TITLE: Multiple-Effect Still With Distilland Recirculation Prioritization

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No 60/565,237, filed April 24, 2004 by the present inventor.

FEDERALLY SPONSORED RESEARCH: Not Applicable

SEQUENCE LISTING OR PROGRAM: Not Applicable

BACKGROUND OF THE INVENTION - FIELD OF THE INVENTION

This invention relates to a multiple-effect still using recirculation of a controlled portion of the distilland in a given effect in order to maintain proper distilland wetting of the heat exchanger elements within that effect, plus a blow-down stream for removal of excess distilland from an effect, where higher priority is given the allocation of distilland to the recirculation stream than to the blow-down stream.

BACKGROUND OF THE INVENTION - PRIOR ART

Multiple-effect stills are an established art, featuring many variations of implementation. The primary heat exchangers of such stills may be tubes of the horizontal or vertical type, bundled tubes or concentric tubes, or plates. Flow of distilland between effects may be forward (from higher temperature to lower temperature effects) or reverse (from lower temperature to higher temperate effects). Stills may be designed for optimization with a variety of distillands, including items such as sea water, brackish water, oil-well produced water, coal bed methane-well produced water, alcohol, and to foodstuffs such as fats or juices. This invention is suitable for use' with a wide variety of multiple-effect stills in which distilland flows downward on a heat exchanger surface. There is a problem associated with the proper design and operation of a still; the problem is in maintaining uniform wetting of the heat exchangers of the still. This problem is common to all stills featuring a downward, gravitational flow of distilland across heat exchanger surfaces. The solution is to provide a certain minimal flow rate of distilland across the heat exchanger surface in order to keep the entire surface fully wetted. Loss of performance and serious scaling problems can occur if adequate wetting is not maintained. A typical rule of thumb used in seawater distillation calls for 1 gallon per minute of flow for each inch of diameter of a heat exchanger tube or each 3 inches or so of linear length at the top of a plate heat exchanger. As the distilland flows from effect to effect, a portion of it is removed as an evaporate, reducing the volume of the remaining distilland. In many designs, the normal flow of distilland from effect to effect is less than this minimum flow rate required for adequate heat exchanger wetting. This problem tends to be more pronounced at the colder end of a still, where much of the supply distilland has already been evaporated.

It is known in the state of the art to recirculate distilland within an effect in order to provide for adequate heat exchanger wetting. After flowing down a heat exchanger surface, the distilland accumulates at the bottom of the effect. It leaves the effect through a first outlet port, is pressurized by a pump, reenters the effect through an inlet port, flows to a point above the heat exchangers, is applied to the upper surface of the heat exchangers, and then flows down the heat exchanger as recirculated distilland. A portion of the distilland is evaporated during each pass. A portion of the recirculating distilland is blown-down at a flow splitter placed in the stream after pump pressurization. The blow-down distilland flows to the next effect except in the case of the last effect, from where it is removed from the still. Each effect also has a supply inlet port to receive blow-down from a previous effect as its feed distilland, except in the case of the first effect, in which the supply stream to the still comprises the feed distilland. Under stable operating conditions, the incoming feed distilland mass into an effect will equal the blow-down mass from the effect plus the mass of the evaporated distilland. Most of the evaporated distilland is condensed as product; a smaller portion is used to entrain non-condensable gasses and carry them off. The difficulty with the above art is in control of the fraction of recirculating distilland split off as blow-down. Typically a proportioning valve is placed after the splitter and in the blow-down stream. Adjustment of this valve controls the fraction of recirculating distilland split off as distilland. This valve will have different settings depending on the operating conditions of the still and possibly of wear rates of the motor. Instrumentation must be provided to indicate proper flow. This instrumentation must be manually monitored or a complex electronic control system must be implemented. A system which can eliminate the proportioning valve is desirable.

Examples of the Prior Art:

Greenfield et al. in their US Patent number 3,947,326 disclose a multi-effect still with distilland recirculation. The pressurized distilland after a recirculation pump is split into a recirculating stream and a blow-down stream into the next effect. This differs from our invention, which does not have splitters in the recirculation streams and which uses separate blow-down ports out of the effects.

