WO1988004193A1

WO1988004193A1 - Liquid purification system

Info

Publication number: WO1988004193A1
Application number: PCT/US1987/003173
Authority: WO
Inventors: Frank W. Hoffman
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-12-03
Filing date: 1987-12-02
Publication date: 1988-06-16
Anticipated expiration: 1989-06-03
Also published as: AU1052688A

Abstract

A liquid purification system (70) comprising means (4) for receiving a first liquid (32) containing impurities, refrigeration means (1) in communication with the liquid receiving means (4) having means (2) for compressing a refrigerant, and at least two sets of heat exchange means (31, 35) and (38, 41) in series adapted to receive the refrigerant from the compressing means (2). Each set of heat exchange means (31, 35) and (38, 41) has liquid evaporation means (31, 38) in communication with the liquid (32, 39) containing impurities for inducing a phase change in the liquid, wherein the phase change includes a vapor phase (8) essentially free of impurities, vapor phase condensation means (35, 41) connected to the liquid evaporation means (31, 38) for condensing the vapor phase (8) in communication therewith to produce an essentially pure liquid (36, 42) and expansion means (33, 40) between the liquid evaporation (31, 38) and condensation means (35, 40) for expanding the refrigerant as it is conveyed between the liquid evaporation means (31, 38) and the vapor phase condensation means (35, 41). The system (70) also includes means (3) for conveying the refrigerant from the heat exchange means (31, 35) and (38, 41) back to the compressing means (2).

Description

LIQUID PURIFICATION SYSTEM

BACKGROUND OF THE INVENTION

The subject of this invention is the separation of impurities from liquids employing phase changes in the liquid and refrigerative circuits. Such impurities may be solid, liquid, or gaseous in nature. Materials to be removed may be dissolved as a solute, dispersed as a suspension, emulsified, such as an oil in water, or blended as two or more liquids.

Phase changes may include any combination of changes of state between the solid, liquid, and vapor phases and are not limite to a single change of state but may include several phase changes of a mixed variety. With respect to the refrigeration circuitry, any suitable refrigerant may be used such as the freons, ethane, propane, n-butane, ammonia, carbon dioxide, ethylene, propylene, and others. It is also possible to use the same liquid for the refrigerant as the liquid being purified. For example, pure water could be used in the refrigerative circuit when purifying seawater.

Separation and purification of liquids is required in many industries over a broad range of applications. This invention applies to, but is not limited to, desalination of seawater, ultrapurification of water, purification of industrial wastestreams, drying of numerous chemical residues and sludges, drying of chemicals, processing of raw materials, food preparation, processing, concentration, sterilization, and many others.

Current technology for liquid purification and separation can be grouped under three headings: chemical methods, filtration systems, and phase change techniques. Chemical methods are costly, time consuming, usually require much space and manpower, plus utilize chemicals other than those of interest. Filtration systems have a limited ability to separate, are prone to clogging and fouling, and change characteristics during their period of use. Phase change systems include distillation, vapor compression, and freezing. Such systems are simpler and usually more effective than chemistry and filtration. Among the phase change techniques, there are advantages and disadvantages to each. Most notable perhaps is the range of temperature over which the process operates. Lower temperatures are desirable because the rates of fouling, scaling, and corrosion are drastically reduced. Such operation then means little or no pretreatment of feed- waters and a much extended life for the equipment itself. It should be noted here that while this invention allows greatly enhanced and practical operation at much lower temperatures, the inventive aspects are independent of temperature range and give improved performance in all cases.

In phase exchange systems, of fundamental concern to the end user are capital cost and cost of operation. This invention significantly lowers both the cost of the equipment as well as the expense or running it.

SUMMARY OF THE INVENTION

For the purposes of discussion and illustration of the inventive aspects and advantages of this system, the example used will be. that of purification of water using a freon refrigeration circuit with a piston type mechanical compressor. The inventive aspects are independent of the nature of the refrigerant, type of compression, be it mechanical, electrical, chemical, or thermal, and process application.

Consider a single stage vacuum distillation process in which the phase change driving force is provided by a freon refrigeration system compressing a closed circuit with a liquid refrigerant metering device, an evaporative heat exchanger, a refrigerant vapor compressor, and a condensing heat exchanger. The condenser is immersed in the water to be purified and the evaporator located such that it may condense the vapor produced by the boiling water and collect the purified water. The reaction must take place in an environment of suitable pressure such that the water will boil and furthermore appropriate means must be provided to remove the excess heat energy introduced into the system by the refrigeration circuit. The following numerical example illustrates a single stage system using a commercially available compressor with the resultant, capacity expected and energy consumed.

