Hydrocracking and Hydrotreating

8 Coal Liquifaction by Rapid Gas Phase Hydrogenation

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MEYER STEINBERG and PETER FALLON Department of Applied Science, Brookhaven National Laboratory, Upton, Ν. Y. 11973

The Increased awareness in this nation's dependence on foreign sources of crude oil has generated much interest in de veloping and u t i l i z i n g U.S. domestic energy resources. Since coal is in such great abundance, it is of particular interest to find an economical method of converting it to a low sulfur al ternate fuel capable of being transported through existing pipe lines and used in existing equipment. This would have the three fold effect of 1) revitalizing the coal industry, 2) providing a greater domestic supply of fuel to the power industry, and 3) i n creasing the availability of feedstock to the petrochemical indus try. A number of coal gasification processes have been developed to the point where large scale application has become feasible. The development of the liquifaction of coal has lagged and when taken together with the advantages of handling, transporting, and higher conversion efficiency, the incentive to perform additional work in converting coal to liquid products becomes apparent. The technology for liquid phase coal hydrogenation has been in existence for a number of years. (J.) For example, during World War II Germany produced much of its liquid fuel via the Bergius Process. This involves forming a slurry of coal, oil and catalyst and heating it under high pressure (10,000 psi) for up to an hour while purging the mixture with hydrogen. Being essen tially a batch process involving the s o l i d , l i q u i d , and gas phase, a more direct less stringent method of hydrogenation was desired. In 1962, Schroeder (2) described a process whereby dry particles of coal entrained in a stream of hydrogen at total pressures in the range of from 500 to 6000 psi are rapidly heated to tempera tures in the range of 600 to 1000°C; the resulting stream contain ing the organic products is then quickly cooled to below the re action temperature and the products separated. This system i s claimed to u t i l i z e less hydrogen than the liquid phase method be cause the liquids produced are primarily unsaturated aromatics rather than saturated paraffins and cycloparaffins. Conversion of almost 50% of the coal (on a moisture and ash free basis) to 123 Ward and Qader; Hydrocracking and Hydrotreating ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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HYDROCRACKING A N D H Y D R O T R E A T I N G

l i q u i d p r o d u c t s , was obtained a t pressures above 2000 p s i , and temperatures i n the order o f 5 0 0 ° C , and when a molybdenum c a t a l y s t was used. Outside t h i s patent l i t e r a t u r e there appears to be no f u r t h e r information on t h i s system. Moseley and Paterson (3) p e r formed experiments s i m i l a r to Schroeder but were p r i m a r i l y i n t e r ested i n the production of methane. The fact that they d i d not r e p o r t the p r o d u c t i o n o f any l i q u i d product i s probably due p r i m a r i l y to the higher temperatures u t i l i z e d , 9 0 0 - 9 2 5 ° C Qader, et a l . , (4) reported work on h y d r o g é n a t i o n i n a d i l u t e phase free f a l l r e a c t o r at temperatures i n the order o f 5 1 5 ° C , pressures of 2000 p s i and w i t h a heavy dose of c a t a l y s t , 15% stannous c h l o r i d e by weight o f c o a l . Up to 75% conversion was r e ported with a product d i s t r i b u t i o n of 43% o i l , 32% gas and 25% char. The residence time o f the c o a l feed p a r t i c l e s was estimated to be i n the order of seconds, however, no measurement was made and aromatics were reported a f t e r f u r t h e r h y d r o r e f i n i n g i n a second stage h y d r o g é n a t i o n . More r e c e n t l y , Yavorsky, et a l . , (5) have r e p o r t e d on the development o f a l i q u i d phase ( s o l v e n t ) h y d r o g é n a t i o n of c o a l i n a h i g h l y t u r b u l e n t tubular r e a c t o r i n the presence of a packed s o l i d catalyst. Residence times i n the order o f s e v e r a l minutes were r e p o r t e d and an o i l product i s formed. This system i s a c o n s i d e r a b l e improvement over the Bergius Process since reduced p r e s s u r e , i n the order o f 4000 p s i and lower temperature o f 4 5 0 ° C are employed. From t h i s l i t e r a t u r e i t thus appears that coal can be converted to l i q u i d and gaseous hydrocarbon products i n substant i a l y i e l d s by means of an elevated temperature ( 4 5 0 - 9 5 0 ° C ) high pressure (2000-6000 p s i ) contact of coal with hydrogen gas. L i quid hydrocarbons are favored i n the lower temperature range and shorter contact time ( i n the order o f seconds), while gaseous hydrocarbons increase i n y i e l d a t the higher temperatures and longer residence times. The b a s i c mechanism appears to i n v o l v e a thermally induced opening o f the polymeric aromatic hydrocarbon s t r u c t u r e i n c o a l , a l l o w i n g h y d r o g é n a t i o n to occur i n a hydrogen atmosphere, followed by a r a p i d removal o f the l i q u i d and gaseous hydrocarbon products formed so that e i t h e r f u r t h e r dehydrogenation, r e p o l y m e r i z a t i o n and c a r b o n i z a t i o n i s prevented or further excess i v e h y d r o g é n a t i o n i s minimized. The purpose o f the present experiments i s to s u b s t a n t i a t e previous work and to gather a d d i t i o n a l process and k i n e t i c i n f o r mation e s p e c i a l l y i n a n o n - l i q u i d non-catalyzed gas phase hydrog é n a t i o n system. S p e c i a l e f f o r t was made to o b t a i n o v e r a l l mat e r i a l balances i n determining y i e l d s . A f t e r a few p r e l i m i n a r y runs using a caking eastern bituminous coal and s e v e r a l r e a c t o r c o n f i g u r a t i o n s , i t was found that b e t t e r r e s u l t s could be obtained w i t h a non-caking coal i n an e n t r a i n e d down-flow tubular r e a c t o r . The coal was dropped down i n t o a tubul a r r e a c t o r through which hydrogen was passed down-flow and ent r a i n e d and c a r r i e d the coal down through the heated tube.

