WO2010071605A1

WO2010071605A1 - System for separation of volatile components from solution

Info

Publication number: WO2010071605A1
Application number: PCT/SG2009/000401
Authority: WO
Inventors: Tong Zhou; Lai Yee Loke; Ooi Lin Lum
Original assignee: Hyflux Membrane Manufacturing (S) Pte Ltd
Priority date: 2008-12-17
Filing date: 2009-11-02
Publication date: 2010-06-24
Also published as: TWI415666B; GB2478467A; GB2478467B; TW201023960A; GB201109866D0

Abstract

There is disclosed a separation system for separation of a volatile component from a feed solution such as the separation of water vapor from a saline solution, such as seawater. The separation system includes a plurality of hollow fiber membranes being selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of the feed solution. The system also includes a heat source capable of heating the feed solution on one side of the hollow fiber membranes to create the volatile phase that passes through the hollow fiber membranes. A heat exchange means is provided to condense the volatile component and which is configured to capture the heat of condensation, and wherein the heat exchange means are thermally coupled to the heat source to thereby drive or supplement the heat source with the heat of condensation.

Description

SYSTEM FOR SEPARATION OF VOLATILE COMPONENTS FROM

SOLUTION

Technical Field

The present invention relates to system, method and apparatus for separating a volatile component from a solution.

Background

Membrane distillation is a membrane-based process in which water vapor is transported through the pores of hydrophobic membranes by a water vapor pressure driving force that is provided by temperature, vacuum and/or solute concentration differences across the membrane. The main characteristics of a membrane in a membrane distillation system is that the membrane desirably be porous and resistant to wetting by the process fluids, and desirably will not substantially alter the vapor- liquid equilibrium of the different components in the process fluids. Desirably, no capillary condensation should take place inside the pores of the membrane. For each component in the system, the driving force that drives the component through the membrane is the partial pressure gradient that exists between the two sides of a membrane .

In contrast to conventional evaporation processes, membrane distillation processes have the advantage of being capable of working effectively at relatively low operation temperatures, hence consuming relatively less energy compared to conventional distillation processes. Using low-grade energy sources, such as waste heat and solar energy, to drive the membrane distillation system makes it energy-efficient and environmental-friendly. However, a problem with the use of low-grade energy- sources is that the thermal energy of the low-grade energy source may vary over time. For example, a low- grade energy source such as a solar energy source will vary over the period of a day depending on the time of day and weather conditions. Furthermore, a low-grade energy source such as a waste heat source from, for example, a chemical production process will vary according to the process conditions in the process plant at any given time. Hence, it can be difficult to adapt a membrane distillation process due to the lack of consistency in the amount of energy being delivered by the low-grade heat source.

Composite membrane distillation systems with multiple stage cell configurations aiming to use the heat of condensation to supplement a heat source for generating the vapor phase of the volatile component face certain limitations. The limited heat duty from each stage used to provide the temperature gradient across another stage may result in a large number of optimum stages required before a reasonable yield of distillate can be achieved.

The structure of each membrane stage also limits the available surface and restricts flexibility for heat exchange. Control of heat duty is restricted because each membrane module and the heat exchanger are one and the same unit at each stage. Alternative heat exchanger configurations, which may optimize heat flow, are not permissible in such a case. Conventional membrane distillation systems face largely the same restriction; both the membrane function and heat exchanger function cannot be easily individualized, leading to a fixed surface area for heat exchange.

There is a need to provide a system or apparatus or process for separating a volatile component from a solution that overcomes, or at least ameliorates, the disadvantages described above.

Summary of invention

According to a first aspect of the invention, there is provided a separation system for separation of a volatile component from a feed solution, the system comprising: a plurality of hollow fiber membranes being selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of said feed solution; a heat source capable of heating the feed solution on one side of the hollow fiber membranes to create the volatile phase that passes through said hollow fiber membranes; and a heat exchange means to condense the volatile component and which is configured to capture the heat of condensation, said heat exchange means being thermally coupled to said heat source to thereby drive or supplement the heat source with said heat of condensation.

Advantageously, the use of the hollow fiber membranes provide a compact configuration with a relatively high surface area compared to flat-sheet membranes, to separate the volatile component from the feed solution. Advantageously, the heat exchange means utilizes the heat of condensation to drive or supplement the energy provided by the heat source in the separation process, thereby saving on energy input into the whole system. Advantageously, the heat exchange means reduces the consumption of cooling medium required to condense the volatile component. Accordingly, the combination of the hollow fiber membranes and the heat exchangers to recover the heat of condensation provide a system that is thermally efficient in separating volatile components from solution.

In one embodiment, there is provided a desalination system comprising: a plurality of hollow fiber membranes being selectively permeable to allow water vapor to pass therethrough while substantially preventing passage of saline water; a heat source capable of heating the saline water on one side of the hollow fiber membranes to create the water vapor phase that passes through said hollow fiber membranes; and a heat exchange means to condense the water vapor phase and which is configured to capture the heat of condensation, said heat exchange means being thermally coupled to said heat source to thereby drive or supplement the heat source with said heat of condensation.

According to a second aspect, there is provided a separation apparatus for separation of a volatile component from a feed solution, the apparatus comprising: a plurality of hollow fiber membranes that are selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of said feed solution; a plurality of hollow fiber modules, each module having an enclosed chamber with a subset of said plural hollow fiber membranes extending through the chamber, and an inlet for transmission of feed solution to one side of said hollow fiber membranes; a plurality of heat exchangers for fluid communication with the volatile component for capturing the heat of condensation of the volatile component as it condenses to a liquid, said heat exchangers being capable of being thermally coupled to said heat source to thereby drive or supplement the heat source with said heat of condensation.

