CA1205930A - Method for the purification of condensates from the sulfate process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Condensates from the sulfate process are freed from a main portion of the pollutants and foul components by being passed with an oxygen-containing gas concurrently through a column containing activated carbon.

Description

`` ~205~3~
-r~he present invention relates to a method for the purification o~ condensates from the sulfa~e process.

In the production of cellulose according to the sulfate process the fibrous raw material (generally wood chips) i~ treated under high pressure and high temperature with so called white liquor~ the 'active chemicals of which are so-dium hydroxide and sodium sulfide (l,Z). Additionally, some manufacturers use other chemicals, e.g. polysulfide.
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After digestion the' fibrous materi~l (sulfate cellulose or kraft pulp) is separated from the dissolved organi~ material and the inorganic chemicals. Because of its colour the liquid phase is now called black liquor.
Said black liquor is now concentxated to such a high solids content that it can ~e burnt. Heat is, thus, released (possibly electric power can be obtained), and inorganic chemicals are recovered and may be converted to white liquor chemicals, which is necessary ~o make the process economical. Some volatile compounds (turpentine methanol and others) and air are 'removed separately from the digester or are separated from the black liquor when it is subjected to reduced pressure (as compared to the digestion pressure).
Aftex cooling, certain of said components will leave with the water steam to ~e condensed. The water insoluble phase is generally called sulfate turpentine. The condensates from this process are generally called digester condensates.

Concen'tration of the black liquor, including the ' 3Q washing water from the'displacement of black liquor from the cellulose,'is generally carried out in a multi-step evaporation plant, partly in a vacuum. Evaporated water is discharged as con-~Z05~3~ -, . .
densates. To~ether with any water from the vacuum pump of the plant, the wa-ter dlscharged at this sta~e is called evapora-tion condensa-tes. Apart from the portion origina-ting from indirect heatin~ with live steam, said evaporation condensates are also more or less polluted In some plants part of the s-team from the digèstion process is used for pre-evaporation of the black liquor. Such pre-evaporation results in condensates considered as something between digester and evaporation condensatesO rrhe different kinds of stripping steam and condensates can be lin~ed to-;
s ~ ~o~ , A.'s gether in various manh~Es in the process~ ~L~Y:, th~t is~
of no concern in this connection~

A soap (sulfate soap) is separated from black liquor. It consists mainly of saponified fatty acids and rosin acids (abietic acids), but it also contains unsaponified com-pounds. Said soap is generally recovered and typically con-verted to so called tall oil. The evaporation condensates will also contain more or less finely divided "oil", i.e. a water insoluble phase that is disadvantageous to any fur-ther treatment of the condensates.

It is common to all the above mentioned condensates that they contain various volatile substances, some of which are lower sulfur containing compounds. Especially during evapo-ration foul and toxic gases, e.g. hydro~en sulfide (H2S), methyl mercap~an (C~3S~I) and the like, escape from the black liquor, and quite a significant portion of these wiil be found in the condensates. Thus, said condensates have an obnoxious smell and are toxic.

Their content of methanol and the ]ike makes condensate effluents a pollution load on the recipient because dis-solved organic material will cause a chemical oxygen demand(COD) and a biochemical oxygen demand (BOV). These three factors, namelythe obnoxious smell, the toxicity to aquatic life, and the organlc~` load cause the c~nden,~tes from the . ' ~
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kraft process to represen-t a serious environmental hazard.
The possibili-ties of reusing unpurified condensates as a - source of li~uor in the process are also limited because they release toxic and foul gases and contain finely divided oils.

The methods in use today for puri:Fying the condensates from the kraft process are primarily based on the removal of volatile compounds (among others methanol and substances having an obnoxious smell) by the`aid of steam and air (stripping) (2a,2b). The removed gases/vapours are typical-ly destroyed by burning together with any non-condensable gases from the digestion and recovery process.

A process of purification based on decomposition by the aid of microorganisms is also used (3).
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It is an object of the`present inven-tion to remove the foul substances in the condensates by combined oxidation and ad-sorption after a preceding removal of finely divided oil, inter alia by coalescence when that is necessary. A smallpilot plant has been tested for a few years in a Norwegian kraft mill for examination of various types of condensates, catalysts, life times and possib~e means of regeneration of the catalysts and so on.