Ward and Qader; Hydrocracking and Hydrotreating ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8.

STEINBERG

A N D

F A L L O N

Coal Liquifaction

D e s c r i p t i o n of Equipment, and Experimental

125 and A n a l y t i c Procedures

The experimental equipment (Figure 1) was designed to r a p i d l y heat c o a l i n the presence o f hydrogen a t elevated temperature and pressure, to maintain these conditions long enough f o r hydrogénat i o n to take p l a c e , to cool the products preventing f u r t h e r react i o n and f i n a l l y to separate and c o l l e c t the products. Hydrogen i s s u p p l i e d from standard 200 cubic f o o t c y l i n d e r s and the operat i n g pressure i s c o n t r o l l e d by a 4000 p s i pressure r e g u l a t o r . Argon i s s u p p l i e d a t low pressure to f l u s h the system before and a f t e r each run and to a c t i v a t e the normally c l o s e d pneumatic v a l v e which shuts the hydrogen supply o f f i n case o f sudden l o s s o f pressure. The hydrogen can then pass through e i t h e r or both o f two 5000 p s i s e r v i c e rotameters. The gas from one meter goes to the top o f the r e a c t o r through an e l e c t r i c a l l y heated preheater cons t r u c t e d from a ten foot length of 1/4 inch tubing formed i n t o a 6 i n c h diameter c o i l . Since heating i s by d i r e c t connection to a v a r i a b l e v o l t a g e transformer, one s i d e o f the system i s e l e c t r i c a l l y i s o l a t e d from the other. Gas from the other meter i s used to c o o l the l i n e between the r e a c t o r and the c o a l feeder and enters j u s t below the feeder. Experience has shown that even w i t h a non-caking c o a l , t h i s s e c t i o n w i l l become plugged i f the temperature i s not c o n t r o l l e d . This i s probably caused by a combinat i o n o f reduced cross s e c t i o n , small p a r t i c l e s i z e and d e v o l a t i l i z a t i o n o f the c o a l . The r e a c t o r column i s f a b r i c a t e d from an e i g h t foot length of 1 i n . O.D. χ 0.120 i n . w a l l thickness type 304 s t a i n l e s s s t e e l tubing. The c o a l feeder i s a p r e s s u r i z e d 40 gram c a p a c i t y hopper having an o r i f i c e i n the base and a tapered plug operated by an electromagnet a c t i n g through a non-magnetic s t a i n l e s s s t e e l w a l l of the hopper. The feed r a t e i s c o n t r o l l e d by both a p u l s e gener a t o r operating an e l e c t r o n i c r e l a y which actuates the s o l e n o i d and by c o n t r o l l i n g the distance the tapered plug i s r a i s e d o f f the seat during i t s v e r t i c a l c y c l i n g . A North Dakota l i g n i t e and a New Mexico sub-bituminous c o a l were mainly used i n these experiments. Coal p r e p a r a t i o n c o n s i s t e d o f s i z e reduction and d r y i n g . I d e a l l y , the p a r t i c l e s i z e should f a l l somewhere between the l a r g e s t s i z e with which the maximum r a t e o f hydrogénation takes place and a s i z e l a r g e enough so that i n t e r - p a r t i c l e a t t r a c t i o n and agglomeration does not occur. With the b a l l m i l l used i n these experiments f o r s i z e r e d u c t i o n , a l a r g e q u a n t i t y of f i n e s were produced when g r i n d i n g to a maximum p a r t i c l e s i z e o f £ 50 or ^ 150 μ . The i n s i d e of the feeder had to be coated with a fluorοcarbon r e l e a s e agent which was baked i n p l a c e . The coal was magnetically a g i t a t e d by pieces of a finned rod attached to the tapered plug to prevent aggregation and c l o g ging o f the feeder. To prevent o x i d a t i o n a l l g r i n d i n g was conducted i n an i n e r t atmosphere. Drying i n a i r a t temperatures s l i g h t l y above 100°C was a l s o found to cause o x i d a t i o n and f o r t h i s reason drying and storage was performed i n a vacuum oven.


