Adiabatic reactor temperature rise

The commercial trickle-bed reactors, such as hydrodesulfurization and hydrocracking reactors, are often operated adiabatically. The temperature rise in such reactors is often controlled by the additions of a quench fluid (normally hydrogen) at one or more positions along the length of the reactor. A schematic of an adiabatic trickle-bed HDS reactor with a single quench is shown in Fig. 4-7. 

Figure 3.8 shows methane conversion, hydrogen yield and adiabatic temperature rise of a methane partial oxidation reactor as a function of the O/C ratio (here expressed as the air ratio, A.) [66]. Owing to coke formation below O/C = 1.2 (A, = 0.3) and it being more pronounced below 0/C = l (2,=0.25), the reactor temperature rises only moderately until O/C reaches 1.2. Beyond this value the adiabatic... 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

In a typical adiabatic polymerization, approximately 20 wt % aqueous acrylamide is charged into a stainless steel reactor equipped with agitation, condenser, and cooling jacket or coils. To initiate the polymerization, an aqueous solution of sodium bisulfite [7631-90-5] is added, followed by the addition of a solution of ammonium persulfate [7727-54-0] N2HgS20g. As the polymerization proceeds, the temperature rises to about 90°C, and then begins to fall at the end of the polymerization. The molecular weight obtained depends primarily on the initiator concentration employed. 

TABLE 23-2 Multibed Reactors, Adiabatic Temperature Rises ... 

Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

The chapter by Haynes et al. describes the pilot work using Raney nickel catalysts with gas recycle for reactor temperature control. Gas recycle provides dilution of the carbon oxides in the feed gas to the methanator, hence simulating methanation of dilute CO-containing gases which under adiabatic conditions gives a permissible temperature rise. This and the next two papers basically treat this approach, the hallmark of first-generation methanation processes. 

Kinetically Limited Process. Basically, this system limits the temperature rise of each adiabatically operated reactor to safe levels by using high enough space velocities to ensure only partial approach to equilibrium. The exit gases from each reactor are cooled in external waste heat boilers, then passed forward to the next reactor, and so forth. This resembles the equilibrium-limited reactor system as shown in Figure 8, except, of course, that the catalyst beds are much smaller. 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

Figure 8.22. Schematic drawing of an adiabatic two-bed radial flow reactor. There are three inlets and one outlet. The major inlet comes in from the top (left) and follows the high-pressure shell (which it cools) to the bottom, where it is heated by the gas leaving the reactor bottom (left). Additional gas is added at this point (bottom right) and it then flows along the center, where even more gas is added. The gas is then let into the first bed (A) where it flows radially inward and reacts adiabatically whereby it is heated and approaches equilibrium (B). It is then cooled in the upper heat exchanger and move on to the second bed (C) where it again reacts adiabatically, leading to a temperature rise, and makes a new approach to equilibrium (D). (Courtesy of Haldor Topspe AS.)...

In order to show how specific guidelines for the reactor layout can be derived, the maximum allowable micro-channel radius giving a temperature rise of less than 10 K was computed for different values of the adiabatic temperature rise and different reaction times. For this purpose, properties of nitrogen at 300 °C and 1 atm and a Nusselt number of 3.66 were assumed. The Nusselt number is a dimensionless heat transfer coefficient, defined as... 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Substantial heat-transfer intensification was also described for a special micro heat exchanger reactor [104]. By appropriate distribution of the gas-coolant stream, the axial temperature gradient can be decreased considerably, even under conditions corresponding to very large adiabatic temperature rises, e.g. of about 1400 °C. 

GP 1] [R 1] Numerical simulations prove that isothermal processing is possible in micro reactors even under severe reaction conditions which correspond to an adiabatic temperature rise up to 1400 °C [98]. 

The temperature rise due to this exothermic reaction then approaches the adiabatic temperature rise. The final steady state is always characterized by conditions T = T, and c = 0. A batch reactor, in which a zero order reaction is carried out, always has a unique and stable mode of operation. This is also true for any batch and semibatch reactor with any order or combination of reactions. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

This is the temperature rise which would occur if the reactor were designed and operated as a single adiabatic stage. 

---