Process control, automatic composition

The schemes used for reactor control depend on the process and the type of reactor. If a reliable on-line analyser is available, and the reactor dynamics are suitable, the product composition can be monitored continuously and the reactor conditions and feed flows controlled automatically to maintain the desired product composition and yield. More often, the operator is the final link in the control loop, adjusting the controller set points to maintain the product within specification, based on periodic laboratory analyses. 

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... 

As defined here, automatic process control always implies the use of a feedback. This means that the control instrument is continuously monitoring certain output variables of the controlled process, such as a temperature, a pressure, or a composition, and is also compeuing this output with some preestablished desired value, which is considered a reference, or a set point, of the controlled variable. An error that is indicated by the compeuison is used by the instrument to compute a correction to the setting of the process control valve or other final control element in order to adjust the value of the outpirt variable to its desired level and maintain it there. 

A new class of microwires can be assembled by dielectrophoresis from suspensions of metallic nanoparticles. The wires are formed in the gaps between planar electrodes and can grow faster than 50 pm per second to lengths exceeding 5 mm. They have good ohmic conductance and automatically form electrical connections to conductive islands or particles. The thickness and the fractal dimensicai of the wires can be controlled, and composite wires with a metallic core surrounded by a latex shell can be assembled. The simple assembly process and their high surface-to-volume ratio make these structures promising for wet electronic and bioelectronic circuits. 

Iron sintering mix control and composition stabilization. For an efficient sintering process, a constant and optimized basicity of raw mix without short and long term fluctuations is a must. Achieving real-time automatic process control without human factor influence requires on-line elemental composition data. Figure 8.29a presents typical breakdown spectra of the sintering mix and the results of industrial LIBS unit test data, where laboratory CaO control data are compared with online analyzer readings. One hundred and forty samples have been taken from conveyer belt and send to laboratory for control analysis. It was found that the correlation of... 

Fluorescence spectrometers are widely used in the metal industry. Frequently, parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard sample that is contained in the sample holder. In this way the intensity of the X-ray tube can be monitored. Frequently, a measurement is terminated when a preset number of comits for the reference sample has been obtained. The corresponding number of counts from the other detectors can then be directly used for a pai allel assessment of the elemental composition of the sample. With a sequential spectrometer, a number of selected elements are measured sequentially by turning the crystal and the detector to preset positions. With computer steering the measurement process is automatic. This type of instrument is well suited for varying types of analysis, whereas parallel spectrometers are more suited to continuous control operation of, for example, a steel mill in near-real time. 

Processes must be controlled for quality purposes. Most major chemical processes have automatic computerized data logging and many parameters are automatically recorded and available for study. These can include set points, process measurements such as flow rate, temperature, pressure, stirring speed, and so forth or product parameters such as composition, density, clarity, and so forth, especially when the product is measured on-line. 

Some special requirements of continuous systems are (1) Metering the feed. A continuous system must be fed at a precise, uniform rate. (See Sec. 21.) (2) Dust collection. This is a necessary part of most diy-processing systems. Filters are available that can effectively remove dust down to 10 mg/m or less, and operate automatically. (Dust collection is covered in Sec. 17.) (3) Ondine analysis. For more precise operation, on-line analysis of product particle size and composition may be desirable. (4) Computer control. SiiTuilation can aid in optimizing system design and computer control. 

II processes are subject to disturbances that tend to change operating conditions, compositions, and physical properties of the streams. In order to minimize the ill effects that could result from such disturbances, chemical plants are implemented with substantial amounts of instrumentation and automatic control equipment. In critical cases and in especially large plants, moreover, the instrumentation is computer monitored for convenience, safety, and optimization. 

Control in one form or another is an essential part of any chemical engineering operation. In all processes, there arises the necessity of keeping flows, pressures, temperatures, compositions, etc. within certain limits for reasons of safety or specification. It is self-evident that automatic control is highly desirable, as manual operation would necessitate continuous monitoring of the controlled variable by a human operator and the efficiency of observation of the operator would inevitably fall off with time. Furthermore, fluctuations in the controlled variable may be too rapid and frequent for manual adjustment to suffice. 

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. 

The chromatographic equipment which is responsible for the separation includes the pump, and in many systems a column oven. The parameters which affect the separation are the flow rate, the solvent composition and the LC gradients. Many different software packages are available which allow completely unattended automatic chromatography. Such systems also include control of the sample injection process. 

Instruments which can monitor the important process variables during plant operation must be specified. These instruments must be capable of measuring the variables and should have an acceptable accuracy and repeatability of measurement, usually the latter attribute is more important than the former on chemical plant measurements. The instruments may be used for manual measurements or included in automatic control loops. Automatic alarms may also be required to indicate deviations outside acceptable limits. If possible, direct measurement of the process variable should be made, however it is often easier to measure a dependent variable, e.g. temperature measured as an indication of composition for distillation column top product. 

However, when the pressure, composition, and temperature controllers are put on automatic, the three-stage process can be made closedloop-stable. Figure 6.20 shows the response to ramped increases in the setpoints of the inlet reactor temperature controllers 3.1 K for Ti, 4.6 K for T2 and 5.8 K for T3. The production rate is increased by 25%, and the new steady-state exit temperature is 510.9 K. When the setpoints of the inlet reactor temperature controllers are reduced (4.4 K for 1, 5.9 K for T2, and 7.1 K for T3), the production rate is decreased by 25% and the new steady-state exit temperature is 487.3 K. 

There being two components and two phases in the following streams at the bottom, feed and overhead, Gibbs s law states that only two of the three variables (pressure, temperature, and composition) are independent. Therefore, the number of independent variables is only 11. The number of defining equations is two (the conservation of mass and energy), and, therefore, the number of degrees of freedom for this process is 11 - 2 = 9. Consequently, not more than nine automatic controllers can be placed on this process. 

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