PBMRs reactors

This led to higher C2 yields, while the CH4 permeability was also kept low, and the contact time was high enough. Removal of the desired products helped the distributed feed PBMR reactor more than the conventional co-feed PFR reactor, i.e. a synergy between the two membrane functions was observed. Unfortunately a membrane with the desired characteristics of selective removal of C2 species at high temperature is unknown. 

The PBMR reactor core is basically a long right circular cylinder with a fuel effective height of 11.0 m and a diameter of 3.7 m. Twenty-four reactivity control system rod holes are equally spaced outside the core. For shutdown purposes and for minor reactivity adjustments, two diverse reactivity control systems are used. 

Most research reports involve an inert, selective membrane that encloses a PB of catalyst particles, a packed-bed membrane reactor (PBMR). It must be noted that the catalyst bed can also be fluidized or fixed, but types other than PBs are rarely found in literature. The following are the advantages of this type of reactor ... 

Alternative reactor types are possible for the VHTR. China s HTR-10 [35] and South Africa s pebble bed modular reactor (PBMR) [41] adopted major elements of pebble bed reactor design including fuel element from the past German experience. The fuel cycles might be thorium- or plutonium-based or potentially use mixed oxide (MOX) fuel. 

Matzner, D., PBMR project status and the way ahead, in Proc. of the 2nd International Topical Meeting on High Temperature Reactor Technology, Beijing, September 22-24, 2004. 

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

Pebble-Bed Modular Reactor (PBMR) A nuclear reactor technology that utilizes tiny silicon carbide-coated uranium oxide granules sealed in pebbles about the size of oranges, made of graphite. Helium is used as the coolant and energy transfer medium. This containment of the radioactive material in small quantities has the potential to achieve an unprecedented level of safety. This technology may become popular in the development of new nuclear power plants. 

This paper describes previously developed models of the S-I/HyS cycle and a PBMR-268. A general coupling methodology via the IHX is developed, and applied to these models. Finally, two nuclear reactor driven transient scenarios are considered. 

A PBMR is a thermal reactor, thus delayed neutrons are the important factor in reactor response. A thermal reactor has a time constant of about 55 seconds. In the chemical plant, Section 2 and Section 3 have different response times. Section 2 has a response time on the order of 20 seconds, whereas Section 3 has a response time on the order of 500 seconds. The limiting reaction rate in the chemical plant is that of Section 3. Since the chemical plant is composed of cyclic processes, we know that the slowest reaction rate will occur in Section 3, the HI decomposition section. The response rate of Section 3 provides at least a first-order approximation of the overall plant response. 

A transient control volume model of the S-I and HyS cycle is presented. An important conclusion based on the results of this model is that the rate-limiting step of the entire S-I cycle is the HI decomposition section. In the HyS cycle, the rate-limiting step is the H2S04 decomposition. A generalised methodology for coupling these thermochemical cycle models to a nuclear reactor model is overviewed. The models were coupled to a THERMIX-DIREKT thermal model of a PBMR-268 and a point kinetics model. Key assumptions in the PBMR-268 model include flattening of the core and parallelisation of the flow channels. 

Several reactors are candidates for use as a high temperature heat source for the S-I cycle. Candidates include the modular helium reactor (MHR) and pebble bed modular reactor (PBMR). One of the most thoroughly investigated candidates is the PBMR. Recent work has been performed in benchmarking the THERMIX code to the PBMR-268 design (Reitsma, 2004 Seker, 2005). 

Safely implementing a thermochemical nuclear hydrogen generation scheme requires a robust understanding of the interaction between the nuclear plant and the chemical plant. In turn, this requires robust models of the chemical plant, reactor thermal-hydraulics and reactor physics. Efforts have been conducted in both the transient modelling of the sulphur-iodine (S-I) and hybrid sulphur (HyS) thermochemical cycles, as well as coupling to models of the pebble bed modular reactor (PBMR-268) (Brown, 2009). 

Accident scenarios initiated in the PBMR plant have been described and thoroughly modelled as benchmark problems (Reitsma, 2004). While modelling these scenarios in a coupled nuclear reactor/ chemical plant scheme is interesting, it should be noted that in most of these scenarios the nuclear... 

