The present invention relates to a plant and a process for production of high pressure steam for use in an industrial process. The high pressure steam may be used in a variety of industrial processes including drying or concentrating of a product by evaporation of a solvent, distillation, sterilisation, cleaning, oil refinery processes, district heating etc. Background art

Plants and processes for production of a high pressure steam are abundant in industry.

Conventionally, steam is produced in a boiler or steam generator by applying heat energy to water. In industry the boiler is usually a stationary steam engine which requires a firebox or a furnace in order to burn fuel and generate heat. The generated heat is transferred to water to generate steam by a process known as boiling. The saturated steam thus produced can then immediately be used to produce power via a turbine or alternatively be superheated to a higher temperature. Superheating of the steam reduces sus- pended water content making a given volume of steam produce more work and creates a greater temperature gradient.

Steam may be used in power plants to generate electricity. An example of a steam power plant is disclosed in US 3,195,306, according to which steam is generated in a furnace and subsequently superheated before it is delivered to a vapour turbine that drives a generator. A second stream of vapour is also used to drive a gas turbine, which is axially connected compressor and a generator. The compressor increases the pressure of the air that enters the furnace to improve the combustion. The vapour that leaves the first and the second turbine is condensed in a traditional fashion using a cooling medium.

US 3,398,534 discloses an industrial system, which uses a turbo- compressor unit driven by steam to produce gas compressed to a high pressure. The turbo-compressor comprises a turbine axially connected to two compressors. The compressors have associated with them a cooling device, such as an intercooler, which is effective to cool the gas from the first com- pressor before it enters the second compressor. The gas may be air, which in a distillation process is used to separate out oxygen from the nitrogen in the air. The oxygen may be may be introduced into the furnace for improved combustion.

CN 102772907 discloses a continuous crystallisation evaporation recovery equipment, in which the steam from the evaporation is treated in a turbine compressor to increase the temperature before the steam is entered into the crystallisation apparatus again. By the raising of the temperature the amount of steam is reduced.

The present invention provides a more effective production of steam or vapour, which may be used for any industrial purpose. Especially, the present invention uses less energy than conventional plants for production of steam. Summary of the invention

The present invention relates to a plant for production of a high pressure steam for use in an industrial process, comprising

a. a steam source capable of delivering a low pressure steam, b. a first compressor adapted to receiving the steam from the steam source and delivering a steam increased in pressure, c. a final compressor adapted to receiving the steam from the first compressor and delivering a high pressure steam, d. a low pressure boiler adapted to delivering a low pressure steam,

e. a first expander adapted to receiving the low pressure steam from the low pressure boiler and delivering a high pressure steam

f. a high pressure boiler adapted to delivering a higher pressure steam, and

g. a final expander adapted to receiving the higher pressure steam from the high pressure boiler and delivering a high pressure steam,

wherein heat exchangers provide for cooling the steam from the first and the final compressor and heating the steam from the low and high pressure boilers. The plant of the invention compresses steam in two or more steps to obtain a high pressure steam. The compression of the steam results in simultaneous increase of the temperature and pressure. By using an intermediate cooling step the steam is cooled while the pressure is maintained. The heat captured in the cooling process is delivered to a steam obtained from a boiler though a heat exchanger. As a result the steam from the boiler is superheated. The superheating of the steam from the boilers in the heat exchangers provide for an efficient process, due to the fact that the steam is heated in two steps. As may be realized from the Carnot cycle, the use of more steps increases the energy efficiency.

The cooling of the high pressure steam before it is used in an industrial process brings its closer to its saturation point. Therefore, the temperature difference is relatively low between the supplied high pressure steam and the condensing temperature, which increases the energy efficiency. The expanders received the superheated steam from the boilers and work is thus transformed to the system, e.g. to a shaft which may be shared with a driver. The driver and the expanders then in cooperation drive the compressors.

The plant provides high pressure steam from at least three points, i.e. after the cooling of the steam from the final expander and after the first and final expander. The pressures from the different steam outlets may be adjusted to the various applications in a certain industrial application. In a certain embodiment, the high pressure steam for use in an industrial process is collected from one or more of the final compressor, the first expander, and/or the final expander. Preferably, the pressures are adjusted to the same level so that the steam streams may be collected and forwarded to the industrial process in question in a common conduit.

