Reactor feed temperature

In normal operation of ebullated bed reactors, the reactor feed temperature is the control variable. The desired reactor feed temperature depends on both the feed rate and feed composition. The feed temperature is chosen such that the overall heat generation in the reactor is used to elevate the low temperature feed material to the bed temperature during... 

The main limitation of this type of reactor is the gradual accumulation of metals when heavy feedstocks are processed. The metals accumulate in the pores of the catalyst and gradually block access for hydrogenation and desulfurization. The length of operation is then dictated by the metal-holding capacity of the catalyst and the nickel and vanadium content of the feed. As the catalyst deactivates, the reactor feed temperature is gradually increased to maintain conversion. 

T = reactor exit temperature To = reactor feed temperature Tc = coolant temperature... 

Just as we approached reactor control in Chap. 4, we will start by exploring the open-loop effects of thermal feedback. Consider Fig. 5.19, which shows an adiabatic plug-flow reactor with an FEHE system. We have also included two manipulated variables that wall later turn out to be useful to control the reactor. One of these manipulated variables is the heat load to the furnace and the other is the bypass around the preheater. It is clear that the reactor feed temperature is affected by the bypass valve position and the furnace heat load but also by the reactor exit temperature through the heat exchanger. This creates the possibility for multiple steady states. We can visualize the different... 

We start by plotting the temperature rise in the reactor. This is done by integrating the steady-state differential equations that describe the composition and heat effects as functions of the axial position in the reactor. The adiabatic plug-flow reactor gives a unique exit temperature for a given feed temperature. This also means that we get a unique difference between the exit and feed temperatures. The temperature difference has to be less than or equal to the adiabatic temperature rise at a given, constant feed composition. Figure 5.20 show s the fractional temperature rise as a function of the reactor feed temperature for a typical system. 

The next step in the analysis is to seek another functional relationship between the reactor exit temperature and the reactor feed temperature resulting from the heat exchange, bypass, and influence of the furnace. Once we find the second relation we can superimpose it on top of the reactor temperature rise expression shown in Fig. 5.20. Intersections between the two curves constitute open-loop, steady-state solutions to the combined reactor-FEHE system. 

The furnace, finally, provides a constant heat input in manual operation. This heat input gives a constant temperature increase AT,. The functional relation between the reactor feed temperature T0 and the reactor exit temperature T can now be computed ... 

Equation (5.11) represents a straight line in the diagram of fractional temperature rise versus reactor feed temperature. We show three such lines in Fig. 5.21. All lines intersect the temperature rise curve at least once (at a low temperature not shown in Fig. 5.21). It therefore appears that the reactor FEHE can have one, two, or three steady-state solutions for this particular set of reaction kinetics. Furthermore, the intermediate steady state, in the case of three solutions, is open-loop unstable due to the slope condition discussed in Chap. 4. This was verified by Douglas et al. (1962) in a control study of a reactor heat exchange system. 

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... 

We next verify that there exists a hot stable steady state when the system equations have three stationary solutions. With the temperature controller in manual, we lower the bypass rate from 12 percent to 5 percent. According to Fig. 5.21 this should create three steady states and we would expect the intermediate state to be unstable. Figure 5.30 shows what happens when we reduce the bypass rate. The reactor feed temperature goes up initially due to reduced bypassing. This makes the reactor exit temperature drop due to the inverse response. The drop in the reactor exit temperature causes a drop in the feed temperature below the intermediate steady state (point b in Fig. 5.21). However, this state is unstable so the temperatures continue to oscillate. Slowly, but surely, the temperatures trend toward the hot steady state (point c in Fig. 5.21) where the oscillations die out and the reactor remains stable. 

The decision variables used for the optimization of the reactor must be selected among the operating ones. After considering industry requirements, the effect of each of the operating variables on the objective function and the easiness of how these variables can be changed in the plant, the feed flow rate of hydrogen (FAo) and the reactor feed temperature (Tfo) were chosen as the decision ones. Thus the optimization routine searches for the values of FAo and Tfo that, with the current value of o-cresol flow rate, lead to maximal reactor profit. 

Fig. 14 Effect of reactor feed temperature on predicted temperature profiles for a gas-phase tubular reactor. (From Ref. " l)...

Figure 5.2. and SPE charts based on PCA for monitoring a continuous polymerization reactor. A 5% increase in reactor feed temperature is introduced for 60 min at the time instant indicated by a vertical bar on the plot. 

A 5% increase in reactor feed temperature was introduced and maintained for 60 min before returning the feed stream to normal operating conditions. The multivariate charts (Figure 5.2) are the first to detect the disturbance to the reactor operation. The statistic exceeds the 99% confidence interval 25 min after the disturbance was introduced, and the SPE statistic 20 min after the disturbance, a few minutes earlier than the chart. The initiator concentration in the reactor exceeds the statistical limits of the Shewhart chart (Figure 5.3) after 35 min. Reactor temperature and conversion readings exceed the statistical limits after approximately 40 min and the polydispersity measurement exceeds the univariate limit after 44 min. 

Here the first term is the energy required for isothermal chemical reaction at the reactor feed temperature, and the second term is the energy required to heat the reaction mixture, without further reaction, from the inlet temperature to the reactor operating temperature. 

The results for other reactor feed temperatures T can be computed in a similar fashion from the data in the figure. ... 

Enzyme—1.0 FP activity Substrate—SF-HM (76% < 53/x)—v0% pH— 4.05-5.2 Saccharification temperature—50°C. Dilution rate in continuous phase—0.025"J hr. Reactor conditions 4.0 liter glass stirred tank reactor Feed temperature—1°-2°C. 

Runaway reactor] feed temperature too high/[temperature hot spot]. ... 

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. 

This arrangement is similar to that of the interstage cooling structure provided earlier. However in this scenario, a portion of the feed is bypassed and mixed with reactor product after each reaction stage, and there is no need to use dedicated cooling equipment, which lowers the costs of operation. The mixture temperature must lie at an intermediate value between the feed and product temperatures. It follows that an expression for how reactor feed temperatures vary with mixing is thus necessary. 

The problem may be due to catalyst deactivation caused by sulfur precipitation. This is a result of low reactor feed temperatures. Check the operation of the reheat exchanger upstream of the reactor with the reduced temperature rise. Raise the reactor inlet temperature 30°F. After a few days, this will dissipate the offending sulfur deposits. 

ILLUSTRATIVE EXAMPLE 14.4 The average weekly inlet reactor feed temperatures (°C) for six consecutive weeks are... 

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. 

Figure 3. Reactor-temperature as a function of the air flowrate at two reactor feed temperatures...

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