Keen Interest in Fluidization Shawn at I&EC Symposium A STAFF
JL HE birthplace of the annual chemical engineering Christmas symposium spon sored
by
the
AMERICAN
CHEMICAL S O
CIETY'S Division of Industrial and Fnginoering Chemistry was the site of its fifteenth appearance Dec. 28 and 29. The program was an impressive example of the contributions this activity is mak ing to the field of chemical engineering technology. A record-breaking group of 150 registrants a t t e n d e d the 21-paper symposium held a t t h e Massachusetts I nstitute of Technology. N o t only was t h e subject of evident interest to t h e large group attending, b u t the papers as a whole were exceptionally well presented. T h e latter much-appreciated trait w a s per haps an indication t h a t the numerous younger speakers on t h e program a r e profiting from the perennial complaints about, reading from a draft prepared p r i marily for publication. Γη subject m a t t e r the symposium papers could be classified roughly in three cate gories Approximately half were studies of the inherent characteristics of fluidized systems as a chemical engineering unit operations tool. Four of the papers dealt with important practical problems asso ciated with t h e field, such as abrasion problems and development of measuring and testing apparatus, and five papers described actual processes that utilized fiuidized systems to accomplish their in tended purpose. Fluidized
Systems
Studies on fluidized particles, fluid-solid systems, fluid flow, a n d pressure drop occupied the attention of m a n y of the authors. Typical of this thorough a p proach was t h e paper b y \V. K. Lewis, K. R. Gilliland, a n d W. C. Bauer of the Massachusetts Institute of Technology reporting the results of studies of the characteristics of fluidized particles. T h e authors pointed out t h a t the use of fluidized solids h a s developed rapidly, especially in the catalytic cracking of petroleum. Currently this operation is being considered for m a n y other chemical operations including t h e synthesis of liquid hydrocarbons, production of phthalic anhydride, gasification of coal, burning of lime, and the partial oxidation of hydrocarbons. The authors believed that to design fluidization units efficiently for these uses, information of the t y p e presented in their paper would be valuable. It was explained that the flow charac teristics for both batch and continuous fluidization of fine glass spheres was in vestigated. In t h e first case, the solid material remains within the unit with virtually no entrainment and in the 148
RKPORT
second «isi· t h e solid flows through the unit continuously. It was revealed that two tubular Lueite units, 50 inches in height, were used for the batch fluidization studies-. One of these had a diameter of 2.5 inches and the other a diameter of 4.2.r) inches. For the continuous fluidi zation studies a 10 foot high, 1.25 inch inside diameter brass tube was used. The authors believed t h a t on the basis of their work two methods of predicting the operating conditions in a fluidized bed have Ijeen established. Assuming that a given solid is to be fluidized with a given fluid, the diameter or diameter distribu tion, t h e absolute density of the solid, and the physical properties of the solid would be known. From the correlations de veloped l)y t h e authors and presented in the pa.per, it would then be possible to calculate the fraction voids and from t h a t the fluidized density of the solid at a n y superficial velocity desired. An alterna tive procedure would be to employ the data to obtain the fraction voids and velocity corresponding to the quiescent state. This could be used in conjunction with other correlations developed by the authors to estimate the fraction voids as a amotion of the fluidizing velocity. T h o authors declared that in continuous fluidization, t h e action is different because of the fact t h a t both powder and gas are being continuously transported through the unit. In experiments on this type of fluidization, it was found t h a t with a given gas velocity a fairly wide range of solid feed rates could be utilized. I t was also observed .that the concentration of the solid obtained in the fluidized bed was a function of this feed rate which, if in creased, fora given gas velocity ultimately reached a point where an unsteady opera tion w a s obtained. Instead of displaying a steady slate condition the solid concen
tration in the fluidizing unit rose continu ally until the bed was of a high density. A good perspective of fluidization was given in the paper b y G. L. Matheson, W. A. Herbst, and P. H. Holt of the Standard Oil Development Co. In the introduction to the paper, they pointed out the amazing growth of catalytic crack ing capacity from 40,000 barrels a day in 1942 to the estimated 1,000,000 barrels a day figure of 1948. The reason for the ready acceptance of this new technique b y the petroleum industry was twofold, ac cording to the authors. T h e major at traction was the ease with which solids could be transported from vessel to vessel, permitting a very efficient utilization of heat and the ready development of a con tinuous process. Also important was the fact that the extreme turbulence of the solid gas suspension allows excellent heat transfer to and from t h e system. A description of the actions accompany ing the upward journey of a gas through a fluid bed given by the authors pointed out the need for a d e q u a t e information about the density a n d viscosity of the dense phase if the engineering considerations are to be met. It was observed that density is maximum at low gas velocities a n d decreases as the flow of gas is increased. Effect of