According to a third aspect, there is provided a process of separating a volatile component from a feed solution, the process comprising the steps of: heating the feed solution with a heat source; passing a feed solution through one side of a plurality of hollow fiber membranes while a pressure differential exists between said hollow fiber membrane sides to form the volatile component in a volatile phase on the side of the hollow fiber membrane opposite to the side of the feed solution; condensing the volatile component in said volatile phase to thereby capture the heat of condensation; and using the heat of condensation to thereby drive or supplement heat to the heat source. Definitions

The following words and terms used herein shall have the meaning indicated:

The term "thermal duty" in the context of this specification refers to the amount of heat energy during a given period of time that is emitted by a "heat source" or "heat sink" (ie a "heat sink" is an apparatus that utilizes heat) . For example, a thermal duty of a heat exchange means refers to the amount of heat energy during a given period of time that is required to be exchanged from one fluid to another by a heat exchange means .

The term "variable thermal duty" as used herein refers to a thermal duty that varies over time.

The term "hollow fiber membrane" as used herein is intended to refer to a membrane having a hollow inner core surrounded by an enclosed wall. While some hollow fiber membranes may be in the form of a substantially circular shaped tube, the term should not be interpreted as indicating that the hollow fiber membrane is in the form of a circular tube but may have any cross-sectional shape. In this disclosure, the walls of the hollow fiber membranes are at least partially permeable to a selected chemical species. Thus, hollow fibers which are physically permeable (e.g., due to the presence of pores in the hollow fiber walls) and/or hollow fibers that are chemically permeable (e.g., due to the mass transport of a chemical species through the hollow fiber walls) are included within the meaning of this definition.

The term "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. That is, the term "substantially" is to be interpreted as "completely" or "partially". Where necessary, the word "substantially" may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a membrane module will now be disclosed.

In one embodiment, there is disclosed a separation system for separation of a volatile component from a feed solution such as the separation of water vapor from a saline solution, such as seawater. The separation system includes a plurality of hollow fiber membranes being selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of the feed solution. The system also includes a heat source capable of heating the feed solution on one side of the hollow fiber membranes to create the volatile phase that passes through the hollow fiber membranes. A heat exchange means is provided to condense the volatile component and which is configured to capture the heat of condensation, and wherein the heat exchange means are thermally coupled to the heat source to thereby drive or supplement the heat source with the heat of condensation.

The hollow fiber membranes may be hydrophobic polymers. Advantageously, when water is being removed from saline water in a desalination system, the hydrophobic polymers are resistant to wetting by the saline water liquid but permit the water vapor to be transmitted through the polymer, ^■ thereby enhancing the separation of the water vapor from the saline water. Exemplary hydrophobic polymers include poly alkyl acrylate, polydiene, polyolefin, polylactone, polysiloxane, polyoxirane, polypyridine, polycarbonate, poly vinyl acetate, polysulfone, polypropylene (PP), polytetrafluoroethylene (PTFE), polyethylene (PE), polyvinylidenefluoride (PVDF) , polymethylpentene (PMP) , polydimethylsiloxane, polybutadiene, polystyrene, polymethylmethacrylate, perfluoropolymer, poly (2-alkyl or phenyl oxazolines), derivatives, salts and combinations thereof.

Advantageously, the type of hollow fiber membrane can be chosen depending on the volatile component that is to be separated from the feed solution. For example, in another embodiment, there is disclosed a separation system for separation of a volatile component from waste oil. The separation system includes a plurality of hydrophilic tubular stainless steel membranes being selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of the feed solution. The stainless steel membrane prevents the permeation of the waste oil due to its hydrophilic nature, while the evaporated volatile components in a volatile phase pass through the membrane, and to be thereby separated from the waste oil.

The feed solution refers to any liquid solution containing a volatile component. That is, the volatile component is one which may be vaporized and thereby be separated from the liquid solution. For example, the feed solution may be water comprising a volatile organic compound which has a lower boiling point that the water. Another example is where the feed solution may be an aqueous solution containing one or more solutes dissolved therein and in which the solvent, substantially free of the one or more solutes, may be the volatile component. It should also be realized that in some embodiments, removal of the volatile component from the feed solution may be not to captures the volatile component but to concentrate the feed solution. For example, in a fruit juice fabrication process, it may be desirable to remove water from the fruit juice in order to concentrate the fruit juice for transportation.

In one embodiment, the separation system comprises means for altering the thermal duty of the heat exchange means according to the heat duty of the heat source. Such means may include a system of valves that may be opened and closed. A programmable logic controller may be used to control the opening and closing of these valves.

The heat exchange means may comprise at least one heat exchanger. The heat exchangers may be arranged in series or parallel fluid flow with respect to each other. In one embodiment, the heat exchange means comprises plural heat exchangers and the means for altering the thermal duty of the heat exchangers comprises means for altering the number of heat exchangers capable of receiving the volatile component. Each heat exchanger may have at least one valve to the flow of the volatile component. The heat exchanger is capable of receiving the volatile component when the valve is opened. When the valve of a particular heat exchanger is closed, the volatile component will not be able to enter that particular heat exchanger. The volatile component will then be directed to another heat exchanger with an open valve. In this way the number of heat exchangers capable of receiving the volatile component may be controlled.

