METHOD FOR COOLING A PROCESS STREAM

The present invention relates to a method of cooling a process stream, such as a gas or a liquid stream, associated with the treatment of natural gas.

Many industrial plants, such as those involved in the treatment of natural gas, make use of water-based coolant systems for the cooling of process streams. Open recirculating cooling water circuits normally contain an evaporation step in which a portion of the cooling water stream is evaporated in order to cool the cooling water. Such an evaporation step leads to a loss of cooling water from the circuit, which must be replaced with make-up water from an external source.

Water is a scarce resource in certain areas of the world. For industrial plants located in such areas, sufficient water to meet the requirements of the plant may not be available, or it may only be available at a prohibitive cost in terms of the capital expenditure required to drill wells, build purification plants for river-water or desalination plants for sea water. In WO 2007/068733 a process is disclosed for cooling down a hot flue gas stream in which the hot flue gas is cooled by a cooling water stream to condense at least a part of the water vapour in the flue gas stream. The condensed water vapour from the flue gas stream is then combined with the cooling water stream to produce a combined water stream. The combined water stream is then cooled by contact with air from the atmosphere and evaporation of a portion of the combined water stream. In this way, the condensed water vapour from the flue

gas stream is used to replace at least a part of the water lost by evaporation.

Not every process stream to be cooled can provide a source of additional water. There is therefore a need for further improvement in the management of cooling water circuits. In particular there is a need to minimise the demand for water in those areas where it is a scarce and therefore expensive resource, resulting in a reduction in costs, particularly capital expenditure. The present invention seeks to provide a method for cooling a process stream using an open recirculating cooling circuit in which the demand for additional (or make-up coolant) is reduced.

To this end, the invention provides a method of cooling a process stream associated with the treatment of natural gas, the method comprising the steps of: (a) providing a liquid coolant stream in an open recirculating cooling circuit and providing a process stream; (b) evaporating a portion of the liquid coolant stream by contacting the liquid coolant stream with a gaseous stream to provide a cooled liquid coolant stream and a gaseous stream comprising coolant; and (c) heat exchanging the cooled liquid coolant stream against the process stream to provide a cooled process stream, wherein the process stream provided in step (a) is at a first process temperature, the cooled process stream provided in step (c) is at a second process temperature and the portion of liquid coolant stream that is evaporated in step (b) is varied in response to the first process temperature of the process stream to at least reduce the variation of the second process

temperature of the cooled process stream in step (c) below the variation of the first process temperature of the process stream provided in step (a) and preferably also to reduce the variation of the second process temperature of the cooled process stream in step (c) to zero .

Conventional open recirculating cooling circuits are operated to provide a more or less constant cooling duty, corresponding to the peak capacity required of the cooling circuit. This is achieved by providing constant heat rejection by evaporating a fixed proportion of coolant from the coolant stream. In order to maintain a constant level of coolant in the cooling circuit, makeup coolant must therefore be added at a constant rate to replenish the coolant lost by evaporation and/or other coolant losses. The provision of additional coolant adds costs to the process.

However, the cooling duty required of a cooling circuit can change as a result of external conditions, such as the temperature of the process stream to be cooled which can vary with ambient temperature . In the method of the present invention, the cooling duty of the cooling circuit is controlled by varying the portion of gaseous stream comprising coolant which is brought into contact with the liquid coolant stream, and as a result the portion of the coolant stream which is evaporated in step (b) is varied. This enables the heat rejection and thus cooling duty of the circuit to be varied to match the changes in required cooling duty which are due to external conditions. The method of the present invention therefore provides a lower average coolant evaporation rate compared the average evaporation rate in an

uncontrolled cooling circuit. Coolant loss by evaporation is therefore reduced.

In a further embodiment of the present invention, when a constant supply of make-up coolant is provided, a surplus of non-evaporated coolant may be produced and stored during a period of low cooling duty and may be used during a period of high cooling duty. The embodiment of the method therefore requires lower makeup coolant maximum flow, which is specifically advantageous in view of the costs associated with the installed capacity for make-up water treatment.

The method of the present invention is particularly suitable for use in hot climates where the provision of fresh water for use as a coolant is expensive and where there is a significant variance in the cooling duty required of the cooling circuit. In particular, the cooling duty required of the cooling circuit can vary as a result of variations in the temperature of the process stream to be cooled due to ambient conditions. By controlling the evaporation rate of the coolant, the heat rejection can be varied to correspond to the cooling duty required by the process stream at a particular time. By adjusting the cooling duty required of the cooling circuit by controlling the evaporation of the coolant over time, the overall loss of coolant by evaporation from the circuit can be reduced.

