Sequence of distillation

Porter, K. E., and Momoh, S. O., Finding the Optimum Sequence of Distillation Columns—An Equation to Replace the Rules of Thumb (Heuristics), Chem. Engg. J., 46 97, 1991. 

The sequencing of distillation columns and other types of equipment for the separation of multicomponent mixtures has received much attention in recent years. Although one separator of complex design can sometimes be devised to produce more than two produc ts, more... 

It is interesting to reflect on the heuristics for sequencing of distillation columns in Chapter 11. Heuristics 2, 3 and 4 from Section 11.1 tend to minimize the flowrate of nonkey components. Heuristic 1 relates to special circumstances when there is a particularly difficult separation8. 

The embedded model approach represented by problem (17) has been very successful in solving large process problems. Sargent and Sullivan (1979) optimized feed changeover policies for a sequence of distillation columns that included seven control profiles and 50 differential equations. More recently, Mujtaba and Macchietto (1988) used the SPEEDUP implementation of this method for optimal control of plate-to-plate batch distillation columns. 

The significant contribution of the utilities to the total cost of a distillation-based separation sequence provides an incentive to synthesize the sequences of distillation columns which feature heat integration alternatives. 

The superstructure for the illustrative example is shown in Figure 9.2. Note that we have two sequences of distillation columns (i.e., column 1 followed by column 2, and column 3 followed by column 4) since the feed stream has three components. In Figure 9.2, the potential process-process matches and the process-utility matches are indicated. The variables shown in Figure 9.2 are the unknown flow rates for the column sequences (i.e., F, F2,F3, F4) and existence variables for the columns (i.e., yi,y2, y3, Vi) and the process-process matches (i.e., yci,R2, yc2,Ri,yc3,R4, yc, R3)-The temperatures of the condensers and reboilers of each column, as well as the existence variables of the process-utility matches are not shown in Figure 9.2. 

To determine a least total annual cost heat-integrated sequence of distillation columns from the postulated superstructure, we define variables for... 

Early work in process synthesis focused on the solution of specific problems, such as the best sequence of distillation columns to perform separation of components in feedstreams into product streams. Another early problem was the synthesis of heat-exchanger networks. 

Heaven, D.L., Optimum Sequencing of Distillation Columns in Multicomponent Fractionation, M.S. Thesis, University of California, Berkeley, CA., 1970. 

Related Calculations. Sequences of distillation columns are often used for separating multicomponent mixtures. The question arises In what order should the individual components be separated For example, should a given component be separated in the first column, the second column, the... 

C4 and heavier coproducts are further separated in a sequence of distillation steps. Ethane and propane are typically recycle cracked. Refrigeration is supplied by cascade ethylene/propylene systems. 

The reactor effluent is now at 100°F and in a flash drum operating at 450psia we can separate the liquid mixture of aromatics (unreacted toluene, produced benzene, and diphenyl) from the gaseous stream of H2 and CH4. Part of the flash liquid is used to quench the reactor effluent and the rest goed to a sequence of distillation columns. 

The sequence of distillations is composed of three columns. The first, called stabilizer, is needed to stabilize the content of the liquid stream by removing the noncondensible gases (H2 and CH4). Without a stabilizer we cannot achieve the desired benzene composition in the second column. The toluene from the third column is recycled back to the beginning of the plant, while the diphenyl of the bottom product is used as fuel. 

Consider the multicomponent hydrocarbon feed in Fig. 12.2. This mixture is typical of the feed to the recovery section of an alkylation plant. Components are listed in order of decreasing volatility. A sequence of distillation columns... 

Figure 14.1. A simple sequence of distillation columns for ethylene purification.

Fresh feed streams of A and B are fed into a reactor. The reactor effluent, a mixture of unreacted A and B with products M, D, T, and C, is fed into a sequence of distillation columns. The light-out-first (direct) sequence is used, i.e., the lightest component is removed overhead in the first column, the next lightest component is removed overhead in the second column, and so on. The relative volatilities areA>M>D>T>B>C. 

If this procedure starts at point 10, for instance, the states 11, 12, 13,... are found that run towards the intermediate boiler b. A very special situation comes up when the sequence of distillation and reaction steps starts at point A on the chemical equilibrium curve. Here, the equihbrium vapor has the state A. After total condensation the liquid-phase reaction effects a concentration change from back to A. Hence, the concentration change of distillation,, is fully compensated... 

Fractionation of multicomponent zeotropic mixtures (i.e., mixtures without azeotropes) into all constituents can be performed by a sequence of distillation columns. Generally, different sequences of columns are feasible to achieve the same products. However, investment costs and energy demand of the feasible processes may... 

Be able to apply algorithmic methods to determine an optimal sequence of distillation-type separations. 

A. Define. We want to develop a sequence of distillation columns including recycle that we claim can produce 99.7" % pure methanol, methyl butyrate, and toluene. Proof requires running a process simulator to show that the separation is achieved. Note that optimization is not required. 

Synthesis of a separation flowsheet consists of determining the best sequence of distillation columns and complexes that will ensure the obtaining of the set of products of a set quality from the initial mixture. 

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