Advantageously, the number of heat exchangers used may be selected to recover as much latent heat from the volatile components as possible, and thus increase the efficiency of the separation system. More advantageously, the separation system is able to function as a dynamic system, and may be customized depending on the process conditions, more particularly the heat duty of the heat source. This is particularly advantageous when the heat source is variable. With the use of a customizable heat exchange means, the overall thermal energy consumption of the separation system is greatly reduced.

The separation system may comprise at least one hollow fiber module. In one embodiment, the separation system comprises plural hollow fiber modules. Each hollow fiber module comprises a chamber having a subset of plural hollow fiber membranes disposed therein. The plural hollow fiber modules may further be in series fluid flow with respect to each other.

The heat exchange means may be disposed in between the hollow fiber modules in the separation system. In one embodiment, the heat exchange means comprises a plurality of heat exchangers, wherein at least one heat exchanger is disposed between an upstream hollow fiber module and a downstream hollow fiber module in series fluid flow with respect to each other.

Each heat exchanger is capable of capturing the heat of condensation of the volatile phase to heat the feed solution in a hollow fiber module. In one embodiment, the heat exchanger disposed between the upstream and downstream hollow fiber modules is configured to capture the heat of condensation of the volatile phase of the upstream hollow fiber module and use the captured heat to heat the feed solution passing into the downstream hollow fiber module.

The heat source is configured to heat the feed solution passing into the plurality of hollow fiber modules. The heat source may heat all the hollow fiber modules simultaneously, or a combination of hollow fiber modules simultaneously, or each of the hollow fiber modules independently. In one embodiment, the heat source is configured to independently heat the feed solution passing into each of the plurality of hollow fiber modules. Advantageously, each hollow fiber module can be operated individually and the operation parameters can be controlled individually.

The heat source may comprise any suitable heat source. The heat source may be of a constant thermal duty or a variable thermal duty. In one embodiment, the heat source is of a variable thermal duty. For example the heat source with the variable thermal duty may be a waste heat source or a solar heat source or a geothermal heat source .

In one embodiment, the heat source may comprise a waste heat source. Exemplary waste heat sources include flue gases from gas turbines in power plants and incinerators, process gases of chemical and metallurgical operations and waste heat from other industrial processes .

In countries that enjoy a warmer climate, solar energy may be used as the heat source. The solar heat may heat the feed solution to temperatures from about 40 degrees to 95 degrees, from about 50 degrees to about 95 degrees, from about 50 degrees to 75 degrees. In the solar heating system, the solar heat is concentrated onto a heating fluid such as water. The heating fluid is then circulated in vacuum tubes and configured to exchange its heat with the feed solution in a heat exchanger, thereby heating up the feed solution.

In one embodiment, there is disclosed, a separation apparatus for separation of a volatile component from a feed solution. The apparatus comprising: a plurality of hollow fiber membranes modules that are selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of the feed solution; a plurality of hollow fiber modules, each module having an enclosed chamber with a subset of the plural hollow fiber membranes extending through the chamber, and a inlet for transmission of feed solution to one side of the hollow fiber membranes; a heat source thermally coupled to the feed solution; and a plurality of heat exchangers fluidly couple to the volatile components and which are configured to capture the heat of condensation, and wherein the heat exchange is thermally coupled to the heat source to thereby drive or supplement the heat source with the heat of condensation.

The plurality of hollow fiber modules may be in series, parallel or a combination of series and parallel fluid flow with respect to each other. In one embodiment, the plurality of hollow fiber modules are in series fluid flow with respect to each other.

The feed solution may pass through each plurality of hollow fiber modules, or may be able to by-pass at least one hollow fiber module. In one embodiment, the feed solution is able to by-pass one or more of the plurality of hollow fiber modules.

Each hollow fiber module has an enclosed chamber with a subset of the plural hollow fiber membranes extending through the chamber These hollow fiber membranes may be hydrophobic polymeric membrane and may further be selected from the group consisting of polyvinylidene fluoride, polypropylene, polyethylene and polytetrafluoroethylene .

The separation apparatus may further comprise a monitoring means coupled to the heat source for monitoring the heat duty of the heat source. The monitoring means may comprise at least one of or a combination of monitoring devices such as heat sensors, temperature transmitters, temperature sensors or thermocouples, coupled to a controller.

The separation apparatus may further comprise a control device coupled to the monitoring means and the plurality of heat exchangers. The control device may be capable of determining the number of heat exchangers to be used in accordance with the monitored heat duty. Advantageously, the number of heat exchangers to be used can be customized in accordance with the variable heat source. There is further disclosed, a process of separating a volatile component from a feed solution, comprising the steps of: heating the feed solution with the heat source; passing a feed solution through one side of a plurality of hollow fiber membrane while a pressure differential exists between the hollow fiber membrane sides to for the volatile component in a volatile phase on the side of the hollow fiber membrane opposite to the side of the feed solution; condensing the volatile component in said volatile phase to thereby capture the heat of condensation; using the heat of condensation to thereby drive or supplement heat to the heat source.

The process may also comprise the step of varying the flux of the heat source. When the flux of the heat source is varied, the process may further comprise the steps of using a plurality of heat exchangers to capture the heat of condensation. The thermal duty of the heat exchangers may then be altered according to the variable heat duty of the heat source.

The process may further comprise the steps of (i) providing a plurality of hollow fiber modules, each module comprising a chamber having a subset of hollow fiber membranes disposed therein, wherein the hollow fiber modules are in series fluid flow with respect to each other, and (ii) independently heating the feed solution passing into each of the plurality of hollow fiber modules.