In the plant of the present invention steam from a steam source and steam from boilers are treated to produce a high pressure steam. At various points in the process heat is exchanged between two steams with different temperatures. While a single heat exchanger may be sufficient, it is generally preferred that at least a first and a final heat exchanger are provided for heating each of the steams from the low and the high temperature boilers. The heating in two steps increases the overall efficiency of the heat transfer. In the present plant heat exchangers are generally configured to cool the steam stream from the compressors and heat the steam from the boilers. In a suitable embodiment, the first heat exchanger is configured to heat the steam from a boiler and cool the steam from the first compressor, and the final heat exchanger is configured to further heat the steam from the first heat exchanger and cool the steam from the final compressor. The steam emanating from the first compressor is usually cooler than the steam from the final compressor, and a higher efficiency is therefore obtained by first heating the steam from a boiler in a first heat exchanger with the steam from the first expander and then further heating the steam from the first heat ex- changer in a second heat exchanger in a second heat exchanger with the steam from the final compressor.

The cooling of a steam from one or more of the compressors may be performed in a single heat exchanger. However, it is generally suitable to cool the steam in two or more steps to increase the energy efficiency. In a preferred embodiment, at least a high temperature and a low temperature heat exchanger are configured for cooling each of the steams from the first and the final compressor.

In an embodiment in which it is desired to obtain the cooling of the steams from the compressors in two or more steps and simultaneously heat the steam from the low and high pressure boilers, the high temperature heat exchanger is configured to cool the steam from a compressor and heat the steam from a high pressure boiler, and the low temperature heat exchanger is configured to further cool the steam from the high temperature heat exchanger and heat the steam from the low pressure boiler. In a particularly preferred embodiment, the first high temperature heat exchanger is configured to cool the steam from the first compressor and heat the steam from a high pressure boiler, and a first low temperature heat exchanger is configured to further cool the steam from the first high temperature heat exchanger and heat the steam from the low pressure boiler. Furthermore, it is pre- ferred, after compression of the steam from the first low temperature heat exchanger in the final compressor to configure a final high temperature heat exchanger to cool the steam from the final compressor and heat the steam from the first high temperature heat exchanger, and configure a final low temperature heat exchanger to further cool the steam from the final high temperature heat exchanger and heat the steam from the first low tempera- ture heat exchanger.

The compression of the steam in two or more stage allows for cooling to occur between stages, which saves work in the compression process. The use of multiple compression steps brings the compression process closer to the isothermal compression, i.e. compression at constant temperature, which is more efficient. Thus, it is preferred to use one or more intermediate compressors between the first and the final compressor. If intermediate compressors are present, the number may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or more. Between each intermediate compression step the steam is generally cooled.

The overheating of the steam from the boilers in the heat exchangers is transformed to axial energy in the expanders. The plant of the invention comprises at least a first and a final expander, but further intermediate expanders may be present. The intermediate expanders are supplied with over- heated steam from corresponding intermediate boilers. The use of multiple expander steps implies that the decompression is brought closer to the isothermal decompression, which increases the efficiency. Thus, it is preferred to use one or more intermediate expanders between the first and the final expander. If intermediate expanders are present, the number may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or more.

The first and the final compressor may be driven individually by a driver. To simplify the plant and make it less expensive, it is however suitable that the first and the final compressor are driven co-axially by a driver. The design of a multi-stage compressor may be referred to as a compressor train. A compressor train is usually driven by a steam or gas turbine, or electric motor.

The first and the final expander may individually or together drive a generator, a compressor, or any other process requiring energy. To simplify the plant and make more efficient it is however desirable that the first and/or the final expander is provided on the same axis as the first and/or the final compressor, optionally with a gearing or transmission between the expander and compressor section. In such setting the expander will assist the driver in compressing the steam in the compressors. In certain embodiments, a driver may be omitted as the expanders provide sufficient power for the compres- sors to work efficiently. The choice of driver depend to a great extend on the energy source available. Usually, the driver is an internal combustion engine, a gas turbine, a gas engine, a steam turbine and/or an electrical motor. The present plant is particularly design for use of a driver that develops exhaust gas or steam, i.e. an internal combustion machine, a gas turbine, a gas engine, or a steam turbine. When such gas or steam developing driver is used, it is preferred that the boiler is adapted to use the exhaust gas or steam from the driver for boiling the water.

The steam source may be selected in accordance with the surround el- ements of the industry. When the high pressure steam is supplied to a drying or concentrating process it is suitable that the steam source is a steam evaporator or similar equipment that generates stream from drying. Examples of drying or concentration processes includes production of salt crystals from brine, food stuff, polymers, chips from wood or straw, distillation products, plaster/gypsum, etc. The steam from the drying or concentration process enters in o the present plant at the first compressor.

In the event that the high pressure steam generated by the final compressor, the first expander and/or the final expander is delivered to the steam evaporator for concentrating or drying a product by evaporation the pressure may be the same or different, depending on the circumstances. When a multi-effect evaporator facility is used the high pressure steam may enter at various stages or the high pressure steam streams are adjusted to approximately the same pressure and collected before it enters the first stage of the multi-effect evaporators. The first stage may also be referred to as the high pressure steam evaporator. The final stage of the multi-effect evaporator has the lowest pressure and is usually referred to as the low pressure steam evaporator. In a preferred aspect, the steam source for the first compressor is the low pressure steam evaporator.

