The minor equipment that is designed in this project is lean/rich MEA heat exchanger E-114. This heat exchanger is a counter flow shell and tube heat exchanger and is designed to heat up the rich MEA stream flowing from the CO2 absorber to the stripper. The principle that is applied is heat exchange between cold stream and hot stream which in this case the heat energy is transferred from the lean MEA stream to the rich MEA stream. Apart from this, the chemical engineering design for this heat exchanger includes the determination of its dimensions and heat exchange coefficient as well as pressure drop. The mechanical design covers the design of pressure vessel, head, supports and piping. In addition, the operating design which includes the commissioning, start-up, shutdown and maintenance procedures, process control, and HAZOP study is considered. 2.0 Process Description
Figure 2.1 Schematic of rich/lean MEA heat exchange process flow sheet The lean/rich MEA heat exchange process is presented in Figure 2.1. The MEA-2 stream containing rich CO2 is flowing from CO2 absorber and enters the heat exchanger to be heated up from 61°C to 80°C by MEA-7 before entering the stripper. The MEA-7 is then cooled down from 105°C to 84°C when pass through the heat exchanger and recycle back to the CO2 absorber. The cold stream in this case is MEA-2 and MEA-3 while the hot stream is MEA-7 and MEA-8. 3.0 Chemical Engineering Design
3.1 Design Methodology
The rich/lean MEA heat exchanger is a counter flow shell and tube heat exchanger. The chemical engineering design methodology for this heat exchanger includes the following steps of Kern’s method according to Sinnott (2005): (a) assume overall heat transfer coefficient , U; (b) select number of shell and tube passes, calculate ΔTlm, correction factor, F and ΔTm; (c) determine heat transfer area; (d) decide tube size, type and arrangements; (e) Calculate number of tubes; (f) Calculate shell diameter; (g) Estimate tube-side heat transfer coefficient; (h) Decide baffle spacing and estimate shell-side heat transfer coefficient; (i) Calculate overall heat transfer coefficient and (j) Estimate tube and shell side pressure drop. Coolant is the contaminated solution of MEA with CO2 which is corrosive; hence it should flow through tube-side.
3.2 Overall heat transfer coefficient determination
a) Assume overall heat transfer coefficient, U
The hot and cold fluid pass through the heat exchanger is dilute stream with large amount of water, hence, its overall heat transfer unit is initially assumed as water, U = 1000 W/m2.°C. b) Select number of shell and tube passes, calculate ΔTlm, correction factor, F and ΔTm The heat exchanger is cross-flow with 1 shell pass and 2 tube passes. The log mean temperature can be obtained from equation 3.1. ∆Tlm=Th,i-Tc,o-(Th,o-Tc,i)ln(Th,i-Tc,oTh,o-Tc,i) (3.1) R=Th,i-Th,oTc,o-Tc,i 3.2 S=Tc,o-Tc,iTh,i-Tc,i (3.3) Where
* Th,i = 104.65°C is the inlet hot fluid temperature;
* Th,o = 104.65°C is the outlet hot fluid temperature;
* Tc,i = 104.65°C is the inlet cold fluid temperature;
* Tc,o = 104.65°C is the outlet cold fluid temperature.
The ΔTlm = 23.87°C, R = 1.08 and S = 0.43 is calculated from the equation 3.1, 3.2 and 3.3 respectively. In order to obtain the true mean temperature difference, the correction factor has to be found by using Figure 12.29 in Sinnott (2005) or equation 3.4.
The calculated true mean temperature difference is 21.25°C. c) Determine heat transfer area
* q = 8.81×106 W is the heat duty of the heat exchanger obtained from Hysys; * U = 1000 W/m2°C is the initial guess of heat transfer coefficient. Heat transfer area obtained is 414.62 m2.
d) Decide tube size, type and arrangements
The standard tube size with outer diameter 20 mm, inner diameter 18 mm and length 4.88m is used in this heat exchanger. The tube...
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