Distillation columns material balance control

Chapter 14 Distillation-Column Material-Balance Control... 

Adjust the benzene product flow rate. This is the overhead product of the benzene distillation column and depends on the value of the feed flow rate. In the next section we will see how to adjust the overhead flow rate in order to maintain the desired benzene purity and fractional recovery by a material balance controller which manipulates the distillate to feed ratio (D/F control). 

The vast majority of distillation columns use material balance control schemes. Only in circumstances where a satisfactory MB control scheme cannot be devised should an alternative control scheme be considered. 

Control of the column material balemce may be difficult when the distillate stream is small relative to reflux flow. Unless large swings in distillate flow are acceptable, changes in the small distillate flow will have little impact on accumulator level, and schemes 16.4a, b, and e will not maintain a steady accumulator level. In most cases, the level in the accumulator will be allowed to drift and will be periodically adjusted manually by changing reflux, boilup, or condensation rate. However, this mode of control does little to maintain the column material balance, and over a period of time may cause accumulation or depletion of the light components. Scheme 16.4d does not suffer from the above problem and is often favored with small distillate flows (300). 

Buckley, P. S., "Material Balance Control in Distillation Columns, Paper presented at the AIChE Workshop on Industrial Process Control, Tampa, FI. Nov. 11-13, 1974. 

Tolliver and McCune described four alternative material balance control schemes for distillation columns. Each scheme manipulates a different variable to control a temperature point in the column, that is, the D/F ratio, and this provides a structure for categorizing distillation control strategies (Table 5.1). There can be a number of variations of the four basic control schemes. 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

Consider the control of the top-product purity in distillation column T-201 in the DME process. As shown in Example 21.3. the material balance control is achieved by a flow controller on the reflux stream and a level controller on the top product stream From previous operating experience, it has been found that the top-product purity can be measured accurately by monitoring the refractive index (RI) of the liquid product from the top of the column. Stream 10. Using a cascade control system, indicate how the control scheme at the top of T-201 should be modified to include the regulation of the top-product conposition. 

In the previous chapter the procedure for the design of control schemes was discussed. The procedure was illustrated on a reactor with recycle. The selection of appropriate combinations of controlled and manipulated variables was relatively simple, since the interactions were limited. In this chapter the procedure will be applied to a distillation column. This is a unit operation with many interactions between the corrections that are made. Using a basic knowledge of the process dynamics, a basic control scheme is designed. Subsequently, two control schemes will be compared a basic control scheme based on material balance control and a control scheme based on "energy balance control. The distillation column can also be used to demonstrate the optimization of the control scheme. The principle is that the control scheme should be designed in such a way, that an objective function can be maximized. 

Often a goal of a control scheme for a distillation column is to maintain the quahty of the top product on specification while also maintaining the material balance. Material balance control and energy balance control are two control schemes that can achieve this. Different criteria can be considered for the selection of a control scheme. Since the performance of both control schemes has a large impact on the profitable operation of the distillation column, this choice is not trivial. 

The product flow of a distillation column that is controlled by an energy balance control scheme may fluctuate, since it is controlled by a level controller consequently it will affect the downstream process unit. If a distillation column is controlled by a material balance control scheme, the distillate flow is affected by the slow quality control loop and disturbances will be smoothed and only partially propagated to downstream process units. Therefore, when the product flow is not allowed to fluctuate or if the one product flow is much larger than the other, a material balance control scheme is selected. 

In the case of material balance control, the correcting actions of the quality controller on the product flow have no effect on the distillation column, i.e. the top product quality, until the level controller adjusts the reflux ratio. The controller should therefore be carefully tuned, such that the dynamics of the level control loop are reduced to a minimum. If material balance control is applied and the reflux drum is large, the power of control can be increased by keeping the ratio R/D constant with a flow ratio controller, which is adjusted by the level controller as a master controller. In that case the reflux is still the adjustable variable. 

The starting point of any design project is a definition of objectives. For distillation there are many possible approaches, but the one chosen here is one the authors have found broadly usdiil in virtually all kinds of processes. It has three main facets (1) material-balance control, (2) product qi ty control, and (3) satisfaction of constraints. As applied specifically to distillation columns, this philosophy su ests the following ... 

Once the basic concept of material-balance control has been selected for a process, one must apply the same concept to all process steps. It is for this reason that the first step in designing column controls is to determine the material-balance control arrangement. Control in the direction of flow is the most commonly used concept (although the least desirable), and a frequently encountered arrangement is shown on Figure 1.5. Here level in the condensate receiver (also commonly called reflux drum or accumulator) sets the top product, or distillate flow, while the level in the base of the column sets the bottom product flow in other columns base level sets steam or other heat-transfer media to the reboiler, in which case the condensate receiver level sets top product flow. 

Distillation column with material balance control in direction of flow... 

One exception to the mle of averaging level control for surge drums is in the case of distillation column hold-ups. Averaging level control should not be used to control the reflux drum level or the reboiler sump level. Tight level control is required for these vessels to maintain the integrity of the column material balance so that changes in the reflux rate and reboiler duty will have the desired effect on product compositions and yields without introducing additional lag to the system. 

The compositions are controlled by regulating reflux flow and boil-up. The column overall material balance must also be controlled distillation columns have little surge capacity (hold-up) and the flow of distillate and bottom product (and side-streams) must match the feed flows. 

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby "fight" each other. 

Once the large internal flow rates have been set via appropriate control laws, the index of the DAE system (7.21) is well defined, and a state-space realization (ODE representation) of the slow subsystem can be derived. This representation of the slow dynamics of the column can be used for the derivation of a model-based nonlinear controller to govern the input-output behavior of the column, namely to address the control of the product purity and of the overall material balance. To this end, the small distillate and bottoms flow rates as well as the setpoints of the level controllers are available as manipulated inputs. 

Four alternative control schemes are commonly used for distillation column control, as shown in Figure 3.15 through Figure 3.18, respectively. Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. 

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