Carl E. Ehnore in Dimple plate horizontal evaporator effects and method of use, US Patent no. 5,139,620 discloses a multiple effect evaporator using heat exchanger plates with recirculating distilland in an effect. A portion of the recirculating distilland in his invention is extracted from the pressurized (post pump) flow fed to the next effect. Again, this differs from our invention, which does not have splitters in the recirculation streams and which uses separate blow-down ports out of the effects.

The advantages of our invention over both Greenfield and Elmore is that no valving/control system is required to properly apportion the recirculation distilland flow between recirculation and blow-down functions.

BACKGROUND OF THE INVENTION - OBJECTS AND ADVANTAGES This invention eliminates the splitter and proportioning valve by separating the recirculation stream from the blow-down stream. Thus, the entire output of the pump within the recirculating distilland stream can be applied to the heat exchanger surfaces and used to wet them; there is no splitter. Ideally, the pump output flow will by design match the wetting requirements of the heat exchangers and a flow control valve will not be needed within the recirculation stream. The blown-down distilland is removed from an effect by means of a dedicated blow-down outlet port.

Distilland is accumulated in an accumulator at the bottom of an effect chamber. Flow priority is given to recirculation. The recirculation pump is fed by the accumulated distilland. Once the accumulator is sufficiently full to meet the recirculation pump requirements, additional accumulations of distilland are blown-down.

This invention provides for two approaches to implementing blow-down distillation removal: active and passive. Active implementation requires the use of a sensor to determine the height of the distilland in the accumulator and then opening a valve to allow the removal of blown-down distilland through a port and when the level is appropriately high and closing the valve when the level is too low. Thus, priority is given to the recirculating distilland. The only distilland removed from an effect is that which can be done without impacting adequate recirculation flow. The preferred embodiment is of an active system. In a small still where the height of the unit is an issue, this embodiment does not require an external trap as is required in the preferred alternative embodiments. This in turn reduces the total height of a still. In environments where height is not an issue, the other embodiments may have their own distinct advantages for use, each of the embodiments described herein can be the preferred embodiments for a particular application, depending on such issues as the number of effects in a still, the chemical composition and viscosity of the distilland, the fraction of distilland evaporated, the dimensional requirements of the still.

By contrast with active implementation, a passive implementation does not require an active distilland height sensor and associated control valve. Rather, the opening of a dedicated blow-down port is placed at a height within the accumulator so as to blow-down only distilland which is not needed for recirculation. This insures that there is always an adequate supply of distilland for the recirculation flow and allows a pump to be selected whose pressure and volume capabilities match the recirculation requirements. The full flow of the pump output is supplied to the recirculation flow and a flow-control valve is not needed within the flow.

In operation of a passive system, feed distilland coming into the effect is used first to establish the recirculation flow and replace distilland evaporated by operation of the still. Assuming that the incoming feed flow is greater than the quantity of distilland evaporated, an increasing amount of distilland will accumulate in the accumulator until it reaches the inlet of the blow-down outlet port. Outflow capacity of the blow-down outlet port must be adequate to remove all of the excess distilland entering the effect.

SUMMARY

In accordance with the present invention, the effects of a multiple-effect still maintain a sufficient flow of distilland across the tops of the heat exchanger elements (typically tubes or plates) to provide for proper wetting of the elements by adding a recirculation flow of distilland to the supply distilland coming into the effect from an external source, such that the combined flows are adequate to maintain proper wetting of the heat exchanger elements whenever the incoming supply distilland by itself might not be. The recirculating distilland is taken from a distilland accumulator at the bottom of the effect.

Under normal operation, the incoming supply distilland will be greater than what is evaporated in an effect, per the standard multiple-effect process. Hence, a portion of the accumulated distilland needs to be blown down, i.e., removed from the effect.

