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GPA USA

March 2010

USE OF GPA RESEARCH PROGRAM

DATA/SOFTWARE FOR TROUBLE SHOOTING

AND SOLVING OPERATIONAL ISSUES

DURING COMMISSIONING OF ETHANE

RECOVERY UNIT AT KNPC MAA

March 2010

89 th Annual GPA Convention

Kuwait National Petroleum Company



GPA USAMarch 2010 2

MAA ETHANE RECOVERY UNIT

The plant, wholly owned and operated by KNPC Ltd.,

Designed by M/s FLUOR

Constructed by M/s HYUNDAI Corporation,

Licensed by M/s Ortloff Houston, Plant has been commissioned Dec-2008.



GPA USAMarch 2010 3


Feed gas originates from the existing lean oil plant.

The plant is designed to process 1140 (MMscfd ) of

feed gas from Lean Oil Plant (LOP) which contains

around 9% of ethane and 1% propane (max.)

without any other heavier HC. However someequilibrium amount of lean oil is present in the

feed.



GPA USAMarch 2010 4


Plant was operating normal at around 70% capacity,

however whenever Ethane recovery level is

increased or column operating at the design level,

the ovhd system gets cooled as a result of flooding

in the top section of the column, which leads tounstable operations.



GPA USAMarch 2010 5




GPA USAMarch 2010

Ethane Product

Feed

Residue Gas

Recycle Split Vapor (RSV)




GPA USAMarch 2010

Gas Turbines

NGL Extraction 1&2Dehydration



GPA USAMarch 2010 8


Plant was pre-commissioned and as per advise of

Ortloff extensive dry out was carried out. All the

construction debris were removed during pre-

commissioning. Initial startup was very smooth.

Flange leaks were arrested during the cool downand after that unit operations were smooth. Plant

was operating normal around 70% capacity and

column pressure drop was higher compared to

design.



GPA USAMarch 2010 9


However, the equipment vendor has agreed to

operate at these levels. When the feed rate was

increased to design levels, the entire NGL section

was getting cooled as a result expander suction

KOD temperature was coming down and expander speed was getting reduced. Due to this the unit

went into unstable operation and cycling. Pressure

drop across the beds went up to more than 1000 mm

H2O as compared to the design of 50 mm H2O. It was concluded that the Column was flooding and

liquid was entering the cold box from overhead,

which caused the temperature reduction in

expander suction KOD and which was the main

reason for unit going into unstable operations.



GPA USAMarch 2010 10


The flooding may be due to any one the

following:

Freeze due to moisture slip from driers Other impurities

Construction debris

CO2 Freeze in the top of the tower

Benzene freeze or Organic material freeze.

Wrong design of column internal

Change in the mode of operations.




FREEZE DUE TO MOISTURE SLIP FROM

DRIERS

Feed to this plant comes from the Lean Oil

Plant which has driers to remove the

moisture. This unit also has driers to

remove moisture up to 0.1 ppm. Driers aredesigned for 1140 mmscfd and currently the

unit is being operated around 700 mmscfd.

The Performance of the driers were

checked by UOP. Moisture in the driersoutlet is lower than the design and hence it

was not the cause of the problem. Further

more moisture will cause the blockage at

lower capacity itself;. This cause is ruled out




OTHER IMPURITIES

In addition to the obvious need of water

removal from the gas stream to protect fromblockage in the cryogenic sections of a

plant, consideration must be given to the

possible formation of other solids or semi-

solids in the gas stream. Amines, glycols,

and compressor lube oils in the gas stream

can form blockages in the system. Generally

these contaminants will form a blockage

upstream of an expander, in the lower

temperature exchange circuit, or on the

screen ahead of an expander. All the screen

and cold box were opened and no debris




CONSTRUCTION DEBRIS

From the plant data it

was found that delta pwas on the higher side

in the top two beds.