Clutches may be present between the compressor train, the expander train and/or the driver to enable declutching of e.g. the driver when the expander train can be driven entirely by the expander train.

The present invention also relates to a plant for the production of a concentrated or dried product by evaporation of steam, comprising

a steam evaporator adapted for receiving an aqueous feed ma- terial and a steam for heating of the aqueous feed material and adapted for delivering a concentrated or dried product and a steam, a first compressor adapted to receiving the steam from the steam evaporator and delivering a steam increased in pressure

a final compressor adapted to receiving the steam from the first compressor and delivering a steam further increased in pressure to the steam evaporator,

a low pressure boiler adapted to delivering a low pressure steam,

a first expander adapted to receiving the low pressure steam and delivering a steam to the steam evaporator

a high pressure boiler adapted to delivering a high pressure steam,

a final expander adapted to receiving the high pressure steam and delivering a steam to the steam evaporator,

wherein heat exchangers provide for cooling the steam from the first and the final compressor and heating the steam from the low and high pressure boilers.

In a preferred embodiment of the invention the steam source is a flash tank, i.e. the steam results from the evaporation of water in a tank having a pressure lower than the pressure of the water. The resulting low pressure steam is supplied to the first compressor. Any type of cooling medium may be used, such as seawater. The water to be flashed may be delivered from a suitable source. In a preferred embodiment, the flash tank receives water to be flashed from a condenser that condenses the high pressure steam. In this way the water/steam is circulated in a closest loop. The condenser may heat water in a district heating system or any other heating system, when the high pressure steam is condensed.

To increase the efficiency, it is generally desired that a heat exchanger is provided for cooling at least a part of the high pressure steam and heating the low pressure steam before it enters the first expander. The heat exchanger will then bring the high pressure steam closer to its dew point, while heating the low pressure steam before it is received by the first compressor.

While a first and final compressor may be suitable for some industrial process it is generally desirable that one or more intermediate compressors are present between the first and second compressor, so that the steam from the first compressor is further compressed in the one or more intermediate compressors before the steam is delivered to the final compressor. The presence of intermediate compressor(s) approximates the use of the plant more to the isothermal compression, which increases the efficiency.

Between two compressor stages the steam is cooled, including between a first and an intermediate compressor, between two intermediate compressors, or between an intermediate compressor and the final compressor. While any cooling device may be used it is generally suitable that heat exchangers provide for cooling the steam from the one or more intermediate compressor(s) and heating the steam from one or more boilers.

The number of boilers normally equates the number of expanders. Thus, in a certain embodiment, one or more intermediate pressure boilers are adapted to deliver an intermediate pressure steam, which is received by an intermediate expander. However, in some embodiment steam from a boiler may feed two or more expanders.

In certain embodiment of the invention, a heat exchanger is adapted for receiving a part of the steam from the steam source to overheating the steam, and an expander is adapted downstream for the heat exchanger for receiving the overheated steam. This feature may be adopted in the plant when the amount of low pressure steam is in surplus. The steam from the expander may be condensed in a condenser using cooling water or cooling air.

While the heat exchangers provide for the superheating of the steam from the boilers to a certain level, it may be desired to heat the steam further before it enters the expanders. For further heating, a heat source is provided immediately downstream for one or more of the expanders for overheating the steam. The heat source may be gas or oil which is burned to provide the heating. The exhaust gases from the overheating may be used in heating other process elements, such as the high, intermediate, and/or low pressure boilers.

The invention may make use of a low pressure boiler, which may boil water at a pressure lower than the other boilers, e.g. at a pressure of around 1.5 bar - 4.5 bar. The steam produced from the low temperature boiler may be used directly in a multi-effect evaporator in one of the last stages, in which the pressure and temperature are relatively low. The low temperature boiler may obtain the energy from the exhaust gases from the driver as the last boiler before the exhaust gases are liberated to the surroundings or an appropriate exhaust gas treatment device.

The invention also provides for a process for production of a high pressure steam for use in an industrial process, the process comprising the steps of

a. providing a low pressure steam from a steam source, b. compressing the steam from step a in a first compressor, c. cooling the steam from step b in a heat exchanger

d. compressing the steam from step c in a final compressor for the provision of a high pressure steam,

e. providing a steam at a lower pressure from a lower pressure boiler,

f. heating the steam from the lower pressure boiler in a heat exchanger,

g. expanding the steam from step f in a first expander for decreasing the pressure and producing a high pressure steam, h. providing a steam at a higher pressure from a higher pressure boiler,

i. heating the steam from the higher pressure boiler in a heat exchanger,

j. expanding the steam from step i in a final expander for decreasing the pressure and producing a high pressure steam, wherein the steam from the first and the final compression is heat exchanged with the low and the high pressure steam from the boilers, thereby obtaining a temperature reduction of the steam from the first and the final compression and a temperature increase of the steam from the lower and higher pressure boiler.

