FI73254B - FOERFARANDE FOER RENING AV SULFATPROCESSENS KONDENSATER. - Google Patents

Description

Method for purifying condensates in the sulphate process 1 73254

The present invention relates to a method of the type set out in the method of claim 11a.

In the production of cellulose according to the sulphate process, the fibrous raw material (usually wood in the form of wood chips) is treated under high pressure and high temperature in a so-called with a white liquor in which the active chemicals are sodium hydroxide and sodium sulfide (1, 2). In addition, some mills use additional chemicals such as polysulfide and the like.

At the end of the cooking process, the fibrous material (sulphate cellulose or sulphate pulp) is separated from dissolved organic matter and inorganic chemicals. Because of its color, the liquid phase is now called black liquor. The black liquor is concentrated to such a high dry matter content that it can be burned. This releases heat (possibly also electric power) as well as inorganic chemicals that can be converted to white liquor chemicals, which is absolutely necessary for the economy of the process. A number of volatile compounds (turpentine, methanol, etc.) and air are removed from the digester separately or separated from the black liquor by subjecting it to reduced pressure (relative to the digester pressure). After cooling, some of these follow with the condensing water vapor. The water-insoluble phase is commonly referred to as sulfate turpentine. Condensates from this part of the process are commonly referred to as cooking condensates.

Concentration of black liquor (including cellulose-displaced wash water) is usually carried out in a multi-stage evaporator, partly under vacuum. Evaporated water is taken out as condensate. Together with the water possibly coming from the plant's vacuum pump, etc., the water leaving it is called evaporation condensate. With the exception of the part from the direct heating of fresh steam, the evaporation condensates are also more or less impure.

Individual mills use a portion of the steam from the cooking process to pre-evaporate the black liquor. Such pre-evaporation condensates can be considered as an intermediate form of cooking and evaporation condensates. Different types of exhaust steam and condensate as a whole may be related to the process in different ways, but this is not considered relevant in this context.

Soap (sulphate soap) is separated from the black liquor.

This consists mainly of saponified fatty acids and rosin acids (abietic acids), but also contains unsaponifiables. Soap can be recovered and it is usually changed to the so-called tall oil.

Evaporative condensates also contain a more or less fine "oil", i.e. a water-soluble phase, which is a disadvantage in their further processing.

What the condensates mentioned here have in common is that they contain various volatile substances, some of which are lower sulfur-containing compounds. Especially during evaporation, malodorous and toxic gases such as hydrogen sulfide (I ^ S), methyl mercaptan (CH 2 SH) and the like are removed from the black liquor, a considerable part of which remains in the condensate. The condensates are therefore malodorous and toxic.

The concentration of methanol plus other substances means that the removal of condensate loads the discharge water in the form of dissolved organic matter, which is associated with chemical oxygen demand (COD) and biochemical oxygen demand (BOD). These three factors, odor, aquatic toxicity and organic load, mean that the condensates in the sulphate process pose a serious environmental problem. At the same time, the use of impure condensate as a liquid source in the process is also limited because it removes toxic and foul-smelling gases and contains fine oils.

The methods currently used to purify condensates in the sulphate process are primarily based on the removal of volatile compounds (including methanol and odoriferous substances) by steam or air (stripping) (2a, 2b). The removed gases / vapors are usually disposed of by co-incineration with some of the condenser tower gases from the cooking and recovery process.

A purification process based on degradation by microorganisms is also used (3).

The purpose of the present method is to remove malodorous substances from condensates by combined oxidation and adsorption after prior removal of a fine oil, e.g. by means of coalescence when this is necessary. A small test facility has been tested in a sulphate plant for a couple of years to study different types of condensate, catalyst types, catalyst life and regeneration possibilities, etc.

It is known that hydrogen sulfide gas in air is oxidized by a suitable catalyst, such as activated carbon. In this gas reaction, t ^ S is converted to elemental sulfur. The process can be further extended to sulfur dioxide (SO2) »which is also (along with a certain amount of SO2) the end product of the combustion of H2S, sulfur and many other sulfur compounds. It has also been proposed to purify odorous gases with alkali and activated carbon. Hydrogen sulfide dissolves in alkali, e.g. sodium hydroxide, to form sulfides (NaHS and / or Na 2 S in NaOH). In the very strongly basic white liquor of the sulfate process, sodium hydroxide and sodium sulfide are, as mentioned, active cooking chemicals.

4 73254

Air oxidizes such white liquor slowly, and the oxidation products are sodium sulfite, sodium thiosulfate, and sodium sulfate. In the presence of a suitable oxygen scavenger or catalyst, the oxidation time can be significantly reduced to form sodium polysulfide and sodium thiosulfate. Examples are the mixing of a certain amount of black liquor (PFI patent) or the use of a special catalyst mass (Mead Moxy process) (4, 5).

