Chapter 7.docx

Table of Contents 1

Pumps................................................................................................................................. 4 1.1

Pump Types ................................................................................................................. 4

1.1.1

Centrifugal Pumps ............................................................................................... 4

1.1.2

Reciprocating Pumps ........................................................................................... 4

1.1.3

Helical Rotor Pumps ............................................................................................ 4

1.2

Selection Criteria of Pumps......................................................................................... 5

1.3

Pump Sizing Calculation Steps ................................................................................... 5

1.3.1 2

Pump Calculations ............................................................................................... 5

Compressors..................................................................................................................... 12 2.1

Positive Displacement Compressor ........................................................................... 12

2.2

Dynamic compressor ................................................................................................. 12

2.3

Compressor Sizing Calculation Steps ....................................................................... 13

2.3.1

Compressor Calculations ................................................................................... 13

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List of Figures Figure 1: Lean Solvent Pump .................................................................................................... 5 Figure 2: Selection of Pump ...................................................................................................... 6 Figure 3:Rule of thumbs for locating the process equipment .................................................... 7 Figure 4:Estimate Frictional pressure losses ED and ES ............................................................ 7 Figure 5:Efficiency of Pump ...................................................................................................... 8 Figure 6:Efficiency of Electric motor ........................................................................................ 9 Figure 7:Horsepower Selection................................................................................................ 10 Figure 8:Centrifugal Compressor ............................................................................................ 13 Figure 9:Compressor Selection Chart ...................................................................................... 14 Figure 10:Compressibility factor Chart ................................................................................... 15 Figure 11: Hydraulic Efficiency Graph ................................................................................... 16 Figure 12: Compressor Characteristics .................................................................................... 17 Figure 13:Compressor Gear, Bearing, and Seal Efficiencies .................................................. 18 Figure 14:Centrifugal Compressor with intercooling .............................................................. 20

List of Tables Table 1-1:Critical temperature and Critical pressure ............................................................... 14

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CHAPTER 7 PUMP AND COMPRESSOR CALCULATIONS

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1 Pumps Pump is a device that imparts momentum and mechanical energy to the process fluid. Pumps are used to transfer fluid from one location to other. A pump is a device used to raise, compress, or transfer fluids. The motors that power most pumps can be the focus of many best practices. It is common to model the operation of pumps via pump and system curves. Pump curves offer the horsepower, head, and flow rate figures for a specific pump at a constant rpm. System curves describe the capacity and head required by a pump system.

1.1 Pump Types Various types of pumps are used in the chemical industry, including centrifugal, reciprocating, and helical rotor pumps.

1.1.1 Centrifugal Pumps Centrifugal pumps operate by applying a centrifugal force to fluids, many times with the assistance of impellers. These pumps are typically used in moderate to high flow applications with low-pressure head and are very common in chemical process industries. There are three types of centrifugal pumps—radial, mixed, and axial flow pumps. In the radial pumps, pressure is developed completely through a centrifugal force, while in axial pumps pressure is developed by lift generated by the impeller. Mixed flow pumps develop flow through a centrifugal force and the impeller.

1.1.2 Reciprocating Pumps Reciprocating pumps compress liquid in small chambers via pistons or diaphragms. These pumps are typically used in low-flow and high-head applications. Piston pumps may have single or multiple stages and are generally not suitable for transferring toxic or explosive material. Diaphragm pumps are more commonly used for toxic or explosive materials.

1.1.3 Helical Rotor Pumps Helical rotor pumps use a rotor within a helical cavity to develop pressure. These pumps are useful for submersible and waste applications.

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1.2 Selection Criteria of Pumps Many different factors can influence the final choice of the pump for an operation. The major factors that govern the pump selection are following: 

The amount of liquid that must be pumped out.



The properties of the fluid.



Cost and mechanical efficiency of the pump.

1.3 Pump Sizing Calculation Steps 

Select the pump.



Estimate z1 and z2.



Estimate Frictional pressure losses ED and ES.



Calculate Pump Work.



Calculate Pump shaft horsepower & estimate its Efficiency.



Calculate electric-motor horsepower & estimate its Efficiency.



Select a standard electric-motor horsepower.



Calculate NPSH.

1.3.1 Pump Calculations Lean Solvent Pump (P-101) This pump is used for to pump lean solvent (selexol) from stripper to absorber.

Selexol =1566 kg/hr P = 10.1 bar T = 120 C

Selexol =1566 kg/hr P = 1.5 bar T = 120 C P-101

Figure 1: Lean Solvent Pump

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Pump Selection Pump type is selected from figure 2, Capacity = 6.89 gpm Head = 49 ft

Figure 2: Selection of Pump (1)

Hence, the Pump Selected is Marginal Centrifugal Pump. Define the inlet and outlet pressures The inlet pressure is = P1 = 2 bar The outlet pressure is = P2 = 10.1 bar Locate the process equipment Locate the process equipment according to the rule of thumb listed below. As we need to pump the liquid in to the absorber so our process equipment is absorber which is supported by a skirt having skirt height of about 4 ft or 1.21 m.

