Temperature Control with Bottoms Flow Rate

5 Temperature Control with Bottoms Flow Rate 

In the bottoms scheme, a column temperature controller manipulates a control valve in the bottoms line. The reflux drum level controller manipulates a valve in the distillate line. The feed and reflux are on flow rate control. The column base level controller manipulates the valve in the steam line to the reboiler. With this scheme, the separation power base is derived from the ratio of reflux/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

The manipulated bottoms flow rate scheme is not used very frequently because of problems with the column base level control using steam. When a thermosiphon reboller Is used and the steam flow Is Increased, there Is usually a reversal in the column base level response. The column level first rises and then falls. The usual case for using this scheme is when the feed concentration is 95% light key or more. In other words, most of the feed is distilled overhead from a few percent heavies or tars. The MRT point is usually in the reboiler. 

A variation on the scheme is to put the steam on flow rate control and let the column base level controller manipulate the feed rate to the column, similar to the way a maple syrup evaporator is run. 

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In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms flow rate. 

The experiments were performed in a CINC V-02 separator also known as the CS-50 (15). Two Verder VL 500 control peristaltic tube pumps equipped with a double pump head (3,2 x 1,6 x 8R) were used to feed the CCS. In case of the enzymatic reaction, the low mix bottom plate was applied. To operate the reactor at a desired temperature, it was equipped with a jacket which was coimected to a temperature controlled water bath with an accuracy of 0.01°C. The CCS was fed with pure heptane and pure water, both with a flow rate of 6 mL/min. Subsequently, the centrifuge was started (40 Hz, which corresponds to 2400 rpm) and the set-up was allowed to equilibrate for a period of 1 h. At this point, the heptane feed stream was replaced by the organic feed stream (oleic acid (0.6M) and 1-bntanol (0.9M) in heptane). After equilibration for 10 minutes, the reaction in the CCS was started by replacing the water stream with the aqueous feed stream (0.1 M phosphate buffer pH 5.6 containing 1 g/1 of the lipase form Rhizomucor miehei). Samples were taken at regular intervals and analysed by GC. 

Automatic valves are part of a control loop, which is shown in Figure 8.6. The loop contains a primary element, which measures the controlled variable, such as temperature, pressure, flow rate, and liquid level. The operation of a control loop is the same regardless of what variable is controlled. In the case of flow-rate control, the controller obtains the flow rate from transmitter a flow meter and compares the measured flow rate with a value that has been preset in the controller. If the flow rate is greater than the preset value, the controller increases air pressure on top or bottom of a diaphragm in the valve. Then, the valve partially closes to reduce the flow rate. On the other hand, if the flow rate is below the preset value, the controller will act to reduce the air pressure on the diaphragm, and hence the valve opens wider. Electric motors can also operate automatic control valves. 

Fig. 13.9 presents the flowsheet for the separation section of a HDA process. The first column is a stabiliser. The vapour distillate - a mixture of H2, CH4 with traces of benzene and toluene - can be used to hold constant the pressure. The quality of the bottom product is ensured by controlling a sensitive temperature in the top zone (inferential control) with the reboiler duty. Reflux and bottoms flow rates control the levels in reflux drum and sump, respectively. 

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-... 

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. 

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. 

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. 

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