.R. Summers, K.G. Moore, R. Maisonneuve; presented at the AIChE Spring National Meeting, March 12, 2002 http//72.14.253.104/search?q=cache:PbfQ31Aobn0J:www2.tku.edu.tw/~tkjse/4-2/4-2-4.pdf+debutanizer+%22reflux+ratio%22…; Case Studies on Optimum Reflux Ratio of Distillation Towers in Petroleum Refining Processes; by H-J Chen and Y-C Lin; Tamkang Journal of Science and Engineering, vol 4, No. 2 (2001), pages 105-110 Graphical Techniques for Process Engineering; by J.E. Johnson and D.J. Morgan; Chemical Engineering, July 8, 1985, page 72. Chemical Engineers’ Handbook; edited by R.H. Perry et al; 4th Edition, 1963; McGraw-Hill Book Company; page 13-22 Perry’s Chemical Engineers’ Handbook, 7th Ed’n; R.H. Perry, D.W. Green, editors; McGraw-Hill; 1997, page 13-34 McCabe and Smith; W.L. McCabe, J.C. Smith; Unit Operations of Chemical Engineering, 2nd Ed’n; McGraw-Hill Book Company; 1967; page 564 R.E. Treybal; Mass Transfer Operations, 3rd Ed’n; McGraw-Hill Book Company; 1980; page 418


Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53 Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993 Further Advances in Light Hydrocarbon Fractionation; by W. de Villiers, P. Wilkinson, D. Summers; PTQ Summer 2004; www.eptq.com Can We Believe the Simulation Results?; by H.Z. Kister; www.cepmagazine.org; October 2002.( downloaded from http://people.clarkson.edu/~wilcox/Design/distmodl.pdf Rethink Column Internals for Improved Product Separation; by D.L. Love, G. Shiveler, D. Pierce; Hydrocarbon Processing; May 2007; pages 97-105 Ultra-Frac® Technology; product brochure by Koch-Glitsch Revamp, Troubleshooting Optimize NGL Depropanizer Operations; by D.W. Hanson, I. Buttridge; Oil & Gas Journal; August 25, 2003; pages 88-99 Distillation Tower Flooding – More Complex Than You Think; http://goliath.ecnext.com/coms2/summary_0199-1789062_ITM Column Efficiency – What to Expect and Why; by M. Pilling; prepared for the 4th Topical Conference on Separations Science and Technology, Session T1006 – Distillation Hardware and Application I; November, 1999. Control Optimization Saves Money in Distillation; by P. Buzzetta, J. Hall, R. Heider; Hydrocarbon Processing; June 2007, page 58.


Endnotes

1 GPSA Engineering Data Book, 11th Ed’n; Fig. 19-2. 2 Rethink Column Internals for Improved Product Separation; by D.L. Love, G. Shiveler, D. Pierce; Hydrocarbon Processing; May 2007; page 102. 3 GPSA Engineering Data Book, 11th Ed’n; Fig. 19-19. 4 Rethink Column Internals for Improved Product Separation; by D.L. Love, G. Shiveler, D. Pierce; Hydrocarbon Processing; May 2007; pages 97-105. 5 Further Advances in Light Hydrocarbon Fractionation; by W. de Villiers, P. Wilkinson, D. Summers; PTQ Summer 2004; www.eptq.com. 6 pdf+debutanizer+%22reflux+ratio%22…; Case Studies on Optimum Reflux Ratio of Distillation Towers in Petroleum Refining Processes; by H-J Chen and Y-C Lin; Tamkang Journal of Science and Engineering, vol 4, No. 2 (2001), pages 105-110. 7 Revamp, Troubleshooting Optimize NGL Depropanizer Operations; by D.W. Hanson, I. Buttridge; Oil & Gas Journal; August 25, 2003; pages 88-99. 8 http://kolmetz.com/pdf/NovaDepropanizerPaper.pdf; Properly Designed High Performance Trays Increase Column Efficiency and Capacity; by D.R. Summers, K.G. Moore, R. Maisonneuve; presented at the AIChE Spring National Meeting, March 12, 2002. 9Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53. 10 Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53. 11 Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993. 12 Revamp, Troubleshooting Optimize NGL Depropanizer Operations; by D.W. Hanson, I. Buttridge; Oil & Gas Journal; Aug. 25, 2003; page 88-99. 13 Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993. 14 Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53. 15 Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993. 16Column Efficiency – What to Expect and Why; by M. Pilling; prepared for the 4th Topical Conference on Separations Science and Technology, Session T1006 – Distillation Hardware and Application I; November, 1999. 17 Column Efficiency – What to Expect and Why; by M. Pilling; prepared for the 4th Topical Conference on Separations Science and Technology, Session T1006 – Distillation Hardware and Application I; November, 1999. 18 Troubleshooting Distillation Columns by J.J. France, presented in a paper at the AIChE meeting in Houston in March 1993. 19 Distillation Tower Flooding – More Complex Than You Think; http://goliath.ecnext.com/coms2/summary_0199-1789062_ITM 20 Distillation Tower Flooding – More Complex Than You Think; http://goliath.ecnext.com/coms2/summary_0199-1789062_ITM 21 Revamp, Troubleshooting Optimize NGL Depropanizer Operations; by D.W. Hanson, I. Buttridge; Oil & Gas Journal; Aug. 25, 2003; page 88-99. 22 Revamp, Troubleshooting Optimize NGL Depropanizer Operations; by D.W. Hanson, I. Buttridge; Oil & Gas Journal; Aug. 25, 2003; page 88-99. 23 Can We Believe the Simulation Results?; by H.Z. Kister; www.cepmagazine.org; October 2002.( downloaded from http://people.clarkson.edu/~wilcox/Design/distmodl.pdf.


