Apparatus for purifying contaminated liquid

Field of the invention

The present invention relates to an apparatus for purifying contaminated liquid. In particular, although not exclusively, the invention relates to a desalination unit which relies on evaporation of the contaminated liquid and subsequent condensation to separate potable water from impurities. The invention also relates to a method of purifying contaminated liquid. A transportable purification unit is also disclosed. While the invention is often described below in terms of desalination, it is to be understood that the invention is not limited to the mere separation of salt from water but can be applied to contaminated liquid containing many other types of impurities.

Background of the invention

There are several conventional desalination methods. Each of these suffers from disadvantages which prohibit wide scale use. The first of these known methods is reverse osmosis. This technique involves the use of reverse osmosis membranes which separate dissolved solids from water. However, these membranes are particularly expensive which hinders the widespread use of this method.

Another known method of desalination uses distillation of seawater through a process of evaporation and condensation to produce potable water. Such methods typically employ high temperatures, thereby requiring users and maintenance personnel to have special qualifications to operate the equipment eg. boilermaker's certificate. Furthermore, the more advanced techniques of distillation known as flash distillation use multiple stages, pressures and vacuums but still result in low yields.

Evaporative techniques are known by creating small droplets of the contaminated liquid in an airstream. One known method published in US patent no 6,699,369 uses high pressure nozzles to create very small droplets less than 20 microns, in order to get full evaporation of the droplets over a short distance. However, in order to separate the salt crystals from the water vapour, the apparatus requires a filter such as an electrostatic particle filter. The high pressure pumps and nozzles required to create the

fine mist and the need for the particle filter increase the costs associated with such a system.

It is therefore an object of the present invention to provide an alternative apparatus for purifying contaminated liquid which provides the public with a useful economic choice.

The foregoing prior art discussion is not to be taken as an admission of common general knowledge.

Summary of the invention

In accordance with a first aspect of the present invention there is provided an apparatus for purifying contaminated liquid including:

an airflow means for creating an airstream;

a disperser for creating droplets of the contaminated liquid in the airstream;

a heater disposed upstream of the disperser for heating the airstream, wherein the apparatus is operable within a range of predetermined conditions to cause only partial evaporation of water from a substantial proportion of the droplets; and

a condenser for condensing the evaporated water, wherein the condenser is spaced downstream from the heater and/or the disperser such that under the predetermined conditions, the droplets settle out of the air under gravity prior to reaching the condenser.

In a preferred form of the invention, the disperser operates to eject a stream of droplets into the airstream. Accordingly, the apparatus may be provided with nozzles, misters or atomisers. Small droplet size is generally not critical. Therefore, the nozzles may operate at a pressure less than 100 psi. As an addition or alternative to nozzles, the airstream may pass through other types of droplet formation means, including fountains and water curtains.

The airstream may be created by fans which preferably blow air from the disperser to the condenser.

The heater may operate by heating the airstream. In a preferred form of the invention, the heater operates by solar energy. The solar energy may be directly incident onto the apparatus. Alternatively, the airstream may pass through a heat gain heat exchanger which is heated by solar energy. Additionally, the contaminated liquid could be preheated prior to spraying into the apparatus.

The apparatus may include an evaporation chamber including a dispersal portion where droplets are dispersed, and a settling portion where droplets settle onto the floor of the chamber. The settling portion may include drip trays and/or a guttering system to drain away the settled droplets. Preferably, the major axis of the settling portion is arranged substantially horizontally. This results in a greater likelihood of the partially evaporated droplets hitting the floor of the settling portion, prior to reaching the condenser. For the sake of efficiency, there may be two or more vertically stacked evaporation chambers. Preferably there are three evaporation chambers. Each of the evaporation chambers has its own dispersal portion and settling portion. However, there may be a common fan to create the airflow through the three evaporation chambers and a common air return duct. Preferably, the evaporation chambers operate at substantially atmospheric pressure.

