Separation and Recycle

An aspect of the hydroformylation reaction which is of particular importance in continuous commercial operation is the separation of the catalyst from product aldehyde and/or alcohol, together with its recovery and recycle into the reactant stream. This feature is of considerable economic and process importance for cobalt reactions and of extreme economic importance for rhodium reactions. 

Cobalt hydrocarbonyl is a volatile substance of limited stability at or above ambient temperature. Its tendency to decompose at undesirable sites in a process has posed a severe problem for commercial operations. Consequently, the patent literature contains numerous references to a variety of schemes for selectively removing cobalt from product and converting it to a form suitable for catalytic reuse. 

Some of the more prominent schemes for cobalt removal and recycle are  

Reaction with aqueous acid to form cobalt salts suitable for reuse. 

Extraction with aqueous acid accompanied by oxygen (46). 

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Figure 1.2 Process design starts with the reactor. The reactor design dictates the separation and recycle problem. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

In describing reactor performance, selectivity is usually a more meaningful parameter than reactor yield. Reactor yield is based on the reactant fed to the reactor rather than on that which is consumed. Clearly, part of the reactant fed might be material that has been recycled rather than fresh feed. Because of this, reactor yield takes no account of the ability to separate and recycle unconverted raw materials. Reactor yield is only a meaningful parameter when it is not possible for one reason or another to recycle unconverted raw material to the reactor inlet. By constrast, the yield of the overall process is an extremely important parameter when describing the performance of the overall plant, as will be discussed later. 

For all reversible secondary reactions, deliberately feeding BYPRODUCT to the reactor inhibits its formation at the source by shifting the equihbrium of the secondary reaction. This is achieved in practice by separating and recycling BYPRODUCT rather than separating and disposing of it directly. 

The liquid used for the direct heat transfer should be chosen such that it can be separated easily from the reactor product and so recycled with the minimum expense. Use of extraneous materials, i.e., materials that do not already exist in the process, should be avoided because it is often difficult to separate and recycle them with high efficiency. Extraneous material not recycled becomes an effluent problem. As we shall discuss later, the best way to deal with effluent problems is not to create them in the first place. 

In general, heterogeneous catalysts are preferred to homogeneous catalysts because the separation and recycling of homogeneous catalysts often can be very difficult. Loss of homogeneous catalyst not only creates a direct expense through loss of material but also creates an environmental problem. 

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

Some reactions are carried out in the liquid phase in a solvent. If this is the case, then the solvent is separated and recycled in arrangements similar to that shown in Fig. 4.5. 

Figure 4.7 Alternative separation and recycle structures for the production of monochlorodecane.

Sometimes it is extremely difficult to avoid vapor recycles without using very high pressures or very low levels of refrigeration, in which case we must accept the expense of a recycle compressor. However, when synthesizing the separation and recycle configuration, vapor recycles should be avoided, if possible, and liquid recycles used instead. 

Given the choice of a batch rather than continuous process, does this need a different approach to the synthesis of the reaction and separation and recycle system In fact, a different approach is not needed. We start by assuming the process to be continuous and then, if choosing to use batch operation, replace continuous steps by batch steps. It is simpler to start with continuous process operation... 

The normal boiling points of the materials are given in Table 4.6. Synthesize a continuous reaction, separation, and recycle system for the process, bearing in mind that the process will later become batch. 

Having considered the separation and recycling of material, the streams entering and leaving the process can now be established. Figure 4.17 illustrates typical input and output streams. Feed... 

In this case, because there are no raw materials losses in the separation and recycle system, the only yield loss is in the reactor, and the process yield equals the reactor selectivity. 

However, the concentration of impurity in the recycle is varied as shown in Fig. 8.5, so each component cost shows a family of curves when plotted against reactor conversion. Reactor cost (capital only) increases as before with increasing conversion (see Fig. 8.5a). Separation and recycle costs decrease as before (see Fig. 8.56). Figure 8.5c shows the cost of the heat exchanger network and utilities to again decrease with increasing conversion. In Fig. 8.5d, the purge... 

The two inner layers of the onion diagram in Fig. 1.6 (the reaction and separation and recycle systems) produce process waste. The process waste is waste byproducts, purges, etc. 

Separation and recycle systems. Waste is produced from separation and recycle systems through the inadequate recovery and recycling of valuable materials from waste streams. 

If the separation and recycle of unreacted feed material is not a problem, then we don t need to worry too much about trying to squeeze extra conversion from the reactor. 

Let us now turn our attention to losses from the separation and recycle system. 

Minimization of Waste from the Separation and Recycle System... 

Waste also can be minimized if the separation system can be made more efiicient such that useful materials can be separated and recycled more effectively. 

Figure 10.3a shows a simplified fiowsheet for the production of isopropyl alcohol by the direct hydration of propylene. Different reactor technologies are available for the process, and separation and recycle systems vary, but Fig. 10.3a is representative. Propylene... 

Additional separation and recycling. Once the possibilities for recycling streams directly, feed purification, and eliminating the use of extraneous materials for separation that cannot be recycled efiiciently have been exhausted, attention is turned to the fourth option, the degree of material recovery from the waste streams that are left. One very important point which should not be forgotten is that once the waste stream is rejected, any valuable material turns into a liability as an effluent material. The level of recovery in such situations needs careful consideration. It may be economical to carry out additional separation of the valuable material with a view to recycling that additional recovered material, particularly when the cost of downstream effluent treatment is taken into consideration. 

Figure 10.7 shows the basic tradeoff to be considered as additional feed and product materials are recovered from waste streams and recycled. As the fractional recovery increases, the cost of the separation and recycle increases. On the dther hand, the cost of the lost materials decreases. It should be noted that the raw materials cost is a net cost, which means that the cost of lost materials should be adjusted to either... 

The third source of process waste after the reactor and separation and recycle systems is process operations. 

Increasing reactor conversion when separation and recycle of unreacted feed is difficult. 

Increasing process yields through improved separation and recycling. 

Increasing process yields through feed purification to reduce losses in the reactor and separation and recycle system. 

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