It is known that hydrogen sulfide gas in air is oxidized on a suitable catalyst, e.g. activated carbon. In said gas reaction H2S is converted to elemental sulfur. Said process may be extended to sulfur diox~de (S02), which is also (to-gether with some S03) the final product when H2S, sulfur and many other sulfur compounds are burned~ I-t has also been sugges-ted to purify foul gases by the aid of alkali and activa-ted carbon.

Hydrogen sulfide is soluble in alkali, e.g. caustic soda with formation of sulfides(NaHS and /or Na2S in NaOH). In ~- the very stron~ly basic white liquor from the sulfate pro-- ~205~30 ,, ~ . , cess sodium hydroxide and sodium sulfide are as mentioned the active digestion chemieals. Such a white liquor is slowly oxidized by air, and the oxidation pr~ducts will be sodium sulfite~ sodium thiosulfate and sodium sul~ate. In the presence of a suitable oxygen converter or eatalyst the oxidation time may be eonsiderably reduced, and sodium poly-sulfide and sodium thiosulfide are formed. By way of example admixture of a eertain portion of blaek liquor (PFI-patent) or use of a speeial eatalyst mass (Meads Moxy proeess) (4,5) 1~ may be mentioned.

Activated carbon is generally known in the art of purification for the removal of small amoun-ts of eontamin-ants. This is based on the large internal surface area of the substanee~ The surfaee will be blocked by the adsorbed substance, and some kind of regeneration will be neeessary in eases of larger amounts of eontaminants.

According to the present invention there is provided a method for the purification o~ condensates from the sulfate proeess which com-prises passiny the condensates eoncurrelltly with an oxygen-containing gas through a column containing activated earbon.
~, EXPERIMENTS
Polluted and fou`l condensates from a kraft mill are passed with air through a eolumn filled with aetivated earbon.

Preliminary tests showed that 1) The obnoxious smell was removed by said treatment.

2) EIydrogen sulfide and methyl mercaptan were removed (de-monstrated by potensiometrie titrations).
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3) The smell, hydrogen sulfide and methyl mereaptan were not ~rt ~- 4 -lZ~93(;~

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removed, unless sufficient amounts of air were passed with the liquid.

4~ The column lost its efficency after some time.

In addition to the fact that it was possible to remoye the obnoxious smell, hydrogen sulfide, and methyl mercaptan ~y this method, the tests disclosed that what had taken place had to be an air oxidation (items 1, ~, 3) 1~ and that the . .

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column material had been active (item 4), i.e. that a cata-lytic oxidation had occurred.

It was assumed that hydrogen sulfide was oxidized to elemen-tary sulfur, which remained on the column material. This assumption was corroborated by the fact that hot white li-;
quor passed through a column was enriched with polysulfide. (Colour yellow/orange/red, dependent on the concentration. Determined quantitatively by potensiometriG
titration after conversion to an equivalent amount of thio-~- sulfate by sodium sulfite.~ It is known that elementary sulfur is dissolved in sulfide solutions, e.g~ white and green liquor. The test, thus, also showed -thai th~ sulfate process itself provides chemicals that may be used to libe-- 15 rate the column material from the deposited sul~ur.

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The untreated condensate contained oil-li~e droplets of varying amounts and droplet sizes. After havin~ passed through said column the condensate, however, was perfectly - 20 clear. Such oil deposition assumedly must reduce the active surface of the column material. This was corroborated by the fact that the column after losing its efficiency was made efficient again after treatment with steam at approxi-mately l~O til 190C.
Observations of the condensates for some time showed that the amount of oil could var~ within wide limits . Efforts were, thus, made to remove the oil before the condensate reached ~he column. An efficient oil removal was achieved by first removing large droplets that were allowed to float on top of the aquous phase and then removing the finely divided floating oil droplets from the water phase by coa-lescence.

Samples of the cleared condensates after coalescence fil-tra-tion were extracted with organic solvents and -the extracts were examined by gas chromatography. The analyses showed that a large amoun-t of polar and non-polar orqanic com-~ ' ' .