Based on matenal considerations, membrane reactors can be classified into (1) organic-membrane reactors, and (2) inorgamc-membrane reactors, with the latter class subdivided into dense (metals) membrane reactors and porous-membrane reactors Based on membrane type and mode of operation, Tsotsis et al. [15] classified membrane reactors as shown in Table 3. A CMR is a reactor whose permselective membrane is the catalytic type or has a catalyst deposited in or on it. A CNMR contains a catalytic membrane that reactants penetrate from both sides. PBMR and FBMR contain a permselective membrane that is not catalytic the catalyst is present in the form of a packed or a fluidized bed PBCMR and FBCMR differ from the foregoing reactors in that membranes are catalytic. 

Before proceeding further it would be appropriate for our readers to familiarize themselves with the few additional acronyms that will be used in this chapter and which are listed in Table 11.1. They are used to describe some of the most common membrane reactor configurations that have been studied in the technical literature. By far the most commonly referred to reactor is the PBMR, in which the reaction function is provided by a packed bed of catalysts in contact with the membrane. The membrane is not itself catalytic at least not intentionally so. Some of the commonly utilized inorganic and metal membranes, on the other hand, are intrinsically catal) ically active. The PBMR clcissification, therefore, should be assigned with caution. When the packed bed... 

Fig. 11.5. Formaldehyde yields in various reactors under similar experimental conditions. V, conventional plug-flow reactor , a PBMR using a Pd/AljCb dense composite membrane A, a PBMR using a mesoporous AI2O3 membrane , a CMR using a mesoporous AI2O3 membrane catalytically impregnated by a sol-gel technique. Reproduced from Deng and Wu [61] with permission.

Several other recent modelling membrane reactor studies are also worth discussing, Varma and coworkers [127] have analyzed the effect that nonuniform catalyst distribution on the membrane itself (for CMR and PBCMR applications) and in the catalyst bed (for PBMR applications) has on membrane reactor performance. Conventional membrane reactor models were utilized by a number of groups to model their experimental data. Shu and co-workers [33]... 

A schematic of a PBMR, in this case for the water-gas shift reaction, is given in Fig. 14.2. Of course the catalytic reactor and the membrane unit can also be separated from each other, but can still be used to enhance the yield of a catalytic process, as will be shown in Section 14.3 (see also Fig. 14.5). 

The packed bed ceramic membrane reactor configuration (PBMR) has been chosen as the reactor set-up (see Section 14.2.2). In the PBMR configuration three possible sub-configurations can be envisioned for a specific sweep gas in combination with a hydrogen or oxygen selective membrane for the dehydrogenation of ethylbenzene. These sub-configurations are shown in Fig. 14.10. 

Results of the implementation of all four types of membranes in only the first reactor (PBMR) are given in Table 14.8. Implementation of these membranes decreases the performance of the reactor because ... 

In order to explain this, the influence of the kinetics of the main reaction on the performance of the membrane reactor has been studied, for microporous membranes implemented in the second reactor. The reaction rate of the main reaction is successively multiplied by a factor 2 and 10, and as a consequence the reaction equilibrium is reached much faster. Under these circumstances increases are found in both yield and selectivity for the conventional dehydrogenation reactor without membranes. The results of the calculations are presented in Table 14.9 in which the differences in yield and conversion are given in percentage points with respect to the conventional case. The higher yields and conversions for the PBMR compared to the conventional reactor are due to the fact that the conversion is no longer limited by the kinetics, as in the previous cases, but by the permeation of hydrogen. 

Yield and selectivity in a PBMR as a function of reaction kinetics, compared to those in a conventional reactor... 

The most commonly utilized catalytic membrane reactor is the PBMR, in which the membrane provides only the separation function. The reaction function is provided (in catalytic applications) by a packed-bed of catalyst particles placed in the interior or exterior membrane volumes. In the CMR configuration the membrane provides simultaneously the separation and reaction functions. To accomplish this, one could use either an intrinsically catalytic membrane (e.g., zeolite or metallic membrane) or a membrane that has been made catalytic through activation, by introducing catalytic sites by either impregnation or ion exchange. This process concept is finding wider acceptance in the membrane bioreactor area, rather than with the high temperature catalytic reactors. In the latter case, the potential for the catalytic membrane to deactivate and, as a result, to require sub-... 

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