The steam from the steam source is in a first embodiment at a pressure close to the ambient pressure, i.e. around 1013 mbar. It may be higher or lower depending on the specific process conditions. The steam is then compressed in a first compressor to a higher pressure. As an example the pressure of the steam may increased by 0.001 bar to 10 bar in the first com- pressor.

The high pressure steam, which is the output of the present process may be collected from one or more of the final compressor, the first expander, and/or the final expander. The pressure of the output gas streams may be adjusted through process design so that it has similar pressure, which enables easier blending into a common gas stream for further use in the specific industry. As an example the pressure of the high pressure steam may be 2 to 10 bar. Usually, the temperature is adjusted through heat exchangers to a temperature above the saturation point which will ensure that the steam is not condensed in the equipment. On the other hand, the temperature of the steam is not too high above the saturation point because this would make the process less effective. As a guide, the temperature should be 5°C-200°C, such as 10°C to 100°C above the saturation point of the high pressure steam produced by the process.

The steam from the low pressure boiler and the higher pressure boiler each are heat exchanged in a first and a final heat exchanger to provide the superheated steam to the expanders. The pressure of the low pressure boiler may be from 1 to 20 bar, and preferably 2 to 15 bar, such as 3 to 10 bar. Usually, the pressure of the higher pressure boiler is 1 to 50 bar, 2 to 40 and preferably 3 to 30 bar above the pressure of the low pressure boiler.

According to the invention a steam from a boiler is heated in the first heat exchanger and the steam from the first compressor is cooled, and the steam from the first heat exchanger is further heated in a final heat exchanger and the steam from the final compressor is cooled. The heating in two or more steps increases the efficiency of the process. Typically, the temperature of the saturated steam from the low pressure boiler is raised 10°C to 100°C in the first heat exchanger and further 20°C to 200°C in the final heat exchanger.

The cooling of each of the steams from the first and the final compres- sor is performed in at least a high temperature and a low temperature heat exchanger. The two steps cooling of the steam from the compressors ensure a process, which operates closer to the isothermal ideal and thus is more effective. The steam from the first compressor is usually cooled 20°C to 200°C in the first high temperature heat exchanger and further 10 to 100°C in the first low temperature heat exchanger. The temperature of the steam leaving the final compressor is usually higher than the steam from the first compressor and is consequently cooled to a higher degree.

In a preferred embodiment, the boiler is adapted to use the exhaust gas or steam from the driver for boiling the water. The plant may be de- signed so that the exhaust gas first heats the high pressure boiler before it is used to heat any intermediate boiler, if present, and finally heat the low pressure boiler. The exhaust gas may have a temperature of 500°C to 800°C, which in two or more steps may be cooled to 50°C to 300°C before it leaves the process.

In the event the high pressure steam is used in a concentration or drying process it is suitable that the steam source is a steam evaporator. The steam from the steam evaporator will normally be at its saturation point, i.e. a steam at atmospheric pressure will have a temperature of around 100°C.

In a concentration or drying process, the high pressure steam gener- ated by the final compressor, the first expander and/or the final expander may be delivered to the steam evaporator for concentrating or drying a product by evaporation. In a certain aspect of the invention, the steam evaporated during the concentrating or drying is applied as a steam source.

The invention also relate to a process for the production of a high pressure steam for use in an industrial process, said process comprising the steps of:

a. heating an aqueous feed material with steam for the production of a concentrated or dried product and a steam,

b. compressing the steam from step a in a first compressor, c. compressing the steam from step b in a final compressor, and delivering the steam to step a for heating the aqueous feed material,

d. providing a steam at a lower pressure from a lower pressure boiler

e. expanding the lower pressure steam in a first expander for decreasing the pressure before using the steam for heating of the aqueous feed material in step a,

f. providing a higher pressure steam from a higher pressure boiler,

g. expanding the higher pressure steam in a final expander for de- creasing the pressure before using the steam for heating of the aqueous feed material in step a,

wherein the steam from the first and the final compression is heat exchanged with the low and the high pressure steam, thereby ob- taining a temperature reduction of the steam from the first and the final compression and a temperature increase of the steam from the lower and higher pressure boiler.