12~ 306 pounds were present. Corresponding analyses of samples taken aEter the condensates had passed through -the column showec1 that said compounds were almost cornpletely removed by -the treatment. The compounds detec-ted by said method oE~
analysis were assumed to be higher organic sulfur compounds.
Since these compounds as well as hydrogen sulfide and lower organic sulfur compounds are expected to have an acute toxic effect on the life in the recipient, condensate samples were used for mortality tests with salmon fry. I-t was found that the untrea-ted condensates were 3 to 35 times more toxic than the treated on~s.
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The toxicity tests ~96 hours - LC50) were carried out at the Norwegian Institute of Water Research (NIV~). The gas chromatographic investigations were carried out at the Norwegian Central Institute for Indus-trial Research (SI), among others by the aid of a ~lass capillary column and a `
flame ionization detector. The content of low boilin~ orga-nic compounds, e.g. methanol, was studied at -the Norwegian Pulp and Paper Research Institute (PFI) ~y the aid of gas chromatography with a polyalkylene glycol column and a hot wire detector. In addi-tion to methanol there were ound traces of another compound (assessed at less than 1,5% of the amount of methanol). This indicatès that methanol could in pratice be enriched from the purified condensates with-out too great demands on fractionated separa-tion. Examina-tions oE volatiles by head-space-technique showed a re-duction of the num~er and the total content of escaping odourants at 40C (SI).
The pilot plant used is shown in a flow sheet of the accom-panying drawings. Its construction is as follows:

By the aid of valves (1,2) and pumps (Pl,P2) a portion of the flow was taken from the condensate line ~in the embodi-ment shown in the drawing it is taken from the outlet of the evaporation stage). Any solids and larger oil droplets are separated in a preliminary filter (3) having an over-. ~ , .

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flow (4). The coalescence uni-t consists of two parallel coalescence filter cartridges (5~ of the Bdls-ton type. The oxidation is carried out in a 4 m long steel column contain-ing 8 l of catalyst mass. The column receives condensate (alternatively white liquor for the removal of deposited sulfur) via pump (P3) and a flowmeter (9), it receives compressed air via a meter (l8) and steam for regeneration.
The efficiency of the column is monitored by a H2S detector mounted below the cover of the collecting tank and additio-nal control is achieved by sampling for assessment of theodour and for potentiometric titration of H2S and CH3SH.
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-Examination of the Efficiency of the Oxidation Column:
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15 Column material: Activated carbon, 8 litres, ~rain size 0~5-2 r5 mm Type Lurgi Hydraffin LS supra Note: Preliminary tests carried out with inter alia ordinary activated carbon lacking technical specifications and bought in a shop in Oslo yielded corresponding results às regards odour, H2S and CH2SH.
The column was filled on Oc-t. 2nd/ 1979.
Experiments were carried out from Oct. 2nd to Oct. 4th, 1979.
Incoming condensate: EVAPORATION CONDENSATES:
H2S Orl - 0~9 g S/l CH3SH o 0 3 Ei2S + CH3SH O~l - lr2 g S/l pH 7,~6 - 8~0 Outgoing condensate:
H2S + CH3SH O g S/l pH approx. 9~7 Amount of condensate 15 l/h, totally 720 l Amount of air approx. 40 l/h,totally 1800 l Totally removed H2S ~ CH3SH corresponding to 360 ~ S
Regenerated with approx. 5 kg steam (190C, 13 kg/cm Continued Oct. 15 - Oct. 17.

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Incoming condensate: -S 0~0~7 ~ S/l 13SH 0~0,r3 H2S + HC3S11 0~.2 ~ 0 g S/l pH approx~ 8 Outgoing condensateo H2S + CH3SH O g S/l pH approx. 9~.6 Amount of condensate 15 l/h, totally~ ~50 1 Amount of air ~ 40 r~ 1/ (31.0 1 Totally removed l12S -~ CH3SH corresponding to 2 O ~ S
Continued Oct. 22, - Oct. 25, Amoun-t of condensate totally 600 1 Amount of air " 1630 1 ~ pH (in) approx. 8 pH (out) " 9~5 Totally removed H2S + CH3SH corresponding to 310 g S
Amount of condensate after previous steam regeneration:
350 1 + 600 1 = 950 1 Amount of condensate totally: 720 1 + 950 1 = 1470 1 Removed 112S + CH3SH totally corresponding to 54Q g S
Regenerated wi h approx. 6 kg steam (190C, 12~ kg/cm2) Regenerated with approx. 4~1 1 hot white liquor, 30 min.
Drained off Li~uor containing polysulfide. Washed with approximately 40 l water.

