Autothermal operation

One of the advantages of the continuous stirred-tank reactor is the fact that it is ideally suited to autothermal operation. Feed-back of the reaction heat from products to reactants is indeed a feature inherent in the operation of a continuous stirred-tank reactor consisting of a single tank only, because fresh reactants are mixed directly into the products. An important, but less obvious, point about autothermal operation is the existence of two possible stable operating conditions. 

Rate of heat Rate of heat Rate of heat 

the enthalpy of the products of mass flowrate G and specific heat c is measured relative to T0, the inlet temperature of the reactants. The term for rate of heat generation on the left-hand side of this equation varies with the temperature of operation T, as shown in diagram (a) of Fig. 1.20 as T increases, lA increases rapidly at first but then tends to an upper limit as the reactant concentration in the tank approaches zero, corresponding to almost complete conversion. On the other hand, the rate of heat removal by both product outflow and heat transfer is virtually linear, as shown in diagram (b). To satisfy the heat balance equation above, the point representing the actual operating temperature must lie on both the rate of heat production curve and the rate of heat removal line, i.e. at the point of intersection as shown in (c). 

In Fig. 1.20c, it may be seen how more than one stable operating temperature can sometimes occur. If the rate of heat removal is high (line 1), due either to rapid outflow or to a high rate of heat transfer, there is only one point of intersection O, corresponding to a low operating temperature close to the reactant inlet temperature T0 or the cooling medium temperature Tc. With a somewhat smaller flowrate or heat transfer rate (line 2) there are three points of intersection 

Obviously, in designing and operating a stirred-tank reactor it is necessary to be aware of these different operating conditions. Further discussion of the dynamic response and control of an autothermal continuous stirred-tank reactor is given by Westerterp et al.m. 

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The first observation of sensitivity-stability was reported by Liljenroth (1918) in connection with the autothermal operation of ammonia oxidation reactors. Papers of Damkdhler (1937) and Wagner (1945) went unnoticed. At Union Carbide Corp. Perkins (1938) used zero order kinetics to define a safe range for ethylene oxidation in an unpublished report. His result,... 

The recycling of PVC by hydrothermal techniques is described, in which PVC is thermally converted in a steam atmosphere into hydrogen chloride, hydrogen, carbon dioxide, carbon monoxide and some gaseous and liquid hydrocarbons. Whilst gasification with only steam is an endothermic reaction, partial combustion of PVC by the addition of small amounts of air, enables autothermic operation of the process to take place. This work deals... 

Partial methane oxidation comprises very high rates so that high space-time yields can be achieved (see original citations in [3]). Residence times are in the range of a few milliseconds. Based on this and other information, it is believed that syngas facilities can be far smaller and less costly in investment than reforming plants. Industrial partial oxidation plants are on the market, as e.g. provided by the Syntroleum Corporation (Tulsa, OK, USA). Requirements for such processes are operation at elevated pressure, to meet the downstream process requirements, and autothermal operation. 

Flow reversal performance is controlled weakly by the period r. Flow reversal is an autothermal operation and as such exhibits parametric sensitivity. Greater stability can be ensured by the conventional expedient of providing cooling in the catalyst bed. It can also be done through bypassing part of the reactor effluent gas around the recuperator section. 

In the design of processes involving exothermic reactions it is generally desirable to use the energy liberated by reaction at some other point in the process or to make it available for use elsewhere in the industrial plant. For example, it can be used to preheat the feed components, particularly when the reaction takes place at high temperatures and the feed components are supplied at much lower temperatures. The term autothermal operation is applied to modes of processing in which exothermic reactions are carried out such that the energy released by... 

ILLUSTRATION 10.6 AUTOTHERMAL OPERATION OF A REACTOR NETWORK CONSISTING OF A STIRRED TANK REACTOR FOLLOWED BY A PLUG FLOW REACTOR... 

