SE518647C2 - Heat exchanger for recovering heat from contaminated steam generated by e.g. steam dryer for timber, effluent heating plant or black liquor cooker - Google Patents

Abstract

Steam is supplied to the heat exchanger in a downwards direction so that gas phase enriched with volatiles can be removed from the top of the heat exchanger, whilst the purified condensate is removed from the bottom. A process for recovering thermal energy from contaminated steam whilst simultaneously separating volatile organic materials comprises introducing the steam into a heat exchanger and supplying a heat-absorbing medium to this heat exchanger, and then removing this medium along with the heat absorbed by it. Steam is supplied to the heat exchanger in a downwards direction, so that it condenses as it travels upwards, whilst the volatiles are evaporated from the condensated steam, which runs downwards. A gas phase enriched with the evaporated volatiles is removed from the top of the heat exchanger, whilst condensate is removed from the bottom. An Independent claim is also included for the process apparatus used.

Description

It must be possible to be directly subjected to conventional treatment, including biological treatment.

Another example is water evaporation in, for example, black liquor evaporation in the pulp and paper industry.

The process can there replace or supplement the stripping of terpenes made by the condensate in a so-called condensate stripper.

Another application of the process is in the evaporation of wastewater, for example spent oil emulsions where volatile components such as ammonia, ethanol, methanol, acetic acid and the like together with volatile solvents tend to contaminate the condensates from the evaporation. In several cases, the heat exchanger is arranged in the evaporator for heat recovery by so-called mechanical steam compression. The evaporated steam from the wastewater is then compressed in one or more fans or compressors to a higher pressure. The higher pressure results in a higher condensing temperature and allows the steam to condense to heat the evaporator. If the steam enters the bottom of the heat exchanger, the volatile pollutants can bleed out concentrated at the top while the condensate flows out purified from the bottom.

Another object of the invention is to provide a device for carrying out the method.

To achieve these objects, the method and the device have been given the features set out in the appended claims.

The heat exchanger which is used in the method according to the invention or is part of the device according to the invention is of the type which has one or more heat exchanger packages comprising, for example, tubes or lamellae of conventional construction, and of jacket type, ie the heat exchanger has an outer casing or shell. In the usual way, the heat exchanger has an inlet and an outlet for medium, such as district heating water, feed water, etc. which flow through the heat exchanger for absorbing heat. According to a preferred embodiment, the height of the heat exchanger is greater than its width and the packages are arranged 518 647 3 c | o o n u ao concentric around the vertical axis of the heat exchanger. When the contaminated steam is led into the bottom of the heat exchanger, a rectification takes place, ie enrichment in the same way as for distillation. Water vapor condenses on its way upwards while volatile constituents evaporate from the condensate, which flows downwards. The presence of steam also means that certain components are distilled with water vapor, ie their volatility increases in the presence of water vapor. At the top of the heat exchanger is a gas phase, which is enriched in volatile constituents and at the bottom, a condensate, which is poor in similar components. From the top, the gas / vapor mixture can be condensed in a separate condenser or led to combustion. In order to reinforce the process described above, according to a preferred embodiment, elements which increase the effective contact surface between gas and liquid, for example Raschig rings, are arranged in the respective space between the outer wall and the outer limiting surface of the packages. This process essentially contributes to the achievement of the feature according to the invention, namely the obtaining of a condensate with such purity that it can be fed directly into a wastewater treatment plant.

According to a preferred embodiment of the invention, when the steam is introduced into the heat exchanger, it has a pressure of 1-5 bar and a temperature of 110-180 ° C.

The invention will be described in more detail below with reference to a preferred embodiment with simultaneous reference to the accompanying drawing, which is a schematic view of a preferred embodiment of a device according to the invention. Steam from a fuel dryer with a pressure of about 3-4 bar and with a temperature of 140-150 ° C is fed to the bottom of the jacket side of the heat exchanger. At the same time, a heat-absorbing medium, in this case water, is introduced through an inlet (not shown) and allowed to flow through the tubes or lamellae in the heat exchanger packages and taken out through an outlet (not shown) for use as district heating water or feed water. The steam introduced above condenses on its way upwards on the inner surfaces of the jacket 1 3 54 7 Éï * 'z as well as on the surfaces of the heat exchanger packages while more volatile constituents evaporate from the condensate, which flows downwards.

This condensate accumulates on the bottom of the heat exchanger and is extracted in the lower part thereof. The condensate is cooled before it is fed into a conventional municipal wastewater treatment plant. The gas phase enriched in the top of the heat exchanger, which contains volatile constituents, can either be condensed in a separate condenser as shown in the drawing, or led to combustion (not shown).

According to the invention, the essential advantage has surprisingly been obtained by combining distillation-condensation-heat exchange that in one and the same process a condensate is obtained which is of such purity that a water purification plant can be supplied directly, and heat recovery from the polluted steam. In other words, the condensate meets the specifications required for such a plant.

Below are the results from nitrification inhibition tests with nitrifying sludge and test water performed according to modified ISO 9509-1989 (E).

Sample Sample marking: MG 11331 (condensate taken at the bottom of the heat exchanger in accordance with the invention) and MG 11332 (sample taken from 'top condensate').