According to data provided by the manufacturer, a nominally 10 hp discus type piston compressor will provide 261,000 btu/hr of refrigeration when operating between an evaporating temperature of S5°F and a condensing temperature of 70°F with a total power consumption of 4.31 kwh. The amount of heat required to be removed from water vapor to condense it under these conditions is approximately 1056 btu/lb. Calculations show that such a system in simple form would yield 710 gallons per day of purified water at a compressor energy expenditure of approximately 150 kwh/1000 gallons. Commercial data on the compressor also shows the closer the evaporating temperature is to the condensing temperature of the system, the greater the ratio of refrigeration capacity to energy consumption. Analysis of the data in this particular case by extrapolation gives at zero temperature difference between evaporator and condenser yields an upper limit on capacity and a lower limit on energy, namely, 893 gpd at 80 kwh/1000 gallons. The lower limit on energy is due to normal energy losses in any mechanical system and the capacity limit is due to mechanical design of the compressor.

In order to improve both capacity and energy consumption one can go to multiple staging. That is, use of the heat rejected by the condensing product water from the first stage to boil feed water in another system or stage. Multiple staging can substantially increase capacity while reducing energy consumption as shown by the table below:

STAGES: 1 2 3 4 5

CAPACITY: 710 1420 2130 2840 3550 (GPD)

ENERGY: 150 75 50 37.5 30 (KWH/1000 GAL)

STAGE AT: 15 7.5 5 3.75 3 (DEG F)

From an economic point of view capital cost is lower because one gets more water from the same compressor. Since the compressor is operating at the same temperature differential its energy consumption is the same, but because more end product is produced the cost per gallon decreases.

One can see from the data shown that for a fixed evaporator-condenser temperature differential, as the number of stages increases, the temperature difference between stages decreases. Thus, success of staging depends on each stage being able to operate over as low a temperature differenc as practical compatable with heat exchanger requirements. The need for decreased temperature differences is greatly enhanced by the fact that in general compressors have a greater capacity and lower energy consumption if they are operated at a closer evaporating-condensing temperature difference.

To date, multiple staging has been accomplished by circulating liquids which involve circulation pumps and sensible heat changes. In such systems as the temperature difference between stages decreases the amount of liquid to be circulated increases. This increases capital cost because of the added size of the pumping system and increases the operating costs because of the energy required to circulate the heat transfer liσuid. The table below shows the flow of liquid required for the sensible heat exchange of conventional staging where used in the above example.

STAGES: 1 2 3 4 5 FLOW RATE: 35 70 105 140 175 (GPM)

In the present invention, staging can be accomplished directly without the use of a circulating heat transfer liquid and the associated pumping, thereby reducing capital cost and energy. Also, because the heat transfer is accomplished by simultaneous phase changes on either side of the heat exchanger there is a very small sensible heat exchange contribution and therefore each stage can operate over a much smaller temperature difference. For the same operating conditions of the compressor more stages are possible thereby further reducing operating and capital costs. The liquid purification system of the present invention consists of a series of interconnected chambers, each representing one process stage. Within each chamber is a divider to keep the process feed separate from the process product. The arrangement is such that the divider allows free flow of process vapor from the feed region to the product region. The vapor regions are connected in such a manner to each other that the proper pressure equilibrium is established. The feed regions of each chamber are connected such that feed may flow into one chamber and sequentially through each chamber until the concentrated feed is extracted from the last stage. Provision is made to extract the product independently from each chamber and/or combine them if required into a single stream. If required for special separation needs, the product from one chamber may be used as the feed to the next.

The invention employs a refrigerative circuit in which gas generated by evaporation of a refrigerant fluid at a low pressure is compressed in the usual way by an appropriate power driven compressor. Compressed gas is then condensed, by giving out heat to evaporate a portion of the feed water to be purified, but according to this invention instead of the usual known single expansion back to the initial low pressure for re-evaporation it is made to cascade through a series of expansions to progressively diminishing pressures before ultimately reaching the initial low pressure. By appropriate design of control passages and heat exchangers provided in the invention, refrigerant fluid in the downwards cascade is condensed, evaporated, recondensed and re-evaporated and so on for several stages until finally it reaches the bottom pressure and temperature where the final evaporation occurs for return to the compressor.