The volatile component may be separated from the liquid by creating a partial pressure difference between a lumen side and a shell side of the hollow fiber membranes. This may be achieved by connecting a outlet end of the hollow fiber membranes to a negative pressure source. The negative pressure is applied to form a vacuum on the lumen side of the hollow fiber membranes. This pressure difference assists in the removal of a volatile component from the liquid and allows the volatile component to be vaporized at a lower temperature, thereby allowing vaporization using low- grade heat sources.

Exemplary volatile components include water, and organic compounds such as esters, ethers, aldehydes, alcohols, nitriles and unsaturated hydrocarbons (e.g., terpenes) . In one embodiment, the feed solution may be a saline solution such as brackish water or seawater and the volatile component may be water that is evaporated from the saline water as water vapour that is substantially free of salt.

The cooling agent used for condensation the evaporated volatile component of the last stage can be any cooling liquid. In one embodiment, the cooling liquid is water at room temperature (ie about 20 deg. C) . Advantageously, room temperature water is readily available in an industrial plant, and can be recycled or disposed of easily.

Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig. 1 shows a schematic diagram of a membrane distillation process in accordance with one disclosed embodiment .

Fig. 2 shows a schematic diagram of a membrane distillation process in accordance with another disclosed embodiment .

Fig. 3 shows an embodiment of a stage of the membrane distillation process of Fig. 2.

Fig. 4 shows a control diagram for controlling the membrane distillation process of Fig.2.

Fig. 5 and shows an algorithm chart for the operation of the programmable logic controller (PLC) of Fig. 4.

Fig. 6 shows a control diagram for controlling the membrane distillation process of Fig.l.

Fig. 7 and shows an algorithm chart for the operation of the programmable logic controller (PLC) of Fig. 6.

Detailed Description

Fig. 1 shows an energy saving membrane distillation process 100 comprising stages A, B, C, ... n, where A represents the first stage, B represents the second stage and n represents the nth stage. Each stage respectively comprises storage tanks (1OA, 1OB, 1OC, ... 1On) containing seawater, heat exchangers (12A, 12B, 12C, ... 12n) and membrane modules (14A, 14B, 14C, ... 14n) . Each of the membrane modules (14A, 14B, 14C, ... 14n) has a subset of plural hollow fiber membranes (not shown) disposed therein.

Referring to stage A of the membrane distillation process 100, a seawater liquid stored in a storage tank 1OA flows through feed stream HA to a heat exchanger 12A. The seawater feed stream HA is heated up as described below, and leaves the heat exchanger 12A via heated seawater stream 13A.

A heated fluid having a temperature higher than that of the seawater in feed stream HA flows into the heat exchanger 12A via heated fluid stream 16A. The heated fluid passing through stream 16A has been heated by- a solar heat source in which solar energy has been used to heat the heated fluid. The heated fluid serves as a heat energy source for heating the seawater passing through the heat exchanger 12A. After transferring the heat energy contained in the heated fluid to the seawater passing through the heat exchanger 12A, the heated fluid from 16A cools to a lower temperature and leaves the heat exchanger 12A via cooled fluid stream 18A.

The heated seawater 13A, after passing through the heat exchanger 12A, flows into the membrane module 14A which is under negative pressure (ie vacuum) . The membrane module 14A like the other membrane modules (14B, 14C, ... 14n) comprises a chamber with an inlet conduit for receiving the seawater and outlet for allowing seawater to be removed from the chamber. As described above, within the chamber of the module 14A are a plurality of hollow membrane fibers (not shown) which each have an open end at one end of the tube and a closed end at the other end of the tube. The hollow fiber membranes are made of a hydrophobic polymer (polypropylene, polyethylene, polyvinylidene fluoride, or polytetrafluoroethylene) which are permeable to water vapor but generally impermeable to water liquid. A vacuum is applied to the lumen or shell side of the hollow fiber membranes, which also creates a vacuum in the chamber 14A

When the heated seawater stream 13A, enters the chamber of the membrane module 14A, the water evaporates as water vapor that is substantially free of salt from the bulk of the seawater contained therein. The water vapor passes through the wall of the hollow fibers membranes into the lumen of the hollow fiber membranes ^' disposed within membrane module 14A, and then leaves via the open ends of the hollow fiber membranes as vapor stream 17A. The bulk of the seawater, which is now enriched in salt, remains in the chamber of the membrane module 14A and is removed therefrom via product stream 15A. The vapor stream 17A is insulated to prevent heat loss and is maintained at the negative pressure to ensure that the water remains in a vaporized state before reporting to the downstream heat exchanger 12B as will be described further below.

It should be realized that the other membrane modules (14B, 14C, ... 14n) work on the same principle as described above for membrane module 14A.

When in operation, it may be necessary for the seawater to undergo more than one cycle of the distillation process as described above to substantially evaporate the water from the seawater liquid. Therefore, the bulk of the seawater passing through the membrane chamber 14A may be returned to the storage tank 1OA via product stream 15A for further processing as described above, until substantially all the water is removed from the seawater.

When the water vapor from vapor stream 17A condenses into water liquid, the latent heat of condensation is given off within the downstream heat exchanger 12B. This heat of condensation is exchanged with the seawater entering via feed stream HB from storage tank 1OB in stage B. The condensed water liquid from water liquid stream 28A is collected as water that is substantially- free of salt.

The vapor stream 17A functions similar to the heated fluid stream 16A in that the vapor stream 17A provides the heat energy for heating up the seawater passing through the heat exchanger 1OB.

For convenience, the plural repetition of heat of condensation capture followed by subsequent seawater heating to evaporated water as water vapor is not described for the other stages B, C, ... n. However, it should be appreciated that stages B, C, ... n work on the same principle as described above for stage A.