During construction any

leftover material or rust

might have closed the

liquid distributor leading

to liquid accumulationcausing flooding.

Therefore it was decided

to shutdown and open

the column to inspect.

On inspection, it was

found that there was no

construction debris andcolumn was clean as

per Figure -1 . Therefore

the same is not cause

for the flooding.




CO2 FREEZE IN THE TOP OF THE TOWER

Upstream lean oil plant do not have any amine unit, Carbon

di-oxide in the feed stream is not being removed in LOP andthe same is processed in the ERU. This problem is

recognized during the design stage. Accordingly the plant

was designed for three levels of CO2. (2 , 2.5 , 3% ) .

Carbon dioxide can form as a solid in lower temperature

systems. Fig 1 will provide a quick estimate for the possibilityof formation of solid CO2. If operating conditions are in the

Methane liquid region as shown by the insert graph, the

dashed solid-liquid phase equilibrium line is used. For other

conditions the solid isobars define the approximate CO2 vapor

concentration limits. For example a pressure of 300 psia andtemperature at ±170°F, the insert graph shows the operating

conditions to be in the liquid phase region. The dashed solid-

liquid phase equilibrium line indicates that 2.1 mol percent

CO2 in the liquid phase would be likely to form solids.

However, at the same pressure and temperature at ±150°F, the





This chart represents an approximation of CO2 solidformation. Detailed calculations were carried out for CO2

freeze margin at each stages 8.5 ° F (2.0% CO2 at 98% recovery

), 6.6 °F (2.5% CO2 at 94% Recovery ), 6.4 °F (3.0% CO2 at

88% Recovery ) during design stage for each one of the feed

case. These correlations in HYSYS are developed based onthe GPA data. There is enough CO2 freeze margin provided in

the design so that column will not freeze due to CO2. We have

suspected that the higher pressure drop observed may be

due to CO2 freeze. Normally when the column is warmed up

the CO2 freeze temperature goes up and solid formed willslowly dissolved in the process and pressure drop will reduce.

Even after increasing the temp, the pressure drop did not

decrease significantly and in some time the pressure drop

take place in the section where the CO2 freeze margin is on

the higher side and not expected to freeze . Hence the CO2




BENZENE FREEZE IN THE TOP OF THE

TOWER

Ortloff has designed or retrofitted over 100 gas plants since 1993.Benzene freeze is not considered in the design. Furthermore

Benzene level in the LOP plant out let is expected to very low

since most of the heavy hydrocarbons are already removed ( like

c3,c4,c5 ) . A gas plant in Malaysia has experienced similar flooding

conditions in the upper section of their tower due to Benzene freeze.

Benzene contamination was discovered as a factor limiting plantthroughput. A paper was presented at the 83rd Gas Processors

Association Conference in New Orleans.

The solubility of Benzene in NGL OVHD streams is composition

dependent. Following Figure 16-35 shows the solubility of Benzenein Methane and Figure 2 shows the solubility of Benzene in Ethane.

From these graph it is obvious that more Ethane slips in the top less

is the chances of the Benzene formation





TOWER

From the above it is clearly evident that for this column operatingconditions, about 400-500 ppm of Benzene is required for freeze.

From the literature it is noticeable that in order to have Potential

Benzene freeze, the Benzene concentration has to be around 400

ppm at the operating temp of around -100 °C.

In the absence of a quantitative BETX analysis, it was assumed thatvarying Benzene concentration in feed and to estimate the level in

which Benzene concentrations might freeze in the upper section of

the demethanizer. Our calculations for Benzene freeze indicated that

Benzene concentrations around 400 ppm in bed-2 of the

demethanizer will require Benzene concentration greater than 3000

ppmv in the feed. The crystallization point will change when the feedgas composition changes. In general, Benzene is more likely to

freeze in a leaner gas. In a lean gas, the solubility of Benzene is

poorer allowing Benzene to crystallize at higher temperatures. For

example, a Benzene concentration of 400 ppmv in a lean gas (90%

C1, 6.5% C2, 0.4% C3) will freeze at -97 °C. Based on calculations.Cyclohexane's crystallization point seems to be lower than the