In each stage the refrigerant fluid is first condensed and then evaporated, heat being extracted from the refrigerant in the first half-stage and supplied to the refrigerant in the second half-stage. The heat extracted in the first half stage is celivered to feed water to be purified, the boiling temperature of the feed water being below the condensing temperature of the refrigerant. Thus, in the first half stage condensing refrigerant causes pure water vapor to boil off. The pure water vapor is free to pass to the second half-stage, where the refrigerant fluid has been expanded to a lower pressure and temperature than it was in the first half-stage, and where the refrigerant coil is not submerged. Thus the water vapor condenses on the outside surface of the refrigerant coil, re-evaporating refrigerant for conveying to the coil in the next first half stage.

In this way, heat given off in the first condensation of refrigerant after it emerges from the compressor is utilized over and over again for as many times as there are stages - without requiring additional power input. This result is entirely consistent with thermodynamic principles. It is achieved in this invention by the appropriate arranging of control passages and heat exchangers provided for the refrigerant fluid and processed liquid by the invention. The invention also includes a separate secondary heat transfer circuit which rejects energy from the main refrigerant and process fluid circuits so as to maintain the operation at a desired steady condition.

DESCRIPTION OF THE DRAWINGS

The drawing is a schematic illustration of the essential components of the apparatus and process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The operation of the invention is explained with reference to the drawing. The number of stages shown in the drawing is three; seawater is the liquid being purified, Freon .is the refrigerant fluid and the arrangement has a horizontal orientation. None of these choices is to be construed as restrictive in any manner on the scope of the invention. Any values given are examples and not to be considered as operative limitations.

Referring to the drawing, there is shown the liquid purification system 70, comprising chambers 5, 14, and 25 arranged as stages in the system 70. Each chamber is divided by a barrier 6, 16, and 59, respectively, thus dividing the chambers into two half-stage regions. The first half of each chamber will be referred to as feed regions 4, 13, and 21, while the other halves of each chamber will be referred to as product regions 8, 19, and 24. Each chamber 5, 14 and 25 has the feed and product regions connected by a vapor passage 7,16a , and 22, respectively. Each chamber is operatively connected to the atrrosphere via passage 29, a pressure regulating device 28, a passage 27, and passages 26, 20 and 11. In this embodiment the device 28 is a vacuum pump. Passages 11, 20 and 26 are sized so as to allow removal of non-condensable gases from the system yet allow the three chambers to maintain their individual thermodynamic balance. The refrigerative circuit in this example consists of a suction line 3, a compressor 2, a discharge line 1, an initial stage feed heat exchanger 31, a restrictive passage 33, initial stage product heat exchanger 35, a passage 37, a middle stage feed heat exchanger 38, a restrictive passage 40, a middle stage product heat exchanger 41, a passage 43, a final stage feed heat exchanger 44, a restrictive passage 46, a final stage product heat exchanger 48, and a passage 60. For purposes of illustration only and not to be construed as limiting the present invention, the system components are described as follows: Suction line 3-suitable metal tubing, compressor 2-conventional piston type refrigeration compressor, discharge line 1 - suitable metal tubing, heat exchangers 31, 35, 28, 41, 44 and 43 - conventional metal coils of adequate size and surface area, restrictive passages 33, 40 and 46 - orifice plates, and passages 37, 43, 61 and 60 - a metal pipe.

There are two branches to the treated process fluid circuit. Raw process feed enters through passage 30 and accumulates to an appropriate level in feed region 4 as feed pool 32. Next, the partially concentrated feed continues through connector passage 52 to form feed pool 39 in feed region 13 where it is further concentrated and then proceeds through connector passage 53 to become feed pool 45 in the last stage feed region 21. After this further and final concentration, the process fluid is withdrawn from the system by passage 57, pump 56, and passage 55. The product portion of the process fluid circuit is the purified product which accumulates in the product regions 8, 19 and 24 as product pools 36, 42 and 47. Product may be withdrawn through passages 58, 54 and 51 into a common passage 62, pump 50, and passage 49.

To thermodynamically balance the system an auxiliary heat rejection circuit must be employed. It need not be a refrigeration unit as shown here but could be a heat exchanger using cooled liquid circulation, air exchange to the atmosphere or some other means of either condensing or removing excess vapor generated. One could even use a vapor pump to remove the excess vapor directly. Again, for purposes of illustration only and not to be construed as limiting the invention the auxiliary system here consists of a refrigeration compressor 15 discharging compressed gas through line 17 to a condensor 23 where the commercial refrigerant condenses to a liquid. This liquid refrigerant passes through line 18 and is metered through the refrigerant expansion valve 12 into the auxiliary heat exchanger 9 to be evaporated to a gas and returned to compressor 15 through gas line 10. The heat to evaporate the refrigerant in the auxiliary heat exchanger 9 is supplied by condensing product vapor 34.

