Distillation throughput

The ability to handle a wide range of throughput. Many of the alternatives to distillation can only handle low 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... 

In summary, distillation is not well suited for separating either low molar mass materials or high molar mass heat-sensitive materials. However, distillation might still be the best method for these cases, since the basic advantages of distillation (potential for high throughput, any feed concentration and high purity) still prevail. 

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. 

Calculation of the pressure drop and flooding rate is particularly important for vacuum columns, in which the pressure may increase severalfold from the top to the bottom of the column. When a heat-sensitive liquid is distilled, the maximum temperature, and hence the pressure, at the bottom of the column is limited and hence the vapour rate must not exceed a certain value. In a vacuum column, the throughput is very low because of the high specific volume of the vapour, and the liquid reflux rate is generally so low that the liquid flow has little effect on the pressure drop. The pressure drop can be calculated by applying equation 4.15 over a differential height and integrating. Thus ... 

Constraint (3.1) below illustrates the refinery raw materials balance in which throughput to each refinery crude distillation unit p P at plant i I from each crude type cr C CR is equal to the available supply SRe(. 

During the past several years such stills have been largely replaced by tube or pipe stills because of their lower initial cost, greater throughput, and economy of operation. A common type of operation utilizes a two-stage atmospheric and vacuum distillation unit (13). This type of operation has an important advantage—the asphaltic residue remains at the extreme temperature for only a fraction of a minute in the pipe stills as contrasted to several hours in shell-type stills. 

Separation of mists by the distillation process is important to remove the endotoxin or other contaminant. Distillators have their own upper limits of throughput capacity. Usually the more the amount of water generated at the unit period, the more the conductivity increases at the range of maximum capacity. This means that the purity of the water becomes worse. 

Flooding is by far the most common upper capacity limit of a distillation tray. Column diameter is set to ensure the column can achieve the required throughput without flooding. Towers are usually designed to operate at 80 to 90 percent of the flood limit. 

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]. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

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