US2760919A

US2760919A - Vapor-compression distillation method and apparatus

Info

Publication number: US2760919A
Application number: US378141A
Authority: US
Inventors: Jr Allen Latham
Original assignee: BADGER Manufacturing Co
Current assignee: BADGER MANUFACTURING Co
Priority date: 1953-09-02
Filing date: 1953-09-02
Publication date: 1956-08-28
Anticipated expiration: 1973-08-28

Description

Aug. 28, 1956 A. LATHAM, JR-

VAPOR-COMPRESSION DISTILLATION METHOD AND APPARATUS Filed Sept. 2, 1953 w mowmmmmiou w I l I NOL Q Dumu muuz mzuxm Emu.

MUFQUIMMA.

Wmwww I INVENTOR. ALLE N LATHAM, JR.

HTTORNEYS linited States Patent VAPOR-COMPRESSION DISTILLATION METHOD AND APPARATUS Allen Latham, Jr., Jamaica Plain, Mass, assignor, by

mesne asignments, to The Badger Manufacturing Cornpatltly, Cambridge, Mass., a corporation of Massachu se s Application September 2, 1953, Serial No. 378,141

11 Claims. (Cl. 202-75) This invention relates to vapor-compression distillation method and apparatus. In vapor-compression distillation, vapor separated from a boiling solution is compressed to a pressure at which its condensing temperature is above the boiling point of the boiling solution and is condensed in out-of-contact heat exchange relation with the boiling solution, thereby returning to the distillation zone the latent heat of vaporation of the separated vapor so that it is utilized and conserved in the system in the boiling of additional solution.

Particularly in connection with the distillation of aqueous solutions, as for example sea water, the entering solution contains air or other non-condensable gases which during continuance of the distillation become separated from the boiling solution and which, being noncondensable at the temperatures prevailing in the system, do not condense along with the vapor. As a result, there is a tendency for the non-condensable gases to accumulate in the system to such an extent-as to seriously affect its efficiency.

It is an object of this invention to improve upon method and apparatus for vapor-compression distillation as regards the venting of non-condensable gases therefrom. Heretofore, if any means at all has been provided for venting non-condensable gases, such means has been in the form of a vent line whichis in communication with the condenser heat exchanger wherein the compressed vapor condenses and through which the non-condensable gases are continuously vented along with some of the vapor.

According to a principal feature of this invention, the non-condensable gases thatbecome separated from the solution undergoing distillation,.instead of being vented from the system continuously, are caused to be gradually accumulated in the system during periods of predetermined substantial duration and by alternating these periods of gas accumulation with brief periods of predetermined duration during which the non-condensable gases accumulated during the preceding period of gas accumulation are vented as a sudden burst. For effecting such intermittent venting, vent means is employed which comprises a vent line through which the non-condensable gases pass in being vented from the condenser heat exchanger of the system, a valve for controllingthe vent line, and means for opening and closing the valve at predetermined intervals for periods of predetermined duration for effecting the alternate accumulation of the non-condensable gases and their release as a sudden burst following each period of gas accumulation.

By the employment of method and means whereby the non-condensable gases are vented intermittently according to this invention from a distillation system of the vapor compression type, it has been found that considerable improvement in overall efiiciency is made possi ble in that the non-condensable gases may be effectively removed from the system with a very substantial decrease in the loss of steam or other vapor. Loss of uncondensed steam or other vapor from the system seriously Patented Aug. 28, 1956 ice affects its efficiency, both because there is a corresponding loss of yield of recovered distillate and because to the extent that the compressed vapor is not condensed in the condenser heat exchanger its heat of vaporization is lost from the system.

In addition, the venting of the non-condensable gases in the form of intermittent bursts counteracts the occurrence of pockets or other stagnant zones of noncondensable gases and the venting that is accomplished is both effective and reliable. Moreover, the venting of the non-condensable gases intermittently has the aforesaid advantages without'involving any mitigating drawbacks.

A further feature of this invention relates to method steps and apparatus whereby the compressed vapors are caused to become condensed in such a way in relation to the removal of the resulting condensate that the noncondensable gases that become separated from the condensate during the condensation of compressed vapor is kept progressively in motion toward the venting zone from which the non-condensable gases are vented, thereby further minimizing the occurrence of the non-condensable gases in pockets or stagnant zones. Thus, instead of employing a common condenser heat exchanger to which the compressed vapor is directed for distribution and condensation substantially simultaneously throughout, the compressed vapor is directed into what in eifect is a series of small condenser heat exchangers or condensation chambers, through which the vapor is successively passed with only partial condensation in the individual chambers.