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Figure 3:Rule of thumbs for locating the process equipment (1)

Estimation of z1 & z2 Z1 with respect to pump = 0 Z2 = skirt height + height of column Z2 = 1.21 + 9 Z2 = 10.21m Z2 = 34 ft Estimation Frictional pressure losses ED and ES

Figure 4:Estimate Frictional pressure losses ED and ES (1)

So, ES & ED is equal to 0.35.(from figure 4) Calculate the pump work W=

g P1 − P2 (z1 − z2 ) + − (ES − ED ) gC ʃ 7

(2 − 10.1) ∗ 105 (0.35 + 0.35) ∗ 105 9.8 (0 − 10.21) + 𝑊= − 32 1010 1010 𝑊 = −874.40

𝑁𝑚 𝐾𝑔

Calculate the pump shaft horsepower Efficiency of Pump is selected from the figure 5,

Figure 5:Efficiency of Pump (1)

𝑃=

𝑚𝑊 ɳ

𝑃=

1566.15 ∗ 874.40 0.45

P = 3423603.9 J/hr P = 1.27 hp Calculate Electric-Motor Horsepower & Estimate its Efficiency On the basis of horsepower the selected motor is Squirrel Cage Induction Motor having power range of 1 to 5,000 hp from figure 6. Efficiency of motor is selected to be = 0.86

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The power of motor is calculated as = PE = 𝑃𝐸 = 𝑃𝐸 =

𝑃𝑃 ɳ

1.27 0.86

PE = 1.47 hp

Figure 6:Efficiency of Electric motor (1)

Select a Standard Electric Motor Horsepower Hence the selected motor is of 2hp from figure 7.

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Figure 7:Horsepower Selection (1)

Net Positive Suction Head (NPSH) 𝑁𝑃𝑆𝐻 =

1 𝑃𝑎 − 𝑃𝑣 ( − ℎ𝑓𝑠 ) − 𝑍𝑎 𝑔 ʃ

Absolute pressure at the surface of reservoir = Pa = 2 bar = 2*105 Pa Vapor pressure of Selexol = Pv =1000 Pa (2) Friction losses in suction line = hfs = 0 Za = 0 𝑁𝑃𝑆𝐻 =

1 2 ∗ 105 − 103 ( − 0) − 0 9.8 1010

NPSH = 20 m

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Specification Sheet Identification Item

Pump

Item No.

P-101

Type

Centrifugal Function To increase pressure from 2 bar to 10.1 bar

Feed Flow Rate

1566.15 kg/hr

Inlet Pressure

2 bar

Outlet Pressure

10.1 bar

Power of pump

1.27 hp

Power of electric motor

1.47 hp

NPSH

20 m

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2 Compressors A device that pressurizes fluids generally. Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. Compression of gases and vapors is an important operation in chemical and petrochemical plants. It is necessary to be able to specify the proper type of equipment by its characteristic performance. The compression step is conveniently identified for the process design engineer by the principal operation of the equipment: 

Reciprocating.



Centrifugal.



Rotary displacement.



Axial flow.

Compression may be from below atmospheric as in a vacuum pump or above atmospheric as for the majority of process applications. A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a device specifically for compressing air to power other tools.

2.1 Positive Displacement Compressor 

Reciprocating compressors



Ionic liquid piston compressor.



Rotary screw compressors.



Rotary Vane compressors.



Rolling piston.



Scroll compressors.



Diaphragm compressors.

2.2 Dynamic compressor 

Air bubble compressor.



Centrifugal compressors.



Continuous Blade Compressor.



Diagonal or mixed-flow compressors.



Axial-flow compressors.

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2.3 Compressor Sizing Calculation Steps 

Select the compressor type.



Calculate the reduce temperature and pressure and then calculate compressibility at the inlet of compressor.



Calculation of specific volume.



Calculation of volumetric flow rate.



Calculate hydraulic efficiency.



Calculate the work of compressor.



Calculate the discharge temperature.



Calculate the reduce temperature and pressure at outlet conditions.



Calculate the overall compressibility factor.



Calculate the shaft work.



Power calculation.



Standard compressor size selection.

2.3.1 Compressor Calculations Syngas = 1287.84 kg/hr P = 10.1 bar T = 55 C

Syngas = 1287.84 kg/hr P = 50 bar T = 55 C Figure 8:Centrifugal Compressor

Flowrate = 1287.23kg/hr = 273.78 m3/hr P1 = 10.1 bar P2 = 50 bar T1 = 55℃

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Selection of the Compressor Compressor is selected from figure 9.

