Neat Reactive Column

The process considered is the ideal quaternary system with the reversible exothermic reaction occurring on the reactive trays A + B C + D with constant and favorable 

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The TAC of the single neat reactive column is 242,300 per year with an energy cost of 122,800 per year, as shown in Table 4.3. The two-column design with a 10% excess of B has a TAC of 308,100 per year with an energy cost of 147,000 per year. The... 

The one-column and two-column systems are both controllable using several types of control structures. The two-temperature control scheme for the neat reactive column handles most disturbances, but it cannot handle one type of feed composition disturbance. The use of an internal composition measurement provides more robust control. 

The reactive distillation column described in the previous section was designed to operate neat (precisely the correct amounts of reactants are fed to the column to satisfy the stoichiometry of the chemistry and there are only small amounts of unreacted reactants that leave in the streams leaving the column). Only a single column is required, so both capital investment and energy cost are minimized. However, it can be difficult to control a reactive column that operates in this neat mode. The problem is the need to feed in exactly enough of both reactants, down to the last molecule, to make sure that there is no excess of either reactant. If the balance is not absolutely perfect, the reactant that is in excess will gradually buUd up in the column, and it will not be possible to maintain product purities. This build-up may take hours or days, but eventually the column control structure will not be able to hold the products at their specified compositions. 

An alternative to operating neat is to operate the reactive column with an excess of one of the reactants. This eliminates the need to perfectly balance the reactant feeds in the reactive column itself, which makes the control of the reactive column easier. However, this mode of operation may have the disadvantage of requiring the recovery and recycle of the reactant that is in excess. The flowsheet typically consists of a two-column system a reactive column and a recovery column. 

We would expect that a single reactive column that is operated neat will have lower capital investment and energy costs than a two-column system. The purpose of this chapter is to give a quantitative comparison of these two alternative processes. 

Many industrial reactive distillation systems do not use stoichiometric amounts of reactants. An excess (10-20% above the stoichiometric amount) of one of the reactants is fed to the reactive column. There may be kinetic reasons for using an excess in some systems. These include suppressing undesirable side reactions, reducing catalyst requirements, and increasing conversion. However, even in the absence of kinetic reasons, the use of an excess of one of the reactants makes the control problem easier because the fresh feed flowrates of the components do not have to be precisely balanced in the reactive column. Achieving this exact balance may require the use of expensive and high maintenance on-line composition analyzers in some systems. In addition, the variability of product quality may be larger in the neat operation process because there arc fewer manipulated variables available and there is only one column to contain disturbances. 

Composition profiles in the reactive column are provided in Figure 4.5 for the 20% excess case. Note that the concentrations of A throughout the column are much lower than in the neat column (see Fig. 4.2). Figure 4.6 shows the composition profiles in the recovery column, and Figure 4.7 gives the temperature profiles in the two columns. Temperatures are lower in the recovery column because of its lower pressure (1 vs. 8 bar). 

Composition profiles in the reactive column are shown in Figure 4.13. The concentrations of A in the upper part of the column are higher than in the neat case (Fig. 4.2), and the concentrations of A in the lower part of the column are much higher than in the 20% excess B case (Fig. 4.13). 

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. 

In this chapter we take a look at an important example of a reactive distillation column operating in a plantwide environment. The reactive column is part of a multiunit process that includes other columns for recovery of one of the reactants. The process may give the impression that the reactive column is not operating in neat mode because of the need for reactant recovery. We will show that this is really not the case. The recovery of reactant is made necessary by the presence of azeotropes that unavoidably remove one of the reactants from the reactive column. 

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. 

In Chapter 4 we compared the economics of two alternative reactive distillation processes for the quaternary system with the reaction A + B C + D. The first was a single reactive column operating in neat mode. The second process was a two-column flowsheet in which the reactive column was fed with an excess of one of the reactants and the excess was recovered in a second distillation column. The one-column system was shown to be significantly less expensive. 

