CFB reactor

While having some advantages over riser reactors, downer reactors also suffer fixrm some serious shortcomings, such as a low solids holdup in the bed, difficulty in even distribution of injected residual on the catalysts, and a high sensitivity to the structure of the inlet [11,12]. Therefore, the development of a new coupled CFB reactor that can fully utilize the advantages of the riser and the downer is of interest. 

One discouraging problem is the decrease in reactor or combustor performance when a pilot plant is scaled up to a larger commercial plant. These problems can be related to poor gas flow patterns, undesirable solid mixing patterns and physical operating problems (Matsen, 1985). In the synthol CFB reactors constructed in South Africa, first scale-up from the pilot plant increased the gas throughput by a factor of 500. Shingles and McDonald (1988) describe the severe problems initially encountered and their resolution. 

Shingles, T., and McDonald, A. F., Commercial Experience with Synthol CFB Reactors, in Circulating Fluidized Bed Technol. II, (P. Basu, and B. P. Large, eds.), Peigamon Press, Oxford (1988)... 

The rapid thermal processing (RTP) of the company Ensyn Group Inc., situated in Ottawa (Canada). Diverse working process plants for the production of biobased aromates are available of which the largest has a biomass processing capacity of 70 t/day. The heart of the process is a CFB reactor. 

Patience, G. S., Chaouki, J., and Kennedy, G. Solids residence time distribution in CFB reactors, in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), pp. 599-604. Pergamon Press, 1991. 

For CFB reactors further aspects have to be taken into account restricted range of admittable particle properties increased particle attrition decreased suspension-to-wall heat transfer coefficients more complexity in designing and operating the circulating loop higher capital costs. 

Figure 2 illustrates the successive steps in the design strategy of a CFB reactor, which will be applied for the design of a biomass-combustor with a capacity of 2.S lonnes/hr. 

Coal combustion is one of the major applications for CFB reactors. Quoted capacities range from 1 MWo, (labscale) to 700 MW (Soprolif-EDF), It is announced by EDF that capacities will exceed 500-600 MW after the year 2000. 

On this background, a Circulating Fluidised Bed (CFB) reactor for biomass fast pyrolysis was extensively tested by CRES in the fiamework of previous JOULE I and IT Programmes The CFB reaaor configuration conceived is differentiated from... 

Two experiments (RSI, RS2) were performed with DC-II and Swedish wheat straw with particle size in the range of 1 to 2 mm. During these experiments the solids recovery system (cyclone and impinger, Figure 1) did not perform satisfoctorily with the very fragile straw char. The char, exposed to the very harsh envirorunent (fast moving sand particles) of the CFB reactor and atritted to very fine particles, almost submicTon powder, could not be collected by conventional solids recovery systems. Consequently, the char was either adhered with the liquids heavy fraction to the STHE inner tubes or readilly accumulated in the liquid recovery system. 

A significant problem remains the relatively high solids (char) content in the pyrolysis liquids, due to the fiagmentation of char and wood fines in the harsh environment which are subsequently entrained in the gaseous stream, an inherent characteristic encountered in all CFB reactor systems, (ineffective solids recovery system). 

After the second period of the CFB reactor operation (DC-II, DC-III) further amendments in the plant were decided to solve respective problems. 

The primary exploitation of the lean-phase fluidized beds is associated with the circulating fluidized bed (CFB) reactors. 

For the low activity FCC catalysts then available, the bubbling bed design was a decided improvement over the first CFB reactor. Until the mid-1970s, virtually all FCC units maintained a dense phase bubbling or turbulent bed in the reactor vessel. A few of the second generation bubbling bed FCC reactors are still in operation [97]. 

With the introduction of zeolites in the early 1960s, the FCC catalyst activity began to increase steadily [97]. By 1980 many units were again operating in a CFB mode to reduce the residence time of the gas reactants in the reactor. Today, by far the greatest use of CFB reactors is for the FCC process in petroleum refining. 

In the case of high-temperature, fluidized-bed reactors operation at higher pressures has an additional benefit in that the rate of carbon deposition on iron catalysts is proportional to Pgg/P H2 (ref. 2). As the partial pressures increase, the carbon deposition rate decreases. Because the deposited carbon has a high area and acts like a sponge for retaining wax, the rate of wax accumulation on the catalyst also decreases with increasing pressure (ref. 2). The benefit of this is that the catalyst particles are less likely to become "sticky" and result in defluidization of the bed. The CFB reactors at Sasol Two and Three operate at higher pressures than the older units at Sasol One, and the lower deposition rates of carbon and wax on... 

DuPont s new tetrahydrofuran plant, which is slated to start up in the second quarter of 1996 in Asturias, Spain, is based on a new process that is claimed to be environmentally friendly [133], The process is harnessed with a circulating fluidized-bed (CFB) reactor for the partial oxidation of n-butane. The catalyst used for this stage is vanadium phosphorus oxide (VPO). To enhance the catalytic life in the CFB reactor, attrition-resistant catalyst particles have been developed. A highly selective catalyst for hydrogenation of maleic acid to THF also has been developed. This THF conversion reaction is carried out in a bubble column reactor under relatively mild conditions. This process has several significances ... 

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