Distillation operating costs

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

Anhydrous Acetic Acid. In the manufacture of acetic acid by direct oxidation of a petroleum-based feedstock, solvent extraction has been used to separate acetic acid [64-19-7] from the aqueous reaction Hquor containing significant quantities of formic and propionic acids. Isoamyl acetate [123-92-2] is used as solvent to extract nearly all the acetic acid, and some water, from the aqueous feed (236). The extract is then dehydrated by azeotropic distillation using isoamyl acetate as water entrainer (see DISTILLATION, AZEOTROPIC AND EXTRACTIVE). It is claimed that the extraction step in this process affords substantial savings in plant capital investment and operating cost (see Acetic acid and derivatives). A detailed description of various extraction processes is available (237). 

Subsequent separation of this solvent imposes substantial capital and operating cost penalties. A Bayer AG patent (37) claims use of a solvent in which DNT is soluble, but in which the TDA is practically insoluble. This allows separation and recycle of the solvent to the reactor without any distillation process. 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Solvent Recovery The largest current industrial use of pei vapo-ration is the treatment of mixed organic process streams that have become contaminated with small (10 percent) quantities of water. Pei vaporation becomes vei y attractive when dehydrating streams down to less than 1 percent water. The advantages result from the small operating costs relative to distillation and adsorption. Also, distillation is often impossible, since azeotropes commonly form in multicomponent organic/water mixtures. 

An old variation of the conversion type is a catalytic combination unit. Development of this scheme was necessitated by the rising cost of refinery construction after World War II and by the great demand for capital for postwar expansion. The scheme reduced the investment and operating costs for refining equipment. The basic feature of the combination unit lies in the integration of the fractionation facilities of the reduced crude distillation and catalytic cracking sections. 

Replacement of the steam-jet ejector with a vacuum pump. The distillation operation will not be affected. The operating cost of the ejector and the vacuum pump are comparable. However, a capital investment of 75,000 is needed to purchase the pump. For a five-year linear depreciation with negligible salvage value, the annualized fixed cost of the pump is I5,000/year. 

A single-column distillation configuration called Flash Compact System has been proposed which is capable of delivering an equivalent high purity product. The key advantage lies in the lower capital and operating costs. The feed is heated and pre-flashed and then sent to a distillation column as two. separate vapour and liquid feeds. 

Thus, there may be many ways in which the separation can be carried out to produce the same products. The problem is that there may be significant differences in the capital and operating costs between different distillation sequences that can produce the same products. In addition, heat integration may have a significant effect on operating costs. Heat integration of distillation will be considered later in Chapter 21. 

Once a distillation column is in operation, the number of trays is fixed and very few degrees of freedom can be manipulated to minimize operating costs. The reflux ratio frequently is used to control the steady-state operating point. Figure El2.4a shows typical variable cost patterns as a function of the reflux ratio. The optimization of reflux ratio is particularly attractive for columns that operate with... 

A discussion on the relative merits of batch and continuous distillation is given by Ellis(36), who shows that when a large number of plates is used and the reflux ratio approaches the minimum value, then continuous distillation has the lowest reflux requirement and hence operating costs. If a smaller number of plates is used and high purity product is not required, then batch distillation is probably more attractive. 

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