Coolant temperature reactors

Tubular reactors can be simulated using Aspen Plus. Several configurations are available constant-temperature reactor, adiabatic reactor, reactor with constant coolant temperature, reactor with countercurrent flow of coolant, and reactor with co-current flow of coolant. The isothermal reactor cannot be exported into Aspen Dynamics because it is not possible to dynamically control the temperature at all axial positions. Therefore only the last four types will be discussed. 

Results for the two reactors are given in Figure 5.27. In the adiabatic reactor, temperature increases down the reactor as chlorine is consumed and products are formed. In the constant-coolant temperature reactor, temperature reaches a maximum of 411 K at about 0.6 m from the inlet. Note that very little allyl chloride is formed in the cooled reactor because the temperatures are low. In the adiabatic reactor, the high temperatures increase the allyl chloride reaction rates because of the higher activation energy. 

A. 1704. The safety limits for important process variables or parameters shall be stated and justified by the analyses provided in the SAR. Safety limits normally involve operational parameters such as fuel and fuel cladding temperatures, reactor coolant temperature, reactor pressure, reactor power, coolant flow rates and, for pool reactors, the water level above the core. These safety limits are derived primarily from Chapters A.5 (Reactor) and A. 16 (Safety Analysis). 

Figure 5. Experiences at production scale. When (B) the Jacket coolant temperature (- -) dropped due to a plant modification, the reactant flow rate (- ) was increased automatically by the PID controller and maintained the reactor temperature (- in (A)) within 1 % of the setpoint.

The desired product is P, while S is an unwanted by-product. The reaction is carried out in a solution for which the physical properties are independent of temperature and composition. Both reactions are of first-order kinetics with the parameters given in Table 5.3-2 the specific heat of the reaction mixture, c, is 4 kJ kg K , and the density, p, is 1000 kg m . The initial concentration of /I is cao = 1 mol litre and the initial temperature is To = 295 K. The coolant temperature is 345 K for the first period of 1 h, and then it is decreased to 295 K for the subsequent period of 0.5 h. Figs. 5.3-13 and 5.3-14 show temperature and conversion curves for the 63 and 6,300 litres batch reactors, which are typical sizes of pilot and full-scale plants. The overall heat-transfer coefficient was assumed to be 500 W m K. The two reactors behaved very different. The yield of P in a large-scale reactor is significantly lower than that in a pilot scale 1.2 mol % and 38.5 mol %, respectively. Because conversions were commensurate in both reactors, the selectivity of the process in the large reactor was also much lower. 

Runaway criteria developed for plug-flow tubular reactors, which are mathematically isomorphic with batch reactors with a constant coolant temperature, are also included in the tables. They can be considered conservative criteria for batch reactors, which can be operated safer due to manipulation of the coolant temperature. Balakotaiah et al. (1995) showed that in practice safe and runaway regions overlap for the three types of reactors for homogeneous reactions (1) batch reactor (BR), and, equivalently, plug-flow reactor (PFR), (2) CSTR, and (3) continuously operated bubble column reactor (BCR). 

Consider a simple first-order exothermie reaction, A —> B, carried out in a single, constant-volume, continuous stirred-tank reactor (Fig. 3.12), with constant jacket coolant temperature, where r = - k Ca,. 

The heat transfer coefficient, U, to the reactor wall (cal/h cm K) and the coolant temperature is Tj. The nitrobenzene feed rate is Nafo (mol/h), with hydrogen in very large excess. The heat of reaction is AH (cal/mol). The heat capacity of hydrogen is Cp (cal/mol K). 

Attainment of reactor outlet coolant temperature of 950°C (April, 2004)... 

The HTTR is an experimental helium-cooled 30 MW(t) reactor. The HTTR is not designed for electrical power production, but its high temperature process heat capability makes it worthy of inclusion here. Construction started in March 1991 [47] and first criticality is expected in 1998 [48]. The prismatic graphite core of the HTTR is contained in a steel pressure vessel 13.3 m in height and 5.5 m in diameter. The reactor outlet coolant temperature is 850°C under normal rated operation and 950°C under high temperature test operation. The HTTR has a primary helium coolant loop with an intermediate helium-helium heat exchanger and a pressurized water cooler in parallel. The reactor is thus capable of providing... 

