METHOD AND APPARATUS FOR SEPARATING NON-CONDENSABLE GASES FROM VAPOR

Abstract: The invention relates to a method and apparatus in which non-condensable gases are discharged from a condenser by bringing a mixture of the vapor and non-condensable gases, by means of cooling, into contact with an absorption medium maintained at a constant temperature, the vapor pressure of said absorption medium being lower than the vapor pressure in the gas mixture to be treated, whereby a portion of the vapor absorbs into the absorption medium, the partial pressure of the non-condensable gases in the mixture rises and the volume of gas to be removed from the process by some previously known pumping method decreases. An amount of vapor corresponding to the amount of absorbed vapor is vaporized from the absorption medium at a slightly higher temperature, and this vapor is condensed in, for example, the same condenser from which the non-condensable gases were removed.

Description: Method and apparatus for separating non-condensable gases from vapor The present invention relates to a method and apparatus for separating non-condensable gases from vapor by isother¬ mal absorption. It is thereby possible to intensify the deaeration of condensers and to decrease the amount of pumping energy required. Removal of non-condensable gases from condensing vapor is a significant problem in all vapor treating processes. The most common problem of this type is the removal of leakage air from the condensers of condensing power plants. Corre¬ spondingly, in sea water distillation plants, the removal of air dissolved in water is prerequisite for the operation of the process. The lower the pressure is at which the vapor is condensed, the more distinct the consequences of this problem are. According to the prior art, entrained non-condensable gases are separable from the vapor in the condenser by cooling the mixture of the .vapor and gases entrained therewith. Thus, the vapor pressure reduces while the partial pressure of the non-condensable gases rises. In this manner, the portion of vapor in the mixture is decreased, which reduces the total amount of gas to be pumped away from the underpressure in connection with deaeration. The condenser designed for cooling of vapor has to meet exacting demands to ensure that the flow velocity of the gas mixture is maintained sufficient in order to prevent the accumulation of non-condensable gases and to keep the flow losses small enough. If the gas mixture is cooled, for example, by 2°C from 25°C, the vapor pressure will reduce from 31.7 mbar to 28.1 mbar. If the total pressure at the same time reduces by 1 mbar because of flow losses, the partial pressure of the non-condensable gases will be 2.6 mbar. In this case, over ten times of the flow volume of vapor has to be removed with non-condensable gases. According to the invention, the partial pressure of the non- condensable gases can be raised by means of isothermal absorption. The gas mixture discharged from the condenser can be brought into contact with an absorption medium, in which medium the vapor pressure is low in comparison with the vapor pressure in the gas mixture. As such absorption mediums can serve aqueous solutions of several well soluble salts such as, for example, sodium hydroxide, lithium- bromide, lithium chloride, calcium chloride, potassium carbonate and potassium acetate. Thereby, vapor is absorbed from the gas mixture into the absorption medium and the partial pressure of the non-condensable gases rises corres¬ pondingly. When the non-condensable gases are removed from this state, only a small amount of vapor has to be pumped from the underpressure. An embodiment of the invention will be described, by way of example, in more detail in the following, with reference to the accompanying flow diagram. Vapor 16 of some process is condensed in a condenser 10, and the flow 17 of non- condensable gases of the vapor and the vapor entrained with said non-condensable gases then flow to an absorber 11, in which they come into contact with an absorption mediu . When absorbing to a solution, vapor gives its absorption heat to said solution, whereby the temperature of the solution tends to rise and the vapor pressure thereof tends to increase correspondingly. In order to prevent such from occurring, the absorption medium is cooled in an indirect heat transfer means 22 by a coolant such as, for example, cooling water, similarly to the coooling in the absorber of an absorption heat pump. Thus, the absorption of vapor take place substantially isothermally and the vapor pressure of the solution remains low. If the absorption medium is, for example, a solution of 30% sodium hydroxide at a temperature of 25°C, the vapor pressure thereof is about 14 mbar. When water vapor is absorbed into this solution at the temperature of 25 β C and the flow loss is assumed to be 1 mbar, which is the same as in the previous example, the partial pressure of non- condensable gases will rise to 16.7 mbar. Only 0.8 of the flow volume of vapor has to be removed with non-conden¬ sable gases in this case. This essentially reduced gas flow 18 is removed from the absorber by some previously known pumping method 14. The absorption medium is diluted when vapor absorbs there¬ into. It is concentrated by vaporizing a corresponding amount of vapor from this absorption medium at a slightly higher temperature. For this purpose, the absorption medium is pumped by pump 15 into the vapor generator 12, in which vapor generator an amount of vapor corresponding to the absorbed amount of water is vaporized from said absorption medium at a higher temperature by heating the medium through a heat exchanger 21 in a manner similar to that used in the vapor generator of an absorption heat pump. The released vapor can be condensed in a separate condenser 13. The temperature can also be so regulated that the pressure of the releasing vapor equals that of the original, absorbed vapor, in which case this revaporized vapor 20 can be condensed in the same condenser 10 from which the mixture of the vapor and non-condensable gases was removed. In order to provide a sufficient heat and material transfer surface for the vapor and the absorption medium and, on the other hand, to ensure effective cooling and heating of the absorption medium, it is possible to construct both the absorber and the generator of horizontal tubes, on the outer surface whereof the absorption medium flows in the form of a thin film.

Claims: We claim: 1. A method of separating non-condensable gases from vapor, characterized in that a mixture of the non-condensable gases and vapor is brought into contact with an absorption medium, the vapor pressure of which medium is lower than the vapor pressure of the gas mixture and the temperature of which medium is, by means of indirect cooling, kept approximately constant, whereby a portion of the vapor is absorbed into said absorption medium and the remaining mixture of gases, containing more non-condensable gases than the original mixture of gases and vapor, is pumped away from the process. 2. A method as claimed in claim 1, characterized in that an amount of vapor, corresponding to the amount of vapor absorbed to the absorption medium, is vaporized therefrom at a temperature higher than the absorption temperature and in that this vapor is condensed in either a separate condenser or the same condenser from which the mixture of non-condensable gases and vapor was removed. 3. An apparatus for applying the method as claimed in claim 1 or 2, for removal of non-condensable gases from a condenser, characterized in that the apparatus comprises an absorber, vapor generator and pumping means, which are so disposed that the gas space of the absorber is, on one hand, in communication with the gas discharge conduit of the condenser and, on the other hand, in communication with said pumping means, and the gas space of the vapor generator is in communication with the vapor space of said condenser or a separate condenser. 4. An apparatus as claimed in claim 3, characterized in that the absorber and the vapor generator include heat exchangers constructed of substantially horizontal tubes, on the surface of which the absorption medium is made to flow in the form of a thin film.

Inventors: SAARI RISTO, SAARI, RISTO
Applicants: INVENTIO OY
Application year: 1989
Main classification: B01D53
IPC codes: B01D5, B01D53