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A n a l y s i s o f the c o a l used i n these experiments and of a t y p i c a l char from the l i g n i t e c o a l produced during a s e r i e s of runs through the hydrogenator are shown i n Table 1. The analyses are on a m o i s t u r e - f r e e b a s i s and were obtained by the U . S . Bureau of Mines u s i n g standard ASTM a n a l y t i c a l procedures. A f t e r the c o a l passed through the r e a c t o r , the char and any unreacted c o a l were c o l l e c t e d i n an a i r cooled tubular trap cont a i n i n g a s t a i n l e s s s t e e l wool f i l t e r attached to the base o f the column. The q u a n t i t y c o l l e c t e d i s determined by weighing the trap before and a f t e r each r u n . The products pass through the trap along with the excess hydrogen which i s s t i l l a t elevated temperature and r e a c t o r p r e s sure. These gases then pass through a double U-tube water cooled trap ( 2 5 - 3 0 ° C ) , which removes any o i l s produced and most o f the moisture. The o i l deposits on the w a l l s o f the trap and due to the r e l a t i v e l y small q u a n t i t y produced and the l a r g e trap surface a r e a , c o l l e c t i o n becomes d i f f i c u l t . By c l e a n i n g w i t h a cotton swab, the o i l c o l l e c t e d appears to be l i g h t i n c o l o r and r e l a t i v e l y low i n v i s c o s i t y , s i m i l a r to a motor o i l . The q u a n t i t y p r o duced i s determined by weighing the trap before and a f t e r the r u n . The gases are then passed through a metering valve reducing the pressure to atmospheric. The remaining l i q u i d s contained i n the gases are c o l l e c t e d i n a l c o h o l - d r y i c e cooled glass trap ( - 7 2 ° C ) . Samples o f the hydrocarbons c o l l e c t e d i n t h i s trap have c o n s i s t e n t l y been analyzed to contain over 98% benzene, the remainder being toluene w i t h a t r a c e o f xylene. The q u a n t i t y produced i s determined by weighing the trap before and a f t e r the run and by volume i n a small graduate attached to the glass t r a p . Water from the r e a c t o r u s u a l l y c o l l e c t i n t h i s trap and appears as a separate l a y e r i n the graduate. This water and that c o l l e c t e d i n the U-tube trap have been analyzed on a Beckman Carbon A n a l y z e r and shows some i n s i g n i f i c a n t amount o f d i s s o l v e d hydrocarbons. The l i q u i d hydrocarbons c o l l e c t e d i n t h i s trap are analyzed by gas chromatography u s i n g a column o f 50-80 mesh Porapak Q and a flame i r o n i z a t i o n detector. The chromatograph i s c a l i b r a t e d to detect benzene, toluene, xylene C6 l i q u i d s ) and a l l the l i g h t hydrocarbon gases, methane, ethane, propane, hexane and pentane C5 gases). The remaining gas i s then vented to the atmosphere. At about two-thirds o f the time p e r i o d through the r u n , a 300 cc gas sample i s taken i n a g l a s s sampling v e s s e l i n s e r t e d i n the vent l i n e . Due to the l i m i t e d capacity of the c o a l feed hopper, the duration o f most runs was i n the order of 10 to 15 minutes. Steady s t a t e operation i s u s u a l l y obtained q u i c k l y s i n c e i n about one or two minutes a t the flow r a t e s used s e v e r a l r e a c t o r volume changes have o c c u r r e d . The gas sample i s analyzed i n the same manner as the l i q u i d sample mentioned above. A s i d e from excess hydrogen, the gas sample u s u a l l y contains mostly methane and ethane, the methane being present i n approximately twice the abundance as the ethane. The next g r e a t e s t c o n s t i t u e n t i s u s u a l l y benzene i n the vapor phase. There appears to be very l i t t l e C3, C4, and C5 hydrocarbons p r e s e n t . P e r i o d i c a l l y these same samples


8.

