Chemical Characterization of Wood Pyrolysis Oils Obtained in a

Chapter 19

Downloaded by UNIV OF TEXAS AT AUSTIN on August 26, 2017 | http://pubs.acs.org Publication Date: September 30, 1988 | doi: 10.1021/bk-1988-0376.ch019

Chemical Characterization of Wood Pyrolysis Oils Obtained in a Vacuum-Pyrolysis Multiple-Hearth Reactor H. Pakdel and Christian Roy Chemical Engineering Department, Université Laval, Pavillion Adrien-Pouliot, Sainte-Foy, Quebec F1K 7P4, Canada A multiple hearth reactor has been used to produce high yield of pyrolysis o i l from aspen poplar. The Process Development Unit (P.D.U.) has the capability of achieving a fair fractionation of wood oils by using six heat exchangers (Primary Condensing Unit, P.C.U.) and a series of cooling trap receivers (Secondary Condensing Unit, S.C.U.) at the outlets of the reactor. While the weight average molecular weights (Mw) of the recovered compounds in the P.C.U. were 342, 528, 572, 393, 233 and 123 from top to bottom of the reactor, the low molecular weight compounds with Mw = 100 or below were recovered in the S.C.U., which contained at least 90% of the total water. Silica-gel column chromatography enabled us to fractionate the oil from P.D.U. into fourteen fractions. Aromatic hydrocarbons were collected in Fraction 1 (F1) and F2 followed by elution of moderately polar compounds in F3 to F11 with about 2335% of P.C.U. o i l which can be fully characterized. 8.1% sugar in P.C.U., mainly glucose, was found in F13. Usefulness of H-FTNMR and infrared spectroscopy was shown for preliminary characterization of the F12, F13 and F14. Overall 27.89% of the P.C.U. including water and low molecular weight carboxylic acids have been measured and identified so far. 1

The i d e n t i f i c a t i o n and extraction of valuable chemicals from woodderived o i l s i s a very important goal for the biomass thermochemical conversion industry (IzZ)· Pyrolysis o i l s have been extensively studied and extensive number of compounds have been i d e n t i f i e d (34). However to our knowledge there are only two general methods which have been reported for the fractionation of pyrolysis o i l s

0097-6156/88A)376-0203$06.00A) « 1988 American Chemical Society

Soltes and Milne; Pyrolysis Oils from Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


204

PYROLYSIS OILS FROM BIOMASS

into chemical groups: the* solvent extraction method (5) and the adsorption-ehromatography method (6). The former technique i s rather tedious and quite often the phase separation i s d i f f i c u l t due to the emulsion formation. The y i e l d of extraction strongly depends on the solvent volume and extraction r e p e t i t i o n number. The adsorption-ehromatography method was used f o r t h i s investigation with further modifications which w i l l be discussed i n this paper. Extensive works conducted by d i f f e r e n t authors u t i l i z i n g GC and GC/MS sometimes lead to d i f f e r e n t results which indicate the difficulties of carrying out accurate detailed analysis of the chemical constituents of pyrolysis o i l s . Examples of incomplete or even contradictory results can be found i n the l i t e r a t u r e (3^4) and t h i s paper i n the analysis of vacuum p y r o l y s i s o i l s . Other researchers have studied the functional group d i s t r i b u t i o n in pyrolysis o i l (8). Although those techniques are long and tedious, they w i l l lead to useful information about wood o i l chemistry. The majority of compounds found in pyrolysis o i l s are oxygenated with rather s i m i l a r p o l a r i t y . Therefore t h e i r gas chromatograms, i n general, suffer from low resolution and consequently the quantitative analysis w i l l be less accurate. Although t h i s problem may be p a r t i a l l y obviated by choosing narrow bore and Long c a p i l l a r y columns, but they are very expensive and not very practical. Direct injection of a complex mixture into the gas chromatograph on the other hand tend to deteriorate the column by b u i l d i n g up of non v o l a t i l e matter i n the column i n l e t leading to gradual decomposition of the column stationary phase. Generally gas chromatography has a limited application and i s not meant to be used for very complex and less v o l a t i l e mixtures. GC/MS i s a much more powerful a n a l y t i c a l tool but would not be available in a great majority of cases. Besides i t i s quite c o s t l y and requires s k i l l e d operators for the interpretation of the r e s u l t s . Therefore development of new methods of separation and fractionation i n p a r t i c u l a r are needed. The primary objective of t h i s work i s to develop a separation and fractionation method f o r better and detailed analysis of pyrolysis o i l s and to demonstrate f a i r fractionating c a p a b i l i t y of the multiple hearth vacuum pyrolysis system. This w i l l eventually enable us to make correlations between the o i l properties and pyrolysis operation conditions. Full, characterization of the o i l s w i l l also shed some l i g h t on the possible pyrolysis reaction mechanism and upgrading of o i l s . The secondary objective i s to develop methods for the extraction of valuable chemicals such as s p e c i a l t y and rare chemicals which are i n increasing demand (9). Experimental The wood o i l samples which have been character!zed in t h i s work have been obtained from pyrolysis of Populus deltoïdes (clone D-38) with no bark i n a multiple-hearth vacuum pyrolysis reactor. The Process Development Unit, Fig. 1, (P.D.U.) has been described in d e t a i l by one of the co-authors i n another paper (1). The P.D.U. was tested for the production of high yields of o i l s from wood chips. One objective was to separate the bulk of the aqueous phase from the organic l i q u i d phase by means of