Activated carbon is commonly known in cleaning technology when used to remove small amounts of impurities. This is based on the large inner surface of the substance. The adsorbed substances seal the surface so that some kind of regeneration is required in the case of larger amounts of impurities.

The present method is characterized by what is set forth in the characterizing part of claim 1.

Exam

Impure, foul-smelling condensates from the sulphate plant were passed together with air through a column packed with activated carbon.

Preliminary experiments showed that 1) odor is removed during treatment, 2) hydrogen sulfide and methyl mercaptan were removed (detected by potentiometric titrations), 3) odor, hydrogen sulfide and methyl mercaptan were not removed if there was not enough air in the liquid and 4) the column lost its power after some time.

Thus, in addition to removing the odor, hydrogen sulfide and methyl mercaptan in this process, the experiments showed that air oxidation had to take place (claims 1, 2, 3) and that the column material had acted (claim 4), i.e. it was catalytic oxidation.

73254

It was assumed that hydrogen sulfide had been oxidized to the elemental sulfur remaining in the column material. This assumption was confirmed by enriching the polysulfide in the white liquor passed through the column. (Color yellow / orange / red depending on concentration. Determined by conversion by potentiometric titration to an equivalent amount of thiosulphate with sodium sulphite.) It is known that elemental sulfur is soluble in sulphide solutions such as white liquor and green liquor. Thus, the experiment also showed that the sulfate process itself contains chemicals that can be used to release your columns from the precipitated sulfur.

The untreated condensate contained oily droplets of varying number and droplet size. However, after passing through the column, the condensate was completely clear. The precipitation of such an oil can be expected to reduce the active surface of the column material. This was confirmed by the fact that the column, after losing its power, again became effective after being treated with steam at about 180-190 ° C.

Following the condensates for some time, it was discovered that the amount of oil could vary within wide limits. Therefore, experiments were performed to remove the oil before the condensate entered the column. Efficient oil removal was achieved by first removing large droplets by allowing them to rise to the surface of the aqueous phase and then removing fine suspended oil droplets from the aqueous phase by coalescence.

Samples of water-clear condensates were extracted after coalescence filtration with organic solvents, and the extracts were examined by gas chromatography. Analyzes showed the presence of a very large amount of polar and non-polar organic compounds. Corresponding analyzes of the samples taken after passing through the column showed that these compounds were almost completely removed in 6 73254 treatments. The compounds obtained in this method of analysis were mainly assumed to be higher organic compounds. Because these, as well as hydrogen sulfide and lower organic sulfur compounds, can be expected to have an acute toxic effect on the life of the discharge water, condensate samples were used to study juvenile mortality. It was found that the untreated samples were 3-35 times more toxic than the treated ones.

Toxicity tests (96 hours - LC ^ q) were performed at the Norwegian Water Research Institute (Norsk Institutt for vannforsk-Ning NIVA). Gas chromatographic experiments were performed at the Central Institute for Industrial Research (Sentralinstitutt for industriell forskning SI) e.g. by means of a glass capillary column and a flame ionization detector. The content of low-boiling organic compounds, such as methanol, was examined in PFI using a gas chromatograph equipped with a polyalkylene glycol column and a thermal wire detector. In addition to methanol, small amounts of the second compound were found (calculated as less than 1.5% of the amount of methanol). This indicates that methanol can in practice be enriched from purified condensates without excessive fractional separation requirements. Odor studies using head-space technology showed a reduction in the amount and total concentration of odors leaving at 40 ° C (SI).

The test facility used is shown in the general drawing. Its structure is as follows:

By means of valves 1, 2 and pumps Pi, P2, a partial flow 1 is taken from the condenser line (in the example of the drawing this is taken from the removal of the evaporator station). Any solid particles and larger oil droplets are separated in a pre-filter 3 with an overflow point 4. The coalescence unit consists of two coalescence filter cartridges 5 of the Balston type connected in parallel. Oxidation

II

7 73254 is carried out on a 4 m steel column containing 8 liters of catalyst mass. Condensate (alternatively white liquor for the removal of precipitated sulfur) is fed to the column via pump P3 and a flow meter 9, compressed air via a meter 18 and steam for regeneration. The performance of the column is monitored by an I 2 S detector mounted under the collecting tank lid, and additional monitoring is obtained by sampling for odor evaluation and for I 2 Srn and CH 2 SHm potentiometric titration.

Oxidation column efficiency study

Column material: activated carbon, 8 liters, grain size 0.5-2.5 mm type: Lurgi Hydraffin LS supra note: Preliminary experiments performed e.g. with ordinary activated carbon without technical specifications, which was purchased from an Oslo store, gave similar results in terms of odor, H 2 and CH 2 SH.