Moreover, the means for .drawing otr" condensate from the several chambers of the system is such that neither vapor nor non-condensable gases are removed with the condensate and the non-condensable' gases are kept in motion by being caused to travelwith' the compressed vapor as it is passedifrom chamber to chamber of the series and into the. last chamber of the series from which the non-condensable gases are directed through the vent line by which they are vented from the system as aforesaid. Preferably thenon-condensable gases are directed from the last condensation chamber of the series to a distillate level-control container or an equivalent container or chamber from which they aretaken oh by the vent line.

Further objects, features and advantages of this invention will be apparent in connection with the following more detailed description of this invention in connection with the accompanying drawing which illustrates schematically a typical embodiment of this invention that is comprised in distillation method and apparatus of the vapor compression type.

Referring to the apparatus shown in the drawing, solution to be distilled is heated in the evaporator chamber 1 to separate vapor therefrom by boiling. The vapor resulting from the boiling of thesolution is passed through the liquid-vapor separator 2 and is directed by the low pressure'fline 3 into the compressor 4, which may be of any type adapted to' eifect' the compression of the vapor to a pressurei'at yv ich the condensing temperature of the vapor is'above'the boiling point of the solution undergoing distillation andwhic'h may be operated from any suitable source of power (not shown). The vapor which has been compressed by the compressor 4 is directed by the high pressure vapor line 5 to the condenser heat exchanger means which is indicated generally by the reference character 6 and which will be described more in detail hereinbelow. The condensate formed in the condenser heat exchanger 6 is directed by the draw-oil lines 7, the header 8 and the line 9 to the condensate-collecting and levelcontrol container 10. The condensate is taken from the container 10 by the line'll, which is controlled by the float valve 12 for maintaining constant the level of the condensate in thecontainer 10. Before it leaves the systern the not condensate .is directed through the preheater heat exchanger, which is indicated generally by the reference character 13 and in which the solution that is being fed into the evaporator chamber by the :feed line 14 is preheated.

Hot concentrated solution is taken from the evaporator chamber 1 by the draw-off line 15, the upper end-of which determines the normal liquid level that .is maintained in the evaporator chamber. The line 15 directs the hot concentrated solution into the concentrated solution-collecting container 16 into which the solution that collects in the liquid-vapor separator 2 likewise is directed by the line 17. The hot concentrated solution is taken from the container by the line 18, which is controlled by the float valve .19 for maintaining a constant level in the container 16. Before leaving the system by the line 18 the concentrated solution is passed through the ,preheater heat exchanger 13 for transferring its heat to the incoming feed.

In the system above described the condensation of the compressed vapor in the condenser heat exchanger .supplies heat for boiling the solution in the evaporator chamber 1 because of the heat of vaporization that is returned to the system and in some measure also because of its sensible heat content as it .leaves the compressor. If desired, auxiliary heating means (not shown) maybe employed in order to compensate for heat losses from the system and heat losses ,may also be minimized by covering the evaporator chamber with a layer of thermal insulation material (not shown);

Referring to the condenser heat exchanger 6 that is comprised in the vapor-compression distillation system, it is to be noted that instead of being in the form of a common unit into which compressed vapor is directed, the

pressed vapor is shown as entering .the chamber 6a. The

chambers are connected in series by thevapor header means 2% so as to cause the vapor to pass successively through the chambers 6a, 6'b, 6c and 6x with partial condensation thereof in each of the chambers. While in the drawing four chambers have been shown connected in series for successive passage of vapor therethrough, .a greater or lesser number could be employed as indicated by the break shown in the drawings. For example, va typical preferred construction is to provide ten of (the chambers in a single series.

Each of the chambers 6a, 6b and 6c is separately drained by the lines 7 to the header 8 from which condensate is taken to the condensate-coilecting container 10 by the line 9 after having first passed the liquid seal trap 21 which maintains the liquid seal legs 22 in the lines 7 for efiectively preventing escape of non-condensable gases or vapor through the lines 7.

