Level, pressure, temperature and composition control

Measurement of fractionating column variables must be within certain tolerances of accuracy, speed of response, sensitivity and dependability they must also be representative of the true operating conditions before successful automatic control can be realized. The instrument equipment selected, the installation design and the location of the measuring points determine these requirements. 

Locating temperature, pressure, flow and composition measuring points for automatic control systems depends on the control scheme used and the static and dynamic 

The interaction of temperature, pressure, and composition will differ with location in the column. The selection of a temperature control point in a fractionating column, which is determined assuming that the pressure and composition are constant, may be unsatisfactory when these variables are permitted to vary with changing process conditions, i.e. feed composition changes. The complex effect of all of the sources of disturbances in the form of changing process conditions on the measuring point must be considered for dynamic stability via dynamic simulation. 

Pressure control is a primary requirement for all towers because of its direct influence on the separation process. Columns are typically designed to operate at sub-atmospheric, atmospheric, or above atmospheric pressure. Tower pressure control configurations can also be required to vent varying amounts of inerts from the overhead accumulator. The venting of inerts or maintaining the desired operating pressure is often the cmx of the control problem. 

In a total condensing service, when varying quantities of inerts are present in a pressurized tower, it is often necessary to vent or alternatively inject a blanketing inert gas. This is normally accomplished using a split-range control scheme, as shown in 

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In this chapter, five typical control loops will be analyzed flow, level, pressure, temperature, and composition. The principal dynamic elements of eaeh proeess will be derived and will be related to the closed-loop response. Constraints and nonlinearities will be included, as well as means for coping with them. A few additional comments will serve to distinguish those control problems which are not typical or which appear to cross into other areas. 

A eonsequence of using a bulk property for detection is that this property of the solvent must be controlled very closely the refractive index of the eluant is sensitive to fluctuations in pressure, temperature and composition. Whilst the pressure and composition can be controlled using pulse dampners and reciprocating pumps, the limits of sensitivity and stability of the RI detector are determined by temperature. The temperature must be controlled to +0.0001 K for accepted noise levels. Fluctuations in the RI caused by temperature and noise changes are compensated for by use of a reference cell. 

General guidelines for selection of controller type (P, PI, etc.) and controller settings are available for common process variables such as flow rate, liquid level, gas pressure, temperature, and composition. The general guidelines presented below are useful but they should be used with caution, because exceptions do occur. 

Initially use proportional-only controllers in all loops except flow7 controllers, where the normal tight tuning can be used K = 0.5 and T = 0.3 minutes). Set the gains in all level controllers (except reactors) equal to 2. Adjust the temperature, pressure, and composition controller gains by trial and error to see if you can line out the system with the proposed control structure. If P-only control cannot be made to work, PI will not w7ork either. When stable operation is achieved, add a little reset action to each PI controller (one at a time) to pull the process into the setpoint values. 

To maintain the production rate, product quality, and plant safety requires a data acquisition and control system. This system consists of temperature, pressure, liquid level, flow rate, and composition sensors. Computers record data and may control the process. Modem chemical plants use program logic controllers (PLC) extensively. According to Valle-Riestra [20], instrumentation cost is about 15% of purchased equipment cost for little automatic control, 30% for full automatic control, and 40% for computer control. 

Since process analysis has become so critical to the computers that control optimal operating conditions, the information from the field measurements, such as flow, temperature, pressure, level, compositions, etc. must be reliable and accurate. These measurements must receive adequate maintenance in order for the control to be successful. Compositional information from online analyzers provides more detailed information for process control than conventional pressure, temperature, and level data thus, online analyzers justifiably require more attention than normal. 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

The Tennessee Eastman test-bed problem (Downs Vogel, 1993) involves the control of five unit operations. The simulated plant has 41 process variables and 12 manipulated variables as illustrated in Figure 2, which are nnodeled with 50 state variables. Out of 41 process variables there are 22 controllable outputs including level, pressure, temperature, flow and 19 composition indicators. The chemical reactions are irreversible and occur in the vapor space of the reactor. The formation of an inert byproduct, F, is undesirable. The products G and H accumulate in the reactor. In this paper the desired set point is 50% G and 50% H on a mass basis. By-product F may be present in the product with 97.5% of the product being composed of G and H. 

The ideal variable to measure is one that can be monitored easily, inexpensively, quickly, and accurately. The variables that usually meet these qualifications are pressure, temperature, level, voltage, speed, and weight. When possible the values of other variables are obtained from measurements of these variables. For example, the flow rate of a stream is often determined by measuring the pressure difference across a constriction in a pipeline. However, the correlation between pressure drop and flow is also affected by changes in fluid density, pressure, and composition. If a more accurate measurement is desired the temperature, pressure, and composition may also be measured and a correction applied to the value obtained solely from the pressure difference. To do this would require the addition of an analog or digital computer to control scheme, as well as additional sensing devices. This would mean a considerable increase in cost and complexity, which is unwarranted unless the increase in accuracy is demanded. 

The variables that need to be controlled in chemical processing are temperature, pressure, liquid level, flow rate, flow ratio, composition, and certain physical properties whose magnitudes may be influenced by some of the other variables, for instance, viscosity, vapor pressure, refractive index, etc. When the temperature and pressure are fixed, such properties are measures of composition which may be known exactly upon calibration. Examples of control... 

Controlled variables include product compositions (x,y), column temperatures, column pressure, and the levels in the tower and accumulator. Manipulated variables include reflux flow (L), coolant flow (QT), heating medium flow (Qb or V), and product flows (D,B) and the ratios L/D or V/B. Load and disturbance variables include feed flow rate (F), feed composition (2), steam header pressure, feed enthalpy, environmental conditions (e.g., rain, barometric pressure, and ambient temperature), and coolant temperature. These five single loops can theoretically be configured in 120 different combinations, and selecting the right one is a prerequisite to stability and efficiency. 

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. 

Instruments are used in the chemical industry to measure process variables, such as temperature, pressure, density, viscosity, specific heat, conductivity, pH, humidity, dew point, liquid level, flow rate, chemical composition, and moisture content. By use of instruments having varying degrees of complexity, the values of these variables can be recorded continuously and controlled within narrow limits. 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

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