FI78935C - Method and apparatus for reducing odor and reducing process wave air release from sulphate pulp processes. - Google Patents

Description

χ 78935

The present invention relates to an improved method of collecting and rendering harmless the odorous gases of a process for the production of cellulosic pulp by the sulphate process. The invention also relates to a device for implementing the method.

When preparing sulphate cellulose, the cellulose wood is decomposed e.g. under the influence of sodium sulfide in an alkaline solution, whereby lignin is hydrolyzed. Hydro-10 lysis produces several organic sulfur compounds, such as methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. These compounds are gaseous at ordinary temperatures and have a very strong and unpleasant odor. It is these compounds that cause the characteristic odor, which can most often be felt in the vicinity of sulphate-cellulose mills.

The problem of odor in the sulphate process has been well known since the process was introduced, but only in recent years have there really been attempts to take action to eliminate the problem, and the authorities have increasingly restricted licensing for odorous processes. If not so much has been done in the past on the problem, it is probably due to the fact that odorous substances have been detected in such small concentrations that there are considerable costs involved in deodorizing them. Furthermore, at the low concentrations that occur around factories, odorous substances do not have any obvious harmful effects, only the odor is usually embarrassing.

30 The methods used to decontaminate odorous gases, that is to say odorous gases, usually involve their combustion so that they are converted into sulfur oxides, after which they are then discharged into the atmosphere. Because malodorous gases are formed in several stages of the cellulose process, mainly in the blowing of cellulose cooking, but also in the evaporation of lye, the separation of turpentine, etc. stages, it is important that the odorous gases are collected at each such stage and discharged in a suitable form for disposal.

Furthermore, it is also suitable to recover as much sulfur as possible from the odorous gases. One suitable way to do this is to wash the odorous gases with a white liquor, whereby this receives sulphides and mercaptans. In this way, the sulphidity of the liquor can be adjusted while the sulfur is recovered, and a smaller amount of odorous gases burdens the disposal equipment.

In a known system for removing and disposing of odorous gases, the gases and steam from the blowing of sulphate cooking are heat exchanged with respect to water in a blowing accumulator and from this the condensate is passed to an evaporator, usually a column where it is vaporized with steam, usually fresh. In those cases where a continuous cooking method is used, the cooking pressure is reduced "abruptly" in the usual way and the steam is also passed to the evaporation column. It goes without saying that in cases where only continuous cooking is performed, no blowing battery is required. Usually, the impure condensate from the liquor evaporation of the equipment is also fed to the evaporation column.

The evaporation column is equipped with a partial condenser, from which, on the one hand, the return flow is returned to the column and, on the other hand, the steam phase is led to the destruction of the gas. This steam stage is combined with the gases from the blasting battery and possibly with the odorous gases from the second stage of the process and passed to a disposal furnace, which may, for example, be connected to the shell shell of the plant. It is necessary to dilute the gases with large amounts of air so that they are not an explosive in the gas mixture, and in addition the gas mixture has a high water vapor content. As a result, large amounts of promoter fuel (auxiliary fuel) are needed to ensure combustion of the gases and to achieve a sufficiently high temperature for gas disposal. As a result, the method becomes uneconomical, which is exacerbated today by the sharp rise in oil prices. It is still not possible to recover 3 78935 sulfur by washing the gases with white liquor, as they contain turpentine and methanol, which must not be introduced into the white liquor system. Because the diluted gases take up a large volume, a large-scale pipeline system is also required, which adds to the additional cost.

Another system for removing and disposing of odorous gases includes a conventional type of purge battery and an evaporation column with a partial condenser in the same manner as described above. The batch method 10 further requires a gas accumulator in the form of a gas clock or the like to equalize the gas flows of the batch blowing blows. The odorous gases are discharged in a plant lime kiln to which the gases are led from a gas accumulator (batch process), a condenser for continuous cooking under reduced pressure, a steam column condenser and lye evaporation, and possibly other odorous gas sources in the equipment. However, the combustion of sulfur compounds in a lime kiln produces sulfur dioxide, which is largely bound by lime and returned to the chemical system of the plant as an inefficient ballast in the form of sulphate. This adds e.g. energy costs.

The present invention overcomes the above drawbacks and provides a method and apparatus for collecting and rendering harmless the odorous gases of a sulfate pulp mill, whereby the method can be implemented with significantly lower energy consumption and greater certainty and easier control than before. According to this method, sulfur can also be better returned to the equipment and the sulfidity can be better controlled in the equipment.

