Evaporator Control Systems

Boer HJ (1995) Mass flow controlled evaporation system. J Phys IV5 C5-961-966... 

The hydrolysate obtained by acid posthydrolysis with 2% sulfuric acid (15 min) was centrifuged as described above. To adjust pH of the hydrolysate to 5.5, NaOH, CaO, or Ca(OH)2 was added. When needed, precipitates formed were removed by centrifugation as described above. To obtain a fermentation medium with a higher monosaccharide content, a concentration step (1.8-fold) was carried out in an evaporation system comprising a Syncore orbital shaker equipped with four evaporation flasks, a vacuum pump VAC v-500, and a vacuum controller B-721 (all from Biichi, Flawil, Switzerland). The operational conditions were as follows lower plate temperature, 100°C upper plate temperature, 70°C pressure, 200 mbar stirring, 175 rpm volume per flask, 100 mL. Under these conditions, the concentrated hydrolysates were obtained in about 3 h. 

As previously stated, the purpose of the solvent evaporation systems is to remove the solvent from the lacquer particle without deforming the spherical shape of the particle. The loss of the liquid level in the evaporators will result in a very sensitive lacquer and could result in thermal ignition of any residue on the evaporator walls. Level controls are interlocked with the steam supply to the evaporators to prevent thermal ignition from occurring. 

Fong, J.W. Microencapsulation by solvent evaporation and organic phase separation processes. In Controlled Release Systems Fabrication Technology Hsieh, D., Ed. CRC Press Boca Raton, 1988 1, 81-108. 

Many of the problems in operation and control of a given evaporator system will be specific to the application. However, all systems need to answer such questions as how to evaluate performance, how best to schedule periodic boil outs (cleaning), how to measure and control variables typified by temperature, pressure, fluid level, fluid flow rate, composition, etc., and how to detect faults in evaporator operation quickly and efficiently. ... 

To, L.C. Robust non-linear control of industrial evaporation systems. World Scientific River Edge, NJ, 1999. Dukelow, S.G. The Control of Boilers The Instrument Society of America Research Triangle Park, NC, 1991. Equipment testing procedures committee. In Evaporators A Guide to Performance Evaluation AIChE Equipment Testing Procedure American Institute of Chemical Engineers New York, 1978. 

In addition to the above mentioned merits, it also helped in identifying new variables that were not detected before, such as controlling the power output, initial charge and tungsten boat dimensions used for each evaporation, system geometry and preventive maintenance schedules. 

From the process viewpoint, the two parameters that should be regulated are the concentration and flow rate of the bottoms product. If the composition of the feed stream is constant, good control of the feed rate and the evaporation rate will give the desired concentrated product at the proper production rate (see Fig. 1). Of course, the method of control can depend upon the evaporator type and method of operation. When evaporation rate is to be maintained at a constant rate, a steam flow controller is generally used. Steam flow control usually is accomplished by throttling the steam which results in a loss of temperature difference. Steam may, therefore, be uncontrolled to achieve maximum capacity. Steam pressure controllers may be used to protect the equipment or to assure substantially constant temperatures in the front end of a multistage evaporation system. Constant temperatures in the later effects of the evaporator can be controlled with a pressure controller on the last effect. 

Any control system which is properly designed to control the evaporation process must maintain both an energy and material balance across the evaporator boundaries. The control system must be able to accommodate some fluctuation in the feed flow rate or composition within a specified range, and still enable the evaporation system to perform the required separation with stable operation. The control system should function to reduce heat input with a reduction in feed rate, or change the evaporation rate as changes in the feed composition occur. 

The best control system should be used in the design of evaporator systems. Products which are off-specification require additional time, expense, and energy in reprocessing. A properly designed control system can do much to reduce these wastes, and ensure that the evaporation system uses the optimum energy during normal operation. 

If the system works with a potential gradient less than 20 V/cm the procedure is called low voltage electrophoresis. There are no additional needs to control evaporation of liquid and warming up the filter paper strips. The filter paper strip is wetted with the electrophoresis buffer and positioned into the electrophoresis equipment. The sample is applied either in the form of drops or streaks. Commercially available applicators can be used for this purpose. Streaks are used to improve resolution of two closely moving zones while dots are preferred if repeated application is needed because of a large volume of diluted sample. After electrophoresis the paper strip is removed from the apparatus and dried in a ventilated oven at 100°C. 

As an additional safety, when filters and demineralizers show insuffici it purification of circulating water and of water for release to the oivironmrat, an evaporator system may be incorporated. Such a system is normally a part of the PWR boric acid control circuit. 

The supply rates of the liquid were eontrolled in the range from 0.05 to 1 g/h by a so-called p-Flow mass-flow meter (Bronkhorst, High-Teeh B.V.). At room temperature, the liquid was drawn from a pressurised container with an inert gas blanket and measured by the liquid mass-flow meter. The required flow rate was controlled to the set-point value by a control valve (C), forming an integral part of the liquid flow and earner gas mixing valve (M). The formed mixture was subsequently led into the evaporator to achieve total evaporation (E). This explains the abbreviation of CEM , shown in Fig. 1, viz. Controller-Evaporation-Mixing, the three basic functions of the liquid delivery system. The main features of this liquid delivery system are a) accurately controlled gas/liquid mixture, b) fast response, c) high reproducibility, d) very stable vapour flow, and e) flexible selection of gas/liquid ratio. 

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