The column was filled on October 2, 1979

The experiments were carried out on 2-4 October 1979: EVAPORATIVE CONDENSES Incoming condensate: H2S 0.1 - 0.9 g S / l CH3SH 0 - 0.3 H2S + CH3SH 0.1 - 1.2 g S / l pH 7.6-8 .0 Outgoing condensate: h2s + ch3sh 0 g S / l pH approx. 9.7

Condensate volume 15 1 / h, total 720 1 Air volume approx. 40 "" 1800 1

Removed total H2S + CH3SH corresponding to 360 g S

8 73254

Regenerated approx. 5 kg of steam (190 °, 13 kg / cm2) cont. 15.10.-17.10.

Incoming condensate: H2S 0.1 - 0.7 g S / l CH3SH 0.1-0.3 H2S + CH3SH 0.2 - 1.0 g C / l pH approx. 8 Outgoing condensate: H2S + CH3SH 0 g S / l pH about 9.6

Condensate volume 15 1 / h, total 350 1

Air volume 40 "" 910 1

Removed total H2S + CH3SH corresponding to 230 g S

cont. 22.10.-25.10.

Condensation Total 600 1

Air volume "1630 1 pH (in) approx. 8 pH (out) approx. 9.5

Removed total H2S + CH3SH corresponding to 310 g S

Quantities of condensate after previous steam regeneration: 350 1 + 600 1 = 950 1

Total condensate volumes: 720 1 t- 950 1 = 1470 1

Removed total H2S + CH3SH corresponding to 540 g S Regenerated with approx. 6 kg of steam (190 °, 12.8 kg / cm2) Regenerated with approx. 4.5 1: warm white liquor, 30 min. Removed polysulfide-containing lye. Washed with approx. 40 liters of water cont. 25 February to 27 February 80

Condensation Total 330 1 il.

9 73254

Total air volume 1000 l pH (in) about 7.8 pH (out) about 9.7

Removed total H2S + CH2H2 corresponding to 230 g of S

cont. 4.3.

Condensation Total 470 1

Air volume "1400 1

Removed total H 2 S + CH 2 SH corresponding to 330 g S

560 g S

The series of experiments was terminated after treating a total of 2270 L of crude evaporation condensate by removing H2S + CH3SH corresponding to 1430 g of sulfur.

Tests performed 28.5.-9.10.80: KITCHEN CONDENSES

Column material as above Column packed 21.5.

Incoming condensate: H2S + CH 2 SH very small amounts pH 9.0-9.5 Outgoing condensate: H2S + CH-jSH 0 g / l pH 9.5-10.0

Condensate volume 15 1 / h, total about 5000 1

Air volume 40 "" approx. 13300 1

Experiment decided, the column still effective.

Gas chromatographic analyzes (performed in SI)

Extraction with cyclohexane comprises non-polar compounds followed by acidification and extraction with butyl acetate and derivatized with trimethylsilyl reagent to determine more polar compounds. The first examined with a glass capillary column, the second with a packed column, both using a flame ionization detector.

10 7 325 4

Evaporation condensates from 17.10.79

The amount of both non-polar and polar compounds was very strongly reduced after treatment of the condensates in the oxidation column. The major component of non-polar compounds decreased by about 20,000-fold (from 60 ppm to 3 x 10 ppm). For polar compounds, a reduction of about 600 times with respect to the major component was observed.

Cooking condensates from 16.1980

For non-polar compounds, a reduction of about 160-fold was observed. No compounds were detected in the polar fraction in the samples taken after the oxidation column, which corresponds to at least a 200-fold reduction when the detection limit is used as a base.

Odor analyzes using head-space technology (performed in SI)

Evaporative condensate samples taken before and after the oxidation column were heated for 1 hour at 40 ° C and the head-space was removed, concentrated with activated carbon and analyzed by gas chromatography. There was a very large difference in the samples reported because about half of the compounds in the pre-oxidation samples were missing from the post-oxidation samples. The total concentration is about 4 times higher than in the untreated ones. Some new volatile compounds appear to be formed, but these are present in very small amounts. They appear to be mainly dimethyl sulfide.

Methanol removal and purity study Methanol was removed (amount of about 5 g / l) and analyzed by PFI gas chromatography. The chromatograms showed only very small amounts of the second compound in addition to methanol, and this amount was reduced to less than 1.5% of the amount of methanol.

tl 11 73254

coalescence

The efficiency of coalescence filtration was demonstrated by the fact that 1) “oil” had accumulated at the top of the unit and 2) the condensate was often pale before the unit and clear afterwards.

The oil content of the untreated condensate varied within very wide limits.

Claims (3)

A process for the condensation of condensates from a sulphate process which contain hydrogen sulphide compounds, in which process the condensates are brought into contact with an oxygen-containing gas in the presence of activated carbon. The condensate and decomposition gas are passed downstream through a column packed with activated carbon and, if necessary, the precipitated sulfur is removed in a manner known per se by treatment with a sulphide-containing alkaline solution such as white liquor. Process according to Claim 1, characterized in that the organic constituents absorbed on or onto the catalyst are removed by trituration and optionally disposed of. Process according to Claim 1, characterized in that the condensate is freed of "oils" or solids by conical size and / or filtration.