Preferably, the method of the invention is a continuous method for at least 24 hours and contains at least day and a night, allowing the manipulation of the cooling duty required of the circuit in response to changes in the temperature of the process stream as a result of the ambient temperature. More preferably, the

method is a continuous method with a duration of one week, preferably one month, more preferably 6 months.

The method according can be applied in any process, preferably in a process selected from the group of refinery processes, chemical industry processes, natural gas treatment, liquid natural gas production, exploration and production processes and hydrocarbon synthesis processes.

Step (a) of the method of the present invention provides a coolant stream. This may be any type of coolant normally used in an open recirculating cooling circuit. Preferably, the coolant stream comprises water, more preferably, the coolant stream consists essentially of water. In such cases, the water stream may also contain standard cooling water additives, such as antifoams, antiscalants, biocides and corrosion inhibitors .

The process stream provided may be any liquid or gaseous stream, for example a liquid or gaseous stream from a natural gas treatment plant. It is preferred that the process stream is an amine stream from an acid gas treatment unit or a sweet or sour natural gas stream. The process stream can be provided at a first process temperature, which can be in the range of 40 to 65 "C. In step (b) , a portion of the coolant stream in the open recirculating cooling circuit is evaporated to provide a cooled coolant stream and a gaseous coolant stream. This is done by contacting the coolant stream with a gaseous coolant medium, suitably air from the atmosphere (ambient air), thereby evaporating a portion of the coolant stream to form a gaseous coolant stream and cooling unevaporated coolant to form a cooled coolant stream. Preferably, the coolant stream comprises

water and the cooled coolant stream has a temperature in the range of from 15 to 35 ⁰ C. Without wishing to be bound to a certain theory, it is believed that the cooling effect to produce the cooled coolant stream mainly results from two mechanisms. Part of the cooling is believed to be the result of heat transfer as a result of contacting cooler gaseous coolant (for example air) with the coolant stream. Another part of the cooling is believed to be the result of evaporation of part of the liquid coolant stream. In the event that the gaseous coolant is air, the relative contribution of the two main mechanisms depends on the ambient air conditions, which can be suitably characterised by the temperature (e.g. so-called dry bulb temperature, a term known in the art) and relative humidity of the air. For instance, the evaporation of 1.5 to 2.5% by weight of water in the water coolant stream corresponds to a temperature decrease of 10 °C.

It has been found that a combination of high relative humidity, preferably in the range of from 80 to 100%, more preferably from 85 to 100% and low dry bulb temperature, preferably in the range of 10 to 30 ⁰ C, more preferably from 10 to 25 ⁰ C, results in lower evaporation rates, even as low as 2% or less, towards 1.5%.

It has been found that a combination of low relative humidity, preferably in the range of from 20 to 45%, more preferably from 20 to 35% and high dry bulb temperature, preferably in the range of 30 to 55 ⁰ C, more preferably from 40 to 55 ⁰ C, results in higher evaporation rates, even as high as 2.5% or more. In extreme cases it can occur that the air dry bulb temperature is higher than the liquid coolant

temperature and the cooling effect is mainly the result of evaporation. In the other extreme case, if the ambient air relative humidity increases to values close to 100 %, the evaporation effect becomes negligible. Preferably, the evaporation step (b) occurs in one or more cooling towers. The cooling towers comprise one or more fans. Air may be drawn through the cooling tower by a number of methods . A mechanical draft cooling tower uses power driven fan motors to force or draw air through the tower.

By varying the rate of rotation of the fans or the fan pitch in the cooling towers, the air flow and thus step (b) is controlled. It is preferred that the air flow rate is varied in response to the temperature of the process stream provided in step (a) . For instance, when the coolant stream provided in step (a) is at a first process temperature, the cooling in step (b) can be controlled in response to the first process temperature of the process stream. This can be done by using a temperature sensor to measure the first process temperature of the process stream, for example by placing the sensor in the process stream line or at the inlet of this line to the heat exchanger.

Alternatively, the air flow rate could be varied in response to another external condition, such as ambient temperature, which can directly effect the first process temperature of the process stream and therefore the cooling duty of the cooling circuit in a know manner, which can be calculated for a particular plant construction.