STEINBERG

A N D F A L L O N

127

Coal Liquifaction VENT

VENT

REACTOR

TUBE

H E A T E D IN F O U R TWO FOOT SECTIONS

Figure 1.

Coal hydrogénation tubular reaction experiment

TABLE 1 COAL AND CHAR FEED ANALYSIS % by wt - Moisture Free Basis North Dakota Lignite Hydrogen Carbon

4.29%

New Mexico Sub Bituminous 4.86%

Char from Lignite 2.74%

62.39

64.21

73.63

0.97

1.41

0.85

21.75

12.26

0.37

Sulphur

1.17

0.61

1.42

Ash

9.43

16.65

20.99

100.00

100.00

100.00

Nitrogen Oxygen


HYDROCRACKING AND HYDROTREATTNG

128

were a l s o analyzed by mass spectrometry as a check on the gas chromatography and a l s o to determine the concentration o f cons t i t u e n t s not detected by the gas chromatograph, such as oxides of carbon. Experimental and C a l c u l a t e d Results Table 2 contains a summary of some of the experimental r e sults. Except as otherwise noted, most o f the runs were made u s i n g ground North Dakota l i g n i t e with no a d d i t i o n of c a t a l y s t . The r e a c t o r operating temperature c o n d i t i o n o f 700°C was chosen on the basis that t h i s was s l i g h t l y above 650°C found i n previous experiments to be the temperature below which l i t t l e r e a c t i o n w i t h the l i g n i t e was noted. The 1500 p s i operating pressure i s the upper s a f e t y l i m i t o f the r e a c t o r at the 700°C temperature. Lower pressures tended to r e s u l t i n lower l i q u i d y i e l d s . A thorough i n v e s t i g a t i o n o f the e f f e c t of pressure i s yet to be made. The residence time o f the c o a l p a r t i c l e s i n the r e a c t i o n zone was estimated by combining the l i n e a r gas v e l o c i t y through the tube with the free f a l l v e l o c i t y of the p a r t i c l e s as c a l c u l a t e d from data c o r r e l a t e d by Zenz and Othmer (6). The p a r t i c l e s were assumed to be uniform spheres d i s t r i b u t e d i n a h i g h l y d i l u t e gas phase. A t a f i x e d r e a c t o r operating temperature and p r e s s u r e , the residence time becomes dependent upon the p a r t i c l e s i z e and the gas flow r a t e . Most o f the o r i g i n a l experiments were conducted u s i n g £ 50 micron coal p a r t i c l e s . In an attempt to improve the u n i f o r m i t y of the coal feed flow and decrease the residence time, t h i s was increased to £ 150 microns. As shown i n Table 3, i t was not p o s s i b l e with the frequent s i e v i n g and g r i n d i n g method used to reduce the presence o f s i g n i f i c a n t amounts o f ^ 50 micron p a r ticles. Using Figure 2, which i s a p l o t o f residence time as a funct i o n of coal p a r t i c l e s i z e based on the Zenz and Othmer c o r r e l a t i o n , i t can be shown that due to the r e l a t i v e l y l a r g e q u a n t i t y of fines p r e s e n t , i n c r e a s i n g the maximum s i z e by a f a c t o r of three only reduces the average residence time by about 30%. F i g u r e 2 a l s o shows the e f f e c t o f reducing residence time by i n c r e a s i n g the hydrogen flow r a t e from the minimum used i n these experiments (0.7 gm/min) to the maximum (4.5 gm/min). Brown and Essenhigh (7) have suggested that the free f a l l v e l o c i t y may be much greater than that c a l c u l a t e d here. In t h e i r experiments u s i n g cork p a r t i c l e s f a l l i n g through an a i r f i l l e d tube, the p a r t i c l e cloud acted as a porous p l u g , f a l l i n g a t a r a t e o f approximately 57 f t / m i n . Using the Zenz and Othmer c o r r e l a t i o n , t h i s v e l o c i t y i s c a l c u l a t e d to be only 17 f t / m i n . Thus, the coal residence time may be much shorter than determined from Figure 2. A d d i t i o n a l experimental e f f o r t i s r e q u i r e d to measure the a c t u a l residence i n the system. The hydrogen fed to the system i s much i n excess o f that u t i l i z e d i n the h y d r o g é n a t i o n r e a c t i o n . The net use o f hydrogen



(1) (2) (3) (4) (5)

44.5

45.7 59.9

35 .8 24.1 6.0 34.1

81.5 121.6

35.8 45.7 6.0 34.1

16 20

61.1

21.2 39.9 6.0 32.9

31.8 70.7

21.2 10.6 6.0 32.9

16 17.5

6/7 1500 700 350 1.66 4.14 0.33

Hydrocracking and Hydrotreating

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