This invention provides for removal of the blow-down distilland as a separate stream of flow from the recirculating distilland. Both recirculating distilland and blow-down distilland are removed from the distilland accumulator, but the outlet port/tube for the blow-down distilland stream is high enough within the accumulator so that no blow-down distilland is removed from the accumulator until there is an adequate supply of accumulated distilland for proper operation of a recirculation pump. Once the accumulated distilland reaches the outlet port for the blow-down distillation flow, any additional amounts of accumulated distilland within the accumulator are removed through the port.

In the preferred embodiment, a height sensor within the accumulator causes a valve to open, which allows blow-down distilland to flow directly out of the effect and either into the next effect or out of the still.

DRAWINGS - Figures

FIG 1. is a diagrammatic view of an still with an active blow-down distilland removal apparatus.

FIG. 2. is a diagrammatic view of a still with passive blown-down distilland removal apparatus and a pump to pressurize blow-down distilland.

FIG. 3 is a diagrammatic view of a still with passive blown-down distilland removal apparatus and a trap in the blown-down distilland flow.

FIG. 4 is a diagrammatic view of a still with passive blown-down distilland removal apparatus and a trap plus a venturi in the blown-down distilland flow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

hi FIG. 1 we have a schematic diagram of the preferred embodiment, a still with an active blow-down system, hi this implementation a float is used to determine when the height of distilland within the distilland accumulator at the bottom of the effect is adequate to allow distilland blow-down. The buoyancy of the float provides a direct mechanical means of opening and closing the blow-down valve at the blow-down port. When a blown-down valve is open, distilland referred to as blow-down distilland flows from the higher pressure and temperature effect to the lower pressure and pressure effect.

The still represented in FIG. 1 is essentially the same as that of the multiple-effect still disclosed in my U.S. Patent 5,423,759, Structure For Multiple Effect Distillation, except as modified to take advantage of the features of this invention. The various basic structures and their operations are explained in that patent. Note, though, the scope of the current invention is far broader than that of the earlier patent, which is mentioned only for purposes of illustration.

Thus, the typical structural elements of a multiple-effect still which are illustrated in FIG 1 are all described in detail in the earlier patent.

Normally, internal pumps and active blow-down systems are co-beneficial. Their use in combination can allow the height of a still to be reduced. There are situations requiring completely self-contained systems to be mounted on a skid or in a container. An example of this would be in a skid- or container-mounted system used for emergency water purification in a scene at a disaster, such as flood or hurricane or tsunami. So, FIG. 1, which discloses an active blow-down distilland removal system, also includes internal recirculation pumps.

In this case, a pump is mounted within the accumulator at the bottom of an effect. The pump shown represents a vane pump, which, being positive displacement, does not need a head in the supply line to avoid cavitation. For purposes of illustration, the pump is shown as having a shaft coming out of both sides of the unit. A shaft port is provided in the common walls between effects, with a rotating seal between the shaft and shaft port. A shaft coupler is provided so that a first side shaft segment of one effect can be connected to a second side shaft segment of an adjacent effect through a shaft port. Provision of the shaft coupler also allows for easy assembly and maintenance of the unit. The pumping system as shown allows the pumps for a number of effects to be driven by a single motor and shaft. This is typically cheaper and more energy efficient than providing a dedicated motor for each pump. As illustrated, a bracket is provided to mount the pump to the effect bottom wall. This mounting is considered representative of many variations which should be readily apparent to one skilled in the art. Likewise, as illustrated, the inlet to a pump is well below the normal level of accumulated distilland within the distilland accumulator, where normal level is defined as the level required for distilland accumulation for blow-down to occur.

The output of the pump is conveyed via a tube to the distilland applicator above the heat exchangers; this distilland comprises the recirculation distilland.

A multiple-effect still 1 shows two typical, adjacent effects, effect 2a and effect 2b. Since heat flow is from right to left as shown, an adjacent effect to the right side of an effect is next to the effect and an adjacent effect to the left side is previous to it. Thus, effect 2b is next to effect 2a; likewise, effect 2a is previous to effect 2b.

