Temperature Control with Distillate Flow Rate

2 Temperature Control with Distillate Flow Rate 

In the manipulated distillate scheme, a column temperature controller manipulates a control valve in the distillate line. In other words, the column temperature is the process variable that is controlled to a setpoint, and the controller output is 

Another variation on the manipulated distillate scheme is to use a setpoint for the steam/feed ratio to establish the separation power base. Generally, the feed flow rate signal should be lagged with an 8 to 20 min capacitance lag (filter), so the steam flow is proportional to a trailing average of the feed rate. Sometimes, this falls into the category of model-based predictive control because the McCabe-Thiele model, or a computer simulation model shows that the separation power base can be established by the steam/feed ratio as shown in Table 3.3. 

A less common variation of the distillate scheme is used on the occasion where the first column in a distillation train is for removing light components, and the bottoms flow rate needs to be very steady as the flow rate to a large diameter fractionator as the second tower. In this case, the bottoms flow rate is set manually for the controller that manipulates the bottoms valve. The column-base level controller in the first-column manipulates the feed rate to the first column. 

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An illustration of the use of chromatography in this industry is in the control of distillation towers. Distillation uses the difference in composition between a liquid and the vapor formed from that liquid as the basis for separation. The efficiency of the process is affected by temperature, pressure, feed composition, and feed flow-rate. Chromatography is used to monitor the composition of the feedstock and to apply feedforward control of the heat input (temperature) to the tower, or to monitor and control the composition of the product. In this latter case, the chromatograph output is simply compared with a set point, and the controller (using feedback) manipulates the temperature, pressure, or feed flow-rate by activating the appropriate final operator. Both types of distillation control are widely employed in petroleum refining. 

Possible candidates for manipulated variables are essentially the same as in non-RD, including reflux, distillate flow rate or reflux ratio at the top of the column and heating rate, bottoms flow rate, or reboil ratio at the bottom of the column. In addition, dosing of the reactants can be an interesting choice in RD, if this is compatible with the upstream processing of the plant. Possible candidates for measured variables are either product compositions or column temperatures. However, online measurement of concentrations is usually slow, expensive, and often not very reliable. Therefore inferential control schemes are preferred, where the product compositions are inferred from temperature measurements. However, the relationship between product compositions and column temperatures is frequently non-unique in RD [26, 98] and this can lead to severe problems as will be filustrat-... 

Figure 8.50 gives responses to 20% increases and decreases in the set point of the feed flow controller. The solid lines are increases and the dashed lines are decreases. Stable control is obtained with transients settling out in about an hour. Stage 3 temperature is tightly controlled by manipulating distillate flow rate. Bottoms water composition remains quite close to the desired 12 mol% specification. Distillate water purity remains quite close to the desired 99.5 mol% specification. 

The proposed control structure does not have these problems because the distillate is manipulated to control temperature with the reflux-drum level controlled by reboiler heat input. The effects of distillate flow rate and reboUer heat input do not conflict. Increasing distillate flow rate increases column temperatures because of material balance adjustment. The resulting decrease in reflux-drum level causes the level controller to increase reboiler heat input, which also increases column temperatures. Thus, the effects of these two variables do not conflict. If two input variables have opposite effects, the result can be inverse response, which may explain the dynamic problems found in this RR control stmcture for this particular highly nonlinear column. 

With these two specifications, the distillate flow rate is being used to control the tray 20 temperature. 

Tray 4 temperature on the Lehigh distillation column i controlled by a pneumatic Pf controller with a 2-mipute reset time and a 50 percent proportional band. Temperature controller output (COr) adjusts the Ktpoint of a steam flow controller (reset time 0.1 min and proportional band 100 percent). Column base level is controlled by a pneumatic proportional-only controller setting bottoms product withdrawal rate. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

Alatiqi presented (I EC Process Design Dev. 1986, Vol. 25, p. 762) the transfer functions for a 4 X 4 multivariable complex distillation column with sidestream stripper for separating a ternary mixture into three products. There are four controlled variables purities of the three product streams (jCj, x, and Xjij) and a temperature difference AT to rninirnize energy consumptiou There are four manipulated variables reflux R, heat input to the reboiler, heat input to the stripper reboiler Qg, and flow rate of feed to the stripper Lj. The 4x4 matrix of openloop transfer functions relating controlled and manipulated variables is ... 

As a minimum, a distillation assembly consists of a tower, reboiler, condenser, and overhead accumulator. The bottom of the tower serves as accumulator for the bottoms product. The assembly must be controlled as a whole. Almost invariably, the pressure at either the top or bottom is maintained constant at the top at such a value that the necessary reflux can be condensed with the available coolant at the bottom in order to keep the boiling temperature low enough to prevent product degradation or low enough for the available HTM, and definitely well below the critical pressure of the bottom composition. There still remain a relatively large number of variables so that care must be taken to avoid overspecifying the number and kinds of controls. For instance, it is not possihle to control the flow rates of the feed and the top and bottom products under perturbed conditions without upsetting holdup in the system. 

The control of the separation section is presented in Figure 10.11. Although the flowsheet seems complex, the control is rather simple. The separation must deliver recycle and product streams with the required purity acetic acid (from C-3), vinyl acetate (from C-5) and water (from C-6). Because the distillate streams are recycled within the separation section, their composition is less important. Therefore, columns C-3, C-5 and C-6 are operated at constant reflux, while boilup rates are used to control some temperatures in the lower sections of the column. For the absorption columns C-l and C-4, the flow rates of the absorbent (acetic acid) are kept constant The concentration of C02 in the recycle stream is controlled by changing the amount of gas sent to the C02 removal unit The additional level, temperature and pressure control loops are standard. 

An interchanger would exchange heat between two process streams, such as a pre-heater on a distillation column recovering heat from the bottom stream to the feed stream, or a pre-heater on a boiler recovering heat from the stack gas to the combustion air. In these cases, the flow rates of the two process streams are set by other control objectives and they are not available as manipulated variables. Only one process stream temperature can be controlled, and this should be achieved with a bypass of that stream, as previously discussed. 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

The stream defined below is heated to 100°C to be partially vaporized in a flash drum before entering a distillation column. The fraction vaporized is controlled by the flash drum pressure. Calculate the required pressure at 100°C to have 20% mole vaporization, assuming Raoult s law applies. What are the products flow rates and compositions The constants for the Antoine Equation 2.19 are given for each component, with the pressure in kPa and the temperature in K. 

In the extractive-stripping (ES) mode the extractor and stripper are combined together. The extractor section is contacted with the lean gas and the rich gas flows down into the stripper column coming in contact with countercurrent stripped hydrocarbons from the reboiler at the bottom of the ES column. The recovery of the desired compounds is controlled by the lean gas flow rate, stripper bottom temperatures and operating pressures. The rich solvent leaving from the ES column is expanded to the pressure of operation in the product column, which is essentially a distillation column. Here the... 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

It consisted of a single-pass, five-plate, water-cooled condenser capable of independent flow and temperature control. Heat was supplied to the evaporator reservoir, so that it could be operated at selected temperature levels. The rotating assembly, consisting of a shaft with spacers capable of accepting six disks 12 inches in diameter, of varying thickness and composition, was equipped with a variable speed drive. With the arrangement shown, approximately 2 sq. feet of disk-condenser surface area was available for mass transfer. Tests were performed on this unit at atmospheric pressure, using city water (125 p.p.m. of dissolved solids) as feed, to determine the effect on distillation rate of (1) reservoir tem-... 

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