HonorsThesis

ContinuousDistillationSystemBottomsPumpRe-design

Spring2020

JacobHay

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TableofContentsAbstract ........................................................................................................................................... 3 Objective ......................................................................................................................................... 3 Theoretical Background .................................................................................................................. 4 Current System Design .................................................................................................................... 7 NPSHA Calculation ......................................................................................................................... 10 Proposed Designs .......................................................................................................................... 12 Conclusion ..................................................................................................................................... 16 References .................................................................................................................................... 17

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Abstract

The main learning objective of the continuous distillation experiment in the separation and mass transfer lab

is for students to investigate how system parameters, such as the reflux ratio and steam flow rate, impact

the binary distillation of a mixture of isopropyl alcohol (IPA) and 2-butanol (2-BUT). The bottoms pump in

the continuous distillation system is utilized to keep the contents of the reboiler evenly mixed and to enable

the transfer of the mixture from the reboiler to the bottoms product tank. The current pump was selected

with the functionality to pump the mixture from the reboiler to 3rd floor tankage; however, this operation is

no longer utilized. Due to this reason, the existing pump is oversized for the current operations because the

net positive suction head required (NPSHR) for the pump is larger than the net positive suction head

available (NPSHA) in the system. When the NPSHR is larger than the NPSHA small vapor bubbles are formed at

the suction of the pump and implode once reaching the impeller. This process is called cavitation and it can

cause severe damage to the internals of the pump, which could cause the pump to be inoperable in the

future. For this reason, calculations were completed first to determine the NPSHA in the system. This

parameter along with operating conditions, such as the reboiler temperature and pressure, were jointly

utilized to determine potential pumps that could operate efficiently with the current system configuration.

Additional research was completed on equipment that could further lower the NPSHR by the pump, such as

variable frequency drives, to ensure optimal cavitation free operations.

Objective

To evaluate the criteria and operating requirements for a new bottoms pump. Additionally, acquire

quotations for possible options and present the findings to the department for further analysis to determine

the optimal pump.

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The following considerations must be taken into account when selecting a new pump:

• The NPSHA in the system is small

• The pump must be explosion proof/intrinsically safe

• The pump must be able to withstand operations with a boiling liquid

TheoreticalBackground

CharacteristicCurvesandOperatingPoints

Pump characteristic curves are useful tools that depict the relationship between the pressure head

generated by the pump and the fluid flow rate. Pump curves are regularly used in parallel with system

curves, which show the relationship between the pressure drop across a pipe network and the fluid flow

rate. It is important to note that each individual network configuration has a different system curve. The

point at which a pump and system curve intersect is called the operating point. Figure 1 below illustrates

these relationships.

Figure 1: Pump and System Curve Relationship [5]

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The operating point satisfies the following two requirements:

1. The fluid flow rate going through the pump is the same as the pipe network

2. The pressure generated by the pump is equal to the pressure needed to overcome the friction losses

in the system

Generally speaking, as the friction losses in a system increase, the pump must generate a higher pressure to

overcome those losses, thereby decreasing the fluid flow rate.

NPSHA

The NPSHA in the system can be determined by using equation 1 below.

𝑁𝑃𝑆𝐻! = 𝐻! ±𝐻" −𝐻# +𝐻$ −𝐻$%

Table 1 below describes the above variables.

(1)

Table 1: NPSHA Variables [1]

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The absolute pressure on the surface of the liquid can be measured using pressure gauges with known

atmospheric pressure or can be determined to be atmospheric if the vessel is vented to the atmosphere.

The vertical distance between the surface of the liquid and the suction of the pump, the static head, can be

determined through a physical measurement. Additionally, the velocity head at the pump suction can be

found from the pump manufacturers test curves.

The total friction losses in the suction piping can be found by summing the friction losses from segments of

straight pipe and pipe fittings in the system. To determine the friction losses from the pipe fittings, equation

2 below can be used.

𝐹&'(( = 𝐾)𝑣2

*

Where Kf is the friction loss coefficient for a particular fitting and v is the velocity of the fluid. To determine

the friction losses from straight segments of pipe, equation 3 below can be used.