During distillation non-condensable gases tend to accumulate in the system .as a result of the boiI ll .of the solution. While the non-condensable material may comprise non-condensable vapor, the non-condensable material is referred to elsewhere herein and in the claims as non-condensable gases. The non-condensable gases tend to accumulate in the chamber comprised in the condenser heat exchanger 6. Because none of the noncondensable gases and none of the'vapor can es ape through the lines '7 and because the compressed vapor is passed successively through the chambers, the compressed vapors are kept moving at a velocity that is .sufiicientlyhigh to keep non-condensable gases moving to the last chamber 6x of the series with avoidance of pockets or stagnant zones in which the non-condensable gases might collect with resultant impairment of the heat exchange elficiency of the condenser heat exchanger. The last chamber 6x of the series is directly connected; at'ithe bottom to the line '9 by the line '23 so as not to be affected by the liquid seal for the header 18, thereby providing free discharge 4 from the chamber 6x of both the non-condensable gases and the condensate formed therein.

While only one series of chambers has been shown disposed in only a part of the evaporator chamber 1, it is to be understood that this has been done for the purpose of simplifying the drawings and that ordinarily the evaporator chamber has the condenser heatexchanger chambers distributed more uniformly throughout the body of the solution that is maintained in the evaporator chamber '1. In such case there may -be additional series arrangements of condensation chambers. For example, there may be four groups with each group consisting of a series of ten chambers and with the first chamber of each series connected to the compressor. All chambers of the several series, except for the last chamber, could in such case be drained to the header 8 so as to be conmonly affected by the liquid seal 21. The last chamber in each series could either separately drain into the condensate-collecting container 10 as shown or a common vent header therefor could be provided for directing the distillate and non-condensable gases therefrom into the container 10. The condensation chambers comprised in the condenser heat exchanger 6 may be'of different size as desired. However, by way of further concrete illustration, for providing a vapor-compression distillation .unit having a capacity of about 300 gallons .per day when employing the forty condensation chambers arranged in four groups of ten each, each of the chambers could be in the "form of an elliptical chamber about inch thick whose vertical and horizontal dimensions are about 9 inches and 5 inches respectively.

The non-condensable gases which are carried to the condensate-collectingcontainer 10 are vented therefrom through the line 24 in which there is the valve 25, the opening and closing of which at stated intervals is effected automatically. For example, the valve 25 may be electrically actuated to open it for the venting of gases through the line 24 and outof the system lay-energiz ng a solenoid, the valve normally being kept closed as by a spring or the like. Suitable solenoid or other valves that are actuatable automatically at stated intervals are well known. For control-ling the actuation of the valve 25 a timing device 26 such as an electric clock may be employed which is adapted to close and open the switch 27 that controls the electric circuit 28 from an electric current source (not shown) for energizing the solenoid 29 when the switch 27 closes the circuit.

In the case of a'unit of the capacity and construction above exemplified which maintains :a gauge measure within the vaporization chamber of about 15 pounds per square inch and a gauge pressure of about 27 pounds per square inch within the condenser heat exchange means during distillation of potable water from sea water, typical venting of the non-condensable gases from the system consists in setting the timing device '26 so that the valve '25 remains closed for intervals of ten minutes between which the valve '25 is opened for an interval of five seconds, this cycle "being regularly repeated during continuance of the distillation. Of course, for operation in the manner described the vent line employed as well as the valve that controls it must be of sufiicient capacity to permit the escape in only a few seconds time of .noncondensable gases that "have accumulated over a much longer period.

-It is apparent that in the operationabove described the non-condensable gases are permitted "to gradually accumulate by preventing their escape and then are vented in a sudden burst. By venting the non-condensable .gases in this way, it'has been found that the non-condensable gases are effectively vented and that in so doing the amount of vapor that is carried therewith is very substantially reduced. Thus, in an operation of the character above exemplified the heat loss resulting from entrainment of vapor with vented non-condensable gases can be cutto only about 10% of'that which is required in order to accomplish the venting of non-condensable'gase's'when they are vented by the usual continuous flow method. This constitutes a substantial saving in the amount of heat that is lost from the system as an incident to obtaining efiective venting of the non-condensable gases.

When the venting is accomplished so as to occur in bursts after stated intervals of accumulation of the noncondensable gases, the flow of the non-condensable gases when the venting occurs is of such suddenness and volume as to stir up any pockets of non-condensable gas or other stagnant zones with resultant more effective removal of the non-condensable gases and in practice it has been found that venting of the non-condensable gases according to the invention results in very reliable and efiective venting. It also has been found in practice that except for the above described advantages-and improvements, the alternate gradual build-up and sudden venting of the non-condensable gases has no perceptible efiect in the V distillation operation.