When terms like "low pressure" or "lower pressure" and "high pres- sure" or "higher pressure" are used, these terms are to be understood as relative terms linked to a certain element of the plant or process. Thus, a lower pressure boiler has a pressure lower than a higher pressure boiler, while not necessarily having a lower pressure than upstream or downstream elements.

The term "around" or "about" means that the specific figure it is ad- hered to be 5% higher or 5% lower. Thus, an indicated temperature of 200°C means that the actual temperature may be between 190 and 210°C.

The term "overheating" or "superheating" is used in the present context to describe the heating of a steam above it saturation point. As an example, steam produced from a boiler will normally be at its saturation point. Thus, if the pressure in the boiler is 1 bar (abs), then the temperature of the steam above the surface of the water will be 100°C. Further heating of this steam will lead to an overheated or superheated steam. The heating may be obtained in a heat exchanger by heat exchanging with a medium having a higher temperature.

Summary of the drawings

The present invention will now be described in greater detail based on preferred embodiments with reference to the drawings on which :

Figure 1 shows a flow diagram of the invention as described in ac- cording to which LP (low pressure) steam is introduced to the first compressor and HP (high pressure) steam is produced.

Figure 2 discloses a flow diagram of the invention according to which a drying apparatus supplies the low pressure steam.

Figure 3 shows a flow diagram of a further development of the inven- tion using three or more compressing and/or expanding stages, and Figure 4 shows a flow diagram of a further development using an overheater.

Figure 5 shows a flow diagram of a plant for heating water in district heating system.

Figure 6 shows a flow diagram of a plant as shown in figure 5 with improved efficiency.

Detailed description

Figure 1 shows an embodiment of the invention. In an exemplary embodiment, the low pressure steam from the steam source is provided at 100°C and 1 bar. The steam enters the first compressor 21, which increases the pressure to about 2 bar and the temperature to around 250°C. The steam from the first compressor enters the first high temperature heat exchanger 41, in which it is cooled to around 180°C. In the first low temperature heat exchanger 61 the steam is further cooled to about 156°C. Subsequently the steam is entered into the final compressor 23, which increases the pressure to about 3 bar and the temperature to about 300°C. The steam from the final compressor is introduced in the final high temperature heat exchanger 43, in which the temperature is cooled to about 250°C. In the final low temperature heat exchanger 63, the temperature is further decreased to 180°C. At this temperature the steam is provided for the industrial process in question.

In a low pressure boiler 71, the water is evaporated at e.g. 5 bar to produce a steam having a temperature of around 151°C. The steam from the boiler is further heated in first low temperature heat exchanger 61 to 175°C and final low temperature heat exchanger 73 to 240°C. The overheated steam is introduced into first expander 31. In a high pressure boiler 73, water is evaporated at e.g. 10 bar to produce a steam of 175°C. This steam is heated in a first high temperature heat exchanger 41 to 245°C and further heated to 295°C before it enters the final expander 33. The exit steam from first and final expander 31 and 33 may be adjusted to essentially the same pressure provided by the compressors of around 3 bar delivered to an industrial process requiring high pressure steam.

Figure 2 shows an embodiment of the invention. In an exemplary embodiment, the steam from the steam source, such as steam evaporator 1, is evaporated at around 100°C and around 1 bar. The steam enters the first compressor 21, which increases the pressure to about 2 bar and the temperature to about 250°C. The steam from the first compressor enters the first high temperature heat exchanger 41, in which it is cooled to around 180°C. In the first low temperature heat exchanger 61 the steam is further cooled to about 156°C. Subsequently the steam is entered into the final compressor 23, which increases the pressure to about 3 bar and the temperature to about 300°C. The steam from the final compressor is introduced in the final high temperature heat exchanger 43, in which the temperature is cooled to about 250°C. In the final low temperature heat exchanger 63, the tempera- ture is further decreased to about 180°C. At this temperature the steam is entered into the steam evaporator 1.

The condensate from the steam in the steam evaporator is collected and forwarded to boilers 71-73. In a low pressure boiler 71, the condensate, typically water, is evaporated at e.g. 5 bar to produce a steam having a tem- perature of around 151°C. The steam from the boiler is further heated in first low temperature heat exchanger 61 to around 175°C and final low temperature heat exchanger 73 to around 240°C. The overheated steam is introduced into first expander 31. In a high pressure boiler 73, the condensate is evaporated at e.g. around 10 bar to produce a steam of around 175°C. This steam is heated in a first high temperature heat exchanger 41 to around 245°C and further heated to around 295°C before it enters the final expander 33. The exit steam from first and final expander 31 and 33 is adjusted to essentially the same pressure of around 3 bar and is entered into the steam evaporator 1.

