Energy balance control columns

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. 

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. 

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. 

In Chapter 2 (Section 2.9.2) the steady-state design of a reactor-stripper process was studied. Now we investigate the dynamic controllability of this process. The dynamic model of the reactor is the same as Eqs. (3.9)—(3.11) except there is a second stream entering the reactor, the recycle stream D (kmol/s) from the column with composition. Vj) (mole fraction A). The reactor effluent is F (kmol/s) with composition z (mole fraction A). The reactor component and energy balances are ... 

Consequently, the bottom composition (x) has to be controlled by manipulating the energy balance of the column. The control system computes V based on the equation, V = F(a + b[V/F]), where [V/F] equals the desired ratio of boil-up to feed. 

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. 

This scheme is recommended when the bottom flow is one of the smaller flows in the column, particularly when the bottom flow is less than 20% of the vapor boilup. This scheme has little interaction with the energy balance, as it provides a good range of control with only small changes in the bottom flow. However, the tuning of the sump level loop usually makes this scheme slower than the others. An inverse response is also possible with this sump level control loop. This type of response occurs when an increase in steam flow temporarily causes the sump level to increase before it begins to decrease. If this occurs, the level loop must be detuned even more. 

Scheme 4. This scheme directly mnaipaiaies die material balance by adjusting die bonom product flow. This scheme has the edvantage of little interaction between die material and energy balances, similar to scheme I. However, die sump level loop can significandy merease die ratio of effective dand tima to total lag time in the compositiou loup. Furthermore, the anmp level icop is ofleu subject to an inverse response, which can make the loop difficult to implement or evna impossible to control. Because of these difficulties, this arrangameat is recommended only when the bottom product flow is sigaificandy smaller than other flows in the column aad is less than 20% of die builup from the rehoiler.

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. 

Figure 18.3 shows examples of applying this procedure to benzene-toluene columns with different feed points and different feed compositions. Accordingly, trays 7,10, and 5 or 10 are the best control trays in Fig. 18.3a, b, and c, respectively. Figure 18.4, based on the column in Fig. 18.3a, shows how a variation in control tray temperature affects product composition with a correctly located and an incorrectly located control tray. When the temperature variation is caused by a change of pressure or in the concentration of a nonkey component, it will produce a steady-state offset in product composition. A disturbance in the material or energy balance will cause a similar temperature variation until corrected by the control action in this case, the offset will only be temporary. Figure 18.4 shows that the offset in either case is minimized when the control tray is selected in accordance with Tolliver and McCune s procedure (403). A dynamic analysis by these authors (403) indicated that the control tray thus selected tends to have the fastest, most linear dynamics. 

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.

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 ... 

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. 

Figure 17-4 shows process control instrumentation being used to control each process variable on a column. The level in the bottom of the column and the overhead accumulator must be controlled at 50%. The thermosyphon reboiler maintains the energy balance on the column at a set temperature. The bottom and the top temperatures form a gradient. Flow to the column and reflux lines allows the system to operate in automatic mode. Pressure is held at specified values on the... 

One of the key requirements of the basic column controllers is to maintain the energy balance. Energy enters the column as feed enthalpy and in the reboiler. It leaves as product enthalpy and in the condenser. If we neglect losses these inputs and outputs must balance. 

While we may have some limited control over feed enthalpy, and maybe some control over product enthalpy, the main source of energy is the reboiler and the main sink is the condenser. If the input energy is greater than the output then more vapour will be produced than condensed and the column pressure will rise. By controlling column pressure we therefore maintain the energy balance. 

After maintaining the energy balance, the next prime objective of the basic column controllers is to maintain the material balance. Material enters as feed and leaves as products. If these are not in balance then the inventory in the process will change. This is reflected by changing levels in the reflux drum and/or the column base. By controlling these levels we maintain the material balance across the 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. 

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