𝐹&'(( = 4𝑓∆𝐿𝐷𝑣*

2

Where 𝑓 is the fanning friction factor, ∆𝐿 is the length of pipe, and D is the pipe diameter. For laminar flow,

𝑓 can be calculated by using equation 4 below. For turbulent flow, 𝑓 is given by empirical relationships, such

as the Colebrook equation or moody diagram.

𝑓 = 16/𝑅𝑒

Where Re is the Reynolds number. The absolute vapor pressure of a liquid in the reboiler can be calculated

using equation 5, the Antoine equation, provided below.

log(𝑃$%) = 𝐴 −𝐵

𝐶 + 𝑇

Where A, B, and C are Antoine constants and T is the temperature in degrees Celsius.

(2)

(3)

(4)

(5)

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CurrentSystemDesign

Materials

The two components that are separated in the continuous distillation unit are IPA and 2-BUT. Table 2 below

summarizes their physical properties.

Molar Mass (g/mol)

Density (g/ml)

Boiling Point (oC)

Viscosity (cP)

IPA 60.1 0.71 82.5 2.1 2-BUT 72.122 0.806 100 3.8

Table 2: IPA and 2-BUT Properties

OperatingConditions

Throughout day to day and week to week operations the compositions of IPA and 2-BUT in the feed and

reboiler continually change. For this reason, the operating conditions must also continually change to ensure

effective separation. The only fixed variable in the system is the absolute pressure, which is atmospheric

because the condenser is vented to the atmosphere. The control variables in the system are the steam flow

rate, reflux ratio, and feed/bottoms/distillate flow rates, while the response variables are the fluid

temperature and bottoms/distillate product purities. The range of values for operating conditions relevant

to the bottoms pump analysis are summarized in table 3 below.

Fixed Control Response Pressure 1 atm Steam Flow Rate 60-70 lbs/hr Temperature 90-95 oC

Bottoms Flow Rate 0.6-0.95 kg/min

Table 3: Operating Conditions

The steam flow rate is the primary variable that sets the temperature of the fluid in the reboiler. It is

important to note that during normal operations the liquid in the reboiler is boiling, therefore the bottoms

pump is pumping a boiling liquid.

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EquipmentOverview

The reboiler for the continuous distillation system is a steam jacketed vessel. The vessel is heated with

steam around 150 oC at 65 lbs/hr. Figure 2 below outlines the process flow diagram for the distillation

system.

Figure 3 below shows the system configuration at the bottom of the column.

Figure 2: Continuous Distillation Bottoms System [6]

Figure 3: Distillation Process Flow Diagram

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The current pump is an Eastern Centrichem centrifugal pump (model ECJ3) supplied by Hudson Pump &

Equipment. The pump curve is provided below in figure 4.

From this diagram it can be made known that the NPSHR for the pump is 14ft of water. The general

specifications of the pump are provided below in figure 5.

From the above figure, it is worth mentioning that the ½” inlet and outlet connections of the pump match

the current pipe diameter in the bottoms pipe network.

Figure 4: Centrichem ECJ3 Pump Curve [2]

Figure 5: Centrichem ECJ3 General Specifications [2]

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NPSHACalculation

The NPSHA calculation was completed using the following simplifications and assumptions:

• Since the liquid in the reboiler is boiling, the absolute pressure on the surface of the liquid equals

the absolute vapor pressure of the liquid

• The velocity head at the suction of the pump is negligible

From the above, equation 1 was simplified to equation 6 below.

𝑁𝑃𝑆𝐻! = 𝐻" −𝐻#

The vertical distance between the bottom of the reboiler and the pump suction was measured to be 15”.

Additionally, the minimum operating liquid level in the reboiler is 15”. Therefore, the vertical distance

between the liquid level and the pump suction, HZ, was determined to be 30”.

The total friction losses in the suction piping, HF, was found by summing the friction losses in the segments

of straight pipe and the pipe fittings. Equation 3 was used to find the friction losses in the 10” of 0.5”

diameter straight pipe and equation 2 was used to find the friction losses in the fittings. The Reynolds

number was calculated to be below 2000, therefore the flow is laminar, and equation 4 was used to find the

fanning friction factor. Table 4 below summarizes the type and number of fittings in the suction piping.

