GB2291365A - Treatment of water for use in a cooling tower by passage through a membrane to remove multivalent ions - Google Patents

Abstract

A method of treating water to be fed into an evaporative cooling tower is provided. Water is passed through one or more membranes, which are adapted to remove multivalent ions from the water, and the treated water is then passed to the cooling tower. The association of a membrane filtration unit and a cooling tower is also claimed. As water is evaporated from a cooling tower (1), contaminants in the water replenishing the system could become concentrated in the cooling water. Unless these concentrations are controlled, the cooling water deposits materials (eg scale) on the heat exchange elements in the cooling tower and the condensor (6) in the cooling circuit (5) if the solubilities of various salts are exceeded and becomes increasingly corrosive. Control can be achieved by passing replenishing water (7) through membranes (8) adapted to remove multivalent ions from the water, so that the cooling tower is fed with treated water. <IMAGE>

Description

METHOD OF TREATING COOLING TOWER WATER The present invention relates to a method of treating evaporative cooling tower water and, in particular, to a method of softening water prior to its use in an evaporative cooling tower.

It is common practice in cooling towers to use water as a medium to transport any excess heat from the system. As a consequence, a significant amount of water is lost by evaporation, which needs to be replaced on a continuous basis.

In the United Kingdom water is classified either as "hard" or "soft", depending on the concentration of certain ions present in the water. In hard water, the level of calcium and magnesium salts is relatively high, so that as the water is evaporated to effect cooling, the salts present in the water tend to be deposited on the elements used to effect the heat exchange and in the cooling tower.

Some multivalent salts are relatively insoluble in comparison to monovalent salts, and consequently, these cause the build-up of the scale. This scaling up of the heat exchange elements leads to a reduction in the efficiency of the heat exchange, as well as an increased requirement for cleaning of the heat exchange elements.

Obviously, this can lead to a large increase in the cost of operating the cooling system. Furthermore, scale can protect harmful bacteria from the action of biocides, thus increasing the risk of proliferation of Legionella bacteria.

In order to alleviate this problem in areas where the water is hard, it has been normal practice to soften the water so as to significantly reduce the amount of scaling up, and this is usually effected by an ion exchange process in which the multivalent metal ions are replaced by monovalent metal ions such as sodium. Although this process leads to a significant reduction in the scaling up associated with cooling towers, a waste product is produced from the regeneration of the resin, namely brine, which needs to be disposed of, and which may attract special disposal requirements for environmental protection reasons.

In addition, during such ion exchange treatment, there is a simple exchange of ions, and overall the number of ions does not decrease. Therefore, the high conductivity of a solution containing these ions is maintained. Assuming that the only loss from the system is the water lost by evaporation, the concentration of the ions present in the closed system will increase, leading to a consequential increase in the conductivity of the solution. This conductivity will increase until a preset limit is reached, when the corrosivity and/or scaling propensity of the water is at the maximum consistent with good operation. At this point, a significant portion of the cooling tower water, which can have an ion concentration of up to three times that of the main supply, is run to waste (a process known as "blowdown").

This "blowdown" represents a significant cost in terms of the volume of water wasted and its supply and disposal cost, and the chemicals which have been added to the system, a proportion of which run to waste at "blowdown".

We have now devised an improved method of treating water for use in an evaporative cooling tower, whereby problems in prior art procedures are mitigated or overcome.

The present invention allows a reduction in the waste of chemicals and water, and the production of ion exchange waste brine is avoided.

In accordance with one aspect of the present invention, there is provided a method of treating water for use in a cooling tower, which method comprises passing the water through one or more membranes which selectively remove multivalent ions, and passing the water so obtained into a cooling tower.

Preferably, water to be added to an evaporative cooling tower is passed through a plurality of membranes which selectively remove multivalent ions, and this treated water is then added to the cooling tower to replenish water which has been lost due to the evaporation in the tower, and "blowdown".

According to a further aspect of the present invention, there is provided an apparatus comprising a cooling tower connected to a membrane filtration unit.