An alternative way of controlling the evaporation rate in step (b) is to manipulate the process stream flow through the heat exchanger, preferably by bypassing

a portion of the process stream to combine it with the cooled process stream downstream the heat exchanger. The person skilled in the art will know that the heat transfer in the heat exchanger is so manipulated. Preferably the coolant flow through the heat exchanger is kept constant and the heat exchanger duty variation results in the difference between coolant inlet and outlet temperature. The effect in the cooling tower will be a lower heat rejection and a lower evaporation rate, without having to reduce the airflow brought in contact with the coolant .

It is preferred that the process stream is pre- cooled to the first process temperature by an air cooler. In a preferred embodiment, the process stream is pre-cooled to a first process temperature in the range of 20 to 65 °C prior to heat exchange with the coolant stream in step (c) .

The cooled coolant stream is heat exchanged against the process stream in step (c) . The heat exchange in step (c) may be achieved through direct contact of the process stream and the cooled coolant stream. Alternatively, indirect heat exchange may be used to cool the process stream. For instance, indirect heat exchange can be carried out in well-known equipment including but not restricted to: a shell and tube heat exchanger, such as an EM baffle heat exchanger, a plate and frame heat exchanger or a fin tube heat exchanger.

In heat exchange step (c) the process stream is cooled to provide a cooled process stream, while the temperature of the cooled coolant stream increases to provide the cooling. In a preferred embodiment, the cooled process stream is at a temperature in the range of 20 to 50 °C after the heat exchange step. The cooled

coolant stream is heated to a temperature in the range of 20 to 45 ¹ C.

After heat exchange step (c), at least a part of the coolant stream i.e. the coolant not evaporated in evaporation step (b) can be recirculated and returned for use as at least part of the coolant stream in step (a) .

In order to compensate for the coolant lost from the cooling circuit in evaporation step (b), additional coolant can be provided to the coolant stream in the cooling circuit as a further step (d) . The additional coolant can be provided as a make-up coolant stream, which is preferably combined with the recirculated coolant stream, preferably after heat exchange step (c) . The make-up coolant stream can provide typically as explained before between 1.5 and 2.5% by weight of recirculating coolant in the coolant stream.

It is preferred that the flow of the make-up coolant stream is regulated in response to the rate of evaporation of the portion of the coolant stream evaporated in step (b) . More preferably, the amount of additional coolant provided to the coolant stream in the cooling circuit from the make-up coolant stream in step (d) is equal to the amount of coolant removed from the coolant stream by evaporation and/or other losses in step (b) . Viewed another way, the rate at which the additional coolant is provided by the make-up coolant stream should match the rate at which coolant is lost from the coolant stream. In a further embodiment, it is preferred that the additional coolant (as the make-up coolant stream) is fed from a buffer tank. When the coolant stream comprises water, the water in the buffer tank may be provided from

one or both of a source from within the plant, such as water produced by one or more of the process units, and a source external to the plant, such as an aquifer, river or other water source. It is preferred that the additional coolant provided in step (d) is water from a sulphur recovery reactor, especially a Claus off-gas treating reactor. A Claus off- gas treating reactor can be used as part of a Claus unit for the conversion of unreacted hydrogen sulphide to sulphur dioxide. In the Claus unit hydrogen sulphide is converted to elemental sulphur via the well-known Claus process. The Claus process is a process wherein elemental sulphur is formed by partial oxidation of the H2S using oxygen-containing gas (including pure oxygen) to form SO2, followed by reaction of the SO2 formed with the remaining part of the H2S, in the presence of a catalyst.

The most widely used Claus catalyst is non-promoted spherical activated alumina. The Claus unit suitably comprises a combustion chamber followed by two or more catalyst beds and two or more condensers. The reaction products are cooled in these condensers and liquid elemental sulphur is recovered. Since the yield of elemental sulphur, relative to the hydrogen sulphide introduced, is not quantitative, a minor amount of unreacted hydrogen sulphide, and sulphur dioxide remains in the off-gases from the Claus unit. Therefore, usually an off-gas treating reactor is employed, wherein sulphur dioxide is reduced to hydrogen sulphide in a hydrogenation reaction. A preferred off-gas treating reactor is a so-called SCOT reactor, i.e., Shell Claus

Off-gas Treating reactor, as for example described in the textbook "Gas Purification", Kohl and Nielsen, Gulpf Publishing company, 5 ^th edition.

Water is produced as a by-product of the Claus off- gas treatment process and can be beneficially used as make-up water in the method of the present invention.

