Recycle of reactant

Reactors may be operated batchwise or continuously, e.g. in tubular, tubes in shell (with or without internal catalyst beds), continuous stirred tank or fluidized bed reactors. Continuous reactors generally offer the advantage of low materials inventory and reduced variation of operating parameters. Recycle of reactants, products or of diluent is often used with continuous reactors, possibly in conjunction with an external heat exchanger. 

Note that with separation of reactants from products and recycle of reactant back to the feed, the conversion of the reactant approaches completion, 1 (Figure 42). The... 

In a first attempt consider full recycling of reactants and no losses in products. 

The process flowsheet inside the battery limits (IBL) is at this stage unknown. However, the recycle of reactant may be examined. The patent reveals that the catalyst ensures very fast reaction rate. Conversion above 98% may be achieved in a fluid-bed reactor for residence time of seconds. Thus, recycling propylene is not economical. The same conclusion results for ammonia. The small ammonia excess used is to be neutralized with sulfuric acid (30% solution) giving ammonium sulfate. Oxygen supplied as air is consumed in the main reaction, as well as in the other undesired combustion reactions. 

Increase conversion For chemical processes involving reversible reactions, conversion of reactants to products is limited by thermodynamic equilibrium constraints. Therefore the reactor effluent by necessity contains both reactants and products. Separation and recycle of reactants are essential if the process is to be economically viable. 

In the second case, which is more general for industrial processes, the reaction rate is not large, so complete one-pass conversion of one reactant would require an excessively large reactor. Economics dictate that reactant concentrations must be significant and recycling of reactants is required. Now the separation section must recover both reactants for recycle. 

Therefore, a systemic approach is necessary in designing chemical reactors. In this chapter, we will demonstrate that a minimum reactor volume is required in a recycle system for flexible operation. Moreover, the nominal design should be placed sufficiently far away from the maximum sensitivity region of the relation conversion-reactor volume. It is worthy to note that multiple steady states are possible in the case of several recycles of reactants. The recycle of heat gives an even more complicated behaviour. Moreover, the stability and performance of the reactor system depends also on the control structures of other units. Hence, reactor design and feasibility of the control structures at the plant level should be examined simultaneously. 

To illustrate the procedure, we consider a fairly complex process sketched in Fig. 6.4, which shows the process flowsheet and the nomenclature used. In the continuous stirred-tank reactor, a multicomponent, reversible, second-order reaction occurs in the liquid phase A + B C + D. The component volatilities are such that reactant A is the most volatile, product C is the next most volatile, reactant B has intermediate volatility, and product D is the heaviest component a/ > ac > olb > OiQ. The process flowsheet consists of a reactor that is coupled with a stripping column to keep reactant. A in the system, and two distillation columns to achieve the removal of products C and D and the recovery and recycle of reactant B. 

Recovery and recycling of reactants, intermediates, or API s, in order to be used again, are considered acceptable. However, the use of these materials must be done using validated and documented procedures. If a recovered solvent is to be used in a different process (e.g., to produce a different API), there must be adequate validation and documentation to assure that it can be used without concern of cross contamination. Recovered solvents must meet predetermined specifications. Where recovered or recycled solvents are used in the manufacturing process, their use must be documented. 

Separation and purification of products is a major cost item in industrial chemical processes. It is important not only to isolate the desired product but also to recover the by-products. The economic success of many processes involves finding uses for co-produced materials. Greater selectivity in the reaction will minimize by-product formation and hence reduce the purification requirements. It is often economically desirable to run a reaction at a lower conversion level in order to increase selectivity even though this increases the amount of recycling of reactants. 

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