In accordance with the invention, the steam and condensate of the cooking-o- and pu-control steps of the process are subjected to evaporation or separation by steam, evaporation. the gas stage 35 is passed through a condenser and the non-condensable gases are destroyed by combustion. The invention is characterized in that the gas phase obtained from evaporation is subjected to extensive condensation to a temperature below 50 ° C, 4 78935 so that all the turpentine and methanol condense and that the non-condensable gases liberated from the turpentine and methanol are washed with an alkaline solution, preferably after being combined with the odorous gases of the second stage of the process and then combusted, and that the condensing condensate in the evaporation gas stage is divided mainly into turpentine liquid phase, mainly methancl liquid phase and gas phase, and that this gas phase is further combined , with non-condensable gases before washing and burning, and that methanol and possibly also turpentine replace fuel oil as an auxiliary fuel in the combustion of gases, and that any flue gases leaving the combustion are washed with a base, preferably v alkolipeällä.

The method according to the invention can be applied to continuous as well as batch cooking methods as well as to combinations of the two methods. In the latter two cases, the steam from the batch 20 blowing is condensed in a blowing battery, from which the condensate is passed to an evaporator, i.e. a separator, and the gas from the blowing battery is combined with the gas obtained from condensing the evaporator.

The condensation of the gas phase of evaporation or separation is preferably performed at an exhaust gas temperature below about 40 ° C and most preferably below about 35 ° C. In this case, it is suitable to divide the condenser into a first partial condenser, from which a return stream is taken, which returns to the evaporation column, and a terminal condenser, in which the gas phase is condensed at the above-mentioned temperatures.

The invention is illustrated in more detail in the accompanying drawing, which forms a block diagram of an apparatus for carrying out the method according to the invention. The drawing shows preferred embodiments using a combination of batch and continuous cooking.

According to the drawing, the blowing steam is led from the blowing of the intermittent sulphate cooking along the line 1 to the blowing pressure vessel, i.e. the blowing accumulator 2, where it is exchanged for water and divided into a gas stage exiting via line 3 and condensate exiting via line 4. The gas phase coming from the blowing battery 2 can still be condensed in the surface condenser 5 and led through the water gap 5 to a gas accumulator 7 »which is usually made as a gas clock. The purpose of the gas accumulator 7 is to compensate for fluctuations in the gas supply caused by intermittent cooking and blowing. Then, the gas coming from the gas accumulator 7 after the washing performed in the washing tower 8 can be led to be disposed via the line 9.

The condensate from the blow accumulator 2 passes via line 4 to an evaporator or separator 10, which in most cases has a vapor column of the usual type. Steam from the depressurization is also fed to the evaporation column or separator 10 via line 11 in continuous cooking, and steam, usually fresh steam, is passed through line 12 for evaporation. The incoming condensate can be heat exchanged in the heat exchanger 13 with respect to the pure condensate which exits via the line 14 after evaporation 20.

The gas phase of the evaporator column 10 is passed to a condenser, which is preferably divided into a sub-condenser 15 and a terminal condenser 16. The return from the sub-condenser 15 is led back via line 17 to the evaporator column 10.

In the terminal condenser 16, the gas phase condenses to a large extent mainly dissolved gases of turpentine, methanol, water and part of it into a liquid phase and a gas phase of "non-condensed" gases, i.e. gases condensing below about 50 ° C, suitably below 30 ° C and preferably below about 35 ° C. These gases are led along line 18 to a water trap 19 where the gas is washed and possibly mixed with odorous gases from the second stage of the process, such as gas from a blower battery 2 through line 9 and gas from evaporation of lye, generally labeled by reference numeral 20, wherein this gas is passed through line 21.

6,78935

From the water trap 19, the gas is led through line 22 to the scrubber tower 23, where it is washed with white liquor, which is led from the white liquor tank via line 24 and discharged along line 25. In the washing tower 23, the white liquor ab-5 sorbs a large part of the sulfur compounds contained in the gas, such as hydrogen sulfide, mercaptan, and sulfides, and can be returned to the process, thereby also increasing the sulfidity.

The liquid phase is passed from the terminal condenser 16 via line 2δ 10 to a turpentine decanter 29, where it is divided into a liquid phase, which is mainly turpentine, a liquid phase, which is mainly methanol, and a gas phase, which is connected via line 30. with gas from the terminal 16 condenser 16. In addition to the gas phase obtained from the turpentine decanter, a gas phase is obtained from the degassing of the liquid phase at 31 and 32 from the methanol phase, which is discharged along line 33.

The resulting methanol phase is passed via line 33 to a methane-20 auxiliary tank 34, where it is stored and then passed to combustion chamber 44 as a combustion fuel (auxiliary fuel). The turpentine phase is passed through line 36 to be recovered as the next sulfate turpentine for purification. However, if for economic reasons it is advantageous, it is also possible to use turpentine as an auxiliary fuel for the combustion of gases. It is led in the same way as methanol to the storage tank (not shown) and from there to the combustion chamber 2b.