Effects 2a and 2b share a common wall 4b between them. Shell 6a provides a vapor-tight enclosure for effect 2a, likewise shell 6b provides a vapor-tight enclosure for effect 2b. (Henceforth, discussion will be limited to side a except where appropriate).

Heat exchanger tubes 8a are mounted onto a tube sheet 9a, which also comprises the upper wall of a condensate accumulator 16a. Heat exchanger caps 10a are mounted to the top of the heat exchanger tubes 8a and provide a seal between a condensate accumulator 16a and the interior of the effect 2a bounded by the shell 6a.

A distribution box 24a distributes distilland to the top of external surfaces of the heat exchanger tubes 8a. A gap 1 Ia is provided for distilland to flow out of the distribution box 24a and onto the tubes 8a. Vapor 12a from a previous effect enters the condensate accumulator 16a through a window 14a in the wall 4p between an effect and its previous effect. Distilland accumulates in a distilland accumulator 20a, which comprises that portion of effect 2a between the lower wall of the condensate accumulator 16a and the bottom of shell 6a, as an accumulated distilland 18a. In the operation of the still 1, the vapor 12a condenses on internal surface of the tubes 8a and flows as a condensate 17a into the condensate accumulator 16a and from there through a condensate accumulator port 60a, through a condensate removal tube 62a, and out of the still through a condensate removal port 64a.

The above description is a brief summary of traditional multiple-effect implementation. More specific details and explanation are available in my above referenced patent. The following features represent new art.

A recirculation Pump 38a is mounted to the bottom of shell 6a by means of a pump mounting bracket 40a, which is representative of many alternative mounting methods. The accumulated distilland 18a flows into a pump inlet 42a, is pressurized by the pump 38a, leaves the pump 38a at an outlet port 43a, then is conveyed to distribution box 24a by means of a recirculation distilland tube 44a.

The pump 38a is configured as having shafts coming out of each end. A first shaft 46a extends out the left side as shown and a second shaft 48a extends out the right side as shown. A rotating shaft seal 50a mounted in wall 4a is provided to inhibit the passage of vapors between adjacent effects 2a and 2b.

In effect 2b a shaft connector 52b allows a second shaft 48a of a previous effect 2a to be connected to the first shaft 46b. The structure of effect 2a is similar.

Blow-down distilland flows between the effects 2a and 2b through a distilland port 36a, which is located in the wall 4a. A valve 34a controls the flow of distilland between effects. A float 30a is linked by a linkage 32a to the valve 34a; the linkage 32a is mounted on the wall 4a. The float 30a rises when the accumulated distilland 18a rises in the distilland accumulator 20a, thus, if properly adjusted, causing valve 34a to open and distilland to flow as blow-down distilland into the next effect 2b. It is the operation of the float 30a causing the valve 34a to open which establishes priority between the recirculation distilland flow and the blow-down distilland flow. This requires float 30a to be located sufficiently higher than inlet 42a to the recirculation pump 38a to insure that adequate recirculation flow is available to the inlet of pump 38a. Float 30a must also be positioned low enough so that valve 34a is opened and excess accumulated distilland 18a removed as blow-down distilland before accumulated distilland 18a overfills the distilland accumulator 20a.

In FIG 2, we have an embodiment of a "passive" system. Where height issues are not a concern, a passive blow-down system and external pumps are typically preferred over the previous embodiment because of simper design and maintenance requirements.

Depending on pump design characteristics, a trap may or may not be needed to prevent vapor blow-by between the effects. Some pumps such as rotary vane pumps, for instance, typically provide sufficient natural blockage of vapor flow through so as to eliminate the need for a trap. By contrast, the valves of certain piston pumps might readily open to allow blow-by when the inlet pressure to the pump is sufficiently higher than the outlet pressure. In this case a trap would be needed.

