Oil Production by High-Pressure Thermal Treatment of Black Liquors

Chapter 10

Oil Production by High-Pressure Thermal Treatment of Black Liquors Process Development Studies

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: September 30, 1988 | doi: 10.1021/bk-1988-0376.ch010

P. J. McKeough and A. A. Johansson Laboratory of Fuel Processing and Lubrication Technology, Technical Research Centre of Finland, 02150 Espoo, Finland Liquid-phase treatment of black liquors, from alkaline pulping, at 300 - 350 °C in a reducing atmosphere results in the formation of an o i l - l i k e product, which separates out from an aqueous phase containing the inorganic constituents. The process has several potential forms of application. This study was conducted in support of the development of one such application: a new recovery system for the kraft pulping process. Thermal treatment experiments were performed using different reactant gases with the main objective of elucidating the interactions between the gaseous-phase and aqueous-phase components. This information was used to advantage in the compilation of a process scheme for the recovery of the cooking chemicals from the aqueous phase. A process producing l i q u i d fuels from the organic matter of the black liquors from alkaline pulping is being developed at the Laboratory of Fuel Processing and Lubrication Technology, Technical Research Centre of Finland (VTT). The central operation in the process i s the l i q u i d phase thermal treatment of black liquor at 300 - 350 °C under a r e ducing atmosphere U ) . The treatment results in the formation of a hydrophobic o i l which separates out from an aqueous phase containing the inorganic constituents and some organic matter. The process can be applied in either of two basic ways: - as a method to produce o i l in conjunction with the kraft pulping process, - as an e n t i r e l y new system for recovering the cooking chemicals and energy from kraft spent l i q u o r s . The f i r s t type of application exploits the favourable properties of black liquor as a feedstock for high-pressure conversion. In comparison to s o l i d biomass, the advantages of black liquor as a feedstock include: - no pretreatment required. Black liquor can be d i r e c t l y pumped into the reactor at dry solids concentrations as high as 60 %. 0097-6156/88/0376-0104$06.00/0 Published 1988 American Chemical Society

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


10.

McKEOUGH& JOHANSSON

Oil Production from Black Liquors

105

- black liquor contains alkaline compounds known to catalyze con version reactions, - c e l l u l o s e , the most valuable component of wood, i s not subjected to the conversion process. In this type of a p p l i c a t i o n , the kraft cooking chemicals are re covered using the t r a d i t i o n a l method which i s centred around the Tomlinson recovery b o i l e r . Additional plant fuel (wood waste, peat, coal) is needed. Preliminary economic evaluations have indicated that t h i s type of o i l production process would be economic i f o i l prices were at their 1985 l e v e l . In the second type of a p p l i c a t i o n , the Tomlinson recovery b o i l e r is replaced by a safer and thermally more e f f i c i e n t recovery system, in which the o i l produced by thermal treatment i s used as plant f u e l . This type of process i s , in many respects, similar to the Hydropyrol y s i s Recovery Process developed by the St. Regis Paper Company, USA the essential difference being the use of a reducing atmosphere in the YTT process. The primary objectives of the present study were to elucidate the main interactions between gaseous-phase and aqueous-phase compo nents, and, by applying this knowledge, to establish process schemes for the recovery of the cooking chemicals from the aqueous phase. Because the composition of the aqueous phase i s dependent upon the composition of the reactant gas, several different gases were employed in these experiments. Experimental The kraft black l i q u o r , employed in the experiments, originated from a laboratory cook of Scots pine (Pinus s y l v e s t r i s ) . An analysis of the liquor i s given in Table I. Table I.