Fitting Quantity Kf Value

90o Elbow 1 0.81

Fully Opened Ball Valve 1 0.08

Cross Run-thru 1 0.54

Table 4: System Fittings and Kf Values [3]

(6)

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Since the composition of the fluid in the reboiler continually changes, calculations were completed for a

100% IPA liquid and a 100% 2-BUT liquid to determine the minimum and maximum NPSHA values. The

calculation was first completed in terms of meters of IPA/2-BUT and then converted into feet of water so

that it could be comparable to the provided pump specifications. Table 5 below summarizes the

calculations.

100% IPA 100% 2-BUT

HZ (m) 0.762 0.762

HF (Pa) 49.42 60.52

HF (m) 0.0071 0.0077

NPSHA (m of liquid) 0.7549 0.7543 NPSHA (ft of water) 1.758 1.995

Table 5: NPSHA Calculation

It is evident from the calculations that the static head on the pump primarily determines the NPSHA in this

system. It is important to note that these calculations were made using the minimum operating level in the

reboiler of 15”. Therefore, if the liquid level in the reboiler is greater than 15”, the NPSHA in the system

would be larger than the values reported in the table above. While the composition in the reboiler

continually changes from day to day and week to week operations, the composition of the fluid in the

reboiler during a single run will become purer in 2-BUT because IPA is more volatile. For this reason, the

NPSHA in the system is closer to that of the 100% 2-BUT value reported above. Finally, in comparison to the

NPSHR by the current pump of 14ft of water, the calculated NPSHA values are 7 to 8 times smaller. Thus,

cavitation is expected to occur while the pump is operating.

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ProposedDesigns

After working with Matt Willis and Hayden Chappel, representatives from Hudson Pump & Equipment, three

different pumps were recommended based on our current system specifications. Table 7 below summarizes

the operating range and quoted price for each pump.

Supplier/Type Model Flow Rate Range Pressure Quoted Price

Eastern Centrichem Centrifugal ECD1 Up to 20 GPM - $2,703.00

Seepex Progressive Cavity MD012-12 0.053 – 264 GPM Up to 360 PSI $4,939.71

Pulsafeeder Eclipse External Gear 02 – metallic Up to 0.45 GPM Up to 150 PSI $4,857.00

Table 5: Proposed Pump Specifications

From the provided information, table 8 below outlines the pros and cons for each option.

Supplier/Type Pros Cons Eastern

Centrichem Centrifugal

- Smooth flow - Relatively low cost compared to other options

- Recirculation line required - Piping modifications required

Seepex Progressive

Cavity

- Smooth flow - Handle low suction head conditions better than most pumps

- Rotor and stator contact each other, which can lead to wear - Piping modifications required - Cost likely out of budget

Pulsafeeder Eclipse External

Gear

- Smooth flow - No mechanical seal, which eliminates a potential leak source

- Low viscosity products may cause wear to the gearing - Piping modifications required - Cost likely out of budget

Table 6: Pump Options Comparison

While extensive analysis could be done between the three provided options, the upfront cost of both the

Seepex and Pulsafeeder pumps were too large for the available budget. With both of those options not

feasible, focus was shifted to the Eastern Centrichem centrifugal pump. The major cons presented were that

a recirculation line and piping modifications would be needed. The current piping design has a recirculation

line to the reboiler, therefore this con is not relevant. Additionally, all three options would require piping

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modifications because the pump’s suction/discharge port sizes are different than the current 0.5” diameter

piping, thus this con is not pertinent when compared to the other options. For further analysis, the ECD1

pump curve and port combination options are provided below in figure 6.

The general specifications of the ECD1 pump are provided below in figure 7.

Figure 7: ECD1 General Specifications [2]

The internals for the ECD1 pump are the same for all three suction/discharge port combinations; however,

an important operating variable that varies is the NPSHR. As stated in the theory, the NPSHA needs to be

Figure 6: ECD1 Pump Curve and Port Combinations [2]

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larger than the NPSHR to prevent cavitation. Therefore, the port combination 3 option of a 0.5” suction

connection and 0.375” discharge connection with a NPSHR of 9ft of water would be favorable over the other

two port combination options with NPSHR values of 18ft and 32ft of water. For now, both port combinations

2 and 3 will be considered. Additionally, the port combination 3 option NPSHR is lower than that of the

current ECJ3 pump NPSHR, which is 14ft of water. However, since the NPSHR of 9ft of water is still higher

than the calculated NPSHA values, additional research was completed on equipment that could further lower

the NPSHR for a pump.

