FLUIDIZED
FIXED BE Method for Contacting Solids wit Gases and Vapors C H A R L E S L.T H O M A S A N D J A M E S HOEKSTRA U N I V E R S A L OIL P R O D U C T S C O M P A N Y 310 S O U T H M I C H I G A N A V E . CHICAGO 4,
ILL.
Cracking U n i t Utilizing t h e Fluidized Fixed Bed
T
HE passage of gases or vapors upward (counter to gravity) through a mass of powdered solid produces the "fluidized fixed bed". The term "fluid" is used to convey the fact that the powdered solid becomes mobile or fluid in the presence of the flowing gases or vapors. I n this respect it resembles the fluid catalyst systems that have become important in catalytic cracking ( I ) . All or most of the powdered solid remains in the reaction zone in the fluidized fixed bed. In this respect i t resembles the fixed beds of immobile solid contact agents through which gases or vapors flow. Thus in the fluidized fixed bed both reaction and regeneration may take place in the same vessel as is usual with fixed beds of catalyst. Both in the fluidized catalyst system and in the compact moving-bed catalyst system i t is customary for the catalyst to be regenerated in a vessel which is separated from the reaction zone. If a column of powder is put in a vertical tube and air is passed slowly upward, a small flow of air can be obtained without any visible effect on the powder. If the air rate is increased beyond this rate, small channels are seen to form with a slight expansion of the powder. Further increase in the air rate results in a large expansion of the bed with violent agitation of the powder. This is the fluidized fixed bed condition. Still further increases in the air velocity result in further expansion of the bed, and finally the powder is blown out of thr t,ube. This dcsciiption
indicates that a number of separable phenomena take place, and that the fluidized fixed bed condition obtains over only a part of the range of flow rates. A number of factors determine the useful range of flow rates: (1) The diameter of the tube should be 2 inches or greater. A t smaller diameters wall effects become sufficiently prominent to interfere with smooth operation. (2) The density of the powder is related to the air rate necessary to fluidize it; the greater the density, the greater the air rate needed. Most of our experience has been with powders having apparent bulk densities from 0.4 to 1.0 gram per cc. (3) For a given bulk density as the particle size of the powder increases, the air rate needed to fluidize the powder increases. A particle-size range of 40 to 100 mesh was selected. With the above densities and sizes, the fluidized fixed bed air rate range Tvas about 0.5 to 1.5 feet per second superficial linear velocity (the linear velocity that would obtain in the absence of the pobyder). (4) The ihape of the particles of powder habe an cffect, Plate-shaped particles behave differently from rounded or \pherehke particles. The flow conditions necessary to obtain a desired operation !$ere such that it was necessary t o construct a full-scale glass model to determine the conditions for satisfactory operation and the limits of the desired type of operation. Based on data obtained with the glass model using air, metal units were constructed. The details of construction are given in Figure 1. Es-
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1945
THE PASSAGE of gases or vapors upward (counter t o gravity) a t controlled rates t h r o u g h a mass of powdered solid produces t h e L‘fluidized fixed bed’,. T h e passage of t h e gas or vapor causes t h e powdered solid t o become mobile so t h a t it moves about, resembling an agitated liquid. The particle size of t h e powder is regulated so t h a t l i t t l e of it is carried o u t of t h e reaction zone w i t h t h e gas or vapor. The fluidized fixed bed is particularly adapted t o
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catalytic reactions in which a reaction occurs t h a t i s accompanied by a large heat of reaction. T h e movement of t h e powdered solid tends t o prevent t h e formation of localized high-temperature zones (called “hot spots”). Some examples are given which illustrate t h e application of t h e fluidized fixed bed t o t h e catalytic cracking of gas o i l and a hydrogen transfer treatment of an unsatu rated gasoline.
sentially the unit is a piece of 2-inch standard pipe, 27 inches long. Attached to the lower end is a 20” (total angle) cone, 6 inches long. A small piece of 100-mesh screen is placed in the bottom of the cone to retain the powdered catalyst. Provisions for introducing reactants are made at the bottom of the cone. At the top a 100-mesh screen is used to retain catalyst, and provisions are made for removing reactants. A thermocouple well (l/(-inch, 18-8 chrome-nickel tubing) is inserted a t the top. Junctions are located just above the screen in the cone at the bottom and every 7.5 inches upward to the top of the vessel (a total of five thermocouples). Connections for the thermocouples are indicated at the top of Figure 1. The entire assembly is mounted in a thermostated metal block (8). With this arrangement no separate preheater has been necessary in our work; apparently the circulation of hot catalyst has served as a preheater. Many successful tests have been made with the unit as described. An improvement in the degree of contact between solid and vapor was obtained by inserting baffles or distributor plates. These were plates 1 / 8 inch thick, drilled with ‘/pinch holes’
TUBING
Table I. Temperature Distribution during Regeneration with Air Catalyst Level, In.
a
‘ 20
min.
--
Temperature F 40 min.
40 min.
120 min.
Above catalyat level.