Analysis of kraft black % of dry

Organic matter Total Na NaOH Na S Na30. Na^CO 9

liquor . 1

solids

78.9 18.4 3.0 5.5 0.04 1.1

dry solids content of 30 % A l a t e r batch of l i q u o r , prepared in a similar way, was also analyzed for v o l a t i l e acids (formic and acetic acids) and l a c t i c a c i d , which are present as sodium salts in the l i q u o r . On the basis of the ana lyses i t can be concluded that the contents of these acids in the feed liquor used in the present experiments were approximately 6 % om, 4 % om, and 4 % om respectively, where % om denotes the per centage of black liquor organics. The experiments were conducted in a 1 - l i t r e autoclave. In a typical experiment 500 ml of black liquor was placed in the autoclave and reactant gas was charged at s u f f i c i e n t pressure ( 5 - 9 MPa) to result in a total pressure of about 20 MPa at the reaction tempera ture. The autoclave was then heated at a rate of about 5 ° C / m i n . A fixed time-at-temperature of 45 m i η was chosen for these experiments. 1


106

PYROLYSIS OILS FROM BIOMASS

At the end of the reaction period, the autoclave was rapidly cooled, after which gases were released, measured, and analyzed. The organic phase and the aqueous phase were separately recovered from the autoclave and weighed. The pH of the aqueous phase was measured. In many of the experiments the aqueous phase was also analyzed for formic, a c e t i c , and l a c t i c acids (present as sodium s a l t s ) and for C0 (chemically bound in sodium carbonate or bicarbonate). The former were analyzed by gas chromatography as their benzyl esters (3J, the l a t t e r by measuring the amount of C0 liberated upon a c i d i f i c a t i o n with excess mineral acid. The primary experiments were those employing either carbon monoxide or hydrogen as reducing gas. Two temperatures, 300 °C and 350 °C were investigated. Experiments employing non-reducing carbon dioxide were also conducted in order to assess whether a l k a l i neutral i z a t i o n by the reactant gas plays an important role in the thermal treatment process. One experiment was performed with a reactant gas of composition similar to that of a typical l o w - c a l o r i f i c fuel gas (producer gas) generated by g a s i f i c a t i o n of biomass with a i r . 2


2

Results and Discussion The main experimental

results are presented in Table

II.

Formation of o i l phase. Although this study was not primarily concerned with the oil-forming reactions, several aspects of o i l formation deserve mention. F i r s t l y , in most of these experiments, the organic- phase product separated into two f r a c t i o n s , denoted as "oil" and "bitumen" in Table II. (In more recent experiments, such a separation has been avoided by using a lower temperature during pressure let-down after the reaction.) In two of the present experiments (Expts. 3 and 6), no "oil" fraction was obtained and the apparent "bitumen" y i e l d was exceptionally high. These products were wet agglomerations of poorly-separating s o l i d s . Thus, the presence of a reducing gas was essential for obtaining an o i l - l i k e product, and, furthermore, when using hydrogen, the higher reaction temperature (350 °C) was necessary. The organic-phase product originates, to a large extent, from the l i g n i n fraction of the black liquor. As w i l l be discussed l a t e r , the other black liquor organics, mainly a l i p h a t i c acids, are not converted into organic-phase products ( o i l , bitumen) to any s i g n i f i c a n t extent. Interactions between gaseous and aqueous components. In a l l experiments the thermal treatment led to a decrease in the black liquor a l k a l i n i t y , the f i n a l pHs being in the range 8 - 1 0 (Table II). The chief neutralizing agents were the gases CO and C 0 . The decrease in a l k a l i n i t y was least extensive in experiments employing hydrogen as reactant gas, but i t was nonetheless quite s i g n i f i c a n t , p a r t i c u l a r l y at 350 °C (Expts. 4 and 5). In these cases, the neutralizing agent was C 0 , a product of the thermal decomposition of the organic matter (decarboxylation). The electrolyte systems of kraft black liquor are well known (4·). In p r i n c i p l e this data can be applied to the product mixture of the thermal treatment process, allowing the following conclusions to be drawn: 2

2


10. McKEOUGH& JOHANSSON

Table II.

