Feed temperature control

A graphical explanation for the constant flow and variable feed temperature controlled adiabatic CSTR is given in Figure 9.6.6. 

From equation 7.10, nf= 8. The feed flow and feed temperature control loops remove two of these and the unknown relationship specifying xf (which may be dependent upon the configuration of a plant upstream) reduces nf again by one—giving nf = 5. Hence, we require five control objectives to specify the system uniquely, i.e. to control it adequately. These will be to maintain xD (as a market requirement) and to control S, Pc, and the levels in the base of the column and the reflux accumulator (for operational feasibility). 

Heat energy can be saved by using the hot bottom product to preheat the feed in an economizer (Figure 2.89c). In order to maximize the amount of heat recovered, a VPC is used as the cascade master of the feed temperature controller. The goal of optimization is to keep the bypassed flow at a minimum. Therefore, the VPC is usually set at about 10% of the valve opening. 

We conclude this discussion by showing the closed-loop response to two different disturbances. In Fig. 5.26 we reduce the setpoint of the reactor feed temperature controller. In Fig. 5.27 we reduce the amount of toluene fed to the reactor. The changes shown were the largest that could be handled by the system with the small furnace and the large exchanger without a bypass. The design with the bypass (CS2) and the... 

Elnashaie, S.S.E.R and Abdel-Hakim, M.N., Optimal Feed Temperature Control for Fixed Bed Non-isothermal Catalytic Reactors Experiencing Catalyst Deactivation. A Heterogeneous Model. Computers and Chem. Eng., Vol. 12, pp. 787-790, 1988. 

When the bottoms stream provides the bulk of the column preheat, bottom flow swings may cause fluctuations in feed enthalpy. Unless the feed temperature controller can suppress these rapidly and effectively, the disturbances will reenter the column and interact with the composition controller. In one column controlled with scheme 16.4a, this resulted in severe oscillations of the composition controller. 

From a control point of view, the best control would be to use only the heat input to the last module. However, it is quite small (9.69 kW) compared to the heat inputs in the upstream heaters (37.5, 34.9,25.6, and 16.8 kW) and would not be able to handle large disturbances. A decrease in throughput would drive the heat input in this last heat exchanger to zero, but the composition setpoint would not be maintained because the temperature would be too high (control would be lost). In order to handle a 20% decrease in feed and not encoimter a zero heat input constraint, the heat inputs in the last three heaters must be adjusted. Thus, with the modified process, the output signal from the composition controller changes the setpoints of the feed temperature controllers to the last three modules. The temperature controllers on the feeds to the first two modules have a fixed setpoint of 371 K. 

Most FCC units only have a few independent variables. Typically, these independent variables are the feed rate, feed preheat temperature, reactor/riser temperature, air flow rate to the regenerator, and catalyst activity. The feed rate and air flow rate to the regenerator are set by flow controllers. The feed temperature is set by the feed temperature controller. Catalyst activity is set by catalyst selection and fresh catalyst addition rate. Reactor temperature is controlled by the regenerator slide valve that regulates the catalyst circulation rate. The catalyst circulation rate is not directly measured or controlled. Instead, the unit relies on the heat balance to estimate the catalyst circulation rate. Except for these independent variables, other variables, such as regenerator temperature, degree of conversion, and carbon-on-catalyst, etc., will vary accordingly to keep the FCC unit in heat balance. These variables are dependent variables. 

The output signal of the primary (frequently referred to as the master ) reactor temperature control loop serves as the set point of the secondary (frequently referred to as the slave ) reactor feed temperature control loop. 

Column feed temperature control with economizer and preheater... 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

The reaction occurs at essentially adiabatic conditions with a large temperature rise at the inlet surface of the catalyst. The predominant temperature control is thermal ballast in the form of excess methanol or steam, or both, which is in the feed. If a plant is to produce a product containing 50 to 55% formaldehyde and no more than 1.5% methanol, the amount of steam that can be added is limited, and both excess methanol and steam are needed as ballast. Recycled methanol requited for a 50—55% product is 0.25—0.50 parts per part of fresh methanol (76,77). 

Both control schemes react in a similar manner to disturbances in process fluid feed rate, feed temperature, feed composition, fuel gas heating value, etc. In fact, if the secondary controller is not properly tuned, the cascade control strategy can actually worsen control performance. Therefore, the key to an effective cascade control strategy is the proper selection of the secondary controlled variable considering the source and impact of particular disturbances and the associated process dynamics. 

FC and TC = flow and temperature controllers, respectively SP = setpoint S/F = steam/feed ratio x = multiplication of signals and + = sum of signals, (a) Additive (b) multiphcative and (c) combined additive and multiphcative. 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

The feed system handles the storage, circulation, and temperature control of the whiskey. Siace permeabiUty iacreases with temperature, and considering the heat stabiUty of whiskeys, it is desirable to operate the system above ambient temperatures. Operating at higher temperatures faciUtates temperature control of the process, siace heat losses can be compensated by the addition of heat. 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

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