Isothermal reactors membrane

Recently, Itoh and Govind(ll.) have reported a theoretical study of coupling an exothermic hydrogen oxidation reaction with dehydrogenation of 1-butene in an isothermal palladium membrane reactor. 

An isothermal reactor concept incorporating a ceramic membrane is more attractive compared to an adiabatic reactor concept from a thermodynamic point of view. In this concept we assumed a reactor with reactor tubes located in a direct-fired heater and operated in a cyclic way to remove coke formed on the catalyst. Parallel bed and heaters have been assumed [35-37]. 

It is concluded that a ceramic membrane reactor based on Knudsen diffusion membranes can give improvements in an isothermal reactor concept although the difference in price level between feedstock and product is too small to give an economically viable process. 

Probably the only possibility is the combination of a high driving force (sweep gas or low permeate pressure) and a very high selective membrane. The use of ceramic membranes in an isothermal reactor concept shows better prospects. This process, in combination with high selective membranes and the necessary membrane boundary conditions are being studied, and the results will be reported in future. 

The results found in this study are less promising then those reported in literature [45-49]. There are several reasons for this difference. In some publications experiments have been reported in which process conditions and/or feed compositions have been used that are not realistic or feasible on an industrial scale but do have a large impact on the performance of the membrane reactor. Also, when results are reported from modelling this process, incorrect assumptions were sometimes made, e.g. side-reactions which have a large influence on the performance of this process have been neglected [47]. In other publications a very large heat input is taken, which leads to a more or less isothermal reactor, and as a consequence to higher conversions [45,46,48]. 

Prior to each experimental trial, the reactor is externally preheated to 200°C with inert flow on both the reaction side and the sweep side of the membrane at the desired total flow rate of the experimental trial. The reactants are then introduced to the reaction side maintaining the desired total flow rate. The net reaction that occurs is exothermic causing the reactor temperature (measured by an alumel-chromel thermocouple) to increase. At steady-state, the reactor temperature measures between 400 and 650°C depending on the iC4Hio 02 feed ratio and the level of dilution. This is not an isothermal reactor at steady state, sizable temperature gradients exist within the catalyst bed. The temperature reported here is the temperature at the axis of the reactor where the feed stream meets the catalyst bed. 

By coupling the membrane reactor to other units, it is possible to improve its performance. For example, Bracht et al. [14] considered two-step adiabatic reactors instead of a one-step isothermal reactor as a means of extending membrane life-... 

In FBMRs, isothermal reactor operation can be achieved more easily, leading to a higher selectivity compared with PBMRs [17, 18]. This is also attributed to the mass-transfer limitations between the bubble and emulsion phases. However, the improvement in selectivity of separation due to the inter-phase gas exchange decreases with increasing pressure. Furthermore, through the controlled dosing of air via porous membranes, hot spots can be minimized and the reactor is inherently safer because the oxygen-hydrocarbon feed separation helps to overcome flammability and explosion limits. 

In the following we attempt to describe the acetylcholinesterase/choline acetyltransferase enzyme system inside the neural synaptic cleft in a simple fashion see Figure 4.49. The complete neurocycle of the acetylcholine as a neurotransmitter is simulated in our model as a simple two-enzymes/two-compartments model. Each compartment is described as a constant-flow, constant-volume, isothermal, continuous stirred tank reactor (CSTR). The two compartments (I) and (II) are separated by a nonselective permeable membrane as shown in Figure 4.50. 

Assaf, E.M., Jesus, C.D.F. and Assaf, J.M. (1998) Mathematical modelling of methane steam reforming in a membrane reactor An isothermic model. Brazilian Journal of Chemical Engineering, 15 (2), 160-166. 

In addition to these experiments, a simplified isothermal 1-D dispersed plug-flow reactor model of the membrane reactor was used to carry out theoretical studies [47]. The model used consisted of the following mass balance equations for the feed and sweep sides ... 

Again, the simple isothermal 1-D plug-flow reactor model provides a good basis for quantitative descriptions. This model allows to explore the potential of using series connections of several membrane reactor segments. The corresponding mass balance for a component i and a segment k can be formulated as follows ... 

It is relevant at this point to stress the importance of preserving the nonisothermal condition of the reactor. Most of the modeling studies of membrane reactors assume an isothermal operation. However, as it has been demonstrated experimentally [Becker et al., 1993], this assumption is incorrect and, more often than not, a temperature profile exists along the membrane reactor length. 

As mentioned earlier, most membrane reactor models are based on isothermal macroscopic balances in the axial direction and do not solve the transport equations for the membrane/support matrix. They all account for the effects of membrane permeation through the use of some common relevant parameters (as a permeation term) in the transport equations for both the feed and permeate sides. Those parameters are to be determined experimentally. The above approach, of course, is feasible only when the membrane (or membrane/support) is not catalytic. 

Tsotsis et al. [1992] considered a case where two reaction zones exist in a porous membrane reactor one inside the membrane matrix and the other in the tubular region which is packed with catalyst particles. They presented a packed-bed catalytic membrane tubular reactor model under isothermal and co-current flow conditions. Thus, Equations (10-37), (10-6) and (10-45) ail reduce to the condition... 

A non-isothermal plug-flow membrane reactor on both sides of the membrane has been developed and applied to the methane steam reforming reaction to produce synthesis gas at high temperatures according to [Oeitel et al., 1987]... 

Applying an isothermal and plug>flow membrane reactor (on both sides of the membrane) to the above reactions, Itoh and Xu [1991] concluded that (1) the packed-bed inert membrane reactor gives conversions higher than the equilibrium limits and also performs better than a conventional plug-flow reactor without the use of a permselective membrane and (2) the co-current and counter-current flow configurations give essentially the same conversion. 

Itoh and Govind [1989b] further analyzed an isothermal packed-bed inert membrane reactor but under a counter-current flow configuration. Under the conditions studied, the authors found that the counter-current flow configuration provides a much greater conversion than the co-current flow mode. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Tsotsis et al. [1993a] presented an isothermal packed-bed porous membrane tubular reactor for ethane dehydrogenation to form ethylene... 

Champagnie et al. [1992] adopted the aforementioned model to describe the performance of an isothermal shell-and-tube membrane reactor for ethane dehydrogenation in a co-current flow mode. Using Equation (10-8la) to represent the reaction kinetics and assuming no reactions and pressure drops on both the tube and shell sides, they were... 

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