In Fig. 3 an embodiment of the invention with 3 or more expander and compressor steps is illustrated. The steam from the evaporator 11 is typically produced at 1 bar and 100°C. The steam is introduced into first compressor 21, in which the pressure is increased to about 3 bar and the temperature is typically 300°C. Subsequently, the steam is introduced into a se- ries of heat exchangers, i.e. first high temperature heat exchanger 41 to obtain a temperature of around 201°C, first intermediate temperature heat exchanger 51 to obtain a temperature of around 180°C, and first low temperature heat exchanger 61 to obtain a temperature of around 156°C. In an optional first feed steam heat exchanger 81, the temperature may further be reduced to around 105°C. This steam is introduced to the second or interme- diate compressor 22. The compressor increases the pressure to around 5 bar and the temperature is typically 350°C. Subsequently, the steam is introduced into a series of heat exchangers, i.e. intermediate high temperature heat exchanger 42 to obtain a temperature of around 300°C, intermediate (or second) intermediate temperature heat exchanger 52 to obtain a temperature of around 201°C, and intermediate low temperature heat exchanger 62 to obtain a temperature of around 180°C. In an optional intermediate feed steam heat exchanger 82, the temperature may further be reduced to around 146°C. This steam is introduced to the third or final compressor 23. The compressor increases the pressure to around 7 bar and the temperature is typically around 400°C. Subsequently, the steam is introduced into a series of heat exchangers, i.e. final high temperature heat exchanger 43 to obtain a temperature of around 350°C, final (or second) intermediate temperature heat exchanger 53 to obtain a temperature of around 300°C, and final low temperature heat exchanger 63 to obtain a temperature of 201°C. In an optional final feed steam heat exchanger 83, the temperature may further be reduced to around 180°C. This steam is introduced into the first of a series of steam evaporators 11, 12, and 13.

In the high pressure steam evaporator 13, the steam is introduced at around 7 bar. The condensation of the steam provides for the evaporation of moisture or water from the product being dried or concentrated. The pressure above the surface of the product being treated is maintained at a pressure of 5.5 bar. In a first intermediate pressure steam evaporator the product is further concentrated or dried at a pressure of 4 bar. In a second intermediate steam evaporator 12, the pressure in the concentration or drying chamber is maintained at 2.5 bar. In the low pressure steam evaporator 11, the pressure above the surface of the product being treated is maintained at 1 bar, which provides a temperature of the water being evaporated of 100°C.

Boilers receive some of the condensate from the steam evaporators. Thus, the second intermediate pressure evaporator 12 provides water for boiling in the high pressure boiler 71, whereas the first intermediate pressure evaporator 12 provides water for the intermediate pressure boiler 72 and the high pressure steam evaporator 73 provides water for the high pressure boiler. In an alternative embodiment the second intermediate pressure evapora- tor provides water to the low pressure boiler and the high pressure steam evaporator provides water to the high pressure boiler.

The steam from the boilers is heated in a series of heat exchangers before it is introduced into the expanders. Thus, the steam from the low pressure boiler 71 is boiled at around 5 bar to produce a steam of around 151°C. This steam is heated in a First low temperature heat exchanger 61 to around 175°C, and then in intermediate low temperature heat exchanger 62 to around 196°C, and finally in Final low temperature heat exchanger 63 to around 295°C before it is entered into the fist expander 31. The Expander is adjusted so that the exiting pressure corresponds to the steam entering the low pressure steam evaporator 11. The steam from intermediate pressure boiler 72 is boiled at around 10 bar to produce a steam of around 175°C. This steam is heated in a First intermediate temperature heat exchanger 51 to around 196°C, and then in intermediate temperature heat exchanger 52 to around 295°C, and finally in Final intermediate temperature heat exchanger 53 to around 345°C before it is entered into the second or intermediate expander 32. The Expander is adjusted so that the exiting pressure corresponds to the steam entering the higher pressure steam evaporator 13.

The steam from high pressure boiler 73 is boiled at 15 bar to produce a steam of around 196°C. This steam is heated in a First high temperature heat exchanger 41 to around 295°C, and then in intermediate high temperature heat exchanger 42 to around 345°C, and finally in Final high temperature heat exchanger 43 to around 395°C before it is entered into the third or final expander 33. The Expander is adjusted so that the exiting pressure corresponds to the steam entering the higher pressure steam evaporator 13. The steam from the intermediate and final expander is entered into the higher pressure steam evaporator 13 for heating of the product to be dried or concentrated.