Figure 11.4 shows the single reactive column operating in neat mode with two ternperamres controlled by manipulating the two fresh feeds. The reflux ratio is controlled. Vapor boilup is the production rate handle. Liquid levels are controlled by product removal rates. Column pressure is controlled condenser cooling water in all of the control stmcmres and is not shown in the figures. 

Figure 11.15 shows an alternative control structure for the single reactive column operating in neat mode. Instead of controlling two temperatures by manipulating the two fresh feed-streams and setting the production rate by vapor boilup (Fig. 11.4), this alternative scheme... 

Now that we have demonstrated that the one-column neat reactive distillation system can be controlled but may require a composition measurement to handle all types of disturbances, we want to see how the two-column system with an excess of reactant handles disturbances. The case considered is the 20% excess of B fed to the reactive coliunn. Figure 11.25 contains the flowsheet of the reactive column/iecovery column system. The two product streams are distillate Di from the reactive column containing product C and bottoms B2 from the recovery column containing product D. The distillate D3 from recovery... 

GLS Fluidized with a Stable Level of Catalyst Only the fluid mixture leaves the vessel. Gas and liquid enter at the bottom. Liquid is continuous, gas is dispersed. Particles are larger than in bubble columns, 0.2 to 1.0 mm (0.008 to 0.04 in). Bed expansion is small. Bed temperatures are uniform within 2°C (3.6°F) in medium-size beds, and neat transfer to embedded surfaces is excellent. Catalyst may be bled off and replenished continuously, or reactivated continuously. Figure 23-40 shows such a unit. 

The reactive distillation columns considered in previous chapters were all operated in neat mode. The two reactants are fed in exactly the correct amounts to satisfy the stoichiometry of the reaction. The control system must be able to detect any imbalance, which will inevitably result in a gradual buildup of one of the reactants and a loss of conversion and product purities. 

The last item requires some discussion. One of the important aspects of the design and/ or control of a reactive distillation column operating in the neat mode is the need to balance the stoichiometry. The correct amounts of each reactant must be fed into the system. This usually requires some way of detecting any imbalance by measuring something inside the process that is an indication of a gradual buildup or depletion of one of the reactants. One way to achieve this is to measure an internal composition inside the eolumn. Beeause the composition of A is highest at its feed tray, this loeation is seleeted. 

In Parts I and II we explored the steady-state designs of several ideal hypothetical systems. The following three chapters examine the control of these systems. Chapter 10 considers the four-component quaternary system with the reaction A + B C + D under conditions of neat operation. Chapter 11 looks at control of two-column flowsheets when an excess of one of the reactants is used. Chapter 12 studies the ternary system A + B C, with and without inerts, and the ternary system A B + C. We will illustrate that the chemistry and resulting process structure have important effects on the control structure needed for effective control of reactive distillation columns. 

There are two reasons for the improvement in control. The first has already been discussed the elimination of inverse response. The second is equally important the two temperature controllers in the CS7-RR structure have the same action (direct). This means that when both control loops see a positive vapor boilup disturbance, which increases both tray temperatures, the two controllers will increase both fresh feeds. This helps to maintain the delicate stoichiometric balance between the reactants that is essential for neat operation of a reactive distillation column. Because a reactive distillation column acts like a pure integrator with respect to the reactants, this similar initial response is very important for CS7-RR, where feedstreams are used as manipulated variables. 

Unless we can consume aU of the HHK before it reaches the bottom of the reactive zone and react away aU of the LLK before it reaches the top of the reactive distillation, a single column configuration (e.g.. Fig. 17.5) is not possible. This is typically true for the neat design. 

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. 

The assumptions made in this work include 1) ideal vapor-hquid equilibrium, 2) equal molar feed (neat process), 3) the reactive holdup set by the column diameter, and 4) a sequential approach for optimization. It is interesting to note that the reactive zone can be placed at the upper section, lower section, middle, or both ends of the reactive distillation column, depending on the sequences of the relative volatilities. The principle is actually quite simple place the reactive zone where the reactants are most abundant and introduce the feeds to facilitate the reaction (considering the composition effect). 

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