Purge streams

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. 

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. 

The purge stream is sent to the biological treatment plant. 

Membrane modules have found extensive commercial appHcation in areas where medium purity hydrogen is required, as in ammonia purge streams (191). The first polymer membrane system was developed by Du Pont in the early 1970s. The membranes are typically made of aromatic polyaramide, polyimide, polysulfone, and cellulose acetate supported as spiral-wound hoUow-ftber modules (see Hollow-FIBERMEMBRANEs). 

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. 

The reactor outlet is flashed to remove ethylene which is then compressed and recycled a-olefins are separated from the solvent that contains the catalyst, treated to remove catalyst, and then distilled into commercial fractions. Most of the catalyst in the solvent is recycled but a portion is purged. The catalyst in the purge stream is recovered by reducing the oxidized nickel with boron hydride. 

In almost all modem plants, the ammonia is recovered by condensation and at modern synthesis pressures, ammonia is usually the source of refrigeration required. In order to maintain a high partial pressure of reactants, inerts entering with the make-up gas are normally removed using a purge stream. 

The designer usually wants to specify stream flow rates or parameters in the process, but these may not be directly accessible. For example, the desired separation may be known for a distiUation tower, but the simulation program requires the specification of the number of trays. It is left up to the designer to choose the number of trays that lead to the desired separation. In the example of the purge stream/ reactor impurity, a controller module may be used to adjust the purge rate to achieve the desired reactor impurity. This further complicates the iteration process. 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Gas recirculation in the anunonia synthesis section is necessary because only 9-30% conversion is obtained per pass over tlie catalyst." There are two tj pes of synthesis loops. One type recovers anunonia product before recycle compression. Inerts entering witli tlie makeup gas are removed with a purge stream. The ammonia is recovered by condensation, which requires refrigeration. Since airliydrous anunonia is readily available, it is normally used as tlie refrigerant. 

The concentration of any component in the purge stream will be the same as that in the recycle stream at the point where the purge is taken off. So the required purge rate can be determined from the following relationship ... 

Purge stream flow-rate] x [Specified (desired) recycle inert concentration]... 

If the unit simply divides the inlet stream into two or more outlet streams, each with the same composition as the inlet stream, then the split-fraction coefficient for each component will have the same value as the fractional division of the total stream. A purge stream is an example of this simple division of a process stream into two streams the main stream and the purge. For example, for a purge rate of 10 per cent the split-fraction coefficients for the purge stream would be 0.1. 

Having considered the feed, reaction, separation and recycling of material, the streams entering and leaving the process can be established. Figure 13.16 illustrates typical input and output streams. Feed streams enter the process and product, byproduct and purge streams leave after the separation and recycle system has been established. 

The purge stream containing hydrogen and methane and the byproduct diphenyl will be burned in a furnace and can be attributed with their fuel value given in Table 13.14... 

Another mode of semibatch operation involves the use of a purge stream to remove continuously one or more of the products of a reversible reaction. For example, water may be removed in esterification reactions by the use of a purge stream or by distillation of the reacting mixture. Continuous removal of product(s) increases the net reaction rate by slowing down the reverse reaction. 

The loss of expensive catalyst from the reactor system can be fatal for any process. Physical loss involves the removal of active catalyst from the closed loop of the process. This can include the plating out of metal or oxides on the internal surfaces of the manufacturing plant, failure to recover potentially active catalyst from purge streams and the decomposition of active catalyst by the process of product recovery. The first two can be alleviated to some extent by improvements in catalyst or process design, the last is an intrinsic problem for all manufacturing operations and is the subject of this book. 

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