The samples were stored refrigerated until the time of analysis.

Materials and methods Test method The experiment was performed at room temperature. The method is simplified, as only two sample concentrations were examined.

In short, the experimental principle means that in a number of cylinders, different volumes of sample water and synthetic medium are mixed. The mixtures are diluted to 125 ml with distilled water. Then 125 ml of nitrifying sludge are added and the cylinders are incubated at room temperature for about 4 hours during aeration. Samples for analysis of the sum of nitrite and nitrate are taken from each cylinder both before and after the incubation. The amount of nitrite and nitrate formed in the cylinders containing the sample is compared. with the amount formed in cylinders, which did not contain any sample.

Sample preparation The samples were pH adjusted to about 7.6 before the experiment.

The following test concentrations (vol%) of the sample were examined: 20 and 40.

Graft The nitrifying sludge came from Klagshamns Reningsverk and was aerated until it was used.

Analysis After filtration (Whatman GF / A), the content of the filtrate was analyzed with respect to nitrite and nitrate nitrogen according to Swedish standard SS 028133-2.

Results Table 1 Form of nitrite and nitrate nitrogen formed. Sample compared to blank and inhibitor.

Sample labeling Sample (vol%) Oxide N (mg / l) Inhibition (%) Blind sample 0 29.9 * - MG 11331 20 24.7 17 MG 11331 40 17.3 ** 42 MG 11332 20 0.0 100 MG 11332 40 0.45 ** 98 Inhib *** - 0 100 * Mean value of three blank samples ** Mean value of duplicate samples *** Reference inhibitor: Allyltiourea 8 mg / l The content of suspended substance in the experiment was 1.52 g / l.

The specific nitrification rate of the sludge was measured to be 7.9 mg / N / (g.h). Due to the high nitrification rate in the sludge, the test time was shortened to 2.5 hours.

The results show that samples labeled MG 11331 showed 17% inhibition at 20% admixture and 42% inhibition at 40% admixture. For municipal treatment, a limit of 20% inhibition is stated at 20% admixture, which means that the condensate obtained according to the invention can be final treated in a municipal treatment plant.

On the other hand, samples labeled MG 1.313 caused a very strong inhibition of nitrification against the tested inoculum. At vol% sample mixture, the inhibition was total (100%).

Analysis of the above samples especially with regard to COD (chemical oxygen demand) and BOD (biochemical oxygen demand) was performed: Sample: MG 11331 Analysis quantity Result Unit KRUT code pH 3.6 PH-25 Conductivity 10.3 mS / m COND -25 Biochemical oxygen demand- 193 mg / l BOD7-NAE ning, BOD7 Chemical oxygen demand, 400 mg / 1 CODCR-NH chromate Suspended substances 51 mg / 1 STR-STG Sample: MG 11332 Analysis quantity Result Unit KRUT code pH 3.9 PH -25 Conductivity 18.1 mS / m COND-25 Biochemical oxygen demand- 3400 mg / l BOD7-NAE, BOD7 Chemical oxygen demand, 13700 mg / l CODCR-NH chromate Suspended substances 50 mg / l STR-STG Acceptable values for condensate for biological purification is: COD 1000-5000 mg / l BOD 800-3000 mg / l Conclusion: The bottom condensate meets the criteria above.

Claims (8)

ÅBO an: ~ w sb.o av: u 10 15 20 25 30 35 518 647 o: en »n PATENTKRAV A process for recovering heat energy from polluted steam while simultaneously separating volatile organic substances, in which the steam is introduced into a heat exchanger and a heat-absorbing medium is introduced into the heat exchanger and heat-absorbed medium is withdrawn therefrom, characterized in that the steam is in the heat exchanger, the steam condensing on its way upwards while the volatile substances are evaporated from the condensate, which flows downwards, and a gas phase enriched in said substances is taken out from the top of the heat exchanger and the condensate poor on said substances is taken out from the bottom of the heat exchanger. 2. A method according to claim 1, characterized in that the polluted steam is steam from a dryer for thermal treatment of wood, preferably wood fuel. 3. A method according to claim 1 or 2, characterized in that the heat exchanger is jacketed and that the steam is introduced at the jacket side. Apparatus for carrying out the process according to claim 1 for recovering heat energy from contaminated steam while simultaneously separating volatile organic substances comprising a heat exchanger having an inlet for a heat absorbing medium and an outlet for heat absorbing medium, an inlet for the contaminated the steam, an outlet for condensate and an outlet for the volatiles, characterized in that the inlet for the polluted steam is arranged at the bottom of the heat exchanger and that the outlet for the condensate is arranged at the bottom of the heat exchanger. Device according to claim 4, characterized in that the heat exchanger is jacketed and that the inlet for steam is arranged at the jacket side. Device according to claim 4 or 5, characterized in that the polluted steam is steam from »J 518 647 o u. n n o | oo a dryer for thermal treatment of wood, preferably wood fuel. 7. Device according to any one of claims 4-6, characterized in that the introduced steam has a pressure of 1-5 bar and a temperature of 110-180 ° C. Device according to one of Claims 4 to 7, characterized in that the heat exchanger is included in an evaporator system.