Process control, automatic liquid flow

The great majority of automatic process control systems involve one or more of only five process variables, namely, pressure, temperature, flow rate, composition, and liquid level. Many of these variables are measured by the same kind of instrument, and indeed, all of them under certain circumstances can be evaluated in terms of pressures. Thus temperature can be measured by the pressure exerted by a confined gas in the gas thermometer the differential pressure across a restriction in a flow line is a measure of flow rate the pressure exerted by a boiling liquid mixture... 

Modern cryogenics plants are well-instrumented to indicate process temperatures and pressures, flow rates, liquid levels, and contaminant concentrations. These instruments are used for measurement, control, and safety if a particular variable (temperature, pressure, flow rate) falls outside the control range, a corrective action is initiated automatically and, if necessary, an alarm may also be actuated. Complete alarm systems are now available commercially for use with all the common safety devices. Further, many are designed on the failsafe principle and sound an alarm in the event a power failure occurs. 

The inputs to the polymerization system can be categorized as manipulated variables or disturbance variables. The manipulated variables are those which are adjusted, either automatically or manually, to maintain the controlled variables at their desired values. Common manipulated variables in polymerization processes include coolant or heating medium flow rates, gas or liquid flow rates for pressure control, feed rate of monomer, solvent, or initiator, and agitator speed. 

Vacuum film drying. In this process a film of the liquid polyurethane prepolymer flows over a weir and runs down the side of a supporting column. The weir and face of the column down which the liquid polyurethane flows is subjected to a high vacuum which degasses the liquid polyurethane. An example is the APC vacuum falling film degasser (Automatic Process Control, California, USA). 

Plant or process The controlled device that produces the principal output in an automatic control system. Process control The term used to refer to the control of industrial processes most frequently used in reference to control of temperature, fluid pressure, fluid flow, and liquid level. 

Operating methods for control valves, which are designed to control or direct gas and liquid flow through process systems or fluid-power circuits, range from manual to remote, automatic operation. The key parameters that govern the operation of valves are the speed of the eontrol movement and the impact of speed on the system. This is especially important in process systems. 

The liquid storage processes discussed above could be operated by controlling the liquid level in the tank or by allowing the level to fluctuate without attempting to control it. For the latter case (operation as a surge tank), it may be of interest to predict whether the tank would overflow or run dry for particular variations in the inlet and outlet flow rates. Thus, the dynamics of the process may be important even when automatic control is not utilized. 

Instruments and Controls. Liquid levels in several of the tanks are automatically controlled with pneumatic instruments. Flows of several of the process streams are controlled and some are recorded. Temperatures of the feed and melt water are automatically controlled temperatures are recorded by two 16-point potentiometer recorders. Pressures are measured at several points in the system but all are manually... 

Kikkoman Corp., a soy sauce making company, developed an automatic chemical composition analyzer of soy sauce (Figure 9) [16]. The analyzer consists of an InfraAlyzer 400 or InfraAlyzer 500, a temperature controller, an automatic sampler and pumps. A certain amount of soy sauce collected by the automatic sampler is sent to the NIR analyzer at a constant flow rate by a pump through a temperature controller at 20°C. NIR measurement is made automatically. After the NIR measurement, the sample cell and tube are washed with cleansing liquid. It takes about 3 minutes to analyze one sample including washing process. 

If we use distillate flow from the recycle column to control overhead receiver level, then we see that all of the flows around the liquid recycle loop are set on the basis of level. This violates our original statement to fix the toluene recycle flow. We are then left with the question how to control the overhead receiver level in the recycle column. We can use the fresh makeup toluene feed to control this level since it represents the toluene inventory in the process. Such a scheme limits large flowrate changes to the refining section and automatically ensures the component balance for toluene. 

To maintain the production rate, product quahty, and plant safety requires a data acquisition and control system. This system consists of tenperature, pressure, liquid level, flow rate, and conposition 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 pinchased equipment cost for little automatic control, 30% for full automatic control, and 40% for computer control. 

This is a continuous fluids process of large capacity. Assume it is automatically controlled. From the block flow diagram, the process is comprised of two reactor sections and one liquid separation section. Therefore, from Table 17.3, three operators per shift are required for a moderate-capacity plant. However, this is a large-capacity plant, requiring twice that number or 6 operators per shift and five shifts or a total of 30 shift operators. Also, a large-capacity plant requires one labor-yr each for technical assistance and control laboratory. Using Eq. (17.2), the annual costs are... 

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