Results of autoclave experiments. -temperature: 45 min.

2

1

Experiment

107

Oil ProductionfromBlack Liquors

Time-at-

4

5

6

H

H

co

3

7

8

00 /Η

Reactant gas

CO

CO

H

Temperature, °C Pressure, MPa Yields, % of black liquor organics [% om) oil bitumen C0 5

300 20

350 26

300 18

350 22

350 22

350 24

24.9 35.1 30.1 0.5 •31.8 0.4 1.4 2.0 35.7 4.5 4.5

26.9 25.6 32.1 0.7 -28.6 0.6 1.1 19.0 10.6 3.9 3.5

0 80.7 0.4 -0.5 0 0.1 0.3 10.0 6.2 4.1 5.4

23.2 36.5 2.4 -0.3 0 0.7 0.5 ND ND ND ND

22.6 33.4 2.4 -0.2 0 0.8 0.4 18.0 4.6 3.8 4.0

0 81.4 0.6 0 0 0.7 1.0 ND 1.6 3.9 ND

22.1 34.1 5.2 -0.3 0 1.0 ND 19.0 ND ND ND

9.5

9.4

8.9

8.4

2

2

2

2

2

2

Ί

Producer gas 350 26


2

3

4

9

& CH -gases HS Co /aqueous formic acid acetic acid l a c t i c acid 2

6 7 7 7

Aqueous-phase pH 1

8.5

8.5

60 % C 0 , 40 % H. 2

k

20.2 36.5 3.9 -0.1 -3.8 0.7 0.4 ND ND ND ND 9.1

in gaseous phase in aqueous phase as HCO" or C0 « present as sodium salts ND not determined 5

o

2 14 % CO 51 % N , 2' 3

10.2

345 26

17 % H , 2

18 % CO organic product as o i l layer heavier organic product

6

3

7

1. In a l l cases, sulphur i s present as HS" in the aqueous phase, the f i n a l pHs (8 - 10) being lower than the pK for HS" (~ 13.5) and higher than the pK for hLS (~ 7). 2. HC0 " Predominates over C0 *, p a r t i c u l a r l y at the lower end of the observed pH range. pK f o r HC0 " i s ~ 10. 3. The phenolic hydroxyl groups of the l i g n i n molecules are largely in an unionized condition, i . e . not bound to the sodium ion (pK s: 9.5 - 11). 4. The carboxyl groups, which do not decompose during the thermal treatment, remain ionized (pK s: 3 - 5). The three main fractions of the organic matter of the Black liquor are l i g n i n , a l i p h a t i c acids, and extractives. The a l i p h a t i c acid fraction contains the bulk of the carboxyl groups, but some are also encountered in the other fractions (5_). a

a

3

3

a

3

a

For the case when the neutralizing agent i s C 0 , the main neutralization reactions can be written as follows: 2


108

PYROLYSIS OILS FROM BIOMASS NaOH + C0 NaHS N a 0

2

3

NaHC0 + H S

2

+

1)

3

2

phenolic

2

NaHC0

C0 + H 0 ^

+

Na C0

^

C

0

3

H 0 ^NaHC0

+

2

2

+ C0 + H 0 ^ 2 N a H C 0 2

2)

2

2

3

+ H0

p h e n

oiic

3) 4)

3

For the case when CO i s the neutralizing agent, the equations become: NaOH + CO ^ H C 0 N a

5)

2

NaHS + CO + H 0

—

C 0

>H

1

2

1

)

2°

Sodium formate can also function as a reductant in a s i m i l a r way (8): HC0 Na + 0 2

o r g a n 1 c

m

a

t

t

e

—> NaOH + C 0 —> NaHC0

r

2

13)

3

However, the results of the H /300 °C experiment suggest that the formate present in the black liquor feedstock (at 6 % om concentration) i s not an e f f e c t i v e reductant under these conditions. In addition to i t s being consumed as a reductant, CO i s consumed in partial neutralization of black liquor a l k a l i n i t y (Equations 5 to 9) and in production of H by the water-gas-shift reaction: 2

CO + H 0 ^ C 0 2

2

+ H

14)

2

From the data of Table II, i t has been possible to establish the consumption patterns of CO at 300 °C and 350 °C (Table III).

