WO1990013342A1

WO1990013342A1 - Method of evaporating warm liquid

Info

Publication number: WO1990013342A1
Application number: PCT/FI1990/000129
Authority: WO
Inventors: Risto Saari
Original assignee: Inventio Oy
Priority date: 1989-05-10
Filing date: 1990-05-10
Publication date: 1990-11-15
Also published as: FI892239A; EP0491696A1; FI82609B; FI82609C; FI892239A0; PT93989A

Abstract

A multistage method of evaporating liquid, for example waste waters of industry, by means of the energy included in the liquid in such a way that the vapor (35, 36) vaporizing in each distillation stage (31) of a multistage distillation plant is in its entirety condensed in the heat exchange (37) in the next distillation stage (32), operating at a lower temperature, with the vaporizing liquid, whereby the output of each distillation stage is higher than that of the preceding and the distillation stages may be constructed as gradually enlarging.

Description

METHOD OF EVAPORATING WARM LIQUID

The present invention relates to a multistage distillation method of evaporating liquid by means of the heat energy in the liquid. The invention is suitable, for example, for concentration of warm waste waters and liquids of the industry.

The waste waters of the pulp industry have in many cases a very low content of waste material which are harmful to the environment, and in order to remove harmful substances it is necessary to clean great amounts of water. With conventional cleaning methods the process involves considerable costs, which again may very often prevent the cleaning of waters necessary for the protection of the environment.

Industrial processes, as is well known, consume beside raw material, also energy. Energy is discharged as waste energy from the process usually at a temperature level which is lower than the original level. Said waste energy is most usually combined in the waters being discharged from the process, whereby part of the waters may also contain waste material. The waste waters of the process are thus on one hand warm and on the other hand they should be cleaned.

Cleaning of waste waters would be more economic, if waste heat could be utilized. The problem is that waste heat is combined in the same waters, which should be cleaned, and it is difficult to "remove" it from the waters to enable it to operate as the heat source of a cleaning process. Warm waste water is in a way both the subject and the object of the cleaning process. Conventional distillation methods cannot use waste heat, because their "subject" is the primary energy being fed to the warm end of the process.

By using the present invention, most of the water containing waste heat can be evaporated by the waste heat included in the water itself. If the temperature of the waste water is, for example, 60^βC, it is possible to evaporate almost half of it and more than three fourths of it when the temperature is 80°C, if cooling water of about 15°C is available.

The invention is characterized in that the vapor generating in each distillation stage of a multistage distillation process will be in its entirety condensed with the vaporizing water in a heat exchange in the next distillation stage operating at a lower temperature. As it is appreciated from the description below, it is possible by this method to utilize the heat content of the liquid itself more efficiently than by the conventional methods.

The invention is described below and compared with the known methods by way of example, with reference to the accompanying drawings, which are all simple flow diagrams, and in which: Fig. 1 is a schematic illustration of a known multiple effect distillation method (MED);

Fig. 2 is a schematic diagram of another known method, a multistage flash method (MSF); and Fig. 3 is a schematic illustration of a distillation process in accordance with the invention.

Several different distillation methods have been developed for the requirements of the process industry and especially for the salt discharge from sea water, whereby the energy consumption of distillation has tended to be decreased. The most important of the used methods are the multiple effect distillation method (MED, Fig. 1) and the multistage flash method (MSF, Fig. 2).

In the MED illustration in Fig. 1 a part of the vapor 13 generating in one of the distillation stages 11 of a conventional multiple effect distillation plant is condensed in a preheater 16 of feed water in communication with each distillation stage. The rest of the remaining part 15 of the vapor is condensed in a heat exchanger 17 in heat exchange with the liquid being distilled, which is vaporized in the next distillation stage 12 operating at a lower temperature. Also the distillate 14 condensed in the distillation stage 11 partially vaporizes when flowing to the lower pressure of the next distillation stage 12 and also this vapor is condensed in the preheater 16 of the feeding water.

In this way the energy being released when the vaporizing liquid 19 and also the distillate 14 gradually cools, is used to heat the feeding water close to the operation temperature of the hottest distillation. The actual distillation takes place by means of the primary energy source 18 introduced to the first stage 10 when the primary energy is gradually transferred with the vaporizing and condensing vapor 15 to a lower temperature and finally when it is discharged to the condenser of the last stage.

Fig. 2 illustrates a multistage flash distillation (MSF) which is the most commonly used method for the discharge of salt from sea water. In a way, it is the extreme embodiment of the above-described method. Liquid 29 vaporizes by flashing, merely by means of the heat energy therein, when flowing from one stage 21 to the following stage 22, which has a lower pressure. Vapor is not in this case condensed at all in the heat exchange with the vaporizing liquid. Instead, all the steam 23 generating in a particular distillation stage 21 is condensed in a condenser 26 at this same stage.