To add additional functionality, a variable frequency drive (VFD) can be coupled with the pump to turn down

the speed of the pump. In general, a VFD is a type of motor controller that drives an electric motor by

varying the frequency and voltage supplied to it. When coupled with a pump, a VFD can control the

operating point by increasing/decreasing the power supplied to the pump. As the speed of the pump is

reduced, the pump curve is shifted down. Figure 8 below illustrates the functionality of a VFD.

In the case of the ECD1 pump, a VFD can be utilized to decrease the speed of the pump, which would lower

the NPSHR. Adding the additional functionality of being able to control the speed and therefore the NPSHR of

the pump would ensure cavitation-free operations. Along with this benefit, running the pump at a reduced

speed also has significant economic impacts. Based on affinity laws, the required power is proportional to

the cube of the speed [4]. Therefore, as the motor speed decreases, the power decreases by the cube. On

Figure 8: VFD Pump Curve Comparison

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top of energy savings, operating at a reduced speed can decrease maintenance costs and extend the life of

the pump.

Logistically both the VFD and the pump can be integrated with the current software, DeltaV, that is utilized

to operate both the continuous and batch distillation units. The DeltaV system is configured to communicate

with external devices using an analog 4-20 mA signal. The signal is then sent to a CHARM module that is

installed in the blue wiring cabinet in the control room. The VFD would be controlled with an analog signal,

where a 4-mA signal would correspond to no power being supplied to the pump and a 20-mA signal would

correspond to maximum power being supplied to the pump. The pump would be controlled using a simple

relay for on/off operations.

An additional consideration for the pump and VFD was intrinsically safe operations. The pump and motor

proposed have both been verified to comply with the unit operations laboratory standards. For the VFD a

National Electrical Manufacturers Association (NEMA) 12 enclosure has been advised to meet these

standards.

Table 9 below outlines the quoted prices for the ECD1 pump with both port combinations in consideration.

Prices for each combination with and without the VFD/NEMA wall mount are included.

Port Combination Equipment Quoted Price

0.5” Suction 0.375” Discharge

ECD1 Pump $2,703.00 ECD1 Pump

VFD & NEMA Mount $4,066.21

0.25” Suction 0.25” Discharge

ECD1 Pump $2,703.00 ECD1 Pump

VFD & NEMA Mount $3,836.99

Table 7: ECD1 Combined Quotation

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Conclusion

After reviewing the theoretical calculations and provided options, the most cost-effective, and suitable

option is the ECD1 pump with the 0.5” suction and 0.375” discharge connections coupled with a VFD. The

lower NPSHR benefit of these port combinations far outweighs the additional $200 cost of this option when

compared to the other port combination option. Since normal operations of the continuous distillation

bottoms system requires a relatively low flow rate, the ECD1 pump could be operated far from its maximum

speed using a VFD. This would lower the NPSHR, which would ensure cavitation-free operations, while also

decreasing maintenance and energy costs.

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References

[1] “Understanding Net Positive Suction Head.” Pump School,

www.pumpschool.com/applications/NPSH.pdf

[2] Eastern Centrichem. pulsa.com/wp-content/uploads/2018/10/Eastern-Centrichem-

Brochure.pdf

[3] Friction Losses in Pipe Fittings. www.metropumps.com/ResourcesFrictionLossData.pdf

[4] “VFD for Centrifugal Pumps.” Variable Frequency Drives, http://www.vfds.org/vfd-for-

centrifugal-pumps-662716.html

[5]“FluidFlowTheory.”ChemicalEngineeringattheUniversityofFlorida,

http://ww2.che.ufl.edu/unit-ops-lab//experiments/FF/FF-theory.pdf

[6]“ContinuousDistillationTheory.”ChemicalEngineeringattheUniversityofFlorida,

http://ww2.che.ufl.edu/unit-ops-lab//experiments/Distillation/Distillation-theory.pdf