In some embodiments a valve 92 is positioned in the feed gas stream to direct part of the steam through the heat exchanger 81, 82, and 83 before they are introduced in to the feed steam expander 90. In an exemplary embodiment the steam has a pressure of 1 bar and a temperature of around 100°C. In the First feed steam heat exchanger 81, the feed steam is heated to around 151°C and in the intermediate feed steam heat exchanger 82, the feed steam is heated to around 175°C, and in a final feed steam heat ex- changer 83, the feed steam is heated to around 196°C. The heated feed steam is introduced into the Feed steam expander 90. The exiting steam pressure is around 0.02 bar and the temperature is about around 30°C. In a condenser 91, the stream from the expander 90 is condensed to water is heat exchange with an available medium, such as sea water, air, process wa- ter or the like.

In the embodiments shown in Fig. 1 and 2, M is a driver, preferably a gas engine or gas turbine. The exhaust gas from the gas engine or turbine may be used for at least partly heating the boilers 71 - 73. In a preferred aspect the heating is sequential, i.e. the high pressure boiler is first heated with the exhaust gas from the gas engine. Subsequently, the exhaust gas now decreased in temperature is used to heat the second or intermediate boiler. Finally, the exhaust gas leaving the intermediate pressure boiler is used to heat the low pressure boiler 71.

The compressors and the expanders are shown positioned on the same shaft. While this is a preferred embodiment the person skilled in the art will realise that one or more of the compressors or expanders may be operated individually. Furthermore only a single intermediate compressor and expander is shown. The person skilled in the art will understand that two or more intermediate compressors and or expanders may be used. In addition, the number of compressors does not necessarily need to be the same as the number of expanders. Depending on the specific design, the number of expanders may be lower than, higher than, or identical to the number of compressors used in the plant.

The product being treated in the present invention may be moved be- tween the stages of the steam evaporator 1. Thus, in a preferred embodiment, the product deprived in water leaving the low pressure steam evaporator 11, may be fed into the intermediary pressure steam evaporator 12 and finally into the high pressure steam evaporator 13. The dried or concentrated product leaving the high pressure steam evaporator 13 may be heat ex- changed with the product to be treated entering the low pressure steam evaporator 11.

Figure 4 shown an embodiment, which further elaborate on the embodiment shown in Figure 3. The embodiment of figure 4 discloses a low pressure boiler 74, which may boil water at a pressure lower than the other boilers, e.g. at a pressure of around 2.5 bar. The steam produced from boiler 74 may be used directly in the multi-effect evaporator in one of the last stages, in which the pressure and temperature is relatively low. The low temperature boiler 74 may obtain the energy from the exhaust gases from the driver as the last boiler before the exhaust gases are liberated to the surroundings or an appropriate exhaust gas treatment device.

Figure 4 also shows steam overheaters 93, which provide the steam with a temperature boost just before it enters the expanders. The overheating may involve the use of a combustion process and the exhaust gas from the combustion may be used in heating other process elements, such as the high, intermediate, and/or low pressure boilers.

Figure 5 shows an embodiment of the invention that provides heating of water used for heating houses and industry in a certain district in a so called district heating system. According to this embodiment the low pressure steam is provided by a flash tank 95. In an exemplary embodiment, the pres- sure of the steam is 0.008 bar and the temperature is 5°C. The steam enters the first compressor 21, which increases the pressure to about 0.03 bar and the temperature to around 350°C. The steam from the first compressor enters the first high temperature heat exchanger 41, in which it is cooled to around 185°C. In the first low temperature heat exchanger 61 the steam is further cooled to about 155°C. Subsequently the steam is entered into the final compressor 23, which increases the pressure to about 0.1 bar and the temperature to about 450°C. The steam from the final compressor is introduced in the final high temperature heat exchanger 43, in which the temperature is cooled to about 350°C. In the final low temperature heat ex- changer 63, the temperature is further decreased to 185°C. At this temperature the steam is provided to the condenser 96. The condenser condenses the high pressure steam to a condensate and heats water from a district heating system, typically from 35°C to 40°C. The condensate may obtain a pressure of about 0.07 bar and a temperature of 37°C. A part of the condensate is de- livered to the flash tank 95, which provides for the low pressure steam, which enters into the first compressor. A cooling medium is required in the flashing process, which may be selected as seawater or another available source, such as ammonia, propan, butan etc. used in another process. Typically, the cooling medium leaves the flash tank at a temperature of about 7°C, when the low pressure steam is at around 5°C. Another part of the condensate is forwarded to the boilers 71 and 73.

In a low pressure boiler 71, the water is evaporated at e.g. 5 bar to produce a steam having a temperature of around 150°C. The steam from the boiler is further heated in first low temperature heat exchanger 61 to 180°C and final low temperature heat exchanger 73 to 345°C. The overheated steam is introduced into first expander 31. In a high pressure boiler 73, water is evaporated at e.g. 10 bar to produce a steam of 180°C. This steam is heated in a first high temperature heat exchanger 41 to 345°C and further heated to 445°C before it enters the final expander 33. The pressure of the exit steam from first and final expander 31 and 33 may be adjusted to essentially the same pressure provided by the final compressor of around 0.1 bar, before the steam is mixed with the steam from the final compressor and delivered to the condenser 96.