19.

PAKDEL & ROY

Vacuum-Pyroly sisMultiple-Hearth Reactor


FEEDER

Figure 1. Schematic view of vacuum pyrolysis Process Development Unit (P.D.U.).


205

PYROLYSIS OILS FROM BIOMASS


206

f r a c t i o n a t i o n of the o i l d i r e c t l y at the outlet of the reactor. This was achieved in the following way. The organic vapor product was removed from the reactor through s i x outlet manifolds which corresponded to the s i x heating plates of the reactor, the hearths. The vapors were condensed i n two condensing units named Primary and Secondary Condensing Units (P.C.U., S.C.U.). P.C.U. consisted of s i x heat exchangers i n p a r a l l e l (H-I to H-VI) and S.C.U. consisted of four receivers placed in series (CI to C4). The pyrolysis o i l s which were obtained i n both condensing units were subjected to sequential elution solvent chromatography. The organic fractions were thon analysed as described below. Relationship of a l l samples and designations are described in Table I. One gram of the o i l sample immediate after pyrolysis was transferred into a glass column with 16 mm i . d . packed with 12.5 g of 60 120 mesh s i l i c a - g e l i n petroleum ether (30-60°C b.p.). Fourteen fractions were c o l l e c t e d using d i f f e r e n t solvents as depicted in Table I I . A l l the solvents were d i s t i l l e d before use and the s i l i ca-gel was washed with dichloromethane and dried in a i r . The o i l fractions were dried by rotary evaporator without heat. A l l the y i e l d s are shown in Table II. H-FTNMR spectra of 5% solution in DMSO were recorded on XI, 200 Varian instrument. Gas chromatographic analyses were performed on a 6000 Varian gas chromâtograph with flame ionization detector with two injectors (on column and s p l i t ) . The c a p i l l a r y columns were J & W fused s i l i c a : DB5, 30 m X 0.25 mm i.d. and UB1, 30 m X 0.32 mm i.d. The c a r r i e r gas was He and N 2 as make up gas. The oven temperature was maintained at 50°C for 2 min then programmed to 150°C and 290°C at rates of 4 and 10°C min' respectively. Water's 840 data and chromatography control station with d i g i t a l professional 350 computer and Ι.Λ50 recorder were used as data processor. Various standard mixtures were prepared with the available compounds. Their r e l a t i v e response factors to benzophenone were measured. S i l i c a -gel eluates were added an accurate quantity of benzophenone as internal standard. Their gas chromât o-grams were compared with the standard mixtures for peaks identificat i o n and followed by integrations f o r t h e i r quantifications. Hydrolysis followed by sugar analysis was c a r r i e d out according to the s i i y l a t i o n technique. The procedure can be found elsewhere (10). Gel permeation chromatographic (G.P.C.) analysis of the s i x o i l s from P.C.U. and one o i l from S.C.U. (CI) were performed on ALC/GPC-20I Water's Associate l i q u i d chromâtograph equipped with a model R-401 refractometer. Four 30 cm X 7.8 mm i . d . columns packed with 100, 500, 10 and 10 μ styragel were used i n series. The sam ples were prepared in THF (5*) and 15 μΐ was injected and eluted with THF. The following standards were used to c a l i b r a t e the system: polystyrene (Mw = 4000; 2000; 800 and 600), polyethylene g l y c o l (Mw = 450, 300, 200), guaiacol, syringaldehyde and v a n i l l i n e . l

