The Effect of Catalyst Concentration, Temperature, and Residence

15 The Effect of Catalyst Concentration, Temperature, and Residence Time on the Chemical Structure of Coal Hydropyrolysis Oils Downloaded by UNIV OF LIVERPOOL on July 18, 2016 | http://pubs.acs.org Publication Date: October 14, 1980 | doi: 10.1021/bk-1980-0139.ch015

JOHN R. KERSHAW, GORDON BARRASS, and DAVID G R A Y

Fuel Research Institute of South Africa, P.O. Box 217, Pretoria, South Africa 0001 Three of the most important parameters that affect the hydrogenation of coal and the products obtained are the catalyst, the temperature and the residence time. The effects of these parameters on the chemical nature of the products are still not fully understood. In this paper we have looked firstly at the effect that the catalyst concentration, secondly at the effect that the reactor temperature and finally at the effect that the residence time at temperature have on the chemical structure of the oils (hexane soluble product) produced on hydropyrolysis (dry hydrogenation) of a high volatile bituminous coal. Generally, the hydropyrolysis conditions used in this study resulted in o i l yields that were considerably higher than the asphaltene yields and this study has been limited to the effects that the three reaction conditions have on the chemical nature of the oils produced. For the study of catalyst concentration tin as stannous chloride was used as the catalyst and the concentration range studied was 0 - 15% by weight of the coal. Stannous chloride is one of the best, i f not the best, catalyst for conversion of coal to liquid products. For the study of the effect of the reactor temperature, the temperature range studied was from 400 - 700°C. In the work reported here hydrogenation of coal was carried out in the absence of any vehicle o i l in a semi-continuous reactor which allowed the volatile product to be swept from the reactor by a continuous stream of hydrogen. The great majority of coal hydrogenation oils studied by other workers were produced using a liquid vehicle. The vehicle o i l may have an effect on the chemical reactions taking place and, therefore, on the chemical composition of the product. For example, it has been suggested (1) that the main role of the catalyst in coal hydrogenation where a solvent is used is to hydrogenate the solvent.

0-8412-0587-6/80/47-139-273$05.00/0 © 1980 American Chemical Society Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

274

COAL LIQUEFACTION

FUNDAMENTALS

EXPERIMENTAL

Downloaded by UNIV OF LIVERPOOL on July 18, 2016 | http://pubs.acs.org Publication Date: October 14, 1980 | doi: 10.1021/bk-1980-0139.ch015

Materials The c o a l (0.50 to 0.25 mm f r a c t i o n ) used was from the New Wakefield C o l l i e r y , T r a n s v a a l . A n a l y s i s , a i r d r i e d b a s i s : Moisture 4.9; Ash 14.9; V o l a t i l e Matter 32.8%; dry ash-free b a s i s : C, 79.2; H, 5.4; N, 2.1; S, 2.3%. The c a t a l y s t was a n a l y t i c a l grade stannous c h l o r i d e . Stannous c h l o r i d e was d i s s o l v e d i n water and added to the c o a l as an aequous s o l u t i o n . The r e s u l t a n t s l u r r y was mixed by s t i r r i n g and then d r i e d under n i t r o g e n . Hydrogénation Hydrogénation was c a r r i e d out i n a r e a c t o r s i m i l a r to the "hot-rod" r e a c t o r (2, 3) designed by Hiteshue et a l . The c o a l (25 g) impregnated wTth c a t a l y s t was mixed w i t h sand to l i m i t agglomeration (40 and i s h e l d w i t h i n the r e a c t o r w i t h s t e e l wool p l u g s . The r e a c t o r was heated by d i r e c t r e s i s tance h e a t i n g at a r a t e of 200°C/minute. Hydrogen, at a flow r a t e of 22 £/minute and a pressure of 25 MPa was c o n t i n u o u s l y passed through the f i x e d bed of c o a l / c a t a l y s t / s a n d . The v o l a t i l e products were condensed i n a high-pressure c o l d t r a p . For the study of the e f f e c t of the c a t a l y s t c o n c e n t r a t i o n and the temperat u r e , the time at temperature was 15 minutes. The other c o n d i t i o n s are given i n the R e s u l t s and D i s c u s s i o n s e c t i o n . The product was removed from the cooled r e a c t o r and from the condenser w i t h the a i d of toluene. The s o l i d r e s i d u e was e x t r a c ted w i t h b o i l i n g toluene (250 ml) i n a soxhlet e x t r a c t o r f o r 12 hours. The toluene s o l u t i o n s were combined and the toluene removed under reduced p r e s s u r e . Hexane (250 ml) was added to the e x t r a c t and i t was allowed to stand f o r 24 hours w i t h o c c a s i o n a l shaking. The s o l u t i o n was f i l t e r e d to leave a r e s i d u e ( a s p h a l tene) and the hexane was removed from the f i l t r a t e under reduced pressure to give the o i l . F r a c t i o n a t i o n of the o i l s The o i l s were f r a c t i o n a t e d by a d s o r p t i o n chromatography on s i l i c a g e l . The column was e l u t e d s u c c e s s i v e l y w i t h 40 - 60°C petroleum ether (12 f r a c t i o n s ) , 40 - 60 petroleum ether/toluene ( i n c r e a s i n g p r o p o r t i o n s of t o l u e n e , 5 f r a c t i o n s ) , t o l u e n e , chloroform and methanol. The o i l s were a l s o e x t r a c t e d w i t h 2 M NaOH and 2 M HC1 to give the a c i d s and bases, r e s p e c t i v e l y . Analyses I . r . s p e c t r a were measured as smears on sodium c h l o r i d e p l a t e s or as a s o l u t i o n i n carbon t e t r a c h l o r i d e u s i n g a Perkin-Elmer 567 g r a t i n g spectrophotometer, w h i l e u.v. s p e c t r a were measured as a s o l u t i o n i n hexane ( s p e c t r o s c o p i c grade) u s i n g a Unicam SP 1700 instrument. Fluorescence and phosphorescence s p e c t r a were recorded as d e s c r i b e d elsewhere 05, 6 ) . 'H n.m.r. s p e c t r a were recorded f o r the o i l s i n deuterochloroform or carbon t e t r a c h l o r i d e at 90 MHz w i t h tetramethys i l a n e as an i n t e r n a l standard u s i n g a V a r i a n EM 390 instrument.

Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

15.

KERSHAW ET A L .

Coal Hydropyrolysis

275

Oils


, J

Broadband proton-decoupled pulse F o u r i e r transform C n.m.r. were recorded i n deuterochloroform at 20 MHz u s i n g a V a r i a n CFT-20 spectrometer. Molecular weights were determined by vapour pressure osmometry i n benzene s o l u t i o n u s i n g a Knauer apparatus. 5 concentrat i o n s over the range 1 - 5 g/£ were employed and the molecular weight was obtained by e x t r a p o l a t i o n t o i n f i n i t e d i l u t i o n . The v i s c o s i t i e s o f the o i l s were measured u s i n g a Haake Rotovisco RV3 viscometer w i t h a cone and p l a t e sensor at 20 C. Elemental a n a l y s i s o f the o i l s was c a r r i e d out as f o l l o w s : carbon and hydrogen by micro combustion u s i n g a Perkin-Elmer 240 Elemental Analyzer; sulphur by X-ray f l u o r e s c e n c e u s i n g a T e l s e c Lab X-100 apparatus; n i t r o g e n by chemiluminescence u s i n g a Dohrmann DN-10 apparatus. RESULTS AND

DISCUSSION

The E f f e c t of C a t a l y s t Concentration The f i r s t parameter that was s t u d i e d was the e f f e c t of the c a t a l y s t c o n c e n t r a t i o n . Samples impregnated with 1 , 5 , 10 and 15% t i n as stannous c h l o r i d e and a sample w i t h no c a t a l y s t were hydrogenated at 450°C to i n v e s t i g a t e the e f f e c t that i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n has on the composition o f the o i l (hexane s o l u b l e p o r t i o n ) formed. From e l u t i o n chromatography, the percentage o f a l i p h a t i c hydrocarbons, aromatic hydrocarbons and p o l a r compounds were obtained. The percentage of p o l a r compounds i n the o i l decreased as the c a t a l y s t c o n c e n t r a t i o n increased (see F i g u r e 1) with mainly an increase i n the percentage of aromatic hydrocarbons. There was a l s o a decrease i n the percentage of both acids and bases i n the o i l as the c a t a l y s t c o n c e n t r a t i o n increases as shown i n Figure 1. The amount of sulphur and n i t r o g e n i n the o i l s decreases w i t h c a t a l y s t c o n c e n t r a t i o n . The decrease i n sulphur content i s shown i n F i g u r e 2 while the n i t r o g e n content o f the o i l s decreased from 1.3% t o 0,9% when the c a t a l y s t c o n c e n t r a t i o n was i n creased from 1% to 10%. Thus the r e s u l t s from e l u t i o n chromatography, e x t r a c t i o n with a c i d and base and elemental a n a l y s i s show that as the c a t a l y s t c o n c e n t r a t i o n used i n c r e a s e s , the heteroatom content of the r e s u l t a n t o i l decreases. The f r a c t i o n s from e l u t i o n chromatography were studied by a number of s p e c t r o s c o p i c methods, n.m.r., i . r . , u.v., f l u o rescence and phosphorescence spectroscopy. Equivalent f r a c t i o n s from chromatographic s e p a r a t i o n o f the v a r i o u s o i l s showed no s i g n i f i c a n t d i f f e r e n c e s i n t h e i r s p e c t r a and i t appears that the composition o f the f r a c t i o n s was independent o f the c a t a l y s t concentration used to produce the o i l . Though, as p r e v i o u s l y mentioned the amounts of the v a r i o u s f r a c t i o n s e s p e c i a l l y the p o l a r f r a c t i o n s d i f f e r with the c a t a l y s t c o n c e n t r a t i o n . G.1.C. a n a l y s i s of the saturate f r a c t i o n s a l s o i n d i c a t e d no changes with d i f f e r e n t c a t a l y s t c o n c e n t r a t i o n s .



