Dual temperature control

The control strucmre used in this chapter uses single-end temperature with a reflux-to-feed ratio. If the column were using a dual-temperature control structure, would the turndown problem be inherently solved  

There are two questions associated with this possibility. First, how widely is dual temperature control used Second, does it really solve the turndown problem  

Concerning the first question, there are many more distillation columns that use singleend temperature control than use dual-temperature control. This is the standard control structure in the methanol/water separation. Using the reflux-to-feed ratio scheme and a single temperature result in energy consumption that is almost the minimum possible by using dual-composition control in the methanol/water system. Instrumentation complexity and loop interaction are avoided. So the structure used in this paper is widely applied in industrial columns. 

Concerning the second question, the dual-temperature control structure was tested on this methanol/water column and found to not work. 

The basic problem is that there is only one break in the temperature profile (see Fig. 15.5). SVD analysis shows only one stage with a significant U (see Fig. 15.15). There is a very small hump in the U curve at Stage 10, so this stage is selected to be controlled by manipulating the reflux-to-feed ratio. 

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Recommendation 5. Use dual-temperature control sensors, located as near the loads as possible and tied together by a low-select system, can help productivity. One sensor about 10% into the zone should control piece temperature, and a second sensor about 15% from the zone discharge should prevent overheating. Benefits will be greater if the loads are positioned to the side of the furnace where the sensors are located. 

The electrical parameters of varieties supplied with a liquid solution are lower than those of the varieties supplied with gases, but research in direct methanol fuel cells was done mainly with liquid supply. This type of fuel cell is much simpler to design and operate, inasmuch as neither a special evaporator nor dual temperature control (for the evaporator and for the reaction zone of the fuel cell) is needed. With the supply of a liquid methanol solution, all risk of the membrane drying out close to the anode side is eliminated. The elimination of heat is also easier with cells having liquid solution supply. All subsequent information on direct methanol fuel cells in the present chapter refers to the variety supplied with liquid water-methanol solution. 

The SVD results are similar to the sensitivity results. They suggest that Stage 8 can be controlled by reflux and Stage 29 by heat input. The singular values of the steady-state gain matrix are ai = 0.479 and 02 = 0.166, which gives a condition number CN = 0 102 = 2.88. This indicates that the two temperatures are fairly independent, so a dual-temperature control scheme should be feasible, at least from a steady-state point of view. 

The performance of the dual-temperature control structure is shown in Figure 15.16 for the same sequence of step disturbances imposed on the other structures. The distillate purity xD drops drastically when the reboiler low limit is reached because the reflux cannot be increased quickly enough. 

Figure 15.16 Dual temperature control step responses.

Holding a single-temperature constant near the top of the extractive column maintains acetone product purity, but it does not prevent acetone from moving down the column and appearing in the chloroform product from the solvent recovery column. We need to find a control stmcture that keeps chloroform from leaving out of the top and acetone from leaving out of the bottom. This implies a dual composition or dual temperature control stmcture. 

The design and control of a maximum-boiling azeotropic system has been studied in this chapter. Extractive distillation is shown to be capable of producing quite pure products. A conventional control structure is developed that provides effective disturbance rejection for both production rate and feed composition changes. Dual temperature control is required in the extractive column in order to handle feed composition disturbances. 

The potential for electrochemical corrosion in a boiler results from an inherent thermodynamic instability, with the most common corrosion processes occurring at the boiler metal surface and the metal-BW interface (Helmholtz double layer). These processes may be controlled relatively easily in smaller and simpler design boilers (such as dual-temperature, LPHW heating, and LP steam boiler systems) by the use of various anodic inhibitors. 

Figure 2. IS temperature Controller with dual thermocouple...

In 1981 the Los Alamos National Laboratory investigated for EPA the thermal destruction of wooden boxes treated with penta-chlorophenol (PCP). The incineration system consisted of a dual-chamber, controlled-air incinerator, a spray quench column, a venturi scrubber, and a packed-column acid gas absorber (11). Destruction efficiencies for PCP exceeded 99.99% for combustion chamber temperatures above 980°C, 20% excess air, and a retention time greater than 2.5 s. For these conditions, TCDD and... 

Let us consider a CSTR/separator/recycle system, where the first-order reaction A —> P takes place. Figure 4.3(a) presents the conventional control of the plant. The fresh feed flow rate is kept constant at the value F0. The reactor holdup V is controlled by the effluent. The reaction takes place at a constant temperature, which is achieved by manipulating the utility streams. Dual-composition control of the distillation column ensures the purities of the recycle and product streams. 

The control structure discussed in this section is presented in Figure 4.4(a). The reactor-inlet flow rate is fixed at the value l. Reactor effluent controls the reactor holdup V, while the coolant flow rate controls the reactor temperature. Dual composition control is used for the distillation column. The reactant is fed on level control. For illustration purposes, a buffer vessel was considered. This increases the equipment cost and might be unacceptable due to safety or environmental concerns. An alternative is to feed the reactant in the condenser dram of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected by a change of the holdup. 

The simultaneous control of two compositions or temperatures is called dual composition control. This is ideally what we would like to do in a column because it provides the required separation with the minimum energy consumption. However, many distillation columns operate with only one composition controlled, not two. We call this single-end composition control. 

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