Thus, in FIG 2 a multiple-effect still comprises a multiplicity of effects 202a,b, which comprise a distilland distribution chamber 204a,b, a shell 205 a,b, distilland to be distributed 206a,b, heat exchanger tubes 208a,b, vapor 209a, a condensation chamber 210a,b, an accumulator 212a,b, accumulated distilland 214a,b, an upper tube sheet 218a,b, a lower tube sheet 220a,b, a recirculation distilland input port 222a,b, a vapor outlet port 224a,b, an inter- effect vapor transport tube 228a,b, a vapor inlet port 230a,b, vapor to be condensed 232a,b, a condensate outlet port 234a,b, condensate 236a,b, a condensate outlet tube 238a,b, a recirculation distilland outlet port 258a,b, a recirculation tube 260a,b, a recirculation pump 262a,b, pressurized recirculation distilland 264a,b, pressurized region of the recirculation tube 266a,b, a blow-down level tube 270a,b with opening 272a,b, blow-down distillation 273 a,b, a blow-down outlet port 274a,b, a blow-down outlet tube 276a,b, a blow-down pump 278a,b, a distilland supply region of blown-down outlet tube 280a,b, a supply distilland inlet port 282a,b, and an energy source 288a,b to blow-down pump 278a,b.

An effect 202a is contained by pressure-tight shell 205a, which encompasses all of the internal structures of the effect 202a as shown. Distilland distribution chamber 204a is bounded on its bottom by the upper tube sheet 218a and elsewhere by the upper portions of shell 205a. Distilland 206a freely flows within the distilland distribution chamber 204a. The tops of heat exchanger tubes 208a are above the top of upper tube sheet 218a, providing a path for distilland 206a to reach the various tubes 208a. When the height of distilland 206a within distilland distribution chamber 204a reaches the upper ends of tubes 208a it flows over the tops of the tubes 208a and down the internal walls of the tubes 208a. A portion of the distilland 206a flowing down the tube walls is evaporated as vapor 209a.

Notice that there is a pressure difference between distilland distribution chamber 204a and condensation chamber 210a. Therefore, upper tube sheet 218a must seal tightly against heat exchanger tubes 208a and shell 205a. Likewise, 1here is a pressure difference between accumulator 212a and condensation chamber 210a, so lower tube sheet 220a must seal tightly against heat exchanger tubes 208a and shell 205a.

After a portion of distilland 206a has flowed down the length of heat exchanger tubes 208a it falls to the bottom of the effect 202a where it is accumulated in accumulator 212a. Accumulator 212a comprises all of effect 202a located below lower tube sheet 220a.

Distillation 206a enters distilland distribution chamber 204a through recirculation distilland input port 222a, which is located in a wall of shell 205a. Vapor 209a flows down through the heat exchanger tubes 208a into accumulator 212a, and then through vapor outlet port 224a, which is located in a wall of shell 205a, into inter-effect vapor transport tube 228a, through the vapor inlet port 230b of the next effect 202b and into the condensation chamber 21 Ob of the next effect 202b. Inter-effect vapor transport tube 228 connects at one end to the vapor outlet port 224a of an effect 202a and at the other end to the vapor inlet port 230b of the next effect 202b. After transportation into the next effect, vapor 209a becomes vapor to be condensed 232b. Vapor to be condensed 232a is condensed on the outer surface of heat exchanger tubes 208a as condensate 236a, flows down tubes and accumulates at the bottom of condensation chamber 210a. Accumulated condensate 236a flows out of effect 202a through condensate outlet port 234a which is located in shell 205a just above the top surface of lower tube sheet 220a. Condensate 236a is then removed from the effect 2a through condensate outlet tube 238a, which is connected at condensate outlet port 234a at one end and an application specific interface at the other.

All of the above discussion relates to standard, well-known multiple-effect technology and is provided for reference. The following material represents the new subject matter of the current invention.

As accumulated distilland 214a collects in accumulator 212a, it will flow out of the effect 202a through recirculation distilland outlet port 258a and into recirculation tube 260a and then into the input of recirculation pump 262a. Typically, the flow rate of distilland entering an effect through supply distilland inlet port 282a will be greater than is evaporated within the effect. Hence, in a properly operating still, accumulated distilland 214a will quickly reach and then exceed the pumping capabilities of recirculation pump 262a. When this happens, the excess accumulated distilland 214a will increase in accumulator 212a until it reaches opening 272a of blow-down level tube 270a. The level of opening 272a needs to be high enough above the level of recirculation distilland output port 258a so as to insure that there is always adequate accumulated distilland 214a in accumulator 212a to saturate the pumping capacity of recirculation pump 262a.