Figure 6 discloses an embodiment of the invention as shown in figure 5, further provided with a heat exchanger 97. The heat exchanger 97 provides for heating of the source steam from the flash tank from 5°C to 180°C, while the steam from the final compressor is cooled from 185°C to 50°C. The low pressure steam from the sour steam heat exchanger enters the first compressor 21, which increases the pressure to about 0.03 bar and the tem- perature to around 450°C. The steam from the first compressor enters the first high temperature heat exchanger 41, in which it is cooled to around 185°C. In the first low temperature heat exchanger 61 the steam is further cooled to about 155°C. Subsequently the steam is entered into the final compressor 23, which increases the pressure to about 0.1 bar and the tempera- ture to about 450°C. The steam from the final compressor is introduced in the final low temperature heat exchanger 63, where the temperature is further decreased to 185°C. At this temperature the steam is provided to the source steam heat exchanger 97. After heat exchange with the source steam from the flash tank, the steam is introduced into the condenser, which con- denses the high pressure steam to a condensate and heats water from a district heating system, typically from 35°C to 40°C. The condensate may obtain a pressure of about 0.07 bar and a temperature of 37°C. A part of the condensate is delivered to the flash tank 95, which provides for the low pressure steam, which enters into the first compressor. A cooling medium is required in the flashing process, which may be selected as seawater or another availa- ble source, such as ammonia, propan, butan etc. used in another process. Typically, the cooling medium leaves the flash tank at a temperature of about 7°C, when the low pressure steam is at around 5°C. Another part of the condensate is forwarded to the boilers 71 and 73.

In a low pressure boiler 71, the water is evaporated at e.g. 5 bar to produce a steam having a temperature of around 150°C. The steam from the boiler is further heated in first low temperature heat exchanger 61 to 180°C and final low temperature heat exchanger 73 to 445°C. The overheated steam is introduced into first expander 31. In a high pressure boiler 73, water is evaporated at e.g. 10 bar to produce a steam of 180°C. This steam is heated in a first high temperature heat exchanger 41 to 445°C before it enters the final expander 33. The pressure and the temperature of the exit steam from first and final expander 31 and 33 may be adjusted to essentially the same pressure provided by the final compressor of around 0.1 bar and temperature of 50°C, before the steam is mixed with the steam from the source steam heat exchanger 97 and delivered to the condenser 96.

Figure 7 discloses an embodiment of the invention as shown in figure 5 and 6, further provided with an initial flash tank 98. The initial flash tank provides for initial flashing of the condensate from the condenser. The vapor produced by the flashing is blended with steam from the heat exchanger 61, which has been used for delivering heat to the evaporation. The liquid produced by the flashing is directed to the first flash tank 95. In the exemplary embodiment of Fig. 5 and 6, the steam having a temperature from heat exchanger 61 of 155°C is further cooled to 45 °C in the flash tank. The conden- sate from the condenser 96 is flashed in two steps in the present embodiment. In an initial step the temperature of the condensate is reduced in the initial flash tank 98 to a temperature of 45°C and further reduced in temperature first flash tank 95 to 5°C. The steam generated in the initial flash tank and the cooled steam from the heat exchanger 61 are mixed before they en- ter the final expander 23. Reference numbers on the drawings:

1 steam evaporator

1 1 low pressure steam evaporator

12 intermediate pressure steam evaporator

13 High pressure steam evaporator

21 first compressor

22 intermediate compressor

23 final compressor

31 first expander

32 intermediate expander

33 final expander

41 First high temperature heat exchanger

42 intermediate high temperature heat exchanger

43 final high temperature heat exchanger

51 First intermediate temperature heat exchanger

52 Second (or intermediate) intermediate temperature heat exchanger

53 Final intermediate temperature heat exchanger

61 First low temperature heat exchanger

62 intermediate low temperature heat exchanger

63 Final low temperature heat exchanger

71 low pressure boiler

72 Intermediate pressure boiler

73 high pressure boiler

74 Low pressure boiler

81 First feed steam heat exchanger

82 Intermediate feed steam heat exchanger

83 final feed steam heat exchanger

90 Feed steam expander

91 Condensor

92 Valve

93 Steam overheater

94 Inlet for cooling medium

95 Flash tank

96 Condenser

97 Source steam heat exchanger

98 Initial flash tank

P Dried or concentrated product

c Condensate

M Driver