Column Operation

While dedicated laboratory-scale distillation can be used for solvent recovery, experimental laboratory-scale columns are used for collection of design data. The operational objective is usually to maximize the recovery of a component under the constraint of a desired purity level. The feature of the batch process which distinguishes it from the more familiar continuous counterpart is its inherently transient nature. The continuously changing feed composition must be

Ventilation fan

Ventilation fan

Figure 4 Safety system circuitry.

accounted for in the calculation theory. A variety of operation and completion criteria may be used depending on the process economics, equipment characteristics and product value. Several different basic operational modes are possible:

1. total reflux, with periodic dumping of the accumulated material from the condenser to the distillate tank;

2. constant reflux, with continuous variation of the instantaneous distillate composition, starting above and finishing below the desired product specification;

3. constant composition, with variable reflux ratio in order to keep the instantaneous distillate composition constant.

Operation strategies can take advantage of all three of these operational modes.

Initially the column is operated at total reflux with subsequent product collection at a purity higher than the final product specification. After this initial cut is withdrawn to the product tank, operation is switched to the constant composition mode and the equipment is run until the reflux ratio becomes so high that product collection is minimal. At this point the operation is switched to the constant reflux mode and continued until the average composition of the distillate drops to the desired level, when the equipment is shut down. Variations on this approach are often required due to equipment limitations. If the column is to be operated manually, then it might be difficult to maintain constant distillate composition. Also, if the column consists of less than five ideal stages, it is more advantageous to operate at constant reflux.

Prior to the initiation of any operational procedure, it is necessary to elaborate an operation schedule that can be used as a guide throughout the run. Although the procedures discussed here can be implemented for columns with a very low level of automation, they can also be used in application programs that run as a part of an automated control loop. One can use simplified calculation methods (e.g. McCabe-Thiele) or resort to more extensive numerical computations if the effect of some variables such as the hold-up is to be taken into account. Independent of the calcu-lational basis, better predictions of the composition profiles can be obtained if the stage efficiencies or at least the overall efficiency is known. Efficiencies depend on the physical properties of the system, particularly the viscosity and the relative volatility, but also on the geometric characteristics of the equipment. Determination of the overall and stage efficiencies can be easily accomplished by running the column at total reflux. When the column has reached steady state, the composition profiles within the column must be determined. The overall efficiency is easily calculated by stepping off theoretical stages in the McCabe-Thiele diagram between the equilibrium and the operating line, which at total reflux coincides with y = x. The number of theoretical stages is determined when the bottom composition is crossed. The Murphree efficiency for stage n receiving liquid from stage n — 1 and vapour from stage n + lis defined as:

Eqn [38] is a measure of the degree of separation achieved in the vapour going from stage n + 1 to stage n and can be visualized in the McCabe-Thiele diagram as a segment ratio, as shown in Figure 5. The maximum degree of separation is represented by the difference in the denominator, where the vapour leaving the stage is in equilibrium with the liquid phase of the same stage.

Determination of the vapour-phase composition is often more difficult than liquid and sample ports are generally only provided for the liquid phase. It is therefore useful to define Murphree efficiency of the liquid compositions as:

Once the efficiencies are determined, an operation schedule at constant reflux should be prepared. The operation schedule consists of a family of curves where the reflux ratio is plotted as a function of reboiler composition, holding the distillate composition constant as depicted in Figure 6. The curves can easily be generated in a spreadsheet if the equilibrium curve for the system can be regressed as an analytical form, x = f(y). A distillate composition (xD) is fixed as the fulcrum, around which all the operating lines pivot, as defined for different reflux ratios (RD), as shown in Figure 7. Since the number of ideal stages is known, the bottoms composition (xB) is found for each operating line by stepping off these stages between the equilibrium curve and the operating lines. Once xB is found for a particular operating line, the reflux ratio is changed, thereby defining another operating line, and the procedure is repeated to find the corresponding xB. In this way, a set of data points (Rd, xB) corresponding to a fixed distillate composition xD is determined. The next set is determined by the same procedure, changing the value of xD. These plots of Rd versus xB represent the reflux ratio required to achieve a specified distillate composition at a given composition within the reboiler.

The Rd versus xB curves can either be used in manual operation or integrated into an automated control strategy, where information about the feed drum composition is used to calculate the new required reflux ratio to keep xD constant. The choice of xD will depend on the minimum acceptable purity for the product. Sometimes, even when a higher purity is desired, the operating xD may be imposed by equipment restrictions. The flow rate range of the pumps, for example, might restrict operation to a certain range of the reflux ratio. In this case, switching to a lower distillate composition will allow longer runs, thus increasing the total amount of product.

Stopping criteria for an industrial distillation is generally dictated by economics. Operating costs accumulate continuously with time, because of energy and labour costs, as discussed above.

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