Recycle purges

As discussed in the previous section, a purge can be used to avoid the cost of separating a component from a recycle. Purges can, in principle, be used either with liquid or vapor (gas) recycles. However, purges are most often used to remove low-boiling components from vapor (gas) recycles. 

Ethylene is to be converted by catalytic air oxidation to ethylene oxide. The air and ethylene are mixed in the ratio 10 1 by volume. This mixture is combined with a recycle stream and the two streams are fed to the reactor. Of the ethylene entering the reactor, 40% is converted to ethylene oxide, 20% is converted to carbon dioxide and water, and the rest does not react. The exit gases from the reactor are treated to remove substantially all of the ethylene oxide and water, and the residue recycled. Purging of the recycle is required to avoid accumulation of carbon dioxide and hence maintain a constant feed to the reactor. Calculate the ratio of purge to recycle if not more than 8% of the ethylene fed is lost in the purge. What will be the composition of the corresponding reactor feed gas ... 

OH = (1 - X )c - AToi) = 0.740 mol H2/mol Recycle-Purge Splitting Point Analysis Total Mole Balance input = output... 

The reader should note that four important processing concepts are bypass, recycle, purge, and makeup. With bypass, part of the inlet stream is diverted around... 

Step 1. Synthesis or selecting the structure of the flowsheet identification of the equipment, interconnecting and specifying the initial design values. In this step, a crude approximate flowsheet is created to consider recycles, purges, and possible separation schemes make a simple, linear model to assess the effects of major parameters and structural variations do necessary laboratory work get more details for physical properties, thermodynamics, utilities, and process units write unit models, if necessary and fix the flowsheet layout. 

From the problem statement we know that the diluted solution added to the crystallizer contains 10/11 parts water and 1/11 parts substance w/w (two relationships). Also, H2O, impurities, and glutamic acid are in the same proportion in all streams with the mother liquor (recycle, purge stream 7, and mother liquor in stream 8. From that we get six independent relationships as follows ... 

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). 

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). 

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. 

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). 

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

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.)...

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 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. 

The methyl ethyl ketazine forms an immiscible upper organic layer easily removed by decantation. The lower, aqueous phase, containing acetamide and sodium phosphate, is concentrated to remove water formed in the reaction and is then recycled to the reactor after a purge of water-soluble impurities. Organic by-products are separated from the ketazine layer by distillation. The purified ketazine is then hydrolyzed under pressure (0.2—1.5 MPa (2—15 atm)) to give aqueous hydrazine and methyl ethyl ketone overhead, which is recycled (122). The aqueous hydrazine is concentrated in a final distillation column. 

Maleic anhydride in the product stream is removed and converted to a maleic acid solution in a water scmbbing system. The maleic acid is sent to the hydrogenation to produce THF while the reactor off-gas after scmbbing is sent to the recycle compressor. A small purge stream is sent to incineration. 

---