The condenser may be in the form of a heat exchanger. The heat exchanger may be cooled by contaminated liquid which circulates through the heat exchanger prior to being introduced into the apparatus. This serves to heat up the contaminated liquid to enable greater efficiency. Not all of the contaminated liquid may be introduced into the apparatus. For example, where there is an ample supply of liquid such as seawater, a large proportion of the seawater may be returned to the sea after being passed through the heat exchanger.

It is to be understood that the partial evaporation referred to in the first aspect of the invention above, gives rise to a substantial proportion of the droplets giving up some

of their water molecules (but not all). This is distinct from partial evaporation where some of the water droplets are completely evaporated.

The factors affecting the conditions to which the droplets are subjected include the heat input, the transit time of the droplets and the droplet size. Any one or more of these factors may be varied to adjust the conditions within the apparatus. For example, the heat input may be varied by adjusting the air temperature of the heated airstream. The transit time for the droplet may be varied by adjusting the flow rate for the airstream. The droplet size may be somewhat predetermined according to the selection of spray nozzles. However, a variation in pressure or flow rate may affect the droplet size. On variation of one or more of these factors, the other factors may be adjusted to obtain the predetermined conditions which allow the partial evaporation of the droplets. Preferably the range of predetermined conditions is so as to achieve partial evaporation of the droplets, while accounting for variation in the input factors, e.g. heat input. A control system may be employed in the apparatus to operate the apparatus within the range of predetermined conditions. By way of specific example only, the heat input from the solar powered heat exchanger may be variable and where the heat input increases, the fan speed may be increased to reduce transit time for the droplets within the apparatus so as to maintain partial evaporation of the droplets.

As a result of only partial evaporation, some of the water will be retained within the droplet along with any contaminant. This will give the droplet sufficient weight to fall under gravity so as to avoid hitting the condenser. The operating factors can be adjusted in order to achieve partial evaporation. For example, the control system may include contaminant detection in the water collected from the condenser. If contaminant is detected then it is considered that the droplets are not falling out of the airstream and are hitting the condenser. Accordingly the operating factors may be changed by the control system to avoid this condition.

In accordance with a second aspect of the present invention, there is provided a method of purifying contaminated liquid including:

creating an airstream having droplets of the contaminated liquid;

operating the apparatus within a range of predetermined conditions, including heating of the airstream, to cause only partial evaporation of water from a substantial proportion of the droplets;

condensing the evaporated water at a condenser; and

allowing the partially evaporated droplets to settle out of the air prior to reaching the condenser.

The above invention may employ any of the features set out above in connection with the first aspect of the invention.

In accordance with a third aspect of the invention, there is provided a method of purifying contaminated liquid including:

forming droplets of the contaminated liquid;

partially evaporating water from the droplets;

collecting the partially evaporated droplets; and

condensing the evaporated water.

Preferably the step of collecting involves allowing the partially evaporated droplets to settle under gravity.

The contaminated liquid is understood to include water which is non-potable, in that it includes contaminants such as salt, i.e. seawater or other contaminants such as sewerage, heavy metals, biological material etc. The present invention aims to purify the contaminated liquid to at least the standard of potable water.

In accordance with a fourth aspect of the present invention, there is provided an apparatus for purifying contaminated liquid including:

a plurality of evaporation chambers for at least partially evaporating water from contaminated liquid;

airflow means for creating airflow through the plurality of evaporation chambers, wherein the airflow through the plurality of evaporation chambers is in parallel; and

a single return air duct to create a closed airflow circuit for the apparatus.