Hereinafter the invention will be further illustrated by the following non-limiting drawings.

Figure 1 schematically shows a process scheme in accordance with an embodiment of the present invention. Figure 2 shows a generalised graph of coolant makeup rate versus time for a controlled evaporation rate according to the method of the invention and an uncontrolled evaporation rate.

Figure 1 is a schematic diagram showing a cooling circuit 10, comprising an optional first cooler 50, a heat exchanger 40, a cooling tower 30, a storage tank 20 and an optional second cooler 60.

The cooling circuit 10 is used to cool process stream 52, such a liquid or gaseous stream e.g. from a natural gas treatment plant, refinery or chemical plant. Process stream 52 is preferably an amine stream from an acid gas treatment unit or a sweet or sour natural gas stream.

In the embodiment shown, process stream 52 is heat exchanged against a water coolant stream in heat exchanger 40 to provide a cooled process stream 54. The cooled process stream is provided with a second process temperature in the range of 20 - 50 °C, more preferably 45 "C. It is preferred that the second process temperature is about 3 to 10 °C more than the first cooled coolant temperature. Cooled process stream 54 can then be optionally further cooled in heat exchanger 60 to provide further cooled process stream 62 at a third process temperature. It is preferred that the cooled process stream 54 is heat exchanged against a chilled

water stream or refrigerant stream 64 in heat exchanger 60. Further cooled process stream 54 can then be passed on to other units in the plant for further processing. The method of the present invention is concerned with the operation of open recirculating cooling circuit 10. In particular, cooled coolant water stream 38 is heat exchanged against process stream 52 in heat exchanger 40 to provide coolant water stream 42 and cooled process stream 54. Coolant water stream 42 is then passed to cooling tower 30. Make-up coolant water stream 22 can replenish the cooling water lost by evaporation and other losses in cooling tower 30.

In cooling tower 30, coolant water stream 42 is contacted with air stream 44. A portion of the coolant is evaporated into the air stream to produce gaseous coolant water/air mixture stream 32, while the remaining liquid coolant exits the cooling tower as cooled coolant water stream 38. Besides evaporation losses, cooling towers have other losses, such a so-called blow down and drift losses, represented by stream 36.

In the cooler hours, typically during the night, the first process temperature of process stream 52 at the inlet of heat exchanger 40 can be lower than during the day e.g. during the night at a temperature in the range of from 40 to 50 °C, for example 45 °C and during the day at a temperature in the range of from 55 to 65 °C, for example 60 °C. To maintain a temperature in the range of from 40 to 50°C, e.g. 45 °C in stream 54, the cooling duty on the open recirculating cooling water circuit is therefore lower during the night than during the day. A reduction in the cooling duty of the cooling water circuit 10 allows the heat rejection in the cooling tower 30 to be reduced, hence reducing the

proportion of the cooling water evaporated in cooling tower 30. The proportion of cooling water evaporated can be reduced by reducing the speed or pitch or in some cases stopping fan 34 in cooling tower 30. Alternatively a portion of stream 52 can bypass (55) the heat exchanger 40 to combine downstream the heat exchanger 40 into stream 54, so reducing the cooling duty in the cooling circuit. The heat rejection and thus water evaporation in cooling tower 30 can so be varied without manipulating the airflow through cooling tower 30.

A result of reducing the proportion of cooling water evaporated is that less water will be lost from the cooling water circuit and therefore the amount of make-up water to be added as make-up water stream 22 to replace the water lost by evaporation is reduced.

In the warmer hours, typically during the day, the process stream temperature at the inlet of heat exchanger 40 can be higher than at night e.g. 55 - 65 °C, for example 60 °C. The cooling duty of the open recirculating cooling water circuit is therefore increased. In order to meet the increased cooling duty of the cooling water circuit, the heat rejection and thus the evaporation rate of cooling water in cooling tower 30 is increased. One way in which the first temperature of the process stream can vary in response to ambient conditions is as a result of pre-cooling with air cooler 50. Air cooler 50 is provided upstream of heat exchanger 40. Air cooler 50 can reduce the temperature of hot process stream 48, e.g. from the range of 70 to 140 °C, typically 130 °C to a first process temperature in the range of 20 to 65 °C.