Table III.

Consumption of CO.

% of black liquor organics (% om) CO/300 °C I II III

Neutralization Reduction Shift Total

18 7 7 32

CO/350 °C 3 16 10 29

The total level of CO consumption, about 30 % om, i s equivalent to a H consumption of 2.1 % om, which i s a much higher value than that observed in H experiments: 0.3 - 0.5 % om (Table II). 2


PYROLYSIS OILS FROM BIOMASS

110 Implications for Process Development

The aqueous phase leaving the thermal treatment reactor contains the following compounds: - NaHS ( ^ N a S ) - NaHC0 ( ^ N a C 0 ) - Sodium salts of a l i p h a t i c acids, with formate, acetate, and lactate as three main components. The formate content i s considerable when the reactant gas i s CO. - Other organic compounds in smaller amounts. The organic acid s a l t s and other organic compounds cannot be recycled, in their entirety, to the cooking stage. If not purged from the system, they w i l l quickly build-up in concentration in the cooking liquor and retard the d e l i g n i f i c a t i o n reactions. Furthermore, p a r t i c u l a r l y i f the formate concentration is high, there may be i n s u f f i c i e n t sodium for binding to the hydroxide ion. In f a c t , various p o s s i b i l i t i e s of converting sodium formate d i r e c t l y into sodium hydroxide were investigated in this study, but none of these proved to be technically f e a s i b l e . One example of a workable scheme for the recovery of chemicals is depicted in Figure 1. A part of the aqueous phase leaving the thermal treatment stage i s recycled through a wet oxidation reactor where organic matter i s oxidized to carbonate, C 0 , and H O . The heat produced i s employed in heating the feed stream to tne thermal treater. The rest of the aqueous phase i s recausticized in the conventional way. In addition to the normal components of kraft green l i q u o r , this liquor contains NaHC0 and some organic matter. The presence of organic matter should Be an advantage because, according to the l i t e r a t u r e , a small but s i g n i f i c a n t increase in pulp y i e l d can be expected (2). The presence of NaHC0 w i l l lead to a higher lime requirement in the r e c a u s t i c i z i n g stage. Wet oxidation i s quite an economic process step in this scheme because the feed stream to i t is already at the required temperature and pressure. Sulphide w i l l be oxidized to sulphate, but i t should be reduced again to sulphide in the subsequent heat treatment stage. This l a t t e r reaction w i l l be investigated experimentally in the near future. For reasons of low gas consumption and low gas cost, producer gas is the reducing gas proposed for the scheme of Figure 1. Preliminary economic evaluations of this scheme have indicated that i t would be more economic than the conventional recovery process, p a r t i c u l a r l y i f a higher pulp y i e l d were obtained. This process would also have a higher thermal e f f i c i e n c y than the conventional process. 0


3

2

3

2

3

3

Concluding Remarks The black liquor treatment process being developed at VTT has several promising forms of application. The work described in this paper has furthered the development of one such application: a new recovery system for the kraft pulping process. Current research is being d i rected at gaining a better understanding of the oil-forming reactions occurring during thermal treatment.



Figure

(PEAT, BARK, WASTE WOOD, COAL)

SOLID FUEL

(30 %ds)

BLACK LIQUOR

1.

Prelimary

AIR

GASIFIC ATION

SCRUB BING

THERMAL TREATMENT

GASES

process.

CAUSTICIZΑΊΠΟΝ

LIME KILN *FUEL

scheme for a new kraft recovery

WET OXIDATION

i>

Oil Production by High-Pressure Thermal Treatment of Black Liquors

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