A tank 37 for fuel oil is also provided to start and connect the equipment in the event of disturbances in the process, which is led to the combustion chamber 44 via a line 38 connected to a methanol line 35. The combustion chamber also terminates lines 39 and 40 for main and secondary air.

35 These wires are ordinary wires.

The required cold water is led to the condensers 5 and 16, to the water traps 6 and 19 and to the washing tower 41 along the supply line 4l. Warm water 7 78935 can be obtained from the terminal condenser 16 via line 42 and contaminated water is led from surface condenser 5, water traps 6 and 19 and washing tower 8 via connection lines to drain line 43 and condensate tank. This system is ordinary in structure.

5 Water can then be passed from the condensate tank to the evaporation column 10 to separate odorous contaminants.

The apparatus is also otherwise provided with standard devices for pumping and regulating liquid and gas flows, for measuring flows and temperatures, safety valves for pressure vessels, etc. Such devices are of ordinary construction and it is not clear to a person skilled in the art how to use them and how to adjust them. Therefore, they are not shown in detail in the drawings.

As mentioned above, the drawing shows an apparatus for handling odorous gases from both the batch cooking process and the continuous cooking. It will be clear to a person skilled in the art that if only one type of soup is used, the apparatus 20 can be varied to a corresponding extent. Thus, in a completely continuous method, the blower battery 2, the surface condenser 5, the water trap 6, the gas battery 7 and the scrubber tower 8 are omitted together with the associated lines. The condensate and steam from the reduction in cooking pressure go directly to the steam-25 knuckle column 10. However, in the batch process alone, most of the equipment is needed because the evaporation column is necessary in all conditions.

It is essential for the invention that the condensate from the gas stage coming from the evaporation column 10 is used for a much longer period of time than before and an attempt is made to condense as much gas as possible. After condensation and decantation of the turpentine, it is desirable that only a minimally small amount of gas exists to be introduced into the white liquor scrubber tower 23, and preferably such gases will be "non-condensable" gases, i.e., gases that condense below 50 ° C. , suitably already below 40 ° C and preferably below 35 ° C.

8 78935 Turpentine decanter 29 is necessary to separate turpentine and methanol from the non-condensable gases it contains. Turpentine and methanol must not enter the water trap 19 or the washing tower 23, as they may form explosive mixtures with air. By using a decanter, better controllable conditions during combustion are obtained.

The purpose of the water trap 19 is to combine the odorous gases from the different stages of the apparatus and wash them before they are led to the white liquor scrubber tower 23 · It is not absolutely necessary to use a water trap as better control during disposal.

15 In the white liquor washing tower 23, the odorous gases are washed with white liquor obtained from the white liquor storage tank and then returned there. In this case, the major part of the sulfur in the odorous gases is separated off and returned to the cooking process in active form. This causes e.g.

20 reduction heat savings in the equipment recovery boiler. Further, the sulfidity of the process can be directed in the desired direction. One condition for the use of such a scrubber tower is, as mentioned above, that the methanol and turpentine have been removed from the gas. In practice, white-25 lye is used as the washing liquid in the washing tower because it is available in the process. However, it is precisely what causes the absorption of sulfur compounds that is the alkali content of the white liquor, and therefore, in principle, any alkaline solution is useful as a washing liquid for the absorption of sulfur compounds.

After washing with base, the odorous gases combined are introduced into a combustion chamber 44, which may be of conventional construction. It is preferable to use a flame combustion chamber. The combustion chamber is connected to a conventional steam-35 boiler, in order to recover the heat of combustion. The combustion uses the methanol separated in the decanter 29 as an auxiliary fuel, and in a well-functioning system under unchanged conditions hardly any additional 9 78935 auxiliary fuel is required. However, this is required when starting and stopping the equipment, as well as in the event of disturbances or faults in the process, and therefore the fuel oil tank 37 is connected to the combustion chamber. It is noted that in the manner according to the invention the consumption of fuel oil is considerably reduced. In previously used processes in which diluted odorous gases were burned in a shell boiler, the oil consumption was measured to be about 150 kg / h, but in the process of the present invention, the oil consumption can be limited to about 25 kg / h with the same production.

The separated turpentine is most often led to refining and sold as sulphate turpentine, but if this is not profitable, the turpentine can also be burned as an auxiliary fuel. This, of course, results in a further reduction in oil consumption. Turpentine and methanol can be combined in a known manner in the desired proportions before combustion.

The combustion of sulfur-containing odorous gases produces sulfur oxide, which can be rendered harmless by flue gas cleaning of 20 known types. Sometimes the flue gases are led to the flue gas cleaning of the factory recovery boiler, where they are subjected to normal cleaning. It is also possible to wash some or all of the flue gases with a base, usually a white-25 head. This allows the sulphidity of the process to be further adjusted.

