Feed control

Nmerican Association of Feed Control Officials (AAFCO) Director, AAFCO, College Station, Tex. 

Association of American Feed Control Officials, official pubHcation, AAFCO, Inc., Adanta, Ga., 1992. 

Fig. 10. Wet high, intensity magnetic separator using cryogenically cooled coils and a stationary matrix where A is the feed control for top-fed or retention time control for underfed operation and B is the feed control for underfed or retention time control for top-fed operation.

The control of feedwater into the boiler is automatic. Feedwater will be delivered from a pump with on-off operation or from a continuously running pump delivering water through a modulating feed control valve. In both... 

Figure 5.166. Exothermic semi-batch reactor with feed control.

There are a number of operational aspects of size reduction, the important points being feed control, mill discharge, and thermal effects. 

The main problem with a feed controlled condition is that throughput is determined entirely by what the feed end manages to pull in and this is very sensitive to the conditions in the feed area. For this reason it is better to keep the machine running full so that the conveying characteristics of the screw determine output rather than the combined characteristics of screw, feed roll, feed opening and feed strip. The compression designed into a screw should therefore be sufficient to achieve complete filling at the lowest likely head pressure. 

American Academy of Industrial Hygiene (AAIH), 14 203. See also United States entries U.S. entries American Association of Feed Control Officials (AAFCO), 10 847, 848 American Association of Textile Chemists and Colorists (AATCC), 9 227, 237 American Boiler Manufacturers ... 

Association-dissociation equilibrium, in micellization, 24 128, 129-131 Association for the Advancement of Medical Instrumentation (AAMI) Standards, 26 818-819 Association of American Feed Control Officials (AAFCO), 10 856 Nutrient Profiles, 10 857, 858-859t... 

The advantages of a semi-batch reaction, that is, a better selectivity in the case of multiple reactions or a better control of the reaction course in the case of exothermal reactions, are obtained if the reaction rate is controlled by the progressive addition of one or more reactants. Indeed, this objective can only be achieved if the added reactant is immediately converted and does not accumulate in the reactor [3]. Often a reaction is said to be feed controlled only because a reactant is fed. This is not always the case, since the feed rate must be adapted to the reaction rate, and the concentration of the added compound (B) is maintained at a low level during the reaction. 

In semi-batch operation, many elements determine the process safety. Among them we mention the temperature control strategy, the feed control strategy, and also the choice of reactant(s) to be initially charged and the reactant(s) to be fed. 

These different temperature and feed control strategies and their impact on reactor safety, together with general rules for assessing and improving process safety, are presented below. The choice of the reactor temperature and feed rate is also of primary importance for safety and this point will be discussed in the last section of this chapter. 

An often-used method for the limitation of the heat release rate is an interlock of the feed with the temperature of the reaction mass. This method consists of halting the feed when the temperature reaches a predefined limit. This feed control strategy keeps the reactor temperature under control even in the case of poor dynamic behavior of the reactor temperature control system, should the heat exchange coefficient be lowered (e.g. fouling crusts) or feed rate too high. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

Consider a reaction in a 16 m3 reactor at 100 °C. At this temperature, using a feed time of at least one hour, the reaction is feed-controlled. The feed rate must be adapted to the cooling capacity of the vessel. There are 15 000 kg of reaction mass in the vessel, the specific heat of reaction is 200 kj kg final reaction mass. During the reaction, the heat exchange area remains constant at 20 m2. Ambient pressure is 1013 mbar. 

Feed by portions this method, presented in Section 7.8.1, is obviously only applicable to discontinuous processes as semi-batch. It reduces the amount of reactant present in the reactor, that is, the accumulation and therefore the energy that may be released by the reaction in case of loss of control. The amount allowed in one portion can be determined in such a way that the maximum temperature of the synthesis reaction (MTSR) does not reach a critical level as the maximum temperature for technical reasons (MTT) or the temperature at which secondary reactions become critical (TD24). The difficulty is to ensure that an added portion has reacted away, before adding the next portion. Generally, the feed control is performed by the operator, but can also be automated. 

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