24 Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53. 25 Distillation: Still Towering Over Other Options; by J.G. Kunesh, H.Z. Kister, M.J. Lockett, J.R. Fair; Chemical Engineering Progress; October 1995; pages 43-53. 26 Column Efficiency – What to Expect and Why; by M. Pilling; prepared for the 4th Topical Conference on Separations Science and Technology, Session T1006 – Distillation Hardware and Application I; November, 1999 27 Column Efficiency – What to Expect and Why 28 Ultra-Frac® Technology; product brochure by Koch-Glitsch 29 Graphical Techniques for Process Engineering; by J.E. Johnson and D.J. Morgan; Chemical Engineering, July 8, 1985, page 72. 30 A mol of a component is the amount equal to the molecular weight of that component. A kilogram-mol of methane, for example, weighs 16.04 kg. A pound-mol of methane weighs 16.04 lb. Distillation works on the basis of the mols of each component, not mass units. 31 Momentum, Heat, and Mass Transfer; by C.O. Bennett and J.E. Meyers; McGraw-Hill Book Company, Inc.; 1962; page 617. 32 Graphical Techniques for Process Engineering; by J.E. Johnson and D.J. Morgan; Chemical Engineering, July 8, 1985, page 78. 33 Chemical Engineers’ Handbook; edited by R.H. Perry et al; 4th Edition, 1963; McGraw-Hill Book Company; page 13-22. 34 McCabe and Smith; W.L. McCabe, J.C. Smith; Unit Operations of Chemical Engineering, 2nd Ed’n; McGraw-Hill Book Company; 1967; page 564. 35 Momentum, Heat, and Mass Transfer; by C.O. Bennett and J.E. Meyers; McGraw-Hill Book Company, Inc.; 1962; page 626 36 W.L. McCabe, J.C. Smith; Unit Operations of Chemical Engineering, 2nd Ed’n; McGraw-Hill Book Company; 1967; page 557. 37 W.L. McCabe, J.C. Smith; Unit Operations of Chemical Engineering, 2nd Ed’n; McGraw-Hill Book Company; 1967; page 558. 38 R.E. Treybal; Mass Transfer Operations, 3rd Ed’n; McGraw-Hill Book Company; 1980; page 418. 39 Chemical Engineers’ Handbook; edited by R.H. Perry et al; 4th Edition, 1963; McGraw-Hill Book Company; page 13-26. 40 Adapted from Fig. 13-38 of Perry’s Chemical Engineers’ Handbook, 7th Ed’n; R.H. Perry, D.W. Green, editors; McGraw-Hill; 1997, page 13-34. 41 GPSA Engineering Data Book, 11th Edition; page 19-6 42 From charts in GPSA Engineering Data Book, 11th Ed’n; Chapter 24. 43 GPSA Engineering Data Book, 11th Ed’n; Fig. 19-19. This source has also been used for the efficiency of the depropanizer, debutanizer, butane splitter and condensate stabilizer.

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