The closed airflow circuit creates greater efficiencies than would be obtained if fresh air was continually introduced from the outside environment. With a number of evaporation chambers, only a single airflow means is needed to push air through each of the evaporation chambers in parallel. Furthermore, creating multiple evaporation chambers enables the evaporation chambers to be configured to have a major axis which is substantially horizontal. This enables a settling portion of sufficient length to enable partially evaporated/denuded droplets and contaminants to fall out of the airflow. As an illustration of the efficiencies which can be obtained from a closed circuit, the apparatus may include a heat exchanger which heats up the air prior to entry into the multiple evaporation chambers. The air may be heated to approximately 100 ⁰ C (or in the range 50 ⁰ C to 120 ⁰ C) at which point, the relative humidity in the air may be 2-3%. As the air passes into the evaporation chambers, the humidity is increased by the dispersers which distribute the contaminated liquid in a finally divided form. Water is evaporated from the surface of the droplets. This is understood to be by a mechanism of interaction between the surface of the droplets and the heated air molecules. Towards the downstream end of the evaporation chambers, the air may have lost some of its heat and be at a temperature of 60°C (or in the range of 60°C to 120 ⁰ C) and 100% relative humidity. Under these conditions, it is known that one cubic metre of air holds 125 grams of water. At the downstream end of the evaporation chambers, a condenser condenses the evaporated vapour and the temperature of the air drops to 25-30 ⁰ C and 100% relative humidity. Under these conditions, one cubic metre of air will hold only 15 grams of water. It will be appreciated that air at a lower temperature can hold much less water than air at a higher temperature. Thus, one cubic metre of saturated air at 60 ⁰ C will yield approximately 100 grams of water at the condenser. The air which then passes through the return air duct at 25-30 ⁰ C will still have 100% relative humidity. Depending

on the conditions outside the apparatus, this relative humidity is probably far greater than the air conditions outside, which if in desert conditions, may be only a few percent relative humidity. The air in the return air duct then passes through the heat exchanger where it is heated up to approximately 100 ⁰ C. At this temperature, the water held by the air represents only 2-3% relative humidity. The cycle thus continues.

It may be understood from the above discussion that while there are a number of evaporation chambers, some of the other main elements such as the condenser, the heat exchanger and the airflow means may all be contained in the remainder of the airflow circuit, thus avoiding duplication.

Preferably there are three evaporation chambers which are arranged one on top of each other. These may be arranged as straight longitudinal portions. The return air duct may comprise two end sections and an intermediate elongate section. The end sections may have an inner curved periphery to create laminar flow around the bends. The heat exchanger and the airflow means may be disposed at one of the end sections. The condenser may be disposed at the other of the end sections.

In accordance with a fifth aspect of the present invention, there is provided an apparatus for purifying contaminated liquid including:

one or more evaporation chambers for evaporating water from contaminated liquid;

an airflow means for creating airflow through the one or more evaporation chambers; and

a return air duct to create a closed airflow circuit for the apparatus, wherein the airflow circuit includes two elongate portions and two end sections to create the closed loop, wherein the end sections each have a curved inner periphery.

Preferably, the one or more evaporation chambers comprise one of the longitudinal sections. The return air duct may comprise the two end sections and the other longitudinal section. In a most preferred form of the invention, the configuration is

such that the apparatus can be received into a housing which makes the apparatus transportable. In a most preferred form of the invention, the housing may be a standard sized shipping container. Rails may be provided, enabling the apparatus to slide in and out of the container for maintenance purposes.

In accordance with an sixth aspect of the present invention, there is provided a transportable purification unit including:

an apparatus for purifying contaminated liquid; and

a transportable container, wherein the apparatus is configured to be movable into and out of the transportable container.

Preferably the apparatus and the transportable container have co-operable guide means to enable the apparatus to slide in and out of the transportable container. Thus the internal periphery of the container may be provided with side rails, with rollers provided on the apparatus to move along the rails.

The transportable container is preferably a standard sized shipping container able to be mounted on a semi-trailer. The transportable container may be partitioned to provide a control portion of the container. In this control portion, the control instrumentation may be remote from the remainder of the apparatus.

As used herein, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude other additives, components, integers or steps.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

The invention consists in the foregoing and also envisages constructions of which the following gives examples.