Channeling
The effect of channeling in fluidized beds upon heat transfer was discussed in the paper by Max I^eva, Murray Weintraub, Milton G rummer, a n d Morris Pollchik of the Bureau of Mines. The authors reported the results of their studies upon coal, sand, Fischer-Tropsch fused iron catalyst, and other materials. I t was found that for all t h e substances studied channeling tendencies increased rapidly with decreasing particle diameter. T h e weight-size distribution as expressed by a
E. C darker, Arthur D. Little, Inc., anil chairman of the Section, ACS; J. C. Elgin, Princeton University, and chairman sion of Industrial and Engineering Chemistry: If. C. Whitman
CHEMICAL
AND
ENGINEERING
Northeast of Divi of MÎT
NEWS
uniformity coefficient, however, seemed to have little effect upon channeling. It was also observed that channeling was not sig nificantly affected by variations of t h e vessel's dimensions within stated limits. A study of the isothermal flow charac teristics of solids-gaseous mixture (A1203, S i 0 2 catalyst, and air) was reported in t h e paper by Leonard F a r b a r of the Univer sity of California. T h e author referred to his investigation as a preliminary one and chose to avoid making a n y general con clusions from the results. He did claim t h a t the work showed t h a t the flow char acteristics of gaseous-solids mixtures in which the particle size distribution covers a wide range differ considerably from the flow of mixtures with fixed-size particles. T h e paper by Sabri E r g u n and A. A. Orning of the Carnegie Institute of Technol ogy declared that in the unfluidized state, the relation between gas flow and pressure drop in fluidization columns is similar to t h a t in ordinary packed columns. Meas urements of pressure drop versus gas flow rate in a small fluidized bed used in the authors' investigations showed that ex perimental constants for a pressure drop equation, as applied to the packed col umn, can be used to predict the rate oi bed expansion with increasing gas flow when the pressure gradient reaches the weight of solids per unit volume of bed. T h e y claimed that t h e change from the dominance of viscous forces to that in which kinetic effects predominate is smooth and allows the development of a theoretical equation t h a t holds over wide ranges of fluid flow. T h e derivation of the equation as developed by the authors is parallel to that of t h e Kozeny equation b u t introduces in addition to the viscous forces, consideration of kinetic forces, not covered by Kozeny. Predicting
Pressure
Drops
D . H. Belden and L. S. Kassei of Uni versal Oil Products Co. reported the results of their work on the development of a means to predict pressure drops in vertical transfer lines when relatively large particles are conveyed with air.
T h e authors revealed t.hat the original purpose of t h e work was to aid · in the design of a pilot plant that employed large catalyst particles. As the work progressed such a large number of vari ables h a d to be examined that a correla tion of data was attempted to find out whether they would be applicable for use in the design of units of commercial size of both the fluidized and moving bed types. The authors declared that although the correlations developed by them were interesting, they should not be considered adequate for engineering design. They claimed that further investigatory work is Fieeded in the areas in which the gen eralized correlation developed from their work breaks down. Such studies should obtain more adequate data on lifting velocity so that accurate determination can be made of the part played by the catalyst density in the total pressure drop. T h e importance of pressure drop in formation in the design of pilot plant and commercial scale fluidized systems was again referred to in the paper by Ο. Η. Hariu and M. S. Molstad of the University of Pennsylvania. A means of estimating the pressure drop in the ducts carrying the gas-solid mixture is especially im portant in the design of the smaller equip ment where t h e risers or ducts may be as small as Ο.ό-inch inside diameter. In pipes of this size, the friction loss between the particles and the walls becomes a considerable factor in total pressure drop. T h e a u t h o r s presented measurement data on the total pressure and static pressure drop in the transport of solid particles through vertical glass tubes of 0.267- and 0.532-inch inside diameters. They also made direct measurements of the dispersed solids density and calculated average particle velocity, slip velocity, static pressure drop, and solids friction loss. In full scale fluid catalyst cracking equipment, t h e authors continued, the riser pressure d r o p serves as a good index in estimating the rate of catalyst circula tion. This is a very important considera tion in those systems where part of the reaction takes place in the riser and it is
James R. Killian, president-designate of MIT, addresses I&EC Division at dinner; W. K. Lewis of MIT, dinner speaker; Lincoln T. Work, Metal and Thermit Corp., chairman of fluidisation symposium, who presided at dinner