It will further be appreciated that the process of capturing the heat of condensation in an upstream heat exchange module and then using that captured heat to heat another downstream seawater feed entering a hollow fiber membrane module can be repeated any number of times until this last downstream repetition which is shown as stage n.

In stage n, the vapor stream 17n is the by-products of the membrane distillation process 100 described herein and may be further processed or utilized for other purposes (not described here) .

A second embodiment will now be disclosed with reference to Fig. 2. Fig. 2 has ^'the same technical features as Fig. 1 and are represented by the same numerals but with the prime symbol (M- Fig. 2 shows a membrane distillation process 100' comprising stages A' , B' , C , ... n' , where A' represents the first stage, B' represents the second stage and n' represents the nth stage. Each stage respectively comprises storage tanks (10A', 1OB', 10C, ... 1On') containing seawater, heat exchangers (12A', 12B' , 12C , ... 12n' ) and membrane modules (14A', 14B' , 14C , ... 14n' ) . Each of the membrane modules (14A', 14B' , 14C , ... 14n' ) has a subset of plural hollow fiber membranes (not shown) disposed therein. The stages A', B', C, ... n' and their respective components are the same as those as described above for the process 100 shown in Fig. 1 above.

In this process 100' , the evaporation of water from the seawater liquid, and the subsequent capturing of the heat of condensation to heat the seawater in the downstream stage is the same as that described in stage A of Fig. 1. In addition to the heat of condensation, a supplementary heat source is provided to each of the heat exchangers (12B', 12C , ... 12n' ) in each of the stages B' , C , ... n' , as described below

Referring to stage B' , a heated fluid having a temperature higher than that of the seawater in feed stream HB' flows into the heat exchanger 12B' via heated fluid stream 16B' . The heated fluid stream 16B' serves as a supplementary heat energy source for heating the seawater stream HB' passing through the heat exchanger 12B' . After transferring the heat energy contained in the heated fluid stream 16B' to the seawater stream HB' , the heated fluid from 16B' cools to a lower temperature and leaves the heat exchanger 12B' via cooled fluid stream 18B' .

Referring now to stage A' , the seawater liquid that remains in membrane chamber 14A' after the water is evaporated, may be returned to the storage tank 1OA' via product stream 15A' for further processing. A conduit 2OA' allows seawater in storage tank 1OA' to be fluidly coupled to storage tank 1OB' . In this way, the seawater in stage A' of the membrane distillation process 100' can be distilled further in stage B' .

Similarly for stages B', ... (n-1)', the respective storage tanks (10B', ... 10 (n-1)') are fluidly coupled to their respective downstream storage tanks (10C, ... 1On' ) so that the seawater in each of the stages can be further distilled in the downstream stage.

A third embodiment will now be disclosed with reference to Fig. 3, which has the same technical features as Fig. 1 and are represented by the same numerals but with the double prime symbol C') . Fig. 3 shows stage A' ' , which is an alternate embodiment of a stage A' in Fig. 2. Stage A'' comprises a storage tank 1OA' ' , containing seawater, heat exchangers 12A' ' and 12 B'' and membrane modules (14A"-1, 14A"-2, ... 14A''-n) . The elements in stage A' ' are the same as those in stage A' of Fig. 2 above, and they are described here using the same reference numerals but with a double prime (' ' ) symbol . In this embodiment, there are plural membrane modules (14A''-1, 14A'' -2, ... 14A''-n) in a single stage A' ' . In addition, the membrane chamber of the membrane modules (14A"-1, Ϊ4A"-2, ... 14A"-n) further comprise at least one additional outlet and bypass conduits (26A"-1, 26A"-2, 26A"-3, ...) for allowing the seawater in any of the membrane modules (14A''-1, 14A' ' - 2, ... 14A''-n) to bypass any of the subsequent downstream modules. The advantage of this is that if any one of the modules malfunction, the seawater will be able to bypass the malfunctioning module and continue with the distillation process.

The seawater leaving heat exchanger 12A' ' enters the first membrane module 14A''-1 via conduit 24A'' . It should be realized that the membrane module 14A''-1 works on the same principle as described above in Fig. 1 for membrane module 14A. After being distilled in membrane module 14A''-1, the remaining seawater travels via conduit 24A''-1 to membrane module 14A'' -2 for further distillation.

It should be appreciated that this can be repeated any number of times until this last downstream membrane module 14A''-n, where the remaining seawater can either be collected via conduit 24A''-n, or returned back to the feed stream HA' ' via recycled stream 15A' ' for further processing.

Fig. 4 shows a membrane distillation process 400, which corresponds to stages A' , B' and C of the membrane distillation process of Fig.2, and further comprising programmable logic controllers (PLCs) (3OA, 30B) . The components in Fig. 4 are the same as those in Fig. 2 except that they are labeled with an asterisk (*).

The PLCs (3OA, 30B) respectively connect to the vapor streams (17A*, 17B*) of each^" stage and measure the temperatures therein. The temperatures of vapor streams (17A*, 17B*) are hereinafter respectively denoted as Ti_7A and Ti_7B.

A temperature "T_set-30A" is inputted in the PLC 3OA, wherein T_set-_30A is the minimum temperature required for heating up the seawater passing through the heat exchanger (12B*) . After comparing the measured temperature Ti_7A to the set temperature T_set-30A? the PLC (30A) controls the opening and closing of the valves (Vi₇A, V₃₂A and V_I6B) respectively on the vapor stream 17A*, a bypass stream 32A and the heated fluid stream 16B*. The detailed control will be further described below.