1

3

4

Results and Discussions Pyrolysis o i l , water, char and gas are the wood pyrolysis products. Depending on the liquefaction process, pyrolysis o i l s composition change s i g n i f i c a n t l y . Lack of a standard pyrolysis o i l characteriz ation technique has i n i t i a t e d us at the f i r s t stage to develop a


Soltes and Milne; Pyrolysis Oils from Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1988. Primary Condensing Unit Secondary Condensing Unit Obtained from the s i x hearths Fourteen fractions of H-I to H o i l s by SESC Receiver 1 See Fig. 2

P.C.II. S.C.U HI F l to F14

CI hearth I to hearth-VI

phase

Oil

Aqueous phase

Pyrolysis o i l

PCU o i l

SCU o i l

S o l i d residue

Vacuum p y r o l y s i s

Vacuum pyrolysis

Vacuum p y r o l y s i s

SESC, GPC, GC, NMR

GPC

1R

SESC = Sequential elution s o l i d chromatography; GPC = Gel permeation chromatography; GC - Gas chromatography; NMR ~ Nuclear magnetic resonance; 1R - Infrared

to H- VI

Remarks

Designation

Nature of Sample

Relationship of the samples studied i n the text (See réf. 1 for more d e t a i l s )

Experimental Technique

Table I.



95.37 3.28 28.26

29.09

33.18

0.12

1.44

H-VI

Fraction 12: Fraction 13: Fraction 14:

Fraction 1: Fraction 2-11:

128 ml with petroleum ether. 128 ml each with C H 2 C I 2 / Petroleum ether mixture, from 10 to 100% F2 to F l l respectively. 128 ml with ether. 128 ml with water. 60 ml with 10% formic acid in methanol.

(a) See the text and table I for description of the classes. A l l figures are expressed in weight percentage (as-received o i l basis).

(10* increments) for

92.41 2.79 29.48

35.35

24.24

0.10

0.45

H-V

CH2CI2

91.35 2.24 30.70

35.60

22.32

0.16

0.33

H-IV

90.12 2.34

26.93

36.80

22.99

0.21

0.85

H-III

87.69 1.65

26.28

32.52

26.37

0.23

0.64

H-II

86.87 1.67

27.16

31.06

26.83

0.04

0.11

H-I

TOTAL

FRACTION 12

FRACTION 14

FRACTION 3-11

FRACTION 2

FRACTION 1

FRACTION 13

PYROLYSIS OIL SAMPLES FROM

Table II. Primary Condensing Unit Pyrolytic Oils and the Yield of Various Classes of Compounds Obtained by Silica-Gel Column Chromatographic Analysis (a)


ce

S δ

§

g

Ο

I

Ο

1

00

ο


19.

PAKDEL & ROY

Vacuum-Pyrolysis Multiple-Hearth Reactor

209

sub-fractionation technique followed by quantitative gas chromato graphic analysis. Interestingly, vacuum p y r o l y s i s y i e l d s r e l a t i v e l y low percentage of high molecular weight compounds. The high molecu l a r weight compounds with Mw > 300 represent approximately 25* of the t o t a l vacuum pyrolysis o i l s . It has been suggested that the high molecular weight less v o l a t i l e and presumably more polar com pounds are produced by incomplete thermal degradation of l i g n o c e l l u l o s i c materials (8). The following characterization technique enabled us to f i n d app. 5% oligosaccharides with Mw > 300. Due to GC l i m i t a t i o n , the rest of the high molecular weight compounds can be characterized u t i l i z i n g GPC, M S - M S , HPLC or combination of those. The work i n t h i s l i n e i s presently under investigation i n our laboratory. The s i x o i l s from the P.C.U. (H-I to H-VI) and an o i l sample from the S.C.U. (CI) were analysed by GPC f o r t h e i r molecular weight range d i s t r i b u t i o n . The weight average molecular weights f o r the H~ I to VI were: 342, 528, 572, 393, 233 and 123 respectively. The test f o r CI showed weight average molecular weight of about .100. The molecular weight d i s t r i b u t i o n of CI was as below: Mw^lOO (40*); 100

Chemical Characterization of Wood Pyrolysis Oils Obtained in a

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