Excess Reactant Design

Type Ijf is used to illustrate the design. The design questions then become, 1) which reactant should be in excess (e.g., light reactant or heavy reactant) and (2) what will be the separation sequence for this recycle plant Recall that in the reaction LLK + HHK LK+ 

EFFECTS OF BOILING POINT FIANKINGS ON THE DESIGN OF REACTIVE DISTILLATION 2000- 

Light-direct Light-indirect Heavy-direct Heavy-indirect 

Two column sequences are possible for this ternary separation direct and indirect (Fig. 17.19fl). A similar scenario applies when the light reactant is in excess (Fig. 17.19 7). The two conventional columns must separate the ternary mixture of LLK, LK, and HK. 

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Equimolar reactant feed is assumed, that is, Fqa = Fob. where Fq, is the feed flowrate of reactant i. This implies a neat reactive distillation design, as opposed to an excess reactant design (cf. Chap. 4). 

Note that the neat design is considered here instead of the excess reactant design often seen in methyl acetate hydrolysis. Because the tight product (LK component C) and heavy... 

Thus far, we have considered only the neat design. The excess reactant design may be preferable for systems with high TACs, especially for types I, II, and possibly IIIr. The motivation comes from the fact that some of the reactant concentrations are so low that a large reflux ratio and/or boilup ratio are required to achieved 95% conversion. We can refer to the composition profile of reactants A toward the column base and B toward the top in Figure 17.9 for type I. Other examples are reactant B in the reactive zone (Fig. 17.13) for type IIr and reactant A in the reactive zone (Fig. 17.17) for type III/. The excess reactant design is a simple means to achieve an improved reactant composition profile. 

Figure 17.19 Possible process configurations for type I (LLK + HHK LK + HK) with excess reactant design (a) heavy reactant (HHK) excess and (b) light reactant (LLK) excess.

As discussed in the flnal section of this chapter, an excess reactant design is more attractive than the neat design for some of the systems with difficult relative volatility rankings. 

The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Extensive washing of the resin with a series of appropriate solvents removes excess reactants and impurities from the solid support after each chemical step (Fig. 5). This task is carried out very efficiently by an automated wash station which utilizes two arrays of 48 luer lock needles to simultaneously add solvent to a set of alpha-beta reaction blocks. Microsoft Windows -based control software provides a custom-designed wash profile for each combinatorial experiment. This profile specifies the identity, volume, order and delivery speed of each wash solvent, as well as the agitation speed and time between solventdeliveries. 

Pellets were predried by heating to 150°C for 2 hours under vacuum (<1 micron). Silanization was performed by exposing the pellet to silane vapor for 1 hour and then pumping away the excess reactant. Silane vapor pressure was controlled by thermostatting the degassed liquid reagent. This procedure was designed to prevent polymer formation of the bound silane. 

Suppose an excess of a functional group is obtained by the addition of reactant designated B+B. In this case the two types of polymerizations discussed above become ... 

A schematic of a reactor made from a nonselective membrane for preventing the slip of an excess reactant is shown in Figure 24.2g. The principle of this reactor was outlined before. In the particular design shown, one of the reactants (5) is continuously recirculated on one side of the membrane so that complete conversion of A can be achieved on the opposite side without any slip. We refer to such a catalytic nonselective membrane reactor without packing as a CNMR-E. When packed, it is referred to as a CNMR-P. Another nonselective... 

Tendency of reactants to form thick viscous gel due to reasons such as overheat-ing/overcooling/power failures/addition of excess reactants, etc. The gearbox, coupling and motor must be designed accordingly... 

In the next three chapters we will explore various aspects of the ideal quaternary chemical system introduced in Chapter 1. This system has four components two reactants and two products. The effects of a number of kinetic, vapor-liquid equilibrium, and design parameters on steady-state design are explored in Chapter 2. Detailed economic comparisons of reactive distillation with conventional multiunit processes over a range of chemical equilibrium constants and relative volatilities are covered in Chapter 3. An economic comparison of neat versus excess-reactant reactive distillation designs is discussed in Chapter 4. 

In the two-column process, the recovery column acts concepmally as an on-line analyzer a higher recycle flowrate means that more of the excess reactant is leaving the reactive column, so the fresh feed flowrate of that reactant must be decreased. An effective control structure for the two-column system is to flow control the sum of the recycle stream and the appropriate fresh feedstream. When the recycle flowrate increases, the fresh feed flowrate is decreased to keep the total constant. Thus, the scheme changes the fresh feed flowrate to accommodate changes in the component inventory of the reactant. From a steady-state economic perspective, the two alternative processes (one column and two columns) have different capital investments and different operating costs. From a dynamic perspective, the two processes show different dynamic behavior and require different control structures. The economic design differences are quantitatively explored in this chapter. The control of these types of systems is discussed in Chapter 11. 

Excessive mixing Limit agitator power input and provide proper of reactants or impeller design impurities which, Return process to pilot or development to rede-promotes process to eliminate or minimize this emulsification. problem Poor phase separa- tion resulting in L it shaft speed problems in subse- Monitor shaft speed quent processing, phase separation steps or in down- stream equipment. I" " de-emulsifiers CCPS G-29 Lees 1996... 

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