Coolant, flowrate

A = heat transfer surface area c = specific heat of batch liquid C = coolant specific heat M = weight of batch liquid Tj = initial batch temperature Tj = final batch temperature tj = initial coolant temperature U = overall heat transfer coefficient = coolant flowrate 6 = time... 

For example, with a 50% design conversion, a reactor temperature of 353 K gives a reactor volume of 4.36 m3 with a heat transfer area of 12.4 m2 that requires a jacket temperature of 298.8 K. This is only 4.8 K above the supply coolant temperature of 294 K. The required coolant flowrate is very large (28.8 kg/s). The fraction of the total available AT is... 

Effect Of Throughput Figures 2.3 and 2.4 show what happens when the feed flow-rate is increased by 50% from the base case production rate. The 50% conversion design requires very large coolant flowrates for reactor temperatures above 340 K. At this temperature the reactor volume of 16.2 m3 with a heat transfer area of 29.8 m2 requires a jacket temperature of 297.6 K. The fraction of the total available AT is... 

The coolant flowrate for this case is 11.1 kg/s. This should be compared with the coolant flowrate required in the jacket-cooled configuration of 25.8 kg/s, which is over twice as much. 

The control structure shown in Figure 6.57 is installed on the flowsheet. The feed is flow-controlled. The outlet temperature is controlled by manipulating the coolant flowrate. Note that the OP signal is sent to both of the control valves on the coolant stream, opening and closing them simultaneously. The setup works in the simulations, but it is not what would be used in a real physical system. A pressure-driven simulation in Aspen Plus requires that valves be placed on both the inlet and outlet coolant streams. In a real system, the cooling water would be drawn from a supply header, which operates a fixed pressure. A single control valve would be used, either on the inlet or on the outlet, to manipulate the flowrate of coolant. 

Simulation examples of four types of tubular reactors have been presented in the sections above. The adiabatic and constant-coolant temperature models are easier to set up and seem to run with fewer problems. In the adiabatic reactor the only variable that can be controlled is the inlet temperature. In the cooled reactors a temperature can be controlled by manipulating either the coolant temperature or the coolant flowrate, depending on the model. 

Column pressure is controlled by manipulating coolant flowrate to the condenser. 

The second issue for cooled tubular reactors is how to introduce the coolant. One option is to provide a large flowrate of nearly constant temperature, as in a recirculation loop for a jacketed CSTR. Another option is to use a moderate coolant flowrate in countercurrent operation as in a regular heat exchanger. A third choice is to introduce the coolant cocurrently with the reacting fluids (Borio et al., 1989). This option has some definite benefits for control as shown by Bucala et al. (1992). One of the reasons cocurrent flow is advantageous is that it does not introduce thermal feedback through the coolant. It is always good to avoid positive feedback since it creates nonmonotonic exit temperature responses and the possibility for open-loop unstable steady states. 

It is concluded that the addition of I-action will not improve control and that the only feasible control strategy is to add a heat exchange device. and to manipulate the coolant flowrate for T control. 

Coolant water temperature > design instrument fault/low coolant flowrate/high coolant inlet temperature/cooling tower fault/excess condenser area. 

Reactor instability] control fault/poor controller tuning/wrong type of control/in-suffident heat transfer area/feed temperature exceeds threshold/coolant temperature exceeds threshold/coolant flowrate < threshold/tube diameter too large. [Runaway reactor feed temperature too high/[temperature hot spot] /cooling water too hot/feed temperature too high. 

At a pressure of 0.06 MPa coolant boiling started at a heat flux density of 117000 W/m (coolant flowrate through bundle subassembly being 0.76 m /hr). At a heat flux density less than 133000 W/m there was observed a steady process of heat removal due to coolant boiling. At an increase of the heat flux density up to 150000 W/m the process of boiling became pulsating, flowrate periodically decreased almost to zero, then sharply increased, splashes of wall temperature up to 90 °C were observed. 

Primary Circuit Main Parameters Reactor thermal rating, MW Pressure, MPa Coolant temperature, °C at core inlet at core outlet Coolant flowrate, t/hr... 

Low heat density of the core, enhanced margins of departure from nucleate boiling, self regulation of coolant flowrate throu the fuel assemblies. 

Beyond the importance of the control loops in maintaining steady-state material balance control, assurance of product purity, and safety, they provide focal points for the optimization that will follow the initial PFD synthesis. As described in Chapter 14. the controlled variables are the variables over which we have a choice. We find the best values of these variables through optimization. These loops also provide early clues to the flexibility of the process operation. For exanple, if the feed to the reactor is cut in half, less heat needs to be removed. Therefore, there must be an increase in the tenperature of the cooling medium, which occurs when the coolant flowrate is reduced. Process control can be both very difficult and extremely inportant in biological processes, as demonstrated in Example 12.5. 

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