Accumulated distilland 214a passing through recirculation pump 262a, is pressurized, and is renamed pressurized recirculation distilland 264a. Pressurized recirculation distilland 264a moves out of recirculation pump 262a, into the pressurized region of the recirculation tube 266a, through the recirculation distilland input port 222a, and into the distilland distribution chamber 204a. Upon entering distilland distribution chamber 204, pressurized recirculation distilland 264a becomes distilland to be distributed 206a.

Accumulated distilland 214a enters opening 272a of blow-down level tube 270a and then flows through blow-down level tube 270a, through blow-down distilland port 274a located within shell 205a, into blow-down outlet tube 276a and into blow-down pump 278a. Pump 278a is energized by an energy source 288a and the blown-down accumulated distilland 214a leaves pump 278a, enters the distilland supply region of blown-down outlet tube 280b, and enters the next adjacent effect 202b through a port 282b. Typically, pressurized blow- down distilland will enter directly into the accumulator 212b of the next effect 202b through its supply distilland inlet port 282b, which is connected to the distilland supply region of blown-down outlet tube 280b.

In FIG 3 we have an embodiment of the simplest form a passive blow-down removal system. Those elements common to FIG. 2 share the same numbers with FIG. 2 and the discussion for FIG. 2 applies here as well.

The following features are new to this embodiment.

The blow-down distilland of effect 202a flows into the opening 272a of a blow-down level tube 270a, which then leaves the effect through a blow-down port 274a, flows through a blow-down outlet tube 276a, and then flows directly into the inlet port 282b of the next effect, with the blow-down outlet tube 276a containing a trap 310 placed in the blow-down flow between the two effects 202a and 202b in order to prevent vapor blow-by between the effects. The blow-down transfer pressure (a pressure applied to the blow-down distilland to induce its flow between effects) results from the difference in operating pressures of the effects. This embodiment is suitable only for forward flow implementations, defined as those having distilland flow from the higher temperature effects to lower temperature effects. It is dependent upon the saturation pressure differential between effects to provide for the motive force to move the blow-down distilland between the effects.

The trap is defined as being located at the lowest point or region of the blow-down outlet tube 276a.

The top 312 of the trap 310 should be sufficiently below the entrance to the inlet port 280B of the adjacent, lower pressure effect 202B such that the pressure head due to the height of distilland in the blow-down removal tube between the inlet port 280b and the top 312 of the trap 310 is greater than the greatest operating pressure differential between the two effects. Thus, there is always sufficient distilland in the distilland supply region of the tube to prevent vapors from the higher pressure effect moving directly into the lower pressure effect, which would interfere with proper operation of a multiple-effect still.

In FIG 4 we have an embodiment similar to that of FIG 3 with the addition of an aid to help the blow-down flow between the effects. This aid comprises a venturi 402 spliced into the recirculating tube 260b between the effect recirculation port 258b and the pump 262b inlet . A trap 430 is also placed in the blow-down flow of blow-down outlet tube 276a to prevent vapor blow-by. The blow-down transfer pressure can be significantly increased because of the suction effect of the venturi. Experiments have shown that uncondensed vapors 435 tend to accumulate in the upper region 440 of the suction flow inlet 450 into the venturi 402, and a vapor bleed-off path needs to be provided to bleed off these vapors, typically a tube 470 which moves vapors from the top of upper region 440 into effect 202B and then releases them above the top of accumulated distilland 214b.

CONCLUSIONS, RAMIFICATIONS AND SCOPE

Accordingly, the reader will see that this invention provides a means of insuring an adequate flow of distilland across the heat exchanger elements of an effect while at the same time providing for a simple and reliable means of removing excess, blow-down distilland from an effect; yet this is done without requiring the elaborate instrumentation and control mechanisms of the prior art, wherein the recirculation flows and blow-down flows were separated after the distilland had passed through a distilland pressurization pump.