The practice of this invention is of especial advantage in the case of vapor-compression distillation units which are of medium to small capacity wherein the rates of fluid movement are of such moderation as to be conducive to difficulties because of the occurrence of dead pockets or zones of stagnant non-condensable gases when the conventional practice of continuously venting the noncondensable gases is employed. A unit having the capac ity and construction above described is in this category.

However, this invention may be employed with advantages of the'character above described regardlessv of the size and capacity of the unit. The duration of the intervals during which the non-condensable gasesare alternately accumulated and suddenly released may be varied,

tial benefits, the period of gradual accumulation of the non-condensables is sixty times the venting period. On the other hand, it would still be consistent with. obtaining good results to extend the period for accumulation of condensables to about minutes. If the venting period of 5 seconds is employed under such conditions, the accumulation period is about 240 times the venting period, which is about the upper limit for this relationship if the distillation conditions are to remain substantially uniform. For similar reasons, the period during which non-condensables are permitted to accumulate ordinarily is not more than about minutes. The period of ventingpreferably is brief so that non-condensables will emerge in a sudden burst with a flow great enough to stir up stagnant pockets in regions affected by the opening of the valve in the vent line while at the same time avoiding undue loss of vapor. A burst of about 5 to about 10 seconds ordinarily is suitable for this purpose, the intervening periods during which the non-condensables are permitted to accumulate being about 60 to about 240 times the duration of the bursts.

While this invention has been described as embodied and employed with one type of vapor-compression distillation unit, it is to be understood that this has' been done for purposes of exemplification and that the elements of vapor-compression distillation apparatus in connection with which this invention may be employed may take many different forms. Thus, while a preferred type of condenser heat exhanger for use in the evaporator chamber has been described and illustrated, other condenser heat exchanger constructions may be used. However, in such case the advantages which result from the combination of the condenser heat exchanger means shown and described hereinabove with the venting means of this invention would not be realized; and in such 6 case, or otherwise, the Vent line means could be com nected directly with an appropriate zone on the interior of the condenser heat exchanger. In the operation above described by Way of example, the pressures maintained within the evaporator chamber'and Within the condenser heat exchanger are such that concentrated solution and condensate may be removed without the employment of pump means to do so. However, particularly if the pressures prevailing in the system are lower, or in the case of units of larger size, pump means may be employed. Likewise, it is not essential to utilize the condensate-collecting

containers

10 and 16 or the level control means contained therein. It is also apparent that other means than that shown and described may be employed for effecting intermittent venting of the noncondensable gases at regularly recurring intervals of predetermined duration.

I claim:

1. In a method of distillation wherein vapor is evolved from a solution in a vaporization zone, the evolved vapor is compressed to a pressure at which its condensing temperature is substantially greater than the boiling point of said solution, the compressed vapor is condensed in out-of-contact heat exchange relation with solution in said vaporization zone, non-condensable gases are separated from the condensate resulting from condensation of said vapor, said condensate is directed out of the system, concentrated solution is directed out of the said vaporization zone and out of the system, and fresh solution to be distilled is fed into said vaporization comprising causing the non-condensable gases to become gradually accumulated in the system during periodsof predetermined substantial duration and alternating said periods of gas accumulation with brief periods of predetermined duration during which non-condensable gases accumulated during the preceding period or" gas accumulation are vented as a sudden burst.

2. The method according to claim 1 wherein the compressed vapor is moved successively through a series of chambers with deceleration in said chambers and acceleration during passage from one chamber to the next during condensation thereof in out-of-contact heat exchange relation with said solution toward a venting zone so as to carry said non-condensable gases with said compressed vapors during condensation thereof to said zone, said non-condensable gases being vented from said zone.

3. In a method of distillation wherein vapor is evolved from a solution in a vaporization zone, the evolved vapor is compressed to a pressure at which its condensing temperature is substantially greater than the boiling point of said solution, the compressed vapor is condensed in out-of-contact heat exchange relation with solution in said vaporization Zone, non-condensable gases are separated from the condensate resulting from condensation of said vapor, said condensate is directed out of the system concentrated solution is directed out of the said vaporization zone and out of the system, and fresh solution to be distilled is fed into said vaporization zone; the venting of said non-condensable gases from the system during continuance of distillation by steps comprising alternately accumulating and venting non-condensable gases by alternately closing and opening a valve in a vent line at predetermined regularly timed intervals, the duration of the intervals during which said valve is closed for causing accumulation of said non-condensable gases being at least about 60 times the duration of the intervals during which said valve is open for permitting the ventingof said non-condensable gases and not being longer than about 30 minutes.