TOWER

Hysys was used to predict the concentration of Benzene and cyclo-hexane in the upper section of the demethanizer from a given

specific concentration in the feed. This helped to determined

Benzene freeze at specific Benzene concentration in the feed, similar

to calculating C02 freeze. Liquid, Solid Gas Equilibrium software

provided by the Gas Producers Association is aid in calculating the

temperature and pressure at which Benzene solids will form within amixture.

The GPA has issued a computer package based on the research

results. This GPA computer program will predict the solubility of

carbon dioxide, BTEX, and heavy hydrocarbons in various

hydrocarbon mixtures at the most stringent conditions and the samewas used by M/s. Ortloff.

The analysis of the feed gas indicates that the Benzene molar

fraction is 0.0005. It probably is the most reliable data in the BTEX

group, since it has the highest concentration in the group. Therefore,

it is more reasonable to use Benzene as the key component to

calculate other variables. .





TOWER

Detailed Hydrocarbon analysis is carried out and found that Benzenelevels are very low and it is less than 50 ppm . Therefore the same is

excluded

Ortloff has designed or retrofitted over 100 gas plants since 1993.

Benzene freeze is not considered in the design. Furthermore

Benzene level in the LOP plant out let is expected to very low

since most of the heavy hydrocarbons are already removed ( like

c3,c4,c5 ) . A gas plant in Malaysia has experienced similar flooding

conditions in the upper section of their tower due to Benzene freeze.

Benzene contamination was discovered as a factor limiting plant

throughput. A paper was presented at the 83rd Gas Processors

Association Conference in New Orleans.

The solubility of Benzene in NGL OVHD streams is composition

dependent. Following Figure 16-35 shows the solubility of Benzene

in Methane and Figure 2 shows the solubility of Benzene in Ethane.

From these graph it is obvious that more Ethane slips in the top lessis the chances of the Benzene formation





TOWER





TOWERIn the absence of any of the above issues, it is decided to study the

performance of the Column and its internals under normal operatingconditions.

It was noticed that Bed 3 is not having enough delta-T as per

Design. This clearly indicated that there could be some potential

for flooding or carry-over or blockage, etc.

Therefore, it was decided to carry out a gamma ray scanning acrossthe tower from top to bottom to identify the source of the problem

at various capacities 60, 80 and 100 %.

Gamma ray scanning was carried out at various thruput 60%,

80% and 100%. Scan revealed that at 60% of the design load (

400 kNm3/hr) the column operated stable and no significant

problems were detected .

At about 80% of design capacity (500 KNM3/HR) flooding starts to

occur in BED 3.

Scan taken at plant loads close to 100% of the design (636

KNM3/HR) identified that under these conditions flooding also

occurred in BED 2 and BED 1 at the top of de-methaniser. The scan





TOWER

This area of the column appears to be hydraulically limited. This is the

place where liquid carry over starts and slowly goes to top beds 1 & 2.

Down comers are restricting and hole size in this section may have to be

increased. M/s Ortloff is carrying out further simulation to identify the

issue. Based on findings, the hole sizes may need to be enlarged to

improve the hydraulics.




CONCLUSION

From the above it is clear that we have under gonetypical trouble shooting exercise to solve the

problem. GPA data / software has helped us to

identify and exclude the major issue of heavy

hydrocarbons freeze. GPA data was very usefuland has helped to rule out many potential

issues like CO2 freeze and Benzene freeze.

Based on which we have progressed and

concluded that our problem is from tray internalswhich will be checked and changed, if required.

We thank GPA for providing these important

data and we also thank M/s Ortloff for running

the software to estimate the Benzene freeze



GPA USAMarch 2010

QUESTIONS & ANSWERS

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