Recycling with vapor-liquid-separation

Figure 10.1 shows the nine basic unit operations of the HDA process as described in Douglas (1988) reactor, furnace, vapor-liquid separator, recycle compressor, two heat exchangers, and three distillation columns, Two raw materials, hydrogen and toluene, are converted into the benzene product, with methane and diphenyl produced as by-products. The two vapor-phase reactions are... 

Problem 6.38 The refrigeration cycle in Figure 6-21 shows a vapor compression cycle with recirculation. The vapor-liquid separator after the throttling valve recycles the vapor to the compressor and passes the liquid to the evaporator. This process uses the refrigerant R-i34a with the evaporator temperature set at - 20 C, the condenser at 30 "C, and with compressor efficiency of 75%. Determine the energy and entropy balances and compare the efficiency of this process to the simple vapor-compression cycle without recirculation. 

Reactive distillation columns incorporate both phase separation and chemical reaction in a single unit. In some systems, they have economic advantages over conventional reac-tor/separation/recycle flowsheets, particularly for reversible reactions in which chemical equilibrium constraints limit conversion in a conventional reactor. Because both reaction and separation occur in a single vessel operating at some pressure, the temperatures of reaction and separation are not independent. Therefore, reactive distillation is limited to systems in which the temperatures conducive for reaction are compatible with temperatures conducive for vapor-liquid separation. 

One way to utilize a stabilizer Is illustrated In Figure 5, which is simply the Figure 4 process with the liquids from K and 4 diverted to a stabilizer. The stabilizer could be either refluxed or cold-feed, as a further variation. This process reduces the recycle load significantly in the two lower compression stages, as compared to the previous processes. This process also provides an additional control for the crude oil vapor pressure which can be independently varied, since the fractionator split can be controlled and the fractionator bottom product is blended with the crude stream. It may be desirable to blend this stream into separator 1... 

In the process (Figure 9-12), the feedstock (a vacuum residuum) is mixed with recycle vacuum residue from downstream fractionation, hydrogen-rich recycle gas, and fresh hydrogen. This combined stream is fed into the bottom of the reactor whereby the upward flow expands the catalyst bed. The mixed vapor liquid effluent from the reactor, either goes to flash drum for phase separation or the next reactor. A portion of the hydrogen rich gas is recycled to the reactor. The product oil is cooled and stabilized and the vacuum residue portion is recycled to increase conversion. 

Gas recycles usually involve a liquid-vapor phase separation in which the lighter reactants come off the top of the flash drum as vapor and are compressed so that they can be recycled back to the reactor inlet. A gas recycle almost always implies a gas-phase reaction. The major costs with gas recycle are compressor capital investment and compressor work. 

The most direct way to control the remaining levels would be with the exit valves from the vessels. However, if we do this we see that all of the flows around the liquid recycle loop would be set on the basis of levels, which would lead to undesirable propagation of disturbances. Instead we should control a flow somewhere in this loop. Acetic acid is the main component in the liquid recycle loop. Recycle and fresh acetic acid feed determine the component s composition in the reactor feed. A reasonable choice at this point is to control the total acetic acid feed stream flow into the vaporizer. This means that we can use the fresh acetic acid feed stream to control column base level, since this is an indication of the acetic acid inventory in the process. Vaporizer level is then controlled with the vaporizer steam flow and separator and absorber levels can be controlled with the liquid exit valves from the units. 

Donald F. Othmer while at Eastman Kodak during the 1920 s experimented using salts to concentrate acetic acid (14). He also developed an industrial process for distilling acetone from its azeotrope with methanol by passing a concentrated calcium chloride brine down the rectification column (15). Pure acetone was condensed overhead, and acetone-free methanol was recovered in a separate still from the brine which was then recycled. The improved Othmer recirculation still (16) has been the apparatus generally favored by investigators who have studied the effects of salts on vapor-liquid equilibrium. 

The reactor effiuent passes into a feed-product heat exchanger, where it is partially condensed. After washing with dilute caustic soda to neutralize traces of phosphoric acid, it passes into a second exchanger and on to a high-pressure separator tO give a liquid and a vapor stream. The condensate goes to purification and the vapor to recycle. The vapor is cooled by the recyde-gas cooler and scrubbed with water to remove alcohol. The build-up of impurities like methane and ethane is controlled at this point by venting a small stream of the recycle gas. 

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