Preferably, the membrane filtration unit includes a plurality of membranes through which water is passed, and the treated water then passes from the membrane filtration unit into an evaporative cooling tower.

As will be appreciated by those skilled in the art, it is the multivalent ions present in the hard water, such as calcium and magnesium, which lead to the scaling up of the cooling tower. Consequently, in accordance with the present invention these ions can be removed rather than being replaced as happened in prior art procedures, leading to a reduction in the rate of increase in ion concentration within the cooling water in the tower.

The present invention has a number of advantages over the prior art procedures, which makes it highly suitable for treatment of water for use in cooling towers.

These advantages include: 1. less overall waste volume due to reduced blowdown frequency.

2. less volume discharge of potentially harmful materials which are used to maintain the cooling tower environment.

3. the avoidance of the discharge of any brine ion exchange waste.

4. the use of the membrane(s) also acts to disinfect the water being added to the cooling water in the tower.

5. any bacterial accumulation which normally occurs with ion exchange softeners is avoided.

6. the make-up water added to the system has a reduced conductivity over the mains supply water, and therefore there will be a reduction in the frequency of waste generation from the main cooling water supply.

In accordance with the present invention, one or more membranes are utilised which are adapted to selectively remove multivalent ions from the water as they make contact with it. Membranes which are capable of selectively removing multivalent ions are known, but certain types of membrane are preferred for use in the present invention.

Preferably, the type of membranes used in the present invention are those commonly referred to as nanofiltration membranes" or "softening membranes". Such membranes are available in a number of materials, the most common being based on cellulose acetate or polyamide, although polyvinyl derivatives or some types of ceramic membranes are also suitable.

The configuration of the membrane may be either as "hollow fibres" or "spirally wound", both obtaining a large surface area of membrane in a small volume of plant.

The equivalent of conventional filtration pore size for membrane filtration is the molecular weight cut off (MWCO). This approximates to the molecular weight of molecules or ions which cannot pass through the membrane.

Typically, the MWCO of nanofiltration membranes is greater than 50, but they can be tailored to the required specification.

An equivalent pore size would be of the order of one nanometer, although the solute rejection properties are strongly affected by a surface charge on the membrane. It is this charge which causes the tendency to preferentially reject multivalent ions.

Operating pressures for nanofiltration are usually in the range 3 x 105 to 5 x 105 Pa (3-5 bar), at which pressure a typical filtration rate would be to produce 20 30 litres/metres2/hour.

A typical spirally wound membrane unit with dimensions of 10cm diameter by 100cm length would contain a membrane surface area of 9 to 10 square metres.

It will be appreciated that the configuration and surface area of the membranes used in a system in accordance with the present invention will be dependent upon the particular conditions and characteristics of that system.

For example, it will be necessary to consider the multivalent ion concentration in the mains water supply, as well as the volume of water which is added to the system in any given period in order to maintain the cooling tower in a substantially equilibriate state.

Materials may be added to the water in order to maintain the environment within the cooling tower in a more desirable condition. For example, biocides, anti-scalants and anti-corrosion agents may suitably be added, which act to ensure that the environment within the cooling tower is conducive to the continued efficient operation of the cooling tower without the need for frequent maintenance of the tower. In addition, it is desirable to add materials to prevent the unwelcome existence of bacteria or other microorganisms, which could cause health problems to the surrounding populous.

These materials are normally added to the cooling water when monitoring indicates that the materials' concentration is too low, and after "blowdown".

Biocidal chemicals are preferably added to the system, because the cooling tower water is at a temperature which encourages the proliferation of bacteria, including Legionella Pneumophila. The biocidal chemicals are typically chlorine- or bromine-based, but other types of biocides may also be used, for example those described in the "Expert Committee on Biocides Report", Report of the Expert Advisory Committee on Biocides, Department of Health, HMSO 1989.

It is beneficial also to add chemicals which reduce the corrosivity of the water. As the conductivity in the cooling tower increases, the corrosivity of the water towards metal components increases. Suitable chemicals for this purpose include phosphates, molybdates and nitrites.