The heat removal depends linearly on the difference between the reactor temperature and the coolant temperature since qm = UAS(T - Tm), where the subscript "m" refers to the cooling medium. The heat removal is represented by straight lines on the figure. The heat flow is zero if no heat is removed, which is the case if the coolant temperature is equal to the temperature of the system. Thus, the intersection of a heat removal line with the Y-axis (e.g., Tm,i)... 

In practice, large-scale reactors operate close to adiabatic conditions on loss of cooling which causes maximum increases in temperature. In smaller reactors, the temperature increase depends on the heating of coolant and reactor, and the heat loss to the reactor frame and confined coolant as well. 

The case in Figure 8 represents a reactor with two different cooling systems. In the not recommended case (right) the cooling system presents a feedback loop between a reactor heat rise and the rise in the coolant temperature, which should be avoided. On the left is the recommended system, where the coolant temperature does not depend on the reactor temperature. 

Figure 8. A recommendation to avoid the feedback loop between a reactor heat rise and a rise in coolant temperature.

A well stirred reactor is effecting a first order exothermic reaction with heat transfer under the following conditions x = 1 min Cf = 1 mol/liter Tf = 350, feed temperature Tm = 350, coolant temperature k - exp(25-10000/T), 1/min UA/pVrCp = 1/min AHr/pCp = -200 °K liter/mol Find the steady operating conditions. 

The size and complexity of the N-reactor plant and the limited amount of computing equipment that was available necessitated a judicious use of simplifying assumptions. For instance, primary coolant temperature transport lags were lumped into two groups, one each for the hot and cold loop legs thermodynamic effects in the secondary system condensate headers and surge... 

The idea behind the proposed feedback is the estimation of the generated heat by reaction, then simulations are aimed to show how the reactor temperature stabilization is affected by the initial value of the estimated heat, 7(0). Figure 4 shows the reactor temperature and the computed coolant temperature for several initial values >)(0). Here the estimation parameter was arbitrarily chosen L = 0.5. Note that as the estimated value decreases the convergence to the reference temperature, 283 K, is reached. Figure 5 shows the same effect for the value of the estimation parameter L = 5.0. By comparing both Figures 4 and 5, we can observe that as the value of L increases... 

Fig. 4. Effects of the estimated initial value of the reaction heat for L = 0.5. Dotted-line, 7(0) = 10.0 Solid-line, r)(0) = 5.0 Dash-dotted line, 17(0) = 1.0 Dashed-line, (0) = 0.0. Smaller initial values of the heat reaction lead to better performance than large values, a) Reactor temperature and b) coolant temperature.

It is well known that a nonlinear system with an external periodic disturbance can reach chaotic dynamics. In a CSTR, it has been shown that the variation of the coolant temperature, from a basic self-oscillation state makes the reactor to change from periodic behavior to chaotic one [17]. On the other hand, in [22], it has been shown that it is possible to reach chaotic behavior from an external sine wave disturbance of the coolant flow rate. Note that a periodic disturbance can appear, for instance, when the parameters of the PID controller which manipulates the coolant flow rate are being tuned by using the Ziegler-Nichols rules. The chaotic behavior is difficult to obtain from normal... 

A reactor will be isothermal at the feed inlet temperature Tq if (1) reactions do not generate or absorb significant heat or (2) the reactor is thermostatted by contact with a temperature bath at coolant temperature Tq. For any other situation we will have to solve the energy-balance equation long with the mass balance to find the temperature in the reactor. We therefore must set up these equations for our mixed and unmixed reactors. 

We used the wall temperature in the boundary condition, and this may be different from the coolant temperature T. There may be temperature variations across the wall as well as through the coolant. These are described through the overall heat transfer coefficient U, but in practice all these effects must be considered for a detailed description of the wall-cooled tubular reactor. 

Note that now Tj is a variable that is a function of position Zc in the cooling coif while T, the reactor temperature in the CSTR reactor, is a constant. We can solve this differential equation separately to obtain an average coolant temperature to insert in the reactor energy-balance equation. However, the heat load on the cooling coil can be comphcated to calculate because the heat transfer coefficient may not be constant. 

The forcing variable is the coolant temperature jc2c as in Sincic and Bailey (1977) and more recently in Mankin and Hudson (1984). In eq. (10) jti is a dimensionless reactant concentration while x2 is a dimensionless reactor temperature. These equations hold at the limit of infinite reaction activation energy. All models were thus chosen so that extensive simulation results existed in the literature, and they cover a wide range of lumped reactor types. 

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