276

COAL LIQUEFACTION FUNDAMENTALS

Figure 1.

Variation of polar compounds, acids, and bases with catalyst concentration


15.

KERSHAW E T A L .

Coal Hydropyrolysis

277

Oils

Ί

Η n.m.r. s p e c t r a were recorded f o r the whole o i l s , the a c i d s , bases and n e u t r a l components. The percentage of hydrogens i n aromatic, b e n z y l i c and a l i p h a t i c environments showed no change with c a t a l y s t c o n c e n t r a t i o n s . n.m.r. s p e c t r a were a l s o recorded f o r the o i l s produced. No d i s c e r n i b l e d i f f e r e n c e s could be found between the s p e c t r a of the o i l s . A more d e t a i l e d de s c r i p t i o n of the s p e c t r a of these o i l s i s reported elsewhere (6). The s p e c t r o s c o p i c evidence i n d i c a t e s that the c a t a l y s t con c e n t r a t i o n had very l i t t l e e f f e c t on the "gross" hydrocarbon s t r u c t u r e present and t h i s i s s u b s t a n t i a t e d by the /C atomic r a t i o s of the o i l s which showed no s i g n i f i c a n t change with catalyst concentration. I t was obvious on v i s u a l examination of the o i l s that the greater the c a t a l y s t c o n c e n t r a t i o n used, the l e s s v i s c o u s was the o i l produced. The decrease i n v i s c o s i t y with c a t a l y s t concentra t i o n i s shown i n Figure 3. I t has been reported that the molecular weight of c o a l l i quids a f f e c t s the v i s c o s i t y (7, 8). However, the decrease i n molecular weight that occurred with i n c r e a s i n g c a t a l y s t concen t r a t i o n was r e l a t i v e l y s m a l l , as shown i n Figure 4. We f e e l that t h i s r e l a t i v e l y small change i n molecular weight would not cause such a n o t i c e a b l e change i n v i s c o s i t y unless changes i n the chemical nature of the o i l a l s o c o n t r i b u t e d to the v i s c o s i t y reduction. Sternberg et a l . (7) showed that the presence of asphaltenes i n c o a l - d e r i v e d o i l s caused a marked increase i n the v i s c o s i t y . This group also showed that these asphaltenes were acid-base complexes and that hydrogen bonding occurs between the a c i d i c and b a s i c components of asphaltenes (9, H))· Recent work (8, JM) on c o a l l i q u e f a c t i o n bottoms has shown the importance of hydrogen bonding on the v i s c o s i t y of c o a l l i q u i d s . The r e d u c t i o n of p o l a r compounds i n the o i l w i t h i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n could reduce hydrogen bonding and, t h e r e f o r e , the v i s c o s i t y of the o i l . To f u r t h e r look at t h i s p o s s i b i l i t y , i . r . s p e c t r a were recorded at the same c o n c e n t r a t i o n i n CCl^ f o r each of the o i l s (see Figure 5 ) . The i . r . s p e c t r a showed sharp peaks at 3610 cm" ( f r e e OH), 3550 cm" (2nd f r e e OH) and 3480 cm" (N-H) and a broad peak at ca 3380 cm" which i s assigned to hydrogen bonded OH. This band decreases with i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n (see Figure 5) i n d i c a t i n g that hydro gen bonding i n the o i l decreases with i n c r e a s i n g c a t a l y s t concen t r a t i o n used to produce the o i l . The band at ca 3380 cm" was shown to be due to i n t e r m o l e c u l a r hydrogen bonding (12, 13) by r e c o r d i n g the spectrum of a more d i l u t e s o l u t i o n (using a longer path length c e l l ) , the 3380 cm" band diminished w i t h an i n c r e a s e i n the 3610 cm" peak. Though, as p r e v i o u s l y s t a t e d , the H n.m.r. s p e c t r a of the o i l s were very s i m i l a r , there was one n o t i c e a b l e d i f f e r e n c e with c a t a l y s t concentration and that was i n the p o s i t i o n of the OH peak. As the c a t a l y s t c o n c e n t r a t i o n used to produce the o i l increases then the OH s i g n a l i s s h i f t e d to higher f i e l d (see


H

1

1

1

1

1

1

1


278


COAL LIQUEFACTION FUNDAMENTALS

Figure 3.

Variation in viscosity with catalyst concentration and reactor temperature


15.

KERSHAW E T AL.

279

Coal Hydropyrolysis Oils -τ

1

5

1

10 15 CATALYST C0NC.(WT%0F COAL )

240 H e> LU

280

H

CE

The Effect of Catalyst Concentration, Temperature, and Residence

Recommend Documents