Brief description of the drawings

In order that the invention may be more fully understood, one embodiment will now be described by way of example with reference to the drawings in which:

Figure 1 is a schematic view of the apparatus for purifying contaminated liquid in accordance with a preferred embodiment of the present invention; and

Figure 2 is a longitudinal sectional view of the main body of the apparatus of Figure 1.

Detailed description of the embodiments

Figures 1 and 2 show the main body 10 of the apparatus which includes three elongate evaporation chambers 12 which are vertically stacked one on top of the other. Airflow is driven through the evaporation chambers 12 by airflow means in the form of a bank of fans 14 held within a rack 15. Fan cowling 17 directs the airflow into each of the evaporation chambers 12. Contaminated liquid is introduced into the three evaporation chambers 12 in a finely divided droplet form through an array of spray nozzles 19. In Figure 1 , the introduction is depicted at arrow 16, but realistically this would take place further upstream in the evaporation chambers 12. The feed of contaminated liquid may derive from source water 18 such as seawater. The apparatus is designed to accept a range of source liquids (seawater, brackish well water, grey water, etc.) This source liquid 18 is pumped through pumps 20 through the condenser 22 on route to the evaporation chambers 12. This achieves preheating of the source liquid 18. Not all of the source liquid 18 which is passed through the condenser 22 need be feed through the evaporation chambers 12 and instead may be routed at three-way valve 23 to waste water container 24. The evaporation chambers 12 may also be fed with waste water 24 which has been collected from the base of the evaporation chambers 12. This waste water 24 is circulated through purge pumps 26 and recycling pumps 28. Periodically, the purge pumps 26 may empty the waste water from the waste container 24.

The airstream which passes into the evaporation chambers 12 is preheated through heat gain heat exchanger 30. The circulating liquid in the heat exchanger 30 is heated by solar energy as will be subsequently explained. The action of the heated airstream onto the droplets of contaminated liquid will cause an interaction which results in the partial evaporation of water molecules from the droplets. It is understood that some of the water will remain in the droplets, provided conditions are suitable. Further, given sufficient length from the spray nozzles to the end of the evaporation chamber, the partially evaporated droplets will still possess sufficient weight and will therefore drop under gravity and hit the bottom of the evaporation chambers and be drawn off into waste 24. Accordingly, each of the evaporation chambers 12 may be provided with collection recesses 32 for this purpose.

The saturated airstream will then flow onto the condenser 22 whereupon the water vapour condenses and is collected in fresh water collection tray 34. The collected water is then pumped through drinking water pumps 36 to a potable water collection vessel 38.

Air returns from the condenser 22 to the heat gain heat exchanger 30 via return air duct 13.

Specific aspect of the above system will now be described.

Solar Collector Array

The heat exchanger is heated by solar energy collected by an external array of reflective solar energy collectors 40. The Solar Collector array consists of a series of semi-cylindrical reflector surfaces angled and hinged to concentrate the sun's rays on an energy collection pipe 42 located along the focal axis of each array. Each reflector surface is approximately 3 metres long and 2.5 metres wide. The arrays are pivoted and connected together so as to allow simultaneous tracking of the sun throughout the day. This tracking is automatic and driven by a small electrically activated, hydraulic actuator (not shown). The solar tracking may be powered by a photo-voltaic cell. Correct angle and direction is determined by GPS and a Process Control System (not shown). The

reflector plates are curved and made from shaped polystyrene with a silver film glued to the polystyrene. The curvature has been determined to give a focus at 1.2 meters from the central axis.

The energy collection pipes 42 form part of a heating loop 44 and are filled with a liquid. A high specific-heat liquid is used throughout the heating loop 44, and is circulated using a variable speed pump 46 or pump array, under direct control from the control system. The pump may be a photo-voltaic powered circulation pump. The liquids that can be used include several substances or compounds such as Glycol with a boiling point of 212 ⁰ C, or one of the synthetic brake fluids with a boiling point of ~400°C. Water is unsuitable due to its low boiling point as this would require that the system operate at significantly higher pressures and requires the operator to hold a current valid boiler attendant certificate.