The general operation of the liquid purification process is as follows: a compressed gaseous refrigerant in feed heat exchanger 31 condenses and rejects heat to feed pool 32 causing some feed to vaporize because of the lowered pressure in the chamber. The now liquid refrigerant is metered through the orifice plate 33 into the product heat exhanger 35 which is at a reduced pressure being connected through the remainder of the circuit to the suction line 3 of the compressor 2. Because of the reduced pressure the refrigerant evaporates, taking in heat by condensing the vapor produced from the feed pool 32. The refrigerant vapor then passes through passage 37 into feed exchanger 38. Because the feed pool 39 is at a lower temperature than the feed pool 32, the gaseous refrigerant condenses again rejecting its heat to the feed pool 39. The process repeats itself through successive stages until the final stage where the gaseous refrigerant is directed to the compressor 2 by suction line 3. Below is an example of typical operating conditions for processing impure water using an R-22 refrigerant fluid.

EXAMPLE

Drawing Reference Description Temperature Pressure

31 Initial Feed Heat Exchanger 70°F 121.4 psig

35 Initial Product Heat Exchanger 66 113.2 38 Middle Feed Heat Exchanger 64 109.3

41 Middle Product Heat Exchanger 60 101.6

44 Final Feed Heat Exchanger 58 97.9

48 Final Product Heat Exchanger 54 90.8 1 Suction Line 54 90.8 3 Discharge Line 70 121.4

9 Auxiliary Product Exchanger 66 113.2

23 Auxiliary Condenser 76 134.5

5 Initial Chamber 68°F 17.5 Torr

14 Middle Chamber 62 14.2 25 Final Chamber 56 11.4

30 Feedwater Source 72°F 760 Torr

32 Initial Feedwater Pool 69 17.-5

36 Initial Product Water Pool 65 17.5 39 Middle Feedwater Pool 63 14.2 42 Middle Product Pool 61 14.2

45 Final Feedwater Pool 57 11.4 47 Final Product Pool 55 11.4

49 Product Discharge 55 760 55 Waster Discharge 57 760 A specific embodiment of the invention has been described and shown in the example accompanying the drawing to illustrate the application of the inventive principles.

The invention in its broader aspects is not limited to the described embodiment and departures may be made therefor within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

Claims

What is claimed is:

1. A liquid purification system, comprising: means for receiving a first liquid containing impurities; a refrigeration means in communication with said liquid receiving means, said refrigeration means including means for compressing a refrigerant, at least two sets of heat exchange means in series adapted to receive the refrigerant from said compressing means, each of said sets comprising a liquid evaporation means in communication with the liquid containing impurities for inducing a phase change in the liquid, wherein said phase change includes a vapor phase essentially free of impurities, a vapor phase condensation means connected to said liquid evaporation means in communication with said vapor phase for condensing the vapor phase to produce an essentially pure liquid, and an expansion means between said liquid evaporation means and said condensation means for expanding the refrigerant as it is conveyed between said liquid evaporation means and said vapor phase condensation means; and means for conveying the refrigerant from said heat exchange means back to said compressing means.

2. A system according to Claim 1, further comprising a second heat exchange means in communication with said vapor phase for further cooling and condensing the vapor phase to the pure liquid.

3. A system according to Claim 1, further comprising a pressure control means for controlling the pressure in said liquid receiving means for inducing the phase change in the impure liquid.

4. A process for purifying a liquid containing dissolved impurities, comprising:

(a) conveying a gaseous refrigerant in indirect heat exchange relation to the liquid to simultaneously induce phase changes in the liquid and in the refrigerant, wherein the liquid phase change includes a vapor phase essentially free of impurities and the refrigerant phase change includes a condensate refrigerant;

(b) expanding the condensate and refrigerant;

(c) indirectly cooling the vapor phase with the expanded condensate refrigerant to induce phase changes in the vapor phase and in the expanded refrigerant, wherein the vapor phase is condensed to produce an essentially pure liquid and the expanded refrigerant is evaporated to form a gaseous refrigerant;

(d) repeating steps (a) - (c) at least once employing the gaseous refrigerant produced in step (c) and the liquid containing impurities from step (a); and

(e) compressing the gaseous refrigerant from the last indirect cooling step for recycling to the first indirect heating step (a).