The specific examples demonstrated herein are representative of many broader applications. For instance, still may use heat exchanger plates or tubes without affecting the principles demonstrated here.

The active blow-down removal system of the first embodiment is not limited to a float and valve combination to control removal of the blow-down distillation from an effect. One skilled in the art could readily implement an electronic sensor to determine the height of accumulated distilland, an electrically actuated valve to open the port between the effects, and a control means to relate the sensor signals to the appropriate valve position. The electrical sensor in such a system could measure changes in optical qualities of the distilland based on its height, changes in the resistance or capacitance of the distilland based on its height, or weight of the distilland accumulated. A float could have indirect action, such that an electrical sensing of float height determined the operation of an electrically actuated valve. The art of sensing the height of a liquid in a container is very mature; any means of controlling a valve position as a result of a signal produced in dependence upon the height of the distilland in the accumulator is considered within the scope of this invention. A valve controlled by a float tends to not only be on and off, but also to have a continuum of positions between shut and fully open. However, an electrically operated valve may be made to work properly which is only on or off, with the duty cycle of the on and off times controlling the distilland height.

Although a string of internal pumps may share a common shaft as illustrated, variations on this are also possible. A pump may have a private shaft port into and out of the still, such that it is not share a common drive force with any other pump within the still. Submersible pumps with self-contained motors could be used, such that only electrical wires need to go between the effects or between a pump motor and the outside of the unit. Pumps could be driven with individual hydraulic motors instead by a shaft; the hydraulic lines could be connected in series, with the inlet of one hydraulic motor connected to the outlet of the motor to an adjacent effect, or each hydraulic motor could have its own supply lines connected through the still walls and to the outside hydraulic supply system.

Although this embodiment shows a still with a common wall between adjacent effects, it could readily be adapted to a still with discrete effects, such as shown in the figures.

The embodiment of FIG 2 will work equally with stills sharing walls between effects, as shown in FIG 1, or with separate stills as illustrated in its own Figure. The motivational force for the blow-down pump between the effects can be electrical, hydraulic, or pneumatic, or mechanical. The blow-down distilland outflow from the blow-down pump is shown as entering the next effect into the accumulator from a port in the floor of the shell. However, it may actually be transferred into the next still in a number of locations, such as combining with the recirculation flow after it has gone through the recirculation pump or through a port in the side of the still, or into a distillation blow-down tube before the pump, or distributed directly onto a heat exchanger surface. The significant issue is that it merely be introduced into the next effect at some point.

In Figure 4, the bleed off tube to move uncondensed vapors from the suction flow inlet of the venturi into the effect and above the accumulated distilland is shown as passing straight up through the venturi and blow-down outlet tube and into the effect until it rises above the accumulated distilland. In practice, it is also effective to place a port in the side of the venturi or in an upper portion of the blow-down tube connected to the venturi and near the suction inlet port of the venturi, such that uncondensed vapors pass through the port and then into the effect through tubing which is external to the venturi; it is also effective to have such an external port and tube which by pass the venturi but pass into the blow-down tube above the venturi. Where the tube removing the uncondensed vapors enters the effect is not as significant as insuring that its outlet is high enough that accumulated distilland does not flow into it and thus block the flow of vapor through it.

Also, in certain practical applications of a still, the initial effects may have sufficient distilland flow such that recirculation is not needed, i.e., the blow down of one effect is pressurized and transferred into the next effect so as to provide the total distilland flow onto the heat exchanger elements of the next effect. However, as more and more of the distilland evaporates as it passes through the effects, it may be important to add recirculation to the latter effects. Thus, a system with some effects non-recirculating and others recirculating could be a desirable implementation. If at least one effect of a multiple-effect still uses recirculation of distillation with priority given to recirculation distilland over blow-down distillation, then that still comes under the scope of this invention. Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the illustrative examples given.