Material balance control columns

One of the most common disturbances in the energy balance is changes in environmental conditions, such as a rain shower on an air-cooled condensor. The response to this type of disturbance is different for an energy balance controlled colirrrm and a material balance controlled column. The differences can be explained using the following example. 

Long time response (2) long time response and increasing complexity (3) V/F control is not desirable in general (4) column bottom level control by steam flow is desirable. (Comments are due toT. Umeda). b Direct M.B. = direct material balance control Indirect M.B. indirect material balance control V/F = vapor to feed ratio 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. 

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. 

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. 

If the condenser load is disturbed owing to a heavy rain shower, a sequence of events happens in a column under material balance control. If the condenser coohng increases, the vapor flow in the column will increase (see Fig. 34.3). This will lead to an increase in the level of the reflux drum. Since the column is on material balance control, the liquid flow to the column will increase, as a result of which the ratio of vapor flow to liquid flow VIL remains more or less constant. 

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 ... 

It is important to note that the material-balance controls on any given column must be consistent with the material-balance controls on adjacent process equipment. In most cases material balance will be controlled by so-called averaging liquid-level or pressure controls. 

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... 

Determine holdup volumes required for smooth material-balance control and for liquid-level override controls at each end of the column. 

Make sure that column material-balance controls are properly designed and timed, and that hardware, especially level and flow transmitters and control valves, is in good working condition. If PI level controllers are"used, follow the tuning procedures of Chapter 16 auto overrides or nonlinear controllers should be used. It is usually desirable to cascade level control to flow control, in which case flow measurement should be linear. 

Practically speaking, material-balance control in the direction of flow means that the column must take whatever feed is supplied. This is, however, subject... 

Here top- and bottom-product flows (or their ratios to feed) are available for composition control, but the side draw is needed for column material-balance control. For this column, however, composititxi of the sidestream is of primary importance. Let us assiune that the feed consists of lumped low... 

To illustrate mathematical modeling for column material-balance control, let us first lose the conventional column of Figure 14.1. The feed, Wp, is split by the column into two parts top product, Wp, and bottom product, Wp. It is assumed that vent losses overhead are negligible. It is further assumed that the heat-transfer dynamics of both the condenser and the reboiler are negligible this will be true for most columns. Let us start at the column base and work up. For convenience the equations are written in Laplace transform notation. 

Let us look at a material-balance control scheme that is in the direction of flow. Let feed rate be set by averaging level control of the feed tank, let condensate receiver level set top-product flow, and let column base level set bottom-product flow. We will assume that each level controller is cascaded to the appropriate flow controller. 

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

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. 

The foregoing type of material balance is of large value in determining the best form of automatic control to apply to the column in order to maximize yields. 

---