Because the heat source is solar powered, the heat energy of the heat source may vary over time from day to day. For example, the early and later part of daylight hours has less energy than compared to the middle part of daylight hours. Hence, the heat energy provided by the solar power source is variable with time, which means that the duty of each stage in the process 400 need to be varied according to the particular energy load being imparted at any given time. For example, the higher the temperature of the vapor stream 17A* (ie typically during the middle of daylight hours) , the more heat energy is dissipated upon the condensation of the water vapor contained therein. Therefore, if the temperature Ti_7A is higher than the temperature T_Set-30Ar a supplementary heat source supplied via heated fluid stream 16B* is not required. Accordingly, when the measured temperature Ti_7A is higher than the set temperature T_set-30A, valve Vi_7A opens while valve V32A and valve V_16B close so that the water vapor in the vapor stream 17A* flows to the heat exchanger 12B* of stage B' .

On the other hand, when the measured temperature Ti_7A is lower than the set temperature T_set-₃₀A (ie typically during the the early or later part of daylight hours), it indicates that there is insufficient heat energy contained in the vapor stream 17A*. Therefore, valve Vi_7A closes while valve V_32A opens so that the water vapor in vapor stream 17A* does not enter the heat exchanger 12B* of stage B' . The water vapor in vapor stream 17A* is the by-product that leaves the membrane module 14A* via bypass stream 32A. Furthermore, valve Vi₆B opens so that a supplementary heat source entering via heated fluid stream 16B* is supplied to the heat exchanger 12B* to enable the heating of the seawater in feed stream HB* passing therethrough.

Similarly, a PLC 3OB connects to the vapor stream 17B* of stage B' and controls the opening and closing of the valves (Vi_7B, V₃2B and Vi₆c) after determining the temperature (denoted as "Ti_7B") of the vapor stream 17B*.

An algorithm chart 500 detailing the operation of the PLC (30A) of Fig. 4 is shown in Fig. 5.

As described above, the PLC 3OA is programmed to control the opening and closing of the valves (Vχ_7A, V_32A and VI6B) • When T_i7A is higher than T_set-30A, the PLC 3OA sends an electrical signal for the opening of the valve V_i7A and the closing of the valve V_32A and valve Vi_6B. On the other hand, when T_i7A is lower than T_set-30A^ the PLC 3OA sends an electrical signal for the closing of the valve Vi_7A and the opening of the valve V₃₂A and valve Vi₆B-

Accordingly, the PLC 3OA controls the flow of water vapor from vapor stream 17A* and supplementary heated fluid 16B* into the heat exchanger 12B*.

Fig. 6 shows another controlled membrane distillation process 600, which corresponds to stages A and B of the membrane distillation process of Fig. 1, and further comprising a programmable logic controller (PLC) (40B) and a plurality of heat exchangers (12B*-1, 12B*-2, ... 12B*-N) are connected in series. All other components in Fig. 6 are the same as those in Fig. 1 except that they are labeled with double asterisks (**) .

When in operation, one or more of the heat exchangers (12B*-1, 12B*-2, ... 12B*-N) may be used for heating the seawater feed HB*. The PLC 4OB measures the temperature of the water vapor, liquid water of mixtures thereof in the streams (38-1, 38-2, ... 38-(N-I)) respectively leaving the heat exchangers (12B*-1, 12B*-2, ... 12B*-N) to thereby control the flow of the seawater feed HB* through these heat exchangers (12B*-1, 12B*-2, ... 12B*-N) via the opened valve Vi_1B or to bypass one or more of the heat exchangers (12B*-2, ... 12B*-N) via the bypass streams (50-1, 50-2, ... 50-(N-I)) . The temperatures of the streams (38-1, 38-2, ... 38-(N-I)) are hereinafter respectively denoted as T₃₈-I, T₃₈_₂, - T₃₈-(N-_D •

The control of the flow of the seawater feed HB** is dependent on the measured temperatures T₃₈-I, T₃₈_2, ... T₃₈-(N-_D- The measured temperatires T₃₈-I, T₃₈_₂, .^'.. T₃₈-(N-_D is individually compared to a set temperature "T_set-4₀B" which is inputted in the PLC 4OB. The set temperature T_Set-4₀B is the minimum temperature required for heating up the seawater passing through each of the heat exchangers (12B*-1, 12B*-2, ... 12B*-N) .

The control of the flow of the seawater feed HB** through the heat exchangers (12B*-1, 12B*-2, ... 12B*-N) are facilitated by the opening and closing of the valves (V₁IB, V₅₀-I, V₅₀-2, - V₅₀-(N-I)) respectively along the seawater feed stream HB** and the bypass streams (50-1, 50-2, ... 50-(N-I)) as will be further described below. All valves (Vn_B, V₅₀-I, V₅₀-₂, ... V₅₀-_(N-i)) are closed unless otherwise indicated in the description below.

When in operation, the PLC 4OB measures the temperature T₃₈-_I in stream 38-1 leaving the heat exchanger 12B*-1. If the measured temperature T₃₈-₁ is not more than the set temperature T_set-₄₀B, PLC 4OB sends an electrical signal for the opening of valve V₅₀-_I . This is because there is insufficient heat energy in stream 38-1 for the operation of the rest of the heat exchangers (12B*-2, ... 12B*-N) . The feed HB** therefore flows directly into heat exchanger 12B*-1, bypassing the rest of the heat exchangers (12B*-2, ... 12B*-N) via feed stream 50-1.

On the other hand, if the measured temperature T₃₈-₁ is more than the set temperature T_set-40B, the PLC 4OB sends an electrical signal for the opening of valve Vn₈, and PLC 40B measures the temperature of the fluid leaving the next heat exchanger 12B*-2.

