Fluidized-bed tests

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

Figure 3. Schematic drawing of the fluidized bed test facility suggested by Gwyn (1969).

Pneumatic Conveying Tests. In contrast to fluidized bed tests, no standard equipment exists that simulates the stress on particles in pneumatic conveying lines. There is no friability test quoted in the pertinent literature that is based on a specific pneumatic conveying system. 

FIXED-BED AND FLUIDIZED-BED TESTS In the fixed-bed test, a sample of cracking catalyst was contacted with an FCC feed in a manner similar to the standard microactivity test (MAT) prescribed by ASTM (10). One major difference was that the reactor temperature was increased from 900°F to 960°F to better simulate current commercial operations. 

The fluidized-bed tests were conducted in a 0.5 BPD circulating pilot plant equipped with a 50-ft riser reactor and an on-line debutanizer. The total liquid product from each test period was distilled externally, and the individual cuts were submitted for inspections. 

Apart from the impact and vibration tests dealt with previously, there are several other tests of friability, some of which appear in British Standards dealing with specific materials. Such tests include mixer tests, jet impingement tests, shear tests, tumbler tests and fluidized bed tests. Of these, the shear tests and tumbler tests are only considered worth mentioning here, with more details available from another BMHB publication54. 

The most satisfactory and meaningful catalyst activity tests are those involving the cracking of a standard cracking stock in small-scale static-bed reactors. Small laboratory-scale fluidized-bed test units have been described, but generally speaking these are more difficult to operate satisfactorily than fixed-bed units. Powdered catalysts are generally tested in pelleted form or in a nonfluidized static bed of powder. 

Boerefijn R, Zhang SH, Ghadiri M. Analysis of ISO fluidized bed test for attrition of fluid cracking catalyst particles. In Fan LS, Knowlton TM, eds. Fluidization IX. New York Engineering Foundation, 1998, pp 325-332. 

Physical Properties The free-swelling index (FSl) measures the tendency of a coal to swell when burned or gasified in fixed or fluidized beds. Coals with a high FSl (greater than 4) can usually be expected to cause difficulties in such beds. Detads of the test are given by the ASTM D 720 (American Society for Testing and Materials, op. cit.) and U.S. Bureau of Mines Report of Investigations 3989. 

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

As mentioned in Section 2.2 (Fixed-Bed Reactors) and in the Micro activity test example, even fluid-bed catalysts are tested in fixed-bed reactors when working on a small scale. The reason is that the experimental conditions in laboratory fluidized-bed reactors can not even approach that in production units. Even catalyst particle size must be much smaller to get proper fluidization. The reactors of ARCO (Wachtel, et al, 1972) and that of Kraemer and deLasa (1988) are such attempts. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

Corrective Action Application Circulating fluidized bed incinerators are ready for full-scale testing under the EPA SITE Program. A unit is now in the RCRA permitting process. 

Findings with PDU. Work with the PDU largely paralleled the bench-scale reactor tests there was one important addition—extensive three-phase fluidization studies. As was mentioned, the PDU is equipped with a traversing gamma-ray density detector that is capable of measuring bed density to within dbO.Ol specific gravity units. Thus, we could measure and correlate fluidized bed expansion as a function of liquid and gas velocities and physical properties, and could also determine the... 

The remaining weight of dry sorbents with time is shown in Fig. 2. For every dry sorbent, attrition mainly still occurs in the early stage of fluidization and A1 test on the basis of the weight after 5 hours shows that Als of molecular sieve 5A and molecular sieve 13X presented 2.1 4.0 times higher than those of activated carbon and activated alumina. Therefore, the use of molecular sieve 5A and 13X in a fluidized bed can cause high maintenance cost and problems in the operating the process. 

Measurements at the wall of fluidized beds were made by Krishna et al. ( K7,K11), who used an annular cell with varying radial ratios to test the influence of inner tubes on fluidization. Only at low r jr0 values is the inner wall coefficient appreciably higher than the outer wall coefficient (which is itself somewhat lower than in a pipe bed of the same radius). 

On the basis of different assumptions about the nature of the fluid and solid flow within each phase and between phases as well as about the extent of mixing within each phase, it is possible to develop many different mathematical models of the two phase type. Pyle (119), Rowe (120), and Grace (121) have critically reviewed models of these types. Treatment of these models is clearly beyond the scope of this text. In many cases insufficient data exist to provide critical tests of model validity. This situation is especially true of large scale reactors that are the systems of greatest interest from industry s point of view. The student should understand, however, that there is an ongoing effort to develop mathematical models of fluidized bed reactors that will be useful for design purposes. Our current... 

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