Consider the reaction studied in Illustration 10.1. Autothermal operation is to be achieved using a CSTR with an effective volume of1000 gal followed by a PFR of undetermined volume. Pure species A enters at a rate of 40.0 gal/hr and at a temperature of 20 °C. The overall fraction conversion is to be 0.97. This flow rate and conversion level will suffice to meet the annual production requirement of 2 million lb of B. Both the CSTR and the PFR are to be operated adiabatically. What PFR volume will be required, and what will be the temperature of the effluent stream ... 

SWITGTHERM A catalytic process for oxidizing volatile organic compounds (VOCs). It involves regenerative heat exchange, which permits autothermal operation at VOC concentrations in the range 250 to 650 ppm. Developed in Poland and now used in over 100 installations there. 

The following example illustrates a combination of semibatch and semicontinuous operation for an irreversible reaction, with one reactant added intermittently and the other flowing (bubbling) continuously, that is, a combination of Figures 12.3(a) and 12.4(a). Chen (1983, pp. 168-211, 456-460) gives several examples of other situations, including reversible, series-reversible, and series-parallel reactions, and nonisothermal and autothermal operation. 

The situation at the upper stable point, when reaction proceeds smoothly to completion at a high temperature without an external source of heat, is described as autothermal or autothermic operation. If the volumetric flow rate of reactants, F, is increased, then the slope of the Ql line is increased and the position of the Qq is also slightly changed the principal effect, however, is that, eventually, an upper intersection of the heat generation and heat loss lines is no longer possible and the autothermic reaction is blown out . 

Autothermal operation is not confined to continuous stirred tank reactors (see Sect. 5.4). 

Inspection of Fig. 24, which represents the behaviour of a reversible exothermic reaction with various cooling rates, shows that autothermal operation is possible (point A), but that a somewhat reduced cooling rate results in a greater Qq, and hence conversion (point B) excessive cooling will, of course, quench the reaction (point C). 

The upper points of intersection between the two lines. A, A and A" represent stable autothermal operation of the system with a high... 

Fig. 1.20. Autothermal operation of a continuous stirred-tank reactor...

Under autothermal operation the hot effluent of a fixed-bed reactor is used to heat up the cold feed to the ignition temperature of the catalytic reaction. Since no other addition or removal of heat takes place, autothermal operation is restricted to reaction systems which all together are exothermic. The conventional reactor design consists of an adiabatic packed-bed reactor coupled with a countercurrent heat exchanger (Fig. 23A). 

Figure 27. Methanol synthesis under autothermal operation (reverse flow reactor) A. B) Temperature and conversion pr< period C) Reaction path for reverse flow operation and for a two-stage adiabatic reactor with interstage cooling [46]...

The investigation of the autothermal operation of the hydrogasifier requires a detailed examination of the process heat balance. Concerning the thermobalance and PFB tests, neither the input heat of the electric heating system, nor the heat losses, are known. However, based on the experimental results, it seems that the methanation reaction is a major heat source in the hydrogasifier, followed by other hydrogenation reactions. 

Another example is monolithic-type reactors, which have found their main application in the field of combustion. A monolith bed allows better autothermic operations with a minimal pressure-drop. This concept was used to improve performances in commercial methanol into formaldehyde conversion by adding a... 

Further increases in methane conversion were attained by using an additional bed downstream from the membrane bed. In addition, the reactor temperature was increased so that the second bed operates at temperatures higher than the autothermal operation. This allows for the dry reforming reaction to occur in the second bed thus increasing the conversion of the methane not consumed in the first bed. In this case the highest methane conversion was about 90% with CO and H2 selectivities of about 90% when the external temperature is 700°C. Similar results can be attained without heating if a third feed of O2 is added between the membrane bed and just before the second fixed bed. In this case, the temperature increase is realized by the partial oxidation reaction with no major loss of selectivity. 

Tonkovich et al. [182] succeeded in obtaining a very thick washcoat on a MSR for the catalytic steam reforming of methane. The reactor was built from Inconel , a NiCrMo alloy, and consisted of a slit-like reaction channel with cross-flow combustion channels for autothermal operation... 

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