Interstage Coolers

As has been demonstrated, the acid gas warms significantly upon compression. In the design of a compressor the temperature should not exceed 180°C (350°F). In a multistage compressor the gas must be cooled on the interstage. 

Interstage cooling is usually achieved using aerial coolers. The design of these cooler is such that the exit temperature of the gas is about 40°C (120°F). In warmer climates this design temperature maybe as high as 50° or 55°C. 

In acid gas compression the cooling will also result in a phase change. In the early stages of compression the phase change is the condensation of an aqueous phase. At higher pressure there may also be a liquefaction of the acid gas. 

The design on an aerial cooler is the same as for any heat exchanger. As with the compressor, our design begins with the First Law of Thermodynamics, Equation (6.1). However, in the case of the cooler, there is no shaft work and it is the heat transfer that we wish to calculate. Thus equation (6.1) becomes  

From conservation of energy considerations, and assuming that there is negligible heat loss to the surroundings, the cold fluid gains all of the energy lost by the hot fluid. In terms of enthalpy, this is 

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A mixture of 3,000 scfin, dry basis, (14.7 psia and 60°F), 60% methane and 40% nitrogen is to be compressed from 16 psig to 3500 psig. Suction temperature is 90°E Intercoolers will use 85°F water cooling gas to 90°F, and the installation is essentially at sea level. The gas is saturated with water vapor. Five lb pressure drop is to be allowed for the interstage coolers. 

Line 7-8 represents the suction stroke of the second stage. The volume of the gas has been reduced in the interstage cooler to Vt that which would have been obtained as a result of an isothermal compression to P, 1. 

The same mass of gas passes through each of the cylinders and, therefore, if the interstage coolers are assumed perfectly efficient, the ratio of the volumes of gas admitted to successive cylinders is (P]/P2 ) / The volume of gas admitted to the second cylinder is then ... 

Air at 290 K is compressed from 101.3 kN/m2 to 2065 kN/m2 in a two-stage compressor operating with a mechanical efficiency of 85 per cent. The relation between pressure and volume during the compression stroke and expansion of the clearance gas is PV1-25 = constant. The compression ratio in each of the two cylinders is the same, and the interstage cooler may be assumed 100 per cent efficient. If the clearances in the two cylinders are 4 per cent and 5 per cent respectively, calculate ... 

Why do staged hydrogen compressors need interstage coolers ... 

Tile partially purilied synthesis gas leaves the C02 absorber containing approximately 0.1% CO2 and 0.5% CO. This gas is preheated at the methanator inlet by heat exchange with the synthesis-gas compressor interstage cooler and the primary-shift converter effluent and reacted over a nickel oxide catalyst bed in the methanator. The methanation reactions are highly exothermic and are equilibrium favored by low temperatures and high pressures. 

If necessary, first-stage reactor effluent may be further cooled to 200—250°C by an interstage cooler to prevent homogeneous and unselective oxidation of acrolein taking place in the pipes leading to the second-stage reactor (56,59). 

For the final reactor we begin at Tq 350 K and X = 0.6 and follow the line representing the equation for the energy balance along to the point of intersection with the equilibrium conversion, which is. X = 0.8. Consequently, the final conversion achieved with three reactors and two interstage coolers is (0.95) (0.8) = 0.76. 

What conversion could Im achieved in Example 8-8 if two interstage coolers were available that had the capacity to cool the exit stream to 350°K. Also determine the heat duty of each exchanger for a molar feed rate of A of 40 mol/s. Assume that 95% of equilibrium conversion is achieved in each reactor. The feed temperature to the first reactor is 300 K ... 

The approach to solving the above reactor system is to first develop the exit values for Reactor 1 and then to use these outlet variables as input for Reactor 2. The reactors will usually be operated at different temperature levels with stage 2 cooler than stage 1 so that the equilibrium for aromatic saturation reactions is favorably shifted. The interstage cooler is shown in the diagram for this reason. Furthermore, the second-stage catalyst may be different. 

The conversion-temperature plot for this scheme is shown in Figure 8-6. We see that with three interstage coolers 90% conversion can be achieved compared to an equilibrium conversion of 40% for no interstage cooling. 

Assume that economics dictate that we must run this reaction, to 70% conversion to make a profit. How much heat must be removed In the interstage cooler to be able to achieve this conversion at the exit of the second reactor What are the temperatures at the inlet and outlet of the second reactor ... 

The reaction. is carried out in two long, adiabatic, plug-flow reactors with an interstage cooler between them, as shown. in Figure 633. (Refer to the discussion in Section 1.4,2 for more on interstage cooling.) The feed consists of component A diluted in an inert N2 stream, j =... 

The compression ratio in each of the two cylinders is the same and the interstage cooler may be taken as perfectly efficient. If the clearances in the two cylinders are 4% and 5% respectively, calculate ... 

B. Interstage Coolers. Even with a suction chiller as a precooler, the compression ratio will be limited to about 3.5 before cooling of the gas again becomes necessary. 

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