Recycles feed impurities

Feed impurities. So far only cases in which the feed is pure have been considered. An impurity in the feed opens up further options for recycle structures. The first option in Fig. 4.4a shows the impurity... 

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). 

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 ... 

Now there are two variables in the optimization. These are the reactor conversion (as before) but now also the concentration of IMPURITY in the recycle. For each setting of the IMPURITY concentration in the recycle, a set of trade-offs can be produced analogous to those shown in Figures 13.17 and 13.18. Figure 13.19 shows the tradeoffs for the feed impurity case and a purge with fixed concentration of impurity in the recycle1213. 

Impurities affecting the catalyst must be removed. Evaluate the cost of an extra purification system for feeds, as well as the cost of recycling harmful impurities, including equipment fouling and maintenance. 

Current zeolite catalysts already operate at process temperatures that require minimal external heat addition. Heat integration and heat management will be of increasing concern at the lower benzene to propylene ratios because the cumene synthesis reaction is highly exothermic (AHf= -98 kJ/mole). Recycle, particularly in the alkylation reactor, is likely to become increasingly important as a heat management strategy. The key will be how to limit the build-up of byproducts and feed impurities in these recycle loops, particularly as manufacturers seek cheaper and consequently lower quality feedstocks. As in the case of ethylbenzene, process and catalyst innovations will have to develop concurrently. 

The hydrogen in the vapor stream is a reactant and hence should be recycled to the reactor inlet (see flow diagram). The methane enters the process as a feed impurity and is also a byproduct from... 

Determining which byproducts to recover, purify, and sell is usually more difficult than determining the main product. Some byproducts are produced by the main reaction stoichiometry and are unavoidable unless new chemistry can be found. These stoichiometric byproducts must usually be sold for whatever price they can get otherwise, waste disposal costs will be excessive. Some examples of stoichiometric byproducts are given in Table 6.1. Other byproducts are produced from feed impurities or by nonselective reactions. The decision to recover, purify, and sell recycle or otherwise attenuate or dispose of them as wastes is an important design optimization problem and is discussed in Section 6.4.8. 

The above simple analysis highlights an important issue in process dynamics the influence of positive and negative feedback on system s stability. Instability can occur in recycle systems due to positive feedback when the gain is larger than unity. We may give as example the recycle of energy developed by an exothermal reaction in an adiabatic PFR for feed preheating. Instability may occur because of the exponential increase in reaction rate with the temperature when this cannot be properly controlled (Bildea Dimian, 1998). Another example is the recycle of impurities in a plant with recycles, whose inventory cannot be kept at equilibrium by the separation system (Dimian et al., 2000). 

The first step is performed in liquid phase with air as oxidizing agent under pressures of 3.5-5 atm to maintain liquid conditions. With a cobalt naphthenate-catalyst, temperatures in the range of 120-130 C are adequate, whereas without catalyst the temperatures need to reach 145-150 0. An important feature of the process is the relatively low per-pass conversion of about 15 per cent of the cyclohexane charge. Water formed by the oxidation reaction and impurities in the feedstock such as sulfur-containing compounds and other hydrocarbons are removed azeotropically as reaction proceeds. Unless reaction water is removed, the air-oxidation ceases after about 25-30 per cent conversion. Removal of feed impurities and oxidation by-products results in a clean recycle stream. 

Their physical properties, such as viscosity, density, thermal stability, or surface tension, are important to consider during the design phase of new processes. The demonstration of the thermal and chemical stability of ILs and the recyclability can only be proven through continuous pilot plant runs. On the other hand, the sensitivity to feed impurities that can accumulate in the ionic phase requires feed pretreatments or guard beds for some scheduled applications. 

Reactor effluents are almost never products that meet purity specifications. Besides the products, effluents may contain reactants, inerts, products of undesired side reactions, and feed impurities. Thus, almost every chemical process that involves a chemical reaction section also involves one or more separation sections in addition to one or more recycle streams. A major challenge of process design is to devise an optimal scheme for uniting the reaction and separation functions of a process. This chapter presents many of the considerations involved in that optimization. Although Figure 7.1 shows only one reactor section, multiple reactor sections are sometimes required, with separation sections located between each pair of reactor sections... 

Industries. (Courtesy of CF Industries.) (b) Very simplified flow diagram of an ammonia synthesis plant. Only the synthesis section is discussed in the text. The feed preparation section is more complex and expensive than the synthesis section. The seemingly illogical placement of the chiller and separator so that they process the fresh feed plus the recycle is dictated by the fact that some feed impurities dissolve in the hquid ammonia, and thus are prevented from entering the reactor. The reactor converts only about 15% of the feed on each pass [2]. 

Figure 4.4 Introduction of an impurity with the feed creates further options for recycle structures. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

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... 

A schematic diagram of a six-vessel UOP Cyclesorb process is shown in Figure 15. The UOP Cyclesorb process has four external streams feed and desorbent enter the process, and extract and raffinate leave the process. In addition, the process has four internal recycles dilute raffinate, impure raffinate, impure extract, and dilute extract. Feed and desorbent are fed to the top of each column, and the extract and raffinate are withdrawn from the bottom of each column in a predeterrnined sequence estabUshed by a switching device, the UOP rotary valve. The flow of the internal recycle streams is from the bottom of a column to the top of the same column in the case of dilute extract and impure raffinate and to the top of the next column in the case of dilute raffinate and impure extract. 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

The carbon monoxide product is removed from the top of the column and warmed against recycled high pressure product. The warm low pressure stream is compressed, and the bulk of it is recycled to the system for process use as a reboder medium and as the reflux to the carbon monoxide column the balance is removed as product. The main impurity in the stream is nitrogen from the feed gas. Carbon monoxide purities of 99.8% are commonly obtained from nitrogen-free feedstocks. 

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