The system has an expansion chamber 48 built into the heating loop 44 so that the system does not operate at high pressure. The expansion chamber 48 is located downstream of the heat exchanger 30 so that little heat is lost from the system once acquired.

The amount of heat that enters the heat exchanger 30 is determined by the control system that has the ability to vary the flow rate of the liquid and possibly restrict or add additional solar panels to the circuit as weather and load conditions vary.

Body of Apparatus

The body 10 is in the form of an enclosed steel "shell" constructed to fit inside a shipping container (not shown). This "shell" houses the heat exchanger 30, evaporator chambers 12, condenser 22 and fan bank 14. At the end of the shipping container, a partitioned equipment and control room is provided to house the electrical, control, communication and pumping systems. These are powered from a Photo-Voltaic (PV) array mounted on the roof of the container. There are also various pipes illustrated in Figure 2 which convey liquids to the main components in the shell. Various sensors

such as humidity sensors 27, 29, convey readings to the control room. An access door 31 is provided in the side of the shell.

The energy collected from the external solar array flows through the heat exchanger 30. This consists of a tube and plate style heat exchanger situated in the main airstream within the body 10. Energy is transferred from the liquid via the exchanger 30 into the air. This heated air then flows onto the entrance to the evaporator chambers 12. The heat exchanger 30 may comprise a commercial radiator with a 3 coil copper pipe centre and plastic coated aluminium fins. The fins are coated with plastic to extend the life of the heat exchanger.

The temperature range for the heat exchanger 30 is incoming air at ~30°C and leaving at ~100°C. The relative humidity of the air at these two measuring points will be, (30 ⁰ C), -100% and at 100 ⁰ C the relative humidity will be -10-15%.

The air then enters the evaporation chamber 12 which are in the form of tunnels. The purpose of this stage is to increase the humidity of the 100 ⁰ C air from the low level of -10-15% relative humidity to the desired 100% relative humidity. The effect of raising the relative humidity is to lower the air temperature to ~60°C (although it may be in the range of 50 ⁰ C to 120 ⁰ C). This temperature drop is caused by the energy used in relation to the latent heat of evaporation, although we do achieve a considerable saving in energy by not using all the heat energy to break the small atomic bonds holding the water molecules together in a rough lattice network.

The evaporation chambers 12 may have several components acting as dispersers to achieve the humidification of the air. They are:

A misting system that creates a large surface area for interaction between the heat, water, airflow and salt and other contaminant molecules. The misters are simply a series of spray nozzles, available from several agricultural spray manufacturers, with the nozzles operating at less than 100PSI. The mister units have a ceramic nozzle-head to increase the life of the misters.

The power for the mister pump (not shown) may come from photo-voltaic cells mounted on the roof of the container. The nozzles have the ability to be directed in different directions and this design criterion is used to increase the interaction between the droplets and the airflow.

The droplet size varies with the water pressure and the orifice dimension. The orifice size depends on the particular nozzle selected and we currently use ALBUZ ATR nozzles. The starting droplet size that we can set varies in the range of 81 microns to 252 microns. The water pressure range is from 3 to 5 Bar or 44 to 73 PSI.

A 'Coolgardie Safe' also acts as a humidification medium. The Coolgardie Safe involves a mesh screen over which the contaminated liquid flows to assist with dispersal of the liquid into the airstream. Depending on the location of the safe structures, they may also operate to remove partially evaporated droplets from the airstream.

The internal surface area of the chamber is also wet increasing the air/water surface area.