Air cooler 50 is designed to operate at peak performance corresponding to maximum daytime temperatures e.g. the hottest summer day. When the air cooler is operated at lower than the maximum designed ambient conditions, the cooling provided to warm process stream 48 will increase, lowering the first process temperature of process stream 52 exiting the air cooler. Consequently, in order to maintain a constant temperature in stream 54, the cooling duty required of open recirculating cooling circuit 10 downstream of air cooler 50 decreases, and the heat rejection by evaporation can be reduced by stopping or reducing the fan speed or pitch in cooling tower 30, or via increasing bypass 55 around heat exchanger 40. This method is preferably carried out by monitoring the process stream with a temperature sensor. For instance, a temperature sensor could be placed either in process stream 54 and/or 52, and the evaporation rate of the coolant controlled in response to this temperature measurement.

Operating in this way, air cooler 50 can provide a process stream with a process temperature in the range of 55 - 65 °C, for example 60 °C, during maximum ambient conditions, for instance during the hottest part of a Summer's day, and can provide a process stream with a process temperature in the range of 40 - 50 °C, for example 45 °C, during cooler ambient conditions, for instance at night. The indicated temperature ranges are typical for hot climates, e.g. desert climates, and can be lower by say 20 ⁰ C in colder climates.

In this way, the cooling duty of the open recirculating cooling circuit varies during day/ night cycles, and across the seasons, in accordance with the

process temperature of process stream 52 exiting air cooler 50. By controlling the heat rejection of the cooling circuit 10 by varying the proportion of water lost by evaporation, water loss and therefore the requirement for make-up water, can be kept to a minimum.

The proportion of water evaporated and/or lost in the cooling tower 30 may be higher than the supply of make-up water in make-up water stream 18, and the excess water stored in buffer tank 20 during the night can be used to maintain the amount of water in the cooling water circuit to provide the required cooling during the day.

In a further embodiment also shown in Figure 1, stream of make-up water 22 is provided from a buffer tank 20. Buffer tank 20 is fed by buffer water stream

18. Buffer water stream 18 must provide water to buffer tank 20 at a rate greater than or equal to the average rate of evaporation of water from the cooling tower 30, in order to ensure that a constant level of cooling water in the cooling circuit can be maintained.

Buffer water stream 18 originates from water source stream 12, via junction point 16. Water source stream 12 can be provided from multiple sources, such as a source within the plant, for instance as water condensed from flue gas or from a SCOT reactor, or from a source external to the plant, such as a well, purification unit for river-water or a desalination plant for seawater.

Typically, buffer water stream 18 provides water to buffer tank 20 at a constant rate, for instance because water is supplied from a unit within the plant at a constant rate of operation, or pumped from a well at a constant rate. If water source stream 12 provides water at a rate greater than the average evaporation rate,

excess water can be removed at junction point 16 and supplied to other units in the plant, or disposed of as draw-off stream 14. In an alternative embodiment (not shown) , the excess water can be supplied to other units in the plant directly from buffer tank 20.

Make-up water is drawn from buffer tank 20 as makeup water stream 22 to replace water evaporated and/or lost from the cooling circuit. When the cooling duty of the cooling circuit is at a minimum (for example when temperatures are lower at night), only small quantities of make-up water will be required, or none at all. In this stage of the cycle, the rate of evaporation of water from the cooling circuit is less than the rate at which buffer stream 18 fills buffer tank 20, and the quantity of water in buffer tank 20 will increase.

Buffer tank can therefore store cooling water for use later in the cycle.

When the cooling duty of the cooling circuit increases (for example when temperatures are higher during the day) , the evaporation rate is increased and larger quantities of make-up water will be required to maintain the quantity of cooling water in the cooling circuit at a constant amount. When the rate of evaporation of the cooling water becomes more than the rate at which buffer stream 18 is supplied to buffer tank 20, the quantity of water in the buffer tank will fall.

For illustration purposes only, the presence of water at various points in cooling circuit 10 can be clarified by variables a and (1-a), wherein 0 < a < 1. Referring again to Figure 1, a cooling water stream 42 is provided which is passed to cooling tower 30. In cooling tower 30, the cooling water stream 42 is cooled

to a temperature in the range of 15 to 35 °C by using air from the atmosphere (stream 44) .

A proportion a (H ₂ O) of the cooling water stream is evaporated as vapour stream 32 causing the remaining cooling water stream to cool. The proportion a (H ₂ O) of the water evaporated from cooling tower 30 is dependent upon the flow rate, temperature and the relative humidity of the air received from the atmosphere at that particular moment, the temperature of the coolant in stream 42, and the rate of revolution of the one or more fans 34 drawing the air through the tower via line 44.

