Batch distillation energy requirements

For on-site separation/purification of recovered solvent it is necessary to consider the number and complexity of distillations needed to obtain materials which are suitably pure for reuse. Where mixtures must be separated into individual solvents this can require several distillations, particularly where the solvents form azeotropes - this can significantly add to costs. The major costs associated with solvent purification are normally the capital required for distillation columns, energy and the additional staffing needs to oversee the operation. Where azeotropic distillations are required the cost of distillation columns can be greater than the capital cost of the recovery unit itself and staffing costs can be a significant variable cost (particularly if batch distillation is required). 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

Batoh distillation is frequently used for small-volume products. One column can be used to separate a multicomponent mixture instead of requiring NC — 1 continuous colimms. The energy consumption in batch distillation is usually higher than in continuous, but with small-volume, high-value products energy costs seldom dominate the economics. 

Rigorous and stiff batch distillation models considering mass and energy balances, column holdup and physical properties result in a coupled system of DAEs. Solution of such model equations without any reformulation was developed by Gear (1971) and Hindmarsh (1980) based on Backward Differentiation Formula (BDF). BDF methods are basically predictor-corrector methods. At each step a prediction is made of the differential variable at the next point in time. A correction procedure corrects the prediction. If the difference between the predicted and corrected states is less than the required local error, the step is accepted. Otherwise the step length is reduced and another attempt is made. The step length may also be increased if possible and the order of prediction is changed when this seems useful. 

In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation. 

In comparison to continuous distillation batch distillative separations have the disadvantage of higher thermal loading of the product due to the longer residence time, and the energy requirement is also higher. 

Fig. 2.3.2-16 Comparison of energy demand for batch distillation with upward and downward operating modes as a function of the required purity (the parameter is the relative volatility a, and the constant is the separation yield = 0.999/

Determine the operating time and energy requirements for a batch distillation... 

A5. Which system(s) require less energy for batch distillation than continuous distillation with the same amount of separation ... 

The main distillation types include atmospheric, vacuum, steam, azeotropic, extractive, and pressure distillation [45]. AU of these distillation methods can be carried out in a batch or continuous marmer with the exception of extractive distillation, which is solely continuous by nature. Gomplex solvent systems often require the use of multiple distillation columns in series to purify certain solvents that are not easily separated. The energy consumption in distillation columns can therefore be quite large because of the continuous operation of condensers and reboilers over extended periods of time. In order to cut down on these costs, both vacuum and steam distillation can be employed ]45]. 

Equality constraints h(D°, D°) = 0 may include, for example, a ratio between the amounts of two products, etc. Inequality constraints g(u, D°) < 0 for the overall operation include Equations 7.14-7.18 (the first two of which are easily eliminated when m and H are specified) and possibly bounds on total batch time for individual mixtures, energy utilisation, etc. Any variables of D° and D° which are fixed are simply dropped from the decision variable list. Here, Strategy II was adopted for the multiple duty specification, requiring B0 to be fixed a priori. Similar considerations hold for V, the vapour boilup rate. The batch time is inversely proportional to V for a specified amount of distillate. Also alternatively, for a given batch time, the amount of product is directly proportional to V. This can be further explained through Equations 7.24-7.26) ... 

Fatty acid esters of mono- and polyfunctional alcohols are the workhorses of oleochemistry. In many fields of application fatty acid methyl esters replace fatty adds because they are less corrosive. Chemical reactions can often be carried out under milder conditions. They have lower boiling points and require less energy to distil and to fractionate than the corresponding fatty acids. The elimination of methanol from the reaction products can be more easily achieved than that of water. Therefore fatty acid methyl esters are primarily used for the production of saturated and unsaturated fatty alcohols. Methyl esters are manufactured by acid catalyzed esterification of fatty acids in counter-current reaction columns or by alkaline transesterification starting directly from the triglyceride oils in a batch, semi-batch or continuous process (Figure 9.1.37> ° ° ... 

PV systems can be divided into two categories (i) Batch PV and (ii) Continuous PV. The batch PV system is simple with great flexibility however, a buffer tank is required for batch operation. On the other hand, continuous PV consumes very little energy, operates best with low impurities in the feed, and is best for larger capacities. Vapor phase permeation is preferred for direct feeds from distillation columns or for streams with dissolved solids. 

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