VOLUME
2 7,
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3 *
. J A N U A R Y
17,
1949
Max ports cite
Leva, on coal
Bureau of Mines, re flui€lisation of anthra and of other solid*
necessary to have a measure of the degree of contact between t h e gas and t h e solid. T h e results of a study of the flow of vapors through moving beds of catalyst in which the entire bed moves with re spect to the walls of the container although the moving particles remain in a fixed position relative to each other were pre sented by John Happel of New York Uni versity. This work, in which both a con current and countercurrent flow of air with respect to the bed were used, furnished d a t a t h a t when coupled with material from pre vious investigations permitted the develoj>ment of a correlating equation. This equation, he revealed, was developed by the application of Navier-Stokes dynamic equations to the viscous motion of fluid through an assemblage of uniform spheres. Problems
in Fluidized
Processes
Several essentially technological prolnlems involved in the use of fluidized proc esses were discussed in another group of papers. R. L. Stoker reported tests in which targets of gypsum plaster a n d black iron were blasted with air jets containing entrained silica or synthetic petroleum cracking catalysts. Maximum erosion was found to occur for a given material and jet velocity when the jet impinged on the plane of the specimen at- a flat angle of incidence of about 20° from the parallel. T h e author suggested the use of plaster samples as a convenient device to measure abrasive influences inside fluid catalyst equipment. The following paper, by W. L. Forsythe, Jr., a n d W. II. Hertwig, reported studies of the size re duction of fluidized particles themselves from attrition. Their findings confirmed t h e logical expectation t h a t spheroidal particles resisted size degradation more effectively t h a n irregularly shaped par ticles and that, the latter material ap proached a more regular conformation in 149
u s e b y wearing away of irregularities. B e c a u s e of this effect, resistance to a t t r i tion improves substantially after t h e ir regularly shaped particles have been in use for a relatively short time. The concluding papers of the symposium significantly per tended t h e wide scope of application in chemical processes for which fluidization is inherently suited. H . P . Meissner a n d H. S. Mickley de scribed tests in which sulfuric acid mists a n d a m m o n i u m n i t r a t e smoke were re m o v e d by a fluidized bed of various por ous materials. Efficiency of removal was 9 0 to 9 5 % . Production of carbon mon oxide by reaction of carbon dioxide with fluidized retort coke o r Pennsylvania a n t h r a c i t e coal w a s reported in a paper by W . K . Lewis, E. R . Gilliland, a n d G u y T. M c B r i d e of M I T . This process is of ob vious interest in t h e liquid-fuel-from-coal investigation. P a p e r s b y F. S. W h i t e of the N e w E n g l a n d Lime Co. a n d by C. J. Wall of t h e D o r r Co. a n d W. J. Ash of New E n g l a n d Lime described applications in t h e lime industry relating to the limeb u r n i n g operation and a drying and sizing process, respectively. I n the final paper, W. K. Lewis, E. R. Gilliland, and W . Λ. Reed of M I T collaborated to present a paper on production of carbon monoxide a n d hydrogen by reaction of m e t h a n e with a fluidized bed of copper oxide. At 1650 ° F. a n d stoichiometric proportions 9 4 % of t h e m e t h a n e w a s reacted with a selec tivity to carbon monoxide and hydrogen of92Vc. T h e full texts of those symposium papers found suitable for publication are sched uled for t h e J u n e 1949 issue of l&EC. Education
Trends
T h e newly designated president of M I T , James R . Killian, Jr., discussed t r e n d s in higher education at t h e banquet meeting. He n a m e d t w o current trends which he t e r m e d hazardous. T h e recom mendation for a twofold increase in college enrollments by 1960, made last spring by the President's Commission on Higher E d u c a t i o n , was felt to carry very real d a n g e r s of lowering the quality of educa tion. Similarly, the proposal of the commission and others that large federal funds be used to finance higher education was considered to h a v e grave disad vantages. Such funds are recommended for t h e public institutions, which might increase in consequence so greatly as to u p s e t the balance now existing between the private schools, which a r e sensitive to t h e needs of special groups, a n d the public institutions, the principal motiva tion of which is the public need. T w o trends considered beneficent are a current broadening of t h e base of t h e curriculums a n d t h e growth of sponsored re search. T h e former development has progressed to t h e point, D r . Killian stated, that he believes we are "on t h e verge of a new synthesis of general and specialized education, with a consequent strength 150
ening of b o t h . T h e engineering colleges recognize t h a t their objective is to train top-flight engineers who can likewise be come top-flight citizens—professional men
who h a v e the background, understanding, and public spirit to be leaders in their p r o fessions, their neighborhoods, a n d the na tion."