A proportion b (H ₂ O) of the cooling water stream is lost as blow-down and drift losses. Blow-down water stream and drift losses stream 36 is collected from the bottom of cooling tower 30. Blow-down water stream 36 contains any impurities such as salts and minerals which may have been concentrated in the cooling water by the evaporation of a proportion of the cooling water to the atmosphere. Blow-down water stream 36 is not normally returned to the cooling water circuit due to the increased impurity concentration.

A proportion (1-a-b) (H ₂ O) of the cooling water stream does not evaporate in cooling tower 30 and is passed to heat exchanger 40 as cooled water stream 38. As discussed above, the relative values of variables a, b and (1-a-b) thus vary due to air temperature, air relative humidity and fan speed or fan pitch in cooling tower 30 over a 24-hour period, or with longer seasonal cycles . It is apparent that the proportion of the cooling water stream (a+b) (H ₂ O) removed from the cooling circuit by evaporation must be replaced in order to keep the amount of cooling water in the circuit at a constant

level. Thus, make-up water stream 22 must supply a proportion (a+b) (H ₂ O) of additional cooling water to the system. The proportion of make-up water added as make-up water stream 22 should therefore match the proportion of water removed from the circuit by evaporation, blow down and drift losses. As blow down and drift losses are imposed by other factors and are more or less constant, the rate at which make-up water is added to the circuit is therefore directly proportional to the evaporation rate in cooling tower 30. The addition of make-up water therefore follows a cycle related to the cooling duty (and therefore evaporation rate) of the cooling circuit.

Figure 2 shows a plot comparing the make-up rate over time of an uncontrolled evaporation rate cooling circuit, A, with a controlled evaporation rate cooling circuit, B, operated according to the method of the invention .

In processes in which the amount of coolant evaporated in step (b) is not controlled, the cooling circuit is designed such to meet at least the maximum cooling duty of the cooling circuit e.g. during a hot Summer's day when the first process temperature will be highest (for example the process temperature of process stream 52 exiting air cooler 50) . However, the actual evaporation rate will vary slightly depending upon the air temperature around the cooling tower. The air temperature depends upon the prevailing weather conditions and time of day. In particular, the air temperature normally varies over a 24 hour period from a relatively cool temperature during the night to a relatively hot temperature during the day. This variance in turn causes a variance in the amount of coolant evaporated. The temperature variance

between day and night in some climates may be more than 10 °C, sometimes more than 20 °C, and even more than 30 °C. Thus, the amount of coolant evaporated during night time hours is typically less than the amount of water evaporated during daytime hours.

Thus, in plants in which the evaporation rate of the coolant stream is uncontrolled, the plot of uncontrolled evaporation rate against time shows a cyclic fluctuation in evaporation rate due to the variation in the ambient conditions for an average day/ night cycle. In Figure 2, peaks 1 and Ia for the uncontrolled evaporation rate correspond to maxima in the ambient temperature e.g. during the hottest part of the day. Troughs 2, 2a in the uncontrolled evaporation rate correspond to minima in the ambient temperature, e.g. during the coldest part of the night.

Changes in ambient conditions will inevitably result in a drop in cooling circuit cooling duty below the designed maximum, particularly when an air cooler 50 is placed upstream of heat exchanger 40. Although the maximum evaporation rate 1, Ia is the same for both the controlled and uncontrolled evaporation rates when the ambient conditions are as per design, significant savings in cooling water evaporation (and therefore the demand for make-up water) can be made by utilising the method of the invention. It is possible to maintain a constant process temperature of stream 54 in the uncontrolled option in figure 1, e.g. by controlling the cooling duty of air cooler 50. However, this would leave the duty in heat exchanger 40 uncontrolled and would not result in the desired coolant make-up savings. These savings in cooling water occur by reducing the evaporation rate in response to a decrease in the

temperature of the process stream, which results in a decrease in the cooling duty of the cooling circuit. The method of the invention provides an average controlled evaporation rate which is significantly less than the average uncontrolled evaporation rate. The reduction in water loss from the cooling circuit can be seen in the difference between the uncontrolled make-up rate A ('stream 18 or 22 uncontrolled (A) ' in figure 2) and controlled evaporation rate B (of the invention; 'stream 22 controlled (B) ' and 'stream 18 controlled (B) ' in figure 2) .

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention. For example, a single buffer tank can be used to supply make-up water to multiple open circulating cooling circuits.