Similarly, if the measured temperature T₃₈_₂ is not more than the set temperature T_set-4₀B, PLC 4OB sends an electrical signal for the opening of valve V₅₀_₂. The seawater feed HB** flows directly into the next heat exchanger 12B*-2 via feed stream 50-2 and subsequently into the heat exchanger 12B*-1.

Likewise, if the measured temperature T₃₈-2 is more than the set temperature T_set-40Br the PLC 4OB sends an electrical signal for the opening of valve V_UB, and PLC 4OB measures the temperature of the fluid leaving the next heat exchanger (12B*-3, not shown) .

As described above, if the temperature of the stream 38-(N-I) entering the heat exchanger 12B*-N is not more than the set temperature T_set-4₀B, the PLC 4OB sends an electrical signal for the opening of the valve V₅O-(N-_D- This is so that the seawater feed HB** flows directly into heat exchanger 12B*- (N-I) (not shown) via feed stream 50-(N-I) and subsequently into heat exchangers (12B*-(N-2) (not shown), ... 12B*-2, 12B*-1) but bypasses the heat exchanger 12B*-N.

If the measured temperature T₃₈-(_N-_U is more than the set temperature T_set-_40B/ PLC 40B sends an electrical signal for the opening of valve Vn₈. That is, the seawater feed HB** flows sequentially through all of the heat exchangers (12B*-N, ... 12B*-2, 12B*-1) that connected in series with one another.

Accordingly, the number of heat exchangers (12B*-1, 12B*-2, ... 12B*-N) that connected in series that is to be used can be varied depending on whether there is sufficient heat energy, as indicated by the temperature, for heating up the seawater feed HB**.

Fig. 7 shows an algorithm chart 700 for the operation of the PLC 4OB of Fig. 6. Unless otherwise stated, when in operation, all valves (Vn₈, V_nB, V₅₀-I, V₅o-2/ - V₅₀-(N-_D )r controlled by the PLC 40B (as shown in Fig. 6) are closed.

The PLC 4OB measures the temperature T₃₈-11 of the stream 38-n, wherein n is 1, 2, ... N-I. If the measured temperature is higher than the set temperature T_set-40B, the PLC 4OB sends an electrical signal for the opening of the valve Vn_B.

On the other hand, if the measured temperature T38-_n is lower than the set temperature T_set-4₀Br the PLC 4OB sends an electrical signal for the opening of the valve ^v ₅₀-n_r wherein n is 1, 2, ... N-I.

In accordance with the description of Fig. 6 and Fig. 7 above, it is appreciated that the PLC 4OB enables the adjustment of the total heat exchange surface area that is required in the membrane distillation process 600.

Example 1

The separation system as described was used for concentrating sodium chloride solution by a multi-stage membrane distillation process. Each stage of the membrane distillation process comprises a membrane distillation module and a heat exchanger unit. The feed liquid was sodium chloride solution and the flow rate of the sodium chloride solution to be concentrated was 1000 kilograms per day with an initial concentration of 6%.

The external heating source is only provided to the feed liquid at the first stage of the membrane distillation module. In this example, the external heating source is thermal energy converted from the consumption of electricity. The thermal energy that is provided by the heat source is 5XlO⁸ joules per day.

The concentrated sodium chloride solution is passed through a heat exchanger and is heated up by the external heating source. The solution then enters the membrane distillation module and the water in the solution evaporates to form water vapor. The membrane distillation module comprises a membrane chamber as described in Fig. 1 above. The membrane is made of polypropylene with a diameter of 0.4 to 0.6 millimeters. Each module houses approximately 60,000 to 80,000 fibers.

The evaporated water vapor from the first stage of the membrane distillation module is taken out via the heat exchanger unit, to heat up the feed liquid to a temperature of about 65 degrees, at the second stage of the membrane distillation module.

The feed liquid at the second stage is the remaining solution that did not evaporate in the first stage. After heating up the feed liquid, the water vapor condenses to form water liquid. The amount of water that was evaporated from the first stage was 190 kilograms.

Similarly, the evaporated water vapor from the second stage of the membrane distillation module is taken out via the heat exchanger unit, to heat up the feed liquid to a temperature of about 55 degrees, at the third stage of the membrane distillation module. The amount of water that was evaporated from the second stage was 180 kilograms .

The evaporated water vapor from the third stage membrane distillation module is taken out to heat up the feed liquid to a temperature of about 45 degrees, at the fourth stage of the membrane distillation module. The amount of water that was evaporated from the third stage was 170 kilograms.

At the fourth stage, the evaporated water vapor was cooled with room temperature water. The evaporated water amount from the fourth stage was 160 kilograms.

The final concentration of the sodium chloride solution was 20%. The thermal energy consumption per evaporation of one ton of water is 2.63X10⁶ joules for a single-stage process, 1.35X10⁶ joules for a two-stage process, 0.93X10⁶ joules for a three-stage process and 0.71X10⁶ joules for a four-stage process.

Thus, when compared to a single-stage process, a four-stage process can reduce the energy consumption of a separation system by 73%.

Applications

It will be appreciated that the disclosed system separates the volatile component from a feed solution by a membrane distillation process. Advantageously, hollow fiber membranes are used, which provide a much larger surface area compared to polymer sheet distillation. This increases the flux of the membrane distillation process, leading to a more efficient process and lower costs.

