Reactor cooling control

IB High 1. Control valve fails open 1. Reactor cools, reactant cone 1. Instruct operators and update JFL 1/93... 

Two simple forms of a batch reactor temperature control are possible, in which the reactor is either heated by a controlled supply of steam to the heating jacket, or cooled by a controlled flow of coolant (Fig. 3.18) Other control schemes would be to regulate the reactor flow rate or feed concentration, in order to maintain a given reaction rate (see simulation example SEMIEX). 

Figure 3.18. Reactor with control of temperature by manipulating the flow of cooling water.

The water flow to a chemical reactor cooling coil is controlled by the system shown in Figure 11-4. The flow is measured by a differential pressure (DP) device, the controller decides on an appropriate control strategy, and the control valve manipulates the flow of coolant. Determine the overall failure rate, the unreliability, the reliability, and the MTBF for this system. Assume a 1-yr period of operation. 

Proportional band settings of the reactor temperature controller, circulating jacket water temperature controller, and cooling water flow controller arc 20, 67, and 200, respectively. 

Figure S.2b shows another common system where cascade control is used. The reactor temperature controller is the primary controller the jacket temperature controller is the secondary controller. The reactor temperature control is isolated by the cascade system from disturbances in cooling-water inlet temperature and supply pressure.

Note that in the floating-pressure application, there was only one manipulated variable (cooling-water flow) and one primary controlled variable (valve position). In the reactor temperature-control application, there are two manipulated variables and two controlled variables (temperature and refrigerant valve position). 

Reactor temperature is controlled through a cascade system. Circulating water temperature is controlled by makeup cooling water. The setpoint of this temperature controller is set by the reactor temperature controller. The circulation rate of process liquid through the cooler is flow-controlled. 

The reactor temperature controller (loop 2) is the primary controller, whereas the jacket temperature controller (loop 3) is the secondary controller. The advantage of the cascade control is that the reactor temperature control quickly reacts by the cascade system to disturbances in cooling fluid inlet conditions. The d3mamics of the transfer function G32 is faster than that of G 22-In the CSTR cascade control there are two control loops using two different measurements temperatures T and Tj, but only one manipulated variable Fj. The transfer function of the primary controller is the following ... 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

The polytropic mode this is a combination of different types of control. As an example, the polytropic mode can be used to reduce the initial heat release rate by starting the feed and the reaction, at a lower temperature. The heat of reaction can then be used to heat up the reactor to the desired temperature. During the heating period, different strategies of temperature control can be applied adiabatic heating until a certain temperature level is reached, constant cooling medium temperature (isoperibolic control), or ramped to the desired reaction temperature in the reactor temperature controlled mode. Almost after the... 

In the following, the model-based controller-observer adaptive scheme in [15] is presented. Namely, an observer is designed to estimate the effect of the heat released by the reaction on the reactor temperature dynamics then, this estimate is used by a cascade temperature control scheme, based on the closure of two temperature feedback loops, where the output of the reactor temperature controller becomes the setpoint of the cooling jacket temperature controller. Model-free variants of this control scheme are developed as well. The convergence of the overall controller-observer scheme, in terms of observer estimation errors and controller tracking errors, is proven via a Lyapunov-like argument. Noticeably, the scheme is developed for the general class of irreversible nonchain reactions presented in Sect. 2.5. 

A relay-feedback test on the reactor temperature controller is used to obtain the ultimate gain and frequency (K, = 64 and Pv = 10 min), using a 50 K temperature transmitter span and assuming the maximum cooling water flow is twice the steady-state value. The Tyreus-Luyben settings give oscillatory response, so the controller gain is reduced by factor of 2 (Kc = 10, t = 1320 s). 

There are two controllers. The proportional reactor level control has a gain of 5. The reactor temperature controller is tuned by running a relay-feedback test. The manipulated variable is the cooling water flowrate in the condenser. With a 50-K temperature transmitter span and the cooling water control valve half open at design conditions, the resulting tuning constants are Kc = 4.23 and = 25 min. 

The reactor is the jacket-cooled CSTR with an irreversible, exothermic, liquid-phase reaction A —> B, which was considered in Section 3.1. In that section the flowrate of the cooling water Fj to the jacket was the manipulated variable for the reactor temperature controller (TR <— Fj control). In this section we explore the use of the flowrate of the fresh feed F() to control reactor temperature (TR <— F0 control). 

The flowrate of the cooling/heating medium is usually the manipulated variable that is changed by a reactor temperature controller, either directly or through a... 

To illustrate some of the design and control issues, a vessel size (DR = 2 m, VR = 12.57 m3, jacket heat transfer area Aj = 25.13 m2) and a maximum reactor temperature (7j) ax = 340 K) are selected. The vessel is initially heated with a hot fluid until the reaction begins to generate heat. Then a cold fluid is used. A split-range-heating/ cooling system is used that adds hot or cold water to a circulating-water system, which is assumed to be perfectly mixed at temperature Tj. The setpoint of a reactor temperature controller is ramped up from 300 K to the maximum temperature over some time period. 

Two reactor temperature controllers are now used. The first manipulates the hot and cold streams used to heat or cool the reactor. The second controller manipulates the feed flowrate. More details about this control structure are presented in Section 4.2. 

There are five fundamental differences between CSTRs and tubular reactors. The first is the variation in properties with axial position down the length of the reactor. For example, in an adiabatic reactor with an exothermic irreversible reaction, the maximum temperature occurs at the exit of the reactor under steady-state conditions. However, in a cooled tubular reactor, the peak temperature usually occurs at an intermediate axial position in the reactor. To control this peak temperature, we must be able to measure a number of temperatures along the reactor length. 

The third difference is the issue of heat transfer in nonadiabatic reactors. Ideally we would like to be able to control the temperature at each axial position down the reactor. However, it is mechanically very difficult to achieve independent heat transfer at various axial positions. About all that can be done is to have the cooling/heating medium flow either cocurrent or countercurrent to the direction of the process flow. The only two variables that can be manipulated are the flowrate of the medium and its inlet temperature. The former is the normal manipulated variable. The result is that only a single temperature can be controlled, which can be the peak temperature or the exit temperature. However, because of the significant dynamics of the tubular reactor, the control of these temperatures is sometimes quite difficult and tight control cannot be achieved in the face of load disturbances. 

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