Droplets which fall out of solution are collected in collection troughs 32 in each evaporation chamber and the liquid is recycled through the mister system until the concentration of salts or contaminants in solution becomes greater than a nominated concentration. The liquid is purged at that point. The control system monitors the liquid concentration and manages the purge/recycle points. This may be a return to the source (e.g. ocean) or storage for later disposal (inland evaporation ponds).The concentration and temperature of this discharge can be carefully controlled to suit local environmental requirements. The control system also monitors the air flow rate and the water pressure and varies these with climatic conditions.

The flow of air is controlled by a series of 30, 24 volt variable speed fans 14 located on a framework suspended above the heat gain heat exchanger 30. The

air speed is determined by the fan speed setting and the fan placement. There are 4 fan speed settings. The fan placement is either flat, or upright. The upright position provides slightly higher air speeds for the same fan speed setting. The measured air speed through the evaporation/misting chamber is 0.25ms ^"1 to 2.5 ms ^'1 . The speed and sequencing of the fans is controlled through the feedback loop of the control system allowing variation depending on climatic conditions and conditions within the desalination unit.

The whole process simply operates on the different amount of water evaporated into the air, (relative humidity), at different temperatures. Air at 60 ⁰ C and 100% relative humidity holds about 125-135 grams of water. Air at 100% relative humidity at 25°C holds about 15 grams of water. For every m ³ of air that passes through the system with these temperatures the yield of fresh water is ~100-110 grams. The water quality is nearly pure, with contaminants below detection levels.

Condenser

The condenser 22 is a radiator type heat exchange unit manufactured from 4 coil copper pipe and plastic coated aluminium fins set at 12 fins per inch. The source liquid 18 is circulated through the condenser 22 before being delivered to the misting system. This has the effect of raising the temperature of the liquid entering the evaporation chambers, while setting the operating temperature of the condenser. If ground water at 16 ⁰ C, (e.g. Western Australian wheatbelt groundwater), is the source liquid then that is the temperature of operation of the condenser 22. If seawater at 23°C, (sub-tropical seawater), is used then that is the operating temperature.

As the air passes through the condenser 22 and the air temperature lowers then the water held in the air drops out of solution collecting on the plastic coated fins and runs into a collecting trough 34. The water is pumped from the collecting trough 34 to an outside storage tank 38. The fresh water pump 36 is controlled by the control system and powered from the PV solar system mounted on the container roof.

From the Condenser, the cooler, saturated air then recirculates around the system back to the heat exchanger 30 ready for another circuit.

Instrumentation and Control System

The operation and performance of the apparatus are controlled by the control system.

All aspects of the process from energy collection, desalination, storage, discharge and external conditions are monitored and controlled by the control system. The control system consists of microcomputer hardware, and utilises the latest in instrumentation technology.

Mathematical modelling, supporting data from earlier prototypes, provides an indication of the potential production capacity of the full-scale unit being evaluated. At latitudes similar to Perth, Western Australia, the anticipated averaged yield should be about 5 kilolitres per day of potable water. This system requires a solar collection area of approximately 100 square metres. This will vary in different climatic conditions and with source water supplies.

Control Room

The apparatus is housed in a shipping container. The apparatus itself is mounted on rails that allow the apparatus to be removed from the container for maintenance and other purposes. The container also protects the unit from casual modification and public access.

One end of the container has been separated with insulated sandwich panel and has an external door installed to what is the control room. A source water filter is located in the control room. The filter is a 'netafiri plate filter array that has three plate filter modules. These units have back flush capability. The filters are located above a wash down trough to allow cleaning without making a mess in the control room.

The pumps for the apparatus are also located in the control room. They are setup in five levels to allow easy access. The operation of the pumps is controlled by the control system which is also located on a workstation platform in the control room. Pipes puncture the wall between the control room and the main part of the container, allowing easy connection of the apparatus to the various pumps and controls.

Parameters monitored that are included in the feedback control system include, relative humidity, temperature, air flow velocity, PV power generation, heat collection, pumping conditions, etc to provide best fit calculations. The control system then moves the various sectors to those predicted points and then re-evaluates the performance.