Distillation column throughput

As an example, one very common retrofit situation is the replacement of distillation column internals to improve the performance of the column. The improvement in performance sought is often an increase in the throughput. This calls for existing internals to be removed and then to be replaced with the new internals. Table 2.8 gives typical... 

Distillation columns are expensive items in any plant, and are tricky to control. They should initially be built large enough to accommodate a proposed expansion. The reboilers, condensers, and pumps, however, do not need to be designed to handle any more than the initial throughput. Figure 5-1 shows how the auxiliary system may be expanded by placing similar equipment in parallel when the plant capacity is increased. 

Pervaporation can also be used to unload a distillation column, thereby reducing energy consumption and operating cost and increasing throughput. The example shown in Figure 9.20(c) is for the recovery of pure methanol by pervaporation of a side stream from a column separating a methanol/isobutene/methyl tertiary butyl ether (MTBE) feed mixture [14,15]. 

EBMax is a liquid phase ethylbenzene process that uses Mobil s proprietary MCM-22 zeolite as the catalyst. This process was first commercialized at the Chiba Styrene Monomer Co. in Chiba, Japan in 1995 (16-18). The MCM-22-based catalyst is very stable. Cycle lengths in excess of three years have been achieved commercially. The MCM-22 zeolite catalyst is more monoalkylate selective than large pore zeolites including zeolites beta and Y. This allows the process to use low feed ratios of benzene to ethylene. Typical benzene to ethylene ratios are in the range of 3 to 5. The lower benzene to ethylene ratios reduce the benzene circulation rate which, in turn, improves the efficiency and reduces the throughput of the benzene recovery column. Because the process operates with a reduced benzene circulation rate, plant capacity can be improved without adding distillation capacity. This is an important consideration, since distillation column capacity is a bottleneck in most ethylbenzene process units. The EBMax process operates at low temperatures, and therefore the level of xylenes in the ethylbenzene product is very low, typically less than 10 ppm. 

The diameter of a distillation column operating at a specified gas velocity varies with the square root of the volumetric throughput (ft /s) ... 

A case of practical interest is a chemical reactor coupled with a separation section, from which the unconverted reactants are recovered and recycled. Let s consider the simplest situation, an irreversible reaction A—>B taking place in a CSTR coupled to a distillation column (Fig. 13.5). Here we present results obtained by steady state and dynamic simulation with ASPEN Plus and ASPEN Dynamics. The reader is encouraged to reproduce this example with his/her favourite simulator. The species A and B may be defined as standard components with adapted properties. In this case, we may take as basis the properties of n-propanol and iso-propanol, and assume ideal phase equilibrium. The relative volatility B/A increases at lower pressures, being approximately 1.8 at 0.5 atm. We consider the following data nominal throughput of 100 kmol/hr of pure A, reactor volume 2620 1, and reaction constant =10 s". For stand-alone operation the reaction time and conversion are r= 0.106 hr and = 0.36. 

If the chemical plant must operate over a wide range of throughputs, the distillation columns in the plant must be able to turn down to low throughputs. Hydrauhc limitations will be unavoidably encountered, so the control structure must be able to avoid these limitations. In this chapter, we assume that the minimum vapor rate is the limitation. 

The objective in this chapter is to avoid minimum vapor flow rate limitations. If feed rates to a distillation column are reduced due to a reduction in plant throughput or a diversion of the feed stream to a more economical product (i.e., hydrogen gas to combustion turbine during peak power demand instead of methanol), the distillation control structure needs to be robust enough to handle these disturbances while maintaining product specifications. Three alternative control structures are explored to obtain this objective. 

The simulated reactive distillation unit includes a pre-reactor (adiabatic tubular fixed bed reactor) and the reactive distillation column. The advantage of using a pre-reactor for TAME synthesis is to insure a partial transformation of reactants before RD column, thus increasing the throughput of reaction system. 

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