The disclosed system is comprised of individual stages which can be operated individually. Each stage comprises at least one membrane module and at least one heat exchanger unit. Advantageously, the operational parameters of the system can be controlled individually. It will be appreciated that the heat exchange means of the separation system allows thermal energy consumed in the evaporation process to be recovered by the feed solution. Advantageously, the overall thermal energy- consumption of the separation system is greatly reduced.

It will be appreciated that the heat exchange means comprises at least one heat exchanger. Advantageously, the number of heat exchangers used may be selected to recover as much latent heat from the evaporated volatile components as possible, and thus increase the efficiency of the separation system.

More advantageously, the energy efficient design of the disclosed separation system overcomes the predominant problems associated with conventional membrane distillation processes.

While reasonable efforts have been employed to describe equivalent embodiments of the present invention, it will be apparent to the person skilled in the art after reading the foregoing disclosure, that various other modifications and adaptations of the invention may be made therein without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A separation system for separation of a volatile component from a feed solution, the system comprising: a plurality of hollow fiber membranes being selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially- preventing passage of said feed solution; a heat source capable of heating the feed solution on one side of the hollow fiber membranes to create the volatile phase that passes through said hollow fiber membranes; and a heat exchange means to condense the volatile component and which is configured to capture the heat of condensation, said heat exchange means being thermally coupled to said heat source to thereby drive or supplement the heat source with said heat of condensation.

2. A separation system according to claim 1, wherein said hollow fiber membranes are hydrophobic.

3. A separation system according to claim 2, wherein said hydrophobic hollow fiber membranes are comprised of hydrophobic polymers.

4. A separation system according to claim 1, comprising means for altering thermal duty of the heat exchange means according to the heat duty of the heat source.

5. A separation system according to claim 4, wherein the heat exchange means comprises plural heat exchangers and said means for altering the thermal duty of the heat exchangers comprises means for altering the number of said heat exchangers capable of receiving the volatile component .

6. A separation system according to claim 1, comprising plural hollow fiber modules, each hollow fiber module comprising a chamber having a subset of said plural hollow fiber membranes disposed therein.

7. A separation system according to claim 6, wherein the plural hollow fiber modules are in series fluid flow with respect to each other.

8. A separation system according to claim 6, wherein the heat exchange means comprises a plurality of heat exchangers, wherein at least one heat exchanger is disposed between an upstream hollow fiber module and a downstream hollow fiber module in series fluid flow with respect to each other.

9. A separation system according to claim 8, wherein the heat exchanger disposed between said upstream and downstream hollow fiber modules is configured to capture the heat of condensation of the volatile phase of the upstream hollow fiber module and use said captured heat to heat the feed solution passing into the downstream hollow fiber module.

10. A separation system according to claim 6, wherein the heat source is configured to independently heat the feed solution passing into each of said plurality of hollow fiber modules.

11. A separation system according to claim ,1, wherein the heat source is of a variable thermal flux.

12. A separation system according to claim 1, wherein the variable thermal flux is caused by the heat source comprising at least one of: (i) a waste heat source; (ii) a solar heat source; and (iii) a geothermal heat source.

13. A separation apparatus for separation of a volatile component from a feed solution, the apparatus comprising: a plurality of hollow fiber membranes that are selectively permeable to allow the volatile component in a volatile phase to pass therethrough while substantially preventing passage of said feed solution; a plurality of hollow fiber modules, each module having an enclosed chamber with a subset of said plural hollow fiber membranes extending through the chamber, and an inlet for transmission of feed solution to one side of said hollow fiber membranes; a plurality of heat exchangers for fluid communication with the volatile component for capturing the heat of condensation of the volatile compoment as it condenses to a liquid, said heat exchangers being capable of being thermally coupled to said heat source to thereby drive or supplement the heat source with said heat of condensation.

14. A separation apparatus according to claim 13, wherein said plurality of hollow fiber modules are in series fluid flow with respect to each other.

15. A separation apparatus according to claim 14, wherein the feed solution is able to by-pass one or more of said plurality of hollow fiber modules.

16. A separation apparatus according to claim 13, wherein said hollow fiber membranes are comprised of hydrophobic polymer membranes.

17. A separation apparatus according to claim 16, wherein said hydrophobic polymeric membranes are selected from the group consisting of polyvinylidene fluoride, polypropylene, polytetrafluoroethylene, polyethylene.

18. A separation apparatus according to claim 13 further comprising a monitoring means coupled to the heat source for monitoring the heat duty of the heat source.

19. A separation apparatus according to claim 18 further comprising a control device coupled to the monitoring means and the plurality of heat exchangers, wherein said control device is capable of determining the number of heat exchangers to be used, in accordance with the monitored heat duty.

20. A process of separating a volatile component from a feed solution, comprising the steps of: heating the feed solution with a heat source; passing a feed solution through one side of a plurality of hollow fiber membranes while a pressure differential exists between said hollow fiber membrane sides to form the volatile component in a volatile phase on the side of the hollow fiber membrane opposite to the side of the feed solution; condensing the volatile component in said volatile phase to thereby capture the heat of condensation; and using the heat of condensation to thereby drive or supplement heat to the heat source.

21. A process according to claim 20 comprising the step of varying the flux of said heat source.

22. A process according to claim 21 comprising the steps of: using a plurality of heat exchangers to capture the heat of condensation; and altering the thermal duty of the heat exchangers according to the variable heat duty of the heat source.

23. A process according to claim 20, comprising the steps of: providing a plurality of hollow fiber modules, each module comprising a chamber having a subset of said hollow fiber membranes disposed therein, said hollow fiber modules being in series fluid flow with respect to each other; and independently heating the feed solution passing into each of said plurality of hollow fiber modules.