Purgative

Also, instead of using two separators, a purge can be used (see Fig. 4.2c). Using a purge saves the cost of a separator but incurs raw materials losses and possibly waste treatment and disposal costs. 

This might he worthwhile if the FEED-BYPRODUCT separation is expensive. To use a purge, the FEED and BYPRODUCT must be adjacent to each other in order of volatility (assuming distillation is used as the means of separation). Of course, care should be taken to ensure that the resulting increase in concentration of BYPRODUCT in the reactor does not have an adverse effect on reactor performance. Too much BYPRODUCT might, for example, cause a deterioration in the performance of the catalyst. 

Rather than send the vapor to one of the separation units described above, a purge can be used. This removes the need for a separator but incurs raw material losses. Not only can these material losses be expensive, but they also can create environmental problems. However, another option is to use a combination of a purge with a separator. 

The hydrogen in the vapor stream is a reactant and hence should be recycled to the reactor inlet (Fig. 4.8). The methane enters the process as a feed impurity and is also a byproduct from the primary reaction and must be removed from the process. The hydrogen-methane separation is likely to be expensive, but the methane can be removed from the process by means of a purge (see Fig. 4.8). 

Figure 4.8 A flowsheet for the production of benzene uses a purge to remove the methane, which enters as a feed impurity and also is formed as a byproduct.

Determine the relation between the fraction of vapor from the phase split sent to purge (a) and the fraction of methane in the recycle and purge (y). 

Hydrogen lost in purge = 1554U Hydrogen feed to the process = 1554a + 269.2... 

Total flow rate of purge = 1554a + 81.79a + 287.5 = 1636a +287.5... 

Figure 4.9 shows a plot of Eq. (4.12). As the purge fraction a is increased, the flow rate of purge increases, but the concentration of methane in the purge and recycle decreases. This variation (along with reactor conversion) is an important degree of freedom in the optimization of reaction and separation systems, as we shall see later. 

Figure 4.9 Variation of vapor mole fraction of methane with purge fraction.

Given the estimate of the reactor effluent in Example 4.2 for fraction of methane in the purge of 0.4, calculate the.actual separation in the phase split assuming a temperature in the phase separator of 40°C. Phase equilibrium for this mixture can be represented by the Soave-Redlich-Kwong equation of state. Many computer programs are available commercially to carry out such calculations. 

One further problem remains. Most of the n-butane impurity which enters with the feed enters the vapor phase in the first separator. Thus the n-butane builds up in the recycle unless a purge is provided (see Fig. 4.13a). Finally, the possibility of a nitrogen recycle should be considered to minimize the use of fresh nitrogen (see Fig. 4.136). 

Can the loss of useful material in the purge streams he avoided or reduced by feed purification If the purge is required to remove b5q)roducts formed in the reactor, then this is clearly not possible. 

Can the useful material lost in the purge streams be reduced by additional reaction If the purge stream contains significant quantities of reactants, then placing a reactor and additional separation on the purge can sometimes be justified. This technique is used in some designs of ethylene oxide processes. 

It also should be noted in Fig. 4.4high concentration, then this reduces the loss of valuable raw materials in the... 

As with the case of byproduct losses, another cost needs to be added to the tradeoffs when there is a purge. This is a raw materials efficiency cost due to purge losses. If the PRODUCT formation is constant, this cost can be defined to be ... 

Cost of purge losses = cost of FEED lost to purge — value of purge... 

The purge usually only has value in terms of its fuel value. Alternatively, if the purge must be disposed of by effluent treatment. 

Again, as with the byproduct case, those raw materials costs which are in principle avoidable (i.e., the purge losses) are distinguished from those which are inevitable (i.e., the stoichiometric requirements for FEED entering the process which converts to the desired PRODUCT). Consider the tradeoffs for the reaction in Eq. (8.1), but now with IMPURITY entering with the FEED. 

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... 

Figure 8.5 Cost tradeoffs for processes with a purge when reactor conversion and recycle inert concentration are allowed to vary. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Obviously, the use of purges is not restricted to dealing with impurities. Purges can be used to deal with byproducts also. 

This eliminates the vapor space but sealing the edge can be a problem. Double seals can help and sometimes a fixed roof is also added above the floating roof to help capture any leaks from the seal. However in this case, the space between the fixed and floating roof now breathes and an inert gas purge of this space would typically be used. The inert gas would be vented to atmosphere after treatment. 

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. 

Some small amount of byproduct formation occurs. The principal byproduct is di-isopropyl ether. The reactor product is cooled, and a phase separation of the resulting vapor-liquid mixture produces a vapor containing predominantly propylene and propane and a liquid containing predominantly the other components. Unreacted propylene is recycled to the reactor, and a purge prevents the buildup of propane. The first distillation in Fig. 10.3a (column Cl) removes... 

Feed purification. Impurities that enter with the feed inevitably cause waste. If feed impurities undergo reaction, then this causes waste from the reactor, as already discussed. If the feed impurity does not undergo reaction, then it can be separated out from the process in a number of ways, as discussed in Sec. 4.1. The greatest source of waste occurs when we choose to use a purge. Impurity builds up in the recycle, and we would like it to build up to a high concentration to minimize waste of feed materials and product in the purge. However, two factors limit the extent to which the feed impurity can be allowed to build up ... 

In general, the best way to deal with a feed impurity is to purify the feed before it enters the process. Let us return to the isopropyl alcohol process from Fig. 10.3. Propylene is fed to the process containing propane as a feed impurity. In Fig. 10.3 the propane is removed from the process using a purge. This causes waste of... 

In early designs, the reaction heat typically was removed by cooling water. Crude dichloroethane was withdrawn from the reactor as a liquid, acid-washed to remove ferric chloride, then neutralized with dilute caustic, and purified by distillation. The material used for separation of the ferric chloride can be recycled up to a point, but a purge must be done. This creates waste streams contaminated with chlorinated hydrocarbons which must be treated prior to disposal. 

Perhaps the most extreme situation is encountered with purge streams. Purges are used to deal with both feed impurities and byproducts of reaction. In the preceding section we considered how the size of purges can be reduced in the case of feed impurities by purifying the feed. However, if it is impractical or uneconomical to reduce the purge by feed purification, or the purge is required to remove a byproduct of reaction, then the additional separation can be considered. 

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