Control feed flow

Process industries frequently need to weigh and control the flow rate of bulk material for optimum performance of such devices as grinders or pulverizers, or for controlling additives, eg, to water suppHes. A scale can be installed in a belt conveyor, or a short belt feeder can be mounted on a platform scale. Either can be equipped with controls to maintain the feed rate within limits by controlling the operation of the device feeding the material to the conveyor. Direct mass measurement with a nuclear scale can also be used to measure and control such a continuous stream of material. 

A reflux reduction of 15% is typical. Improved control achieves this by permitting a reduction in the margin of safety that the operators use to handle changes in feed conditions. The key element is the addition of feed-forward capabiUty, which automatically handles changes in feed flow and composition. One of the reasons for increased use of features such as feed-forward control is the reduced cost of computers and online analy2ers. 

A cocurrent evaporator train with its controls is illustrated in Fig. 8-54. The control system applies equally well to countercurrent or mixed-feed evaporators, the princip difference being the tuning of the dynamic compensator/(t), which must be done in the field to minimize the short-term effects of changes in feed flow on product quality. Solid concentration in the product is usually measured as density feedback trim is applied by the AC adjusting slope m of the density function, which is the only term related to x. This recahbrates the system whenever x must move to a new set point. 

Adiabatic. Control gas flow and/or solids feed rate so that the heat of reaction is removed as sensible heat in off gases and solids or heat supphed by gases or solids. 

Flowmeters These are used to measure flocculant addition, underflow, and feed flow rates. For automatic control, the more commonly used devices are magnetic flowmeters and Doppler effect flowmeters. 

The feed flow is often not controlled but is rather on level control from another column or vessel. The liquid product flow s (distillate and bottoms) are often on level rather than flow control. Top vapor product is, however, usually on pressure control. The reflu.x is frequently on FRC, but also may be on column TRC or accumulator level. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

Appropriate design features may include feed-forward temperature control, high temperature alarms, high-temperature cutouts to stop feed flow and open a vent to atmospheric or closed system, adequate temperature monitoring through catalyst beds, etc. 

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

When the set-points for M and conversion are changed again at 600 min the controller predictively increases both the feed flow rate and jacket inlet temperature. Conversion decrrases due to the incaease of feed flow rate but the feed flow rate reaches its upper bound very quickly. Therefore, both inputs are decreased and these bring the conversion and My, to their respective set-points through interactive dynamics. When compared with the other... 

The compositions are controlled by regulating reflux flow and boil-up. The column overall material balance must also be controlled distillation columns have little surge capacity (hold-up) and the flow of distillate and bottom product (and side-streams) must match the feed flows. 

The feed flow-rate is often set by the level controller on a preceding column. It can be independently controlled if the column is fed from a storage or surge tank. 

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 gas metering section is designed to deliver controlled gas flows of C3H6 (Praxair, 99.99%), 02 (Praxair, 99.998%) and He (Praxair, 99.999%) to the reactor system via Brooks 5850 mass flow controllers at a total flow rate 40 ml/min. and latm pressure. Feed gas compositions are C3H6 (40%), 02 (10%) and He (50%) for the steady state reaction. Prior to each experiment, the catalyst was reduced in pure flowing H2 at 34 ml/min for 2 hours at 400 °C. 

Thus, the specific growth rate in a chemostat is controlled by the feed flow rate, since // is equal to D at steady state conditions. Since ft, the specific growth rate, is a function of the substrate concentration, and since fi is also determined by dilution rate, then the flow rate F also determines the outlet substrate concentration S. The last equation is, of course, simply a statement that the quantity of cells produced is proportional to the quantity of substrate consumed, as related by the yield factor Yx/s-... 

Operate the model to include feedback control of the feed flow rate and experiment with various values of KP, T and rF. 

Example 1.4. For the heat exchanger shown in Fig. 1.4, the load disturbances are oil feed flow rate F and oil inlet temperature Tq. The steam flow rate f, is the manipulated variable. The controlled variable is the oil exit temperature T. 

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