Table II. Catalytic Cracking and Hydrogen Transfer in Fluidized Fixed Bed w i t h 40-100 Mesh Silica-Alumina Catalyst Catalyst vol cc. Catalyst wt.:’grame Charging stock type Process period hr. Total time on lest hr. Wt. oil/hr./wt. cabalyat Pressure lb./$q.in. abs. Block temp., F. Regeneration period, hr. Yielda in vol. % ’ charge Gaeoline Naphtha Gal oil Liquid loss Yields in wt. % ’ charge Gasoline Naphtha Gas oil Gas Carbon Hydrogen CYH~ C:H8 C1 Cr Ca paraffins I~O-CIH; n-cdb I~O-CIHI~ n-C:Hlo Gasoline gravity A.P.I. Octane No., F12 Bromine No.‘ Analysis, % Olefins Aromatics Paraffin6 and naphthenes a Bromine No. 51.
1020 593 Gasoline” 1.0 36 0.52 14.7 900 2.67 46.1 14.6 36.1
67.5 {14.1 18.4
21.6 33.3
65.3 (16.8 12.8 5.3 0.16 0.70 2.41 4.31 5.26 0 50 3.57
+ +
0.88
80.9 f
.
.
.
11
8 32 62
14 33 53
e
6 32 62
.
Flgure 1.
Fixed Fluidized Bed U n i t
Their’ diameter wiu such that they fitted snugly in the tube. They were spaced every 2 inches in the reactor. Apparently the ‘/s-inch holes in the baffles break up any large “bubbles” of vapor and any channels that form along the wall just as they would if the catalyst were a liquid. At the same time the I/o-inch holcs are large enough to permit flow of catalyst. REGENERATION
Catalytic solids that have been contacted with hydrocarbons usually become contaminated with a carbonaceous deposit. It is customary to remove these desposits by oxidation with air or flue gas that contains oxygen. When a regeneration is carried out with air in the fluidized fixed bed, there is a rather uniform temperature rise throughout the catalyst zone. Burning seems to occur throughout the catalyst, and the motion of the catalyst seems to quench any hot spots that may form momentarily. This action is in distinct contra& to regenerations of true fixed beds of catalyst. The passage of air through a nonfluidized fixed bed of
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INDUSTRIAL AND ENGINEERING CHEMISTRY
carbonized catalyst causes the formation of a relatively small high-temperature burning zone which is described aptly as a hot spot. This hot spot moves through the catalyst bed in the direction of air flow. There are at least three undesirable characteristics of the nonfluidizpd fixed bed regeneration: (1) Careful control m w t be maintained t o prevent the temperature of the hot spot from rising too high and damaging the catalyst. (2) Catalyst in the regeneration zone but outside the hot spot is idle-i e., serving no useful purpose. (3) The gas for combustion must be preheated to some critical temperature to maintain combustion vithin the hot spot. This reduces the amount of heat that is removed by outgoing gas of a given temperaturr. The fluidized catalyst overcomes the foregoing disadvantages as follows: (1) There is no hot spot within the catalyst. If one tends to form, the moving catalyat quenches it. (2) Essentially all the catalyst is undergoing regeneration at one time. (3) The lower hot portion of the bed of moving catalyst is used to heat the air as it enters the regeneration zone; this eliminates the necessity for outside preheating. The results of a typical regeneration are given in Table I. In this regeneration, cold air was introduced into the bottom of the reactor immediately after it had been used in a test cracking gas oil. The air rate was 12 cubic feet per hour. Temperature surveys were made 20, 40, 80, and 120 minutes after the air was started. It will be noticed that the temperature distribution was distinctly different from the hot spots encountered in the regeneration of a fixed bed. After 120 minutes the regeneration was essentially complete. A total of 40 grams of carbon was removed from 600 grams of cracking catalyst; this regeneration was made with the baffles in the reactor.
RESULTS
Much of our experience with the fluidized fixed bed has been in catalytic cracking and hydrogen transfer. I n these tests the unit has been used without any preheater: The cold oil is pumped directly into the bottom of the unit and is preheated by hot catalyst. This causes the temperature in the cone to be considerably lower than the rest of the catalyst in the unit. This practice has advantages and disadvantages. For our purpose the advantage8 outweighed the disadvantages. A few of the results 0btaine.d with the unit are summarized i n Table 11. Two cracking runs are given, one at 850" and the other at 950" F.; both were made \sith baffles in the reactor. I n addition, a hydrogen transfer run on a gasoline obtained by catalytic cracking is given; it n-as made without baffles in the reactor. These results are not given with the intention of comparing this method of contacting a solid with a vapor or gas with any other methods of cont,acting, but rather to demonstrate that useful results can be obtained by this method. The fluidized fixed bed has certain desirable feat'ures that may be summarized as follows: ( 1 ) The catalyst (or solid) may be used in the form of a powder. ( 2 ) Localized thermal effects in the catalyst, bed are eliminated by the moving catalyst. (3) The catalyst may be used and regenerated in the same vessel, (4) In most, cases no separate preheater is necessary, LITERATURE CITED
(1) Mu:pliree, E. V., Brown, C. L., Fisoher, I I . G. M., Gohr, E, 3.. and Sweeney, U'. J., ISD.ENC.CHEM.,35, 765 (1943). (2) Thomas, C. L., and Egloff, G., "Temperature, Its Measurement. and Control", p. 617, New Yorlc, Reinhold Publishing Gorp., 1941.
COURTESY. STANDARD
Bench-Scale Catalytic Testing Equipment
OIL COMPANY
OF
N. *,
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