4. The method of claim 3 wherein the duration of the interval during which said valve is closed for causing accumulation of said non-condensable gases is from about 5 to about 20 minutes and the duration of the intervals area during which said valve is open for permitting the vent ing ,of said non-condensable gases is at least about seconds. 7

5. The method of claim 3 wherein the period during which said non-condensable gases is vented sis of the order of 5 to 10 seconds.

6. A method of distillation which comprises directing vapor evolved from a solution in a vaporization zone to a compressor for compression therein, directing vapor compressed by said compressor into .out-of-contaot heat exchange relation with solution in said vaporization chamber in a series of condensation .chambers through which said vapor is directed sequentially with partial condensation of said vapor and separation of non-.condensable gases from condensate in each of said chambers, directing condensate from each of said chambers through a liquid seal that prevents escape of non-condensable gases from said condensation chambers except with vapor that is passed from one chamber to the next in said series, thereby moving said gases with said vapors through said chambers to the last chamber of the series, directing solution to be distilled into said vaporization 'zone, wit-hdrawing concentrated solution from said vaporization zone, and venting non-condensable gases from said last chamber of said series intermittently in regularly timed bursts of about 5 to about 10 seconds duration between intervening periods of the order of about 60 to about 240 times the duration of said bursts.

7. Distillation apparatus comprising an evaporator chamber, a compressor, means for directing-vapor from said evaporator chamber to said compressor for compression therein, a condenser heat exchanger within said evaporator chamber adapted to maintain a "fluid therein in out-of-contact heat exchange relation with solution in said evaporator chamber, means for directing compressed vapor from said compressor into said condenser heat exchanger for condensation therein, means for directing condensate'resulting from condensation of vapor in said condenser heat exchanger from said condenser heat exchanger and out of the system, means for withdrawing concentrated solution from said evaporator chamber, means for introducing solution to be distilled into said evaporator chamber, and means for venting non-condensable gases from said condenser heat exchanger; said means for venting non-condensable gases comprising a vent line through which said gases pass in being vented from said condenser heat exchanger, valve means for controlling the venting of said gases through said vent line and means for opening and closing said valve means at predetermined intervals for periods of predetermined duration.

8. Distillation apparatus comprising an evaporator chamber, a compressor, means for directing vapor -from said evaporator chamber to said compressor for compression therein, a condenser heat exchanger within said evaporator chamber adapted to maintain a fluid therein in out-of-contact heat exchange relation with solution in said evaporator chamber, means for directing compressed vapor from said compressor into said condenser heat exchanger for condensation therein, means for directing condensate resulting from condensation of vapor in said condenser heat exchanger from said condenser heat exchanger and out of the system, means for withdrawing concentrated solution from said evaporator chamber, means for introducing solution to 'be distilled into said evaporator chamber, and means for venting non-condensahle gases from said condenser heat exchanger; said means for venting non-condensable gases comprising a vent line through which said gases pass in being vented from said condenser heat-exchanger, valve means in said vent line that is responsive to electrical actuation for eifectingsudden opening and sudden closing of said valve means, a timing device, electric circuit means operatively connected to said valve means, and switch means actuated by said timing device for controlling said electric circuit :means whereby responsive to timing automatically controlled by said timing device said valve means normally maintained'closed and is opened .at regular internals for periods .of predetermined duration, said vent dine and said valve means being adapted. when saidvalve means is suddenly opened to vent non oondensable gases .therethrough .as a sudden burst.

9. .-Dist-illation apparatus comprising an evaporator chamber, a compressor, means for directing vapor from said xevaporator chamber to .said compressor for com- ,pression therein, a condenser heat exchanger within said evaporator chamber adapted to maintain a fluid therein in out-.oflcontact :heat exchange relation with solution in said evaporator chamber, means for directing compressed vapor firom said compressor into said condenser heat exchanger ior condensation therein, means for di- .recting condensate resulting from condensation of vapor in said condenser heat exchanger from .said condenser heat exchanger and out of the system, means for withdrawing concentrated solution from said evaporator chamber, means ,for introducing solution to be distilled into said ievaporator c'hamber, and means for venting inon-condensable gases itrom said condenser heat .exchanger; .said condenser heat exchanger comprising a series .ofchambers adapted :for condensation of vapor therein .and means .for directing vapor directed from said compressor to said condenser heat exchanger successively throughsaid series .of chambers; said means for directing condensate from said condenser heat exchanger comprising draw-off line means for Withdrawing condensate frorn each ot the chambers in said series and means for maintaining aliquid seal in said draw-ofi line means that eifective .to prevent escape of non-condens- Table gases orvapsr -from said chambers of said series vthrough. sa'id draw -ofi line' means except for the chamber at theendjof said seriesremote from the chamber of said series into which .said vapor is initially introduced for sequential travel through said series of chambers; and said means for venting non-.condensable gases comprising a vent line communicating with the chamber at said end of said series for venting non-condensable gases 'tlierefrom'through said line, a valve for controlling flow through said vent line, and automatically operable means for maintaining said valve normally substantially closed and for opening said valve at predetermined intervals and for periods of predetermined duration for venting noncondensa'ble ,gas accumulated While said valve was maintained closed prior to the opening thereof.

