Distillation columns energy balance control

Due to the above limitations, an energy balance control scheme such as scheme 16.7 is not recommended, but some situations exist where this scheme can offer better product composition control than many other alternatives. Superfractionators with a reflux to distillate ratio of 10 to 1 or more are one example. Here, distillate flow may be too small to satisfactorily control either accumulator level or column temperature. The author has experienced a satisfactory operation of scheme 16.7 in a propylene-propane splitter, with intermittent operator intervention to ac ust the material balance. The cycles in reflux and reboil (see above) could be tolerated, as the column was not operating close to its limits. In this column, scheme 16.7 gave tighter composition control than scheme 16.4d. 

Another type of heat exchanger is a kettle reboiler. Reboilers are energy-balance devices attached to distillation columns to help control temperature. Reboilers have two basic designs ... 

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 conclusion, it can be said that the energy balance control scheme exhibits some degree of self-regulation of the outpoint of the distillation column for changes in the feed composition and feed flow. In dealing with this type of disturbances, the energy balance control scheme is favored over the mass balance control scheme. 

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. 

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. 

Forty years ago these computed variables were calculated using pneumatic devices. Today they are much more easily done in the digital control computer. Much more complex types of computed variables can now be calculated. Several variables of a process can be measured, and all the other variables can be calculated from a rigorous model of the process. For example, the nearness to flooding in distillation columns can be calculated from heat input, feed flow rate, and temperature and pressure data. Another application is the calculation of product purities in a distillation column from measurements of several tray temperatures and flow rates by the use of mass and energy balances, physical property data, and vapor-liquid equilibrium information. Successful applications have been reported in the control of polymerization reactors. 

An energy balance scheme as in Fig. 16.7 usually requires continuous adjustment of a product rate (in Fig. 16.7, the distillate rate) by the operator, and a very slow level control action (in Fig. 16.7, accumulator level). Consider a rise in concentration of the light component in the feed. The bottom section temperature will drop, and the temperature controller will raise boilup. Column pressure will rise, and the pressure controller will increase the condensation rate. The accumulator level will rise, and the level controller will pour more reflux into the column. This in turn will reduce control tray temperature, and the temperature controller will raise boilup again. This will continue until reflux and boilup sufiiciently rise to keep the bottom section temperature up. In the meantime, the light component accumulates in the system, and this will cause further increase of reflux and boilup. 

Scheme 2. This scheme indirectly adjusts the material balance through the two level control loops. This arrangement has the advantage of reducing the ratio of effective dead time to total lag time within the composition loop. It has the disadvantage of allowing greater interaction between the material and energy balances because internal reflux is not held constant. This scheme should be considered when the reflux is smaller than other flows in the column and when the reflux to distillate ratio is 0.8 or less. It should also be considered for applications where reducing the ratio of effective dead time to total lag time in the composition loop is a significant and necessary consideration.

The basic components of a plate distillation column include a feed line, feed tray, rectifying or enriching section, stripping section, downcomer, reflux line, energy-balance system, overhead cooling system, condenser, preheater, reboiler, accumulator, feed tank, product tanks, bottom line, top line, side stream, and an advanced instrument control system. Plate columns hold trays that may be bubble-cap, valve, or sieve. Figure 6-19 shows the basic components of a plate distillation column. 

The column at this stage of the control design wdl have two MVs remainmg for use to control the composition of both products. Which these variables are depends on the choice of level control configuration. If the material balance scheme is in place then, usually, distillate flow and reboil are available or, less usually, bottoms flow and reflux. If the energy balance scheme is in place then reboil and reflux are available. 

One of the common applications of MVC is providing dual composition control on distillation columns. It is often the most practical way of resolving interactions. Here we work through a simple example of its design. Figure 12.117 shows the addition of MVC to a column with an energy balance scheme. 

When the same two-product distillation column is viewed in dynamics, the number of degrees of freedom increases from two to five. These three new dynamic degrees of freedom correspond to three new manipulated variables needed to control the integrating, inventory variables within the column that are not fixed by tbe steady-state material and energy balances alone. The inventory variables for this column are condenser level, reboiler level, and the column pressure. 

One of the most difficult aspects of distillation column control is the interaction effects between the material and energy balance and composition controls. Depending on the inventory controls, a heat input or removal can alter both the material draws and the compositions. This interaction can work for us or against us, depending on the control strategy. 

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. 

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