Continued February 25th - 27th, 1~80 ~mount of condensate totally 330 1 ~ount of air " 1000 1 pH (in) approx. 7~8 pH (out) " 9~7 Totally removed H2S + CH3SH corresponding to 230 g/S

Continued March 4th Rn~ount of condensate totally 470 i Amount of air ~ " 1400 1 Totally removed H2S -~ CH3SH corresponding to 330 g/S
- 560 g/S

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~205~30 The series of experiments was ended after the -treatment of totall~ 2270 1 of non-purified evaporation condensate and removal of ~2S ~ CH3SH corresponding to 1430 g sulfur.

Experiment carried out May 28th - Oct. 9th 1980: Di~ester Condensate Column ma-terial as above Column filled May 21st Incoming condensate:
~ H2S + CH3SH traces pH 9~0 - 9 Outgoing condensate:
H2S + CH3SH O ~/
pH 9t`5 10 p Amount of condensate: 15 l/h, totally approx. 5000 1 Amount o air ~0 " " " 13300 1 Experiment completed, column still efficient.

Gas Chromatographic Analyses (carried out by SI) Extraction with cyclohexane involves non-polar compounds, ~ollowe~ by acidification and extraction with butyl acetate and deri~ation with a trimethylsilyl-reagent to determine the more polar compounds. The first mentioned were studied in a ~lass capillary column, the last mentioned in a packed column, both by the aid of a flame ionization detector.

Evaporation Condensate from Oct. 17th, 73 The amounts of both non-polar and polar compounds were very much reduced after the condensate treatment in -the oxida-tion ~olumn. The main component of the non-polar compounds was reduced approximately ?0 000 times ~from 60 ppm to 3.10 3 ppm). A reduction of approximately 600 times was found for the main component of polar compounds.

~205~30 Digester Condensate from early in June 1980 For the non-polar compounds a reduction of approx. 160 times was found. In the polar frac-tion no compounds were found in the sample taken after the oxidation column, which corresponds to a reduction of at least 200 times, based on;
the detection limit.
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Odour Analyses by ~ead-space Technique (carried out by SI) The evaporation condensate samples from upstreams and down-streams the oxidation column were heated to ~0Cfbr 1 hour, and the gas above the liquid (head-space) was extracted, concentrated on activated carbon and analyzed by gas chro-matography. Great difference of the samples was re~orted,approximately hàlf of the compounds from the sample up-streams of the oxidation ~eing absent in the sample down-streams of the oxidation. The total content is approximate-ly 4 times higher in the untreated sample. A few new vola-tile compounds seemed to have been produced, but these werepresent in very small amounts. The main compound present seems to be dimethyl disulfide.

Destillation of Metanol and Determina-tion of its Purity Methanol was destilled off (amounting to approx. 5 g/l) and was analysed by gas chromatography at PFI. The chromato~rams only showed traces of one compound in addition to methanol, ~ the amount of which was assessed to be less than 1~.5% of the amount of metanol.

Coalescence The efficiency of coalescence filtration was proven by the fact that 1) "oil" accumulated on top of the unit and that 2) the condensate was often mil~ upstreams of the unit and clear downstreams. The oil content in the un-treated condensates varies within very wide llmits.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the purification of condensates from the sulfate process which comprises passing the conden-sate and an oxygen-containing gas concurrently through a column containing activated carbon.

2. A method in accordance with claim 1, in which deposited sulphur on the carbon material is removed by treat-ment with a sulfide containing alkaline solution such as white liquor.

3. A method in accordance with claim 1, in which organic components deposited in/on the activated carbon are removed by steam stripping.

4. A method in accordance with claim 1, in which the condensate is previously freed from "oils" or solids.

5. A method in accordance with claim 4 r in which the condensate is freed from "oils" or solids by coalescence and/or filtration.