Figure 9:Compressor Selection Chart (3)

Hence the compressor selected is centrifugal compressor. The advantages of centrifugal compressor are following: 

Can handle wide range of flow rate



Compatible for corrosive fluids



They can deal liquid with large amounts of solids.



Maintenance costs are lower than from other types of compressors.

Calculation of critical temperature and critical pressure of gaseous mixture Table 1-1:Critical temperature and Critical pressure

Components Composition(X) k

Pc(bar)

Tc(K)

X*k

X*Pc

X*Tc

CO

0.26

1.40

35.2

134.4

0.36

9.15

34.94

H2

0.64

1.41

13

33.3

0.90

8.32

21.31

CO2

0.08

1.30

74

304.4

0.10

5.92

24.35

Total

1.37

23.39

80.60

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Calculate the reduce temperature and pressure and then calculate compressibility Reduced temperature: TR = T1/Tc = 3.43 Reduced Pressure: PR = P1/Pc = 0.19 Compressibility: Z = 0.99(from fig 10).

Figure 10:Compressibility factor Chart (1)

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Calculate hydraulic efficiency

Figure 11: Hydraulic Efficiency Graph (3)

From graph the Polytrophic efficiency = ηp= 0.65

n  1 (k  1) / k  n p 𝑛−1 𝑛

= 0.41

Calculate the work done Let assume that initially we have only one stage N=1 n 1   ZR T  p2  n      1  (n  1) / n  p1    '

WPN

' 1

WP1 = 6.05*106 J/kmol. Calculate the discharge temperature

WCN 

R' (TD  T1' ) (k  1) / k

TD = 524 K 16

So, Temperature of gases rise because of compression due to which increase no of stages to achieve the desire pressure. Now Assume number of stages = N = 2 P1 = 168 kPa 1

ps 1  pD  N   ps  p1  1

p2  pD  N   p1  p1 

P2 = (PD/ P1)1/N* P1 P2 = 22.47 bar. ( n 1)/ n ' '  p     ZRT p4 1 2    1   WPN   1  (n  1) / n  p1  p2  0.1 p20.7    

WP2 = 4.41 * 106 J/kmol. Now calculating discharge temperature,

WCN

R'  (TD  T1' ) (k  1) / k

TD = 470K

Figure 12: Compressor Characteristics (1)

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Critical Properties of Gases at outlet Calculate the reduce temperature and pressure and then calculate compressibility. Reduced temperature: TR = T1/Tc = 4.91 Reduced Pressure: PR = P1/Pc = 0.97 Compressibility: ZD = 1.02(from graph) Z = (Z1 + ZD ) / 2 Z=1 Shaft Work Calculation

WCN 

WPN  p s B G

Figure 13:Compressor Gear, Bearing, and Seal Efficiencies (1)

s = 0.98

B = 0.97 G = 0.97 4.41∗10^6

WCN = 0.65∗0.98∗0.97∗0.97 WCN = 7.35*106 J/kmol Shaft Work Power Calculation Gas flowrate = 101.78 kmol/hr PCP = 7.35*106*101.78 18

PCP = 7.48*108 J/hr. PCP = 207.77kW PCP = 154.93 hp Motor Power Calculation 𝑃𝑐𝑝

PE =

ŋ𝐸

ŋE = 0.94 PE = 164.81 hp We select standard motor of 200hp. Heat Load Q = m Cp (T2-T1) Cp = 9.61 kJ/kmol.K Q = 1287.84*9.61*(470-328) Q = 1.66*106 kJ/hr Water Required Cp = 4.18 kj/kmol.K T1 = 25℃ T2 =? ΔH

T = + T. <𝐶𝑝> 𝑅

𝐵

𝐶

𝐷

= A+ 2 T. (τ+1) + 3 T.2 (τ2 + τ +1) + τ T.2

T

τ = T. By iteration the outlet temperature is T water out = 49.3℃ m = Q/ Cp (T2 - T1)

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m = 1.66*106 /4.18*(322-298) m =16547.04 kg /hr.

Figure 14:Centrifugal Compressor with intercooling

Specification Sheet Identification Item

Compressor

Item No.

C-101

Type

Centrifugal Function

To increase pressure from 10.1 bar to 50 bar Feed Flow Rate

1287.84 kg/hr

Inlet Pressure

10.1 bar

Outlet Pressure

50 bar

Power

200 hp

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3 References 1. Silla, Harry. Chemical Process Engineering Design and Economics. USA 2. Jennifer Dyment , Suphat Watanasiri. Acid Gas Cleanin using DEPG Physical Solvents. 3. Compressor Selection Guidelines.

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