Compared to the process in Fig. 1 the preheater 16, 26 has in effect become so large that the actual heat exchanger 17 has diminished away. The first distillation stage 20 has become the last heater of the feed water, which is heated with primary energy 28. The feed water flow has respectively increased so that in MED-distillation it is typically double compared with the output, whereas in MSF- distillation it is approximately ten times. If this water flow cools by vaporizing altogether, for example 56°C, then 10 % thereof will evaporate. The method in accordance with the invention, illustrated in Fig. 3 is the other extremity of the process of Fig. 1, in a way an opposite to the MSF-process of Fig. 2. In this case all the vapor 35 generating in a particular distillation stage 31 is condensed with the vaporizing liquid in a heat exchanger 37 in the next distillation stage operating at a lower temperature. There is neither a preheater 16 nor a condenser 26 of MSF-distillation. Also the expanding vapor 36 from the condensed distillate 33 is condensed in the heat exchanger 37 together with the vaporizing liquid in the next stage.

The first stage 30 may simply be an expansion chamber, in which part of the warm liquid 38 flowing to the process evaporates by flashing and the steam 40 being released is condensed in the next stage 31. The liquid 41 which is cooled in the vaporization flows to the next stage 31, in which due to a lower pressure prevailing in that stage, part of it vaporizes by further flashing to a lower temperature. Here it receives the condensation heat from the vapor 40 of the preceding stage 30, which heat vaporizes a respective amount of new vapor. The remaining liquid 39 flows further to the next stage, wherein the same process takes place again, etc. as is required. In the last stage, the vapor is condensed in a condenser 42, which may, for example, be a jet condenser.

In the described application of an embodiment of the invention, the entire vaporization takes place by means of the heat in the liquid being vaporized, no primary energy is required. The energy being released in the first stage and the energy being released in the cooling of liquid and distillate in each stage are used again the next stage. This means that the output of each stage is more than the production of the preceding stage. In the conventional MED-distillation, which is used as a comparison, the output of each stage is slightly less than the output of the preceding stage and in the MSF-distillation the outputs of each stage are approximately the same. If in the first stage of the process in accordance with the present invention, for example 1 % of the water is vaporizes, and the water thus cools about 6^βC, it vaporizess during its condensation another 1 % of the water in the second stage. If the temperature difference between the stages is 4.5°C, 0.8% is additionally vaporized in the second stage by flashing, in other words altogether 1.8 %. In the third stage a corresponding amount of vapor is vaporized and additionally by flashing 0.8 %, i.e. totally 2.6 %. In a 12-stage distillation plant the vaporization is in the order of the stages 1, 1.8, 2.6, 3.4, 4.2, 5.0, 5.8, 6.6, 7.4, 8.2, 9.0 and 9.8 %, i.e. totally 64.8 %. At the same time the water has cooled 54 %, in other words the same amount as in the example of the above described MSF-distillation. The evaporated amount of water is, however, 6.5 times the amount evaporated with the MSF.

The gradually increasing output may, in principle, be realized in two ways. A heat exchanger of each stage may be constructed larger than that of the preceding stage, whereby the temperature differences remain approximately the same. It is also possible to construct the heat exchangers approximately of the same size, as in the conventional distillation plants, whereby the temperature differences between the stages gradually increase, when the production increases. The first mentioned alternative is more economic, especially when the number of the stages increases.

If there is any extra energy available, it is possible to intensify the evaporation process in accordance with the invention by heating the liquid being evaporated to a temperature, which is higher than the original, before it is conveyed to the process. If, however, the temperature level is not high enough for that it is possible to vaporize liquid by means of extra energy in the first stage of the process and to add thus steam, generating there otherwise by flashing. Part of the liquid may also be refed from the last to the first distillation stage through one or more heat exchangers, which are heated by extra energy and which may thereby utilize the waste heat at different temperature levels.

Claims

1. A multistage distillation method of evaporating warm liquids, characterized in that vapor generating in each distillation stage is totally condensed in a heat exchange with the liquid vaporizing in the next distillation stage operating at a lower temperature.

2. A method of evaporating liquids in accordance with claim 1, characterized in that part of the waste liquid being discharged from the coldest distillation stage is returned to the hottest distillation stage through one or more subsequent heat exchangers operating at different temperature levels, which heat exchangers are heated by means of heat energy being fed from the outside to the process.

3. An apparatus for realizing the method in accordance with claims 1 or 2, characterized in that a heat exchanger (37) of each distillation stage (32) is larger than the heat exchanger (43) of the preceding distillation stage (31) operating at a higher temperature.