Excess reactant operation

Purify the conjugate from excess reactants by dialysis or gel filtration using a desalting resin. Use 0.01 M sodium phosphate, 0.15M NaCl, pH 7.0, as the buffer for either operation. The conjugate may be further purified by removal of unconjugated enzyme using one of the methods described in Section 1.5, this chapter. 

The Merrifield method has a number of attractive characteristics beyond the simple steps outlined here. For example, because the product molecule (such as the monomer, dimer, or trimer) is attached to a solid support, a chemist can apply other operations to the system without fear of losing that product. The reaction system can be washed at almost any point. This property is very useful, for example, because it allows the chemist to remove excess reactant or undesired by-products of the reaction. 

When conversion is incomplete, the excess reactant(s) must be recycled therein lies a major distinction—single-pass vs. recycle operation. Some reactions are conducted in an essentially inert media such as a solvent, which also may be recycled. Finally, reactions are occasionally moderated by dilution with product, which then is recycled. To summarize, reactors may be classified as ... 

NEAT OPERATION VERSUS USING EXCESS REACTANT... 

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. 

Figure 4.16 compares the reactive column composition profiles in the two excess reactant cases with those for base case neat operation. The concentrations of A are lower than the base case when an excess of B is used and larger when an excess of A is used. The opposite is true for the concentrations of B. The concentration of C at the top of the column is lower when an excess of A used. The concentration of D at the bottom of the column is lower when an excess of B used. 

The use of excess reactant has been demonstrated to increase capital and energy costs when compared to neat operation. The dynamic control of these two flowsheets will be compared in Ghapter 11. 

However, the flowsheets for the first and second types of chemistry come in two flavors. The flowsheet can consist of either a single reactive column or a two-column system with a reactive distillation column followed by a recovery column and recycle of excess reactant back to the reactive column. The type of flowsheet depends on whether we want to operate the reactive distillation column neat (i.e., no excess of either reactant). The excess reactant flowsheet has higher capital and operating costs, but its control is easier. The control of this system is considered in Chapter 11. 

The principal reactions are reversible and a mixture of products and reactants is found in the cmde sulfate. High propylene pressure, high sulfuric acid concentration, and low temperature shift the reaction toward diisopropyl sulfate. However, the reaction rate slows as products are formed, and practical reactors operate by using excess sulfuric acid. As the water content in the sulfuric acid feed is increased, more of the hydrolysis reaction (Step 2) occurs in the main reactor. At water concentrations near 20%, diisopropyl sulfate is not found in the reaction mixture. However, efforts to separate the isopropyl alcohol from the sulfuric acid suggest that it may be partially present in an ionic form (56,57). 

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

The law of mass action, the laws of kinetics, and the laws of distillation all operate simultaneously in a process of this type. Esterification can occur only when the concentrations of the acid and alcohol are in excess of equiUbrium values otherwise, hydrolysis must occur. The equations governing the rate of the reaction and the variation of the rate constant (as a function of such variables as temperature, catalyst strength, and proportion of reactants) describe the kinetics of the Hquid-phase reaction. The usual distillation laws must be modified, since most esterifications are somewhat exothermic and reaction is occurring on each plate. Since these kinetic considerations are superimposed on distillation operations, each plate must be treated separately by successive calculations after the extent of conversion has been deterrnined (see Distillation). 

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