Multicomponent feeds

Identify mislocated feed points where the feed, for binary mixtures, is not where the q-line intersects the equilibrium curve. This is not necessarily the case for multicomponent feed, however. 

In some operations, where the top product is required as a vapour, only sufficient liquid is condensed to provide the reflux flow to the column, and the condenser is referred to as a partial condenser. When the liquid is totally condensed, the liquid returned to the column will have the same composition as the top product. In a partial condenser the reflux will be in equilibrium with the vapour leaving the condenser. Virtually pure top and bottom products can be obtained in a single column from a binary feed, but where the feed contains more than two components, only a single pure product can be produced, either from the top or bottom of the column. Several columns will be needed to separate a multicomponent feed into its constituent parts. 

As was mentioned in Section 11.2, in multicomponent distillations it is not possible to obtain more than one pure component, one sharp separation, in a single column. If a multicomponent feed is to be split into two or more virtually pure products, several columns will be needed. Impure products can be taken off as side streams and the removal of a side stream from a stage where a minor component is concentrated will reduce the concentration of that component in the main product. 

Consider a simple process in which a multicomponent feed is allowed to separate into a vapor and a liquid phase with the phases coming to equilibrium, as shown in Figure 4.2. An overall material balance and component material balances can be written as ... 

Floudas, C. A. Separation Synthesis of Multicomponent Feed Streams into Multicomponent Product Streams. AIChE J 33 540-550 (1987). 

Most columns handle multicomponent feeds. But many can be approximated by binary or pseudobinary mixtures. For this example, however, we will make several additional assumptions and idealizations that are sometimes valid but more frequently are only cmde approximations. 

During the last three decades a lot of attention has been on the separation of a single multicomponent feed stream into pure substances using sharp separations. Relative to this effort, there has been less work for nonsharp separation synthesis, as well as the separation of several multicomponent feed streams into several multicomponent streams. 

Given a single multicomponent feed stream of known conditions (i.e. flowrate, composition, temperature and pressure) to be separated into a number of multicomponent products of specified compositions, determine an optimal distillation configuration that performs the desired separation task by allowing the use of nonsharp columns and which satisfies the criterion of minimum total annual cost. 

C. A. Floudas. Separation synthesis of multicomponent feed streams into multicomponent product streams. AlChEJ., 33(4) 540, 1987. 

C. A. Floudas and S. H. Anastasiadis. Synthesis of distillation sequences with several multicomponent feed and product streams. Chem. Eng. Sci., 43(9) 2407, 1988. 

FIGURE 1 Typical temperature profiles for several process heat exchanger applications (a) product cooler (b) feed heater with condensing stream (c) multicomponent feed heater with vaporization and superheating (d) pure-component product condenser (e) multicomponent product condenser (f) typical feed-effluent heat exchanger. 

It is possible to analyze multicomponent feed and copolymer composition data directly to determine the reactivity ratios that apply to a particular system [18]. 

Carry out a partial condensation on a multicomponent feed stream. 

The process is called Gemini because of its ability to produce two products from a multicomponent feed gas. It could simultaneously produce a primary H2 product at a purity of 99.999% with an H2 recovery of 87.1% and a secondary C02 product... 

All cases of practical importance in liquid chromatography deal with the separation of multicomponent feed mixtures. As shown in Chapter 2, the combination of the mass balance equations for the components of the feed, their isotherm equations, and a chromatography model that accounts for the kinetics of mass transfer between the two phases of the system permits the calculation of the individual band profiles of these compounds. To address this problem, we need first to understand, measure, and model the equilibrium isotherms of multicomponent mixtures. These equilibria are more complex than single-component ones, due to the competition between the different components for interaction with the stationary phase, a phenomenon that is imderstood but not yet predictable. We observe that the adsorption isotherms of the different compounds that are simultaneously present in a solution are almost always neither linear nor independent. In a finite-concentration solution, the amount of a component adsorbed at equilib-... 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

These could refer to the main and poisoning reactions, or to reforming or cracking reactions with different compounds in a multicomponent feed stream. Type II selectivity refers to parallel reactions with a single reactant ... 

For multicomponent feeds, specification of two key components and their distribution between distillate and bottoms is accomplished in a variety of ways. Preliminary estimation of the distribution of nonkey components can be sufficiently difficult as to require the iterative procedure indicated in Fig. 12.1. However, generally only two and seldom more than three iterations are necessary. 

The separation of a multicomponent feed produces subgroups or streams of adjacent, ordered components that are either separator feeds or final products. For example, a four-component process feed comprised of four ordered components can produce the 10 different subgroups listed in Table 14.2 from the five different sequences in Fig. 14.4. In general, the total number of different subgroups G, including the process feed, is simply the arithmetic progression... 

A first estimate of whether a multicomponent feed gives a two-phase equilibrium mixture when flashed at a given temperature and pressure can be made by inspecting the K-values. If all X-values are greater than one, the exit phase is superheated vapor above the dew point (the temperature and pressure at which the first drop of condensate forms). If all X-values are less than one, the single exit phase is a subcooled liquid below the bubble point (at which the first bubble of vapor forms). 

When the process feed is a binary mixture and the task is to separate that mixture into two products, a single separation device may suffice if an ESA is used. If an MSA is necessary, an additional separation device will be required to recover the MSA for recycle. For a multicomponent feed that is to be separated into nearly pure components and/or one or... 

Now consider the more general case of the synthesis of all possible ordinary distillation sequences for a multicomponent feed that is to be separated into P final products, which are nearly pure components and/or multicomponent mixtures. The components in the feed are ordered by volatility, with the first component being the most volatile. This order is almost always consistent with that for normal boiling point if the mixture forms nearly ideal liquid solutions, such that Eq. (7.3) applies. Assume that the order of volatility of the components does not change as the sequence proceeds. Furthermore, assume that any multicomponent products contain only components that are adjacent in volatility. For example, suppose that the previously cited mixture of benzene, toluene, and biphenyl is to be separated into toluene and a multicomponent product of benzene and biphenyl. With ordinary distillation, it would be necessary first to produce products of benzene, toluene, and biphenyl, and then blend the benzene and biphenyl. 

The FUG method, which applies to binary and multicomponent feeds, is described in detail by Seader and Henley (1998) and in Perry s Chemical Engineers Handbook (1997). Only the procedure is discussed here. The method involves five steps based on the desired separation of two key components in the feed. It includes an estimation of the separation of the nonkey components. 

Flash Vaporization of Multicomponent Feed. For the feed to the distillation... 

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