honors thesis continuous distillation system bottoms pump
TRANSCRIPT
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