The technical embodiment of the various devices of the apparatus according to the invention is of the usual type, and will be apparent to a person skilled in the art. Problems such as the selection and sizing of the materials of the devices at different stages can also be solved by a qualified professional.

The present invention provides many advantages over previously known methods and apparatus for capturing and rendering odorous gases harmless. By concentrating concentrated gases that have not been diluted with air or steam in a separate furnace, the risk of system malfunctions is reduced because conditions can be more easily controlled. The use of fuel oil 10,78935 as an auxiliary fuel can be greatly reduced due to the use of methanol and possibly also turpentine as an auxiliary fuel. The intake of turpentine in liquid form gives freedom to its use, as it 5 can either be incinerated or cleaned and sold. Thanks to the methanol separation tank, the combustion can also be adjusted more precisely.

Because sulfur can be recovered from odorous gases, the overall economy of the process is improved, while the sulphidity in the equipment can be better controlled. At the same time, the amount of gas to be burned is reduced, which makes it easier to ensure that the odorous gases escape as little as possible.

Finally, the method and apparatus 15 according to the invention also have the advantage that no new structures of individual parts are required, but that these parts are of the usual type. This means significant financial benefits.

Claims (9)

A method for collecting and decontaminating odorous gases from a process for producing a sulphate pulp production process, wherein the steam and condensate are evaporated with fresh steam during the cooking and publishing step of the process, after which the gas phase obtained in the evaporation is condensed, long 11c passage in two stages at atmospheric pressure to a final temperature below 40 ° C so that substantially all of the turpentine and methanol condense, the non-condensable gases liberated from the turpentine and methanol are washed with water and then with an alkaline solution, preferably then white liquor, , when combined with the odorous gases of the other 15 stages of the process, and then combusted, and that the condensate from the advanced condensation of the gas phase from the evaporation is divided into a liquid phase containing mainly turpentine, mainly prevented the methanol-containing liquid phase and the gas phase ik.si 20 and that this gas phase was combined with non-condensable gases from degenerate condensation before being washed and burned in two stages, and that methanol and possibly also turpentine replace the fuel oil with preferably with white liquor, wherein the white liquor obtained from the scrubbing of the gas phase and the flue gases is returned to the cooking stage so that the sulphidity in the apparatus can be adjusted. A method according to claim 1, wherein the cooking process is carried out in batch soup, characterized in that the steam from the blowing stage is condensed in a blasting steam accumulator: condensate is passed to this evaporation k with the new sound coming from the gas. 12,78935 Process according to Claim 1 or 2, characterized in that the gas phase from the evaporation is condensed in two stages to a final stage temperature which is more suitably below 35 ° C. 5 i). Process according to one of Claims 1 to 3, characterized in that the gases from the evaporative gas phase from the evaporative two-stage condensation, which are freed from substantially all of turpentine and methanol, are washed with water before washing 10 together with the odorous gases of the other stages of the process. An apparatus for collecting and rendering harmless the odorous gases of a process for a sulphate pulping process of a cellulosic pulp in a manner according to any one of claims 1 to 4, comprising a process from the cooking and blowing stage steam and a condensate evaporator, condenser evaporator and evaporator evaporator. for the combustion of odorous gases, characterized in that the condenser is dimensioned extensively in two stages to condense and cool the gas phase to a final stage temperature below 40 ° C and that the apparatus further comprises a decanter for dividing the condensate -25 gives mainly methanol, and as a gas phase, as well as for washing the uncondensed gas from the progressive condensation of the washing tower and the gas phase from the decanter with a base, more preferably white liquor, before y the combined gas phases are combusted, as well as equipment for introducing methanol and possibly also turpentine into the combustion of the gases, and for washing the flue gases leaving the combustion of the scrubber with a base, in particular white liquor. Apparatus according to Claim 5, characterized in that the two-stage condenser system is designed to cool the gas phase from the evaporation to a final temperature of less than 40 ° C at the end stage, and is suitable for virgin n below 35 ° C. 13 78935 Apparatus for use in a batch cooking process according to claim 5 or 6, characterized in that it also comprises a blower accumulator for collecting steam from the blowing step and lines for the laubdate 5 . Device according to one of Claims 5 to 7, characterized in that it also comprises water for combining and washing the gases from the continuous condensation of the lock, the gases from the blow-off steam accumulator and other odorous gases from other stages of the process with water before the combined gases are washed away. . Apparatus according to one of Claims 5 to 8, characterized in that the two stages of the condenser apparatus are divided into a pre-condenser, from which the return stream is returned to the evaporator, and a terminal to the evaporator. Device according to one of Claims 5 to 9, characterized in that it also comprises a storage tank for methanol and possibly also turpentine for balancing the flow to the combustion and a tank for auxiliary fuel to be added, and further lines for returning wastewater from the water trap and scrubber via the buffer tank. condensates Tue a. een. 78935