Accident in Crystallization of Alpha, Alpha' Azodiisobntyronitrile J. CARLISLE, Manager, Field Research Section, Electrocheinicals Department, E . I. du Pont de Nemours & Co., Inc., Wilmington, Del. PAUL
I N CONNECTION with an extensive r e search program on aliphatic azonitriles as initiators for vinyl a n d diene polymeriza tions (Brit. AppL N o . 7123/47 and 7124/ 47), several thousand pounds of «,a:'-azodiisobutvronitrile CH, XC-
Oils
I -c:—N=X—οI
Ι
-ex
CII3
were p r e p a r e d on a semiworks scale. Tests m a d e by t h e explosives department of t h e TC. T. du P o n t de N e m o u r s & Co., Inc., t o g e t h e r with several years of experience in handling t h i s material, have indicated it to be nonexplosive. or, α'-A zodiisobutyronitrile decomposes at its melting point (107° C.) with evolution of nitrogen a n d formation of tetramethvlsuccinonitrile (Thie.le a n d Heuser, Ann., 1, 290 [1806]), but storage tests a t ordinary t e m p e r a t u r e s (35° C m a x i m u m ) have shown a loss of only 5 % of the azo-nitrogen in a year. However, on one occasion recrystailization of a 5-pound lot of crude material from acetone solution led to a violent reac tion resulting in considerable damage. Rince α,α'-azodiisobutyron! ;\v has a t t r a c t e d a t t e n t i o n as a blowing agent for elastomers and plastics (PeBell, Goggin, and Gloor, " G e r m a n Plastics P r a c t i c e , " 456 fY. DeBell & Richardson, Inc., W 4 6 \ a description of the circumstances involved is being published so that others may be forewarned by our experience. On the occasion of the accident men tioned a b o v e , 5 pounds of crude (90°T0 c*,a:'-azodiisobutyronitrile was added to 25 p o u n d s to technical-grade acetone in an agitated 10-gallon glass-lined reactor. T h e t e m p e r a t u r e was raised to 5 3 ° C. to dissolve t h e azo compound and then r e duced to 2 1 ° C. to effect crystallization. This mixture was then discharged from t h e r e a c t o r and allowed to s t a n d overnight in a covered vessel. Due to unsatisfactor\ r crystallization, t h e entire b a t c h was re charged t o the reactor. A b o u t half of t h e acetone w a s removed by distillation, a n d the steam on the jacket of t h e vessel was s h u t off. D u r i n g this operation, the re a c t o r w a s vented to t h e . atmosphere through t h e condenser. As the operator walked a w a y after shutting off the steam, the m i x t u r e decomposed violently. T h e cover of t h e reactor was blown off, and t h e glass lining was severely damaged. White C H E M I C A L
smoke was evolved, b u t 1x0 flash was o b served. Fortunately, no o n e w a s injured. In s u b s e q u e n t tests, 5—pound samples from two separate b a t c h e s of 9 0 % azonitrile decomposed rapidly but d i d not ex plode when heated alone i n an uncovered jacketed steel vessel by s t e a m a t a t m o s pheric pressure. T w o s i m i l a r 5-pound samples, each mixed with, one gallon of acetone, decomposed explosively when subjected to t h e s a m e h e a t i n g . On t h e other hand, a 5-pound s a m p l e of 9 0 % azonitrile mixed w i t h one gallon of methanol and h e a t e d similarly led t o a very rapid but nonexplosive decomposition. In s t a n d a r d laboratory tests a t t h e D u Pont Burnside L a b o r a t o r y to m e a s u r e t h e explosive properties of a-,ar'-azodiisobutyronitrile, t h e p r o c e d u r e u s e d w a s : A weighed sample was l o a d e d into a 50cc. b o m b , and a weighed q u a n t i t y of blank fire powder was poured o n top of it. T h e firing head, which h a d t w o firing pins (one insulated for electrical firing), was equipped with fine iron calorimeter wire and 0.1 or 0.2 gram of nitrocellulose. T h e firing head was then screwed in place a n d the. b o m b fired electrically. A piston in the b o m b transmitted t h e p r e s s u r e d e veloped to a q u a r t z piezoelectric gage, t h e o u t p u t of which was amplified and con nected to the plates of a c a t h o d e - r a y oscillograph. From this, records of pres sure and rate of p r e s s u r e rise were o b tained. T e s t s were m a d e o n b o t h p u r e and crude material. In some tests, the s a m p l e was mixed with an equal weight of smokeless powder. I n n o case did. the c o m p o u n d have a positive effect in increasing the r a t e of pressure rise over that of t h e igniter or powder present. Only in the case of p u r e material was there any increase in t h e m a x i m u m pressure of t h e m i x t u r e , a n d this was only slight. F r o m these tests i t w a s concluded t h a t the material need n o t t>e classed a s a n explosive and, for s h i p p i n g , would r e q u i r e no particular safety p r e c a u t i o n s o t h e r than those t a k e n for o r d i n a r y flammable solids. I n closed r e a c t i o n systems, how ever, it is believed t h a t s o m e type of pres sure release mechanism SULCII a s a r u p t u r e disk should b e provided t o h a n d l e a mild rise in pressure. I t is r e c o m m e n d e d t h a t until m o r e information is available, t h e use of acetone as a s o l v e n t for o^a'-azodiisobutyronitrile should be a v o i d e d . AND
E N G I N E E R I N G
NEWS