10."Disitillation apparatus comprising an evaporator chamber, a compressor, means for directing vapor from said evaporator chamber to said compressor for compression therein, a condenser heat exchanger within said evaporator chamber adapted to maintain a fluid therein in out-of-contact heat exchange relation with solution in said'evaporator chamber, means for directing compressed vapor from said compressor into said condenser heat exchanger .for condensation therein, means .for directing condensateresulting from condensation of vapor in said condenser heat exchanger from said condenser heat exchanger and out of the system, means for withdrawing concentrated solution from said evaporator chamber, means .for introducing solution to 'be distilled into said evaporator chamber, and means for venting non-condensable gases from said condenser heat exchanger; said means for directing condensate from said condenser heat exchanger comprising a condensate-collecting container, means for directing condensate and non-.condensable gases from said condenser heat exchanger into said condensate-collecting container and condensate line means for directing condensate from said condensate-collecting container out of the .system; and said means for venting non-.condensable gases comprising a vent line communicating with said condensate-collecting container for venting therethrough non-condensable gases from said condensate-collecting container, a valve for controlling flow through said vent line, and automatically operable means for maintaining said valve normally closed and for opening said valve at predetermined intervals and for periods of predetermined duration for venting non-condensable gases accumulated While said valve was maintained closed prior to the opening thereof.

11. Distillation apparatus according to claim 10 which comprises means for maintaining susbtantially constant the liquid level of condensate in said condensate-collecting container.

References Cited in the file of this patent UNITED STATES PATENTS Heinzerling' Jan. 26, 1897 10 Forbes May 24, Le Sucur Dec. 11, Soderlund Aug. 17, Soderlund Oct. 10, Harmon May 15, Stone July 17, Mason Apr. 2, Sloan et al. Feb. 19, Kirgan May 11, Cowherd Nov. 21, Cleaver et al. J an. 9, Filliung May 15, Latham Mar. 18,

FOREIGN PATENTS France Feb. 22, France Ian. 30,

Claims

1. IN A METHOD OF DISTILLATION WHEREIN VAPOR IS EVOLVED FROM A SOLUTION IN A VAPORIZATION ZONE, THE EVOLVE VAPOR IS COMPRESSED TO A PRESSURE AT WHICH ITS CONDENSING TEMPERATURE IS SUBSTANTIALLY GREATER THAN THE BOILING POINT OF SAID SOLUTION, THE COMPRESSED VAPOR IS CONDENSED IN OUT-OF-CONTACT HEAT EXCHANGE RELATION WITH SOLUTION IN SAID VAPORIZATION ZONE, NON-CONDENSABLE GASES ARE SEPARATED FROM THE CONDENSATE RESULTING FROM CONDENSATION OF SAID VAPOR, SAID CONDENSATE IS DIRECTED OUT OF THE SAID SYSTEM, CONCENTRATED SOLUTION IS DIRECTED OUT OF THE SAID VAPORIZATION ZONE AND OUT OF THE SYSTEM, AND FRESH SOLUTION TO BE DISTILLED IS FED INTO SAID VAPORIZATION ZONE; THE VENTING OF SAID NON-CONDENSABLE GASES FROM THE SYSTEM DURING CONTINUANCE OF DISTILLATION BY STEPS COMPRISING CAUSING THE NON-CONDENSABLE GASES TO BECOME GRADUALLY ACCUMULATED IN THE SYSTEM DURING PERIODS OF PREDETERMINED SUBSTANTIAL DURATION AND ALTERNATING SAID PERIODS OF GAS ACCUMULATION WITH BRIEF PERIODS OF PREDETERMINED DURATION DURING WHICH NON -CONDENSABLE GASES ACCUMULATED DURING THE PRECEDING PERIOD OF GAS ACCUMULATION ARE VENTED AS A SUDDEN BURST.