Even in relatively soft water, there may be a tendency to "scale", as salts are deposited which have exceeded their saturation concentration at the higher concentrations present. Therefore, scale inhibitors or dispersants may be added. for example acrylates and lignosulphonates.

In order that the invention may be more fully understood, it will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram of a system of the present invention: and Figure 2 is a schematic diagram of an arrangement of membranes in accordance with the present invention.

Referring to Figure 1, the system comprises an evaporative cooling tower (1) which provides suitable conditions for the evaporation of water (2), and a cooling water reservoir (3).

A means (4) which enables cooling water from a cooling water reservoir (3) to be extracted and sent to waste is also provided.

In operation, cooling water from the reservoir (3) is pumped around a cooling circuit (5) and passed over the heat exchange elements, commonly in a "condensor" (6), collecting the excess heat therefrom. This excess heat is removed from the system by the evaporation in the cooling tower of a proportion of the cooling water, whilst the remaining cooling water is allowed to flow back into the cooling water reservoir (3). Clearly, as the cooling is effected, in part by evaporation of water, a significant proportion of this water will be vented to the general atmosphere over a period of time, and this water needs to be replaced in order to ensure the continued efficient operation of the cooling tower.

In order to make up for the loss of water as a result of the evaporation in the tower and the water which is passed to waste from the reservoir, an inlet feed line (7) is provided, which is connected to the general mains water supply. This line (7) is provided with a membrane filtration plant (8) having a waste line (9) and a position (10) at which any further materials can be added.

The membrane filtration plant (8) includes a number of separate membrane units which are arranged so as to contact the water passing through the plant (8) from the mains water supply and extract from the water a significant proportion of the multivalent ions present in it, whilst maximising softened water production and minimising waste production. The water is then allowed to pass from the membrane filtration plant (8) via the position (10) to the cooling water reservoir (3) where it is mixed with the main body of cooling water.

The waste material extracted from the mains water supply by the membranes in the membrane filtration plant is allowed to flow via the waste line (9) of said plant (8) to a suitable storage facility or sewer where it is treated in the normal fashion. The concentration of multivalent ions present in this waste material may be up to ten times the concentration of that found in the mains water supply.

The membranes produce a permeate stream (the softened water), and a concentrate stream (the waste). This concentrate preferably passes through further stages of membranes, continuing until it is not cost effective to treat the concentrate further.

A schematic diagram of a desirable arrangement of membranes for use in the present invention is shown in Figure 2. Reference number 11 shows the flow of the softened water to the cooling tower: and 12 the flow of the waste water to disposal.

Claims (13)

CLAIMS:

1. A method of treating water for use in a cooling tower, which method comprises passing water through one or more membranes which remove multivalent ions from the water, and passing the treated water into a cooling tower.

2. A method according to claim 1, wherein the water feeding the cooling tower is passed through one or more membranes before use, so that the cooling tower is fed with treated water.

3. A method according to claim 1 or 2, wherein the treated water replenishes water lost from the cooling tower due to evaporation to the atmosphere and the purging of water to waste.

4. A method according to claim 1, 2 or 3, wherein the or each membrane is a nanofiltration membrane.

5. A method according to any preceding claim, wherein the or each membrane disinfects the water being passed into the cooling water.

6. A method according to any preceding claim, wherein at least one water treatment chemical is added to the cooling tower.

7. A method according to claim 6. wherein the water treatment chemical is a biocide, anti-scalant agent, anticorrosion agent or a mixture of two or more thereof.

8. An apparatus comprising a cooling tower connected to a membrane filtration unit.

9. An apparatus according to claim 8, wherein the membrane filtration unit contains a plurality of membranes.

10. An apparatus according to claim 8 or 9, wherein the or each membrane in the membrane filtration unit is a nanofiltration membrane.

11. An apparatus according to claim 8, 9 or 10, which comprises means to allow water to be fed into the membrane filtration unit and through one or more membranes and passed from the membrane filtration unit into the cooling tower.

12. An apparatus substantially as herein described with reference to the accompanying drawings.

13. A method of treating water for use in a cooling tower substantially as herein described with reference to the accompanying drawings.