A process for producing steam

Abstract

The invention relates to a process for producing steam, comprising: (a) heating a feed water stream (1) in a first heat exchanger (3) to a temperature in a range from 40 to 95 °C; (b) heating the feed water stream (1) in a second heat exchanger (5) to a temperature in a range from 90 to 115 °C; (c) feeding the heated feed water stream into an apparatus for thermal degassing (7), which is operated at a boiling temperature in a range from 90 to 115 °C, thereby obtaining a gas stream (35) and a degassed water stream (9); (d) optionally compressing the degassed water stream (9) to a pressure in a range from 0.9 to 20 bar(abs); (e) feeding the degassed water stream into a flash apparatus (13), in which the degassed water stream is expanded to a pressure in a range from 0.04 to 0.8 bar(abs), so that a part of the water evaporates to form steam (21), or evaporating the degassed water in an evaporator (47); (f) preferably compressing the steam (21) in at least one compressor with at least one compressor stage (23, 25).

Description

[0001] A process for producing steam

[0002] Description

[0003] The invention relates to a process for degassing water and producing steam using the degassed water.

[0004] Steam is a major thermal energy carrier in industry, for example in the chemical industry. Conventional fossil steam production comes along with significant carbon dioxide emissions. Heat pumps are considered to minimize the carbon dioxide emissions with reduced demand of (renewable) electrical power compared to electric boilers.

[0005] Besides thermal energy, the main utility for steam production is demineralized water. The demineralized water generally is relatively cold which increases the energy demand to produce steam. Furthermore, the demineralized water contains dissolved gases which need to be removed. Conventional thermal degassing uses steam produced with fossil resources, which is counterproductive for the energy transition. Hence, the open loop heat pump itself needs to produce steam for the thermal degassing. For an open loop heat pump system, including degassing, using the heat of the waste heat source with the maximum Coefficient of Performance (COP) is crucial.

[0006] Besides thermal degassing, membrane degassing processes are used. When membrane degassing is done using an inert sweep gas like nitrogen, some of the inert gas is left in the steam, which can affect the functionality of downstream equipment like heat exchangers. The water temperature needs to be well below 50 °C because of degradation of the membrane material. Conventional vacuum degassing at temperatures below 25 °C uses energy, e.g. fossil steam to operate the steam ejector, to reach pressures below 40 mbar(abs). Due to the very low pressure, the equipment size is large and expensive.

[0007] An open loop lithium bromide heat pump system with a degassing apparatus is shown in CN-A 109028644 or in CN-U 208588109.

[0008] It is an object of the present invention to provide a process for producing steam with integrated degassing of water, which is an energy efficient process along with a reduction of the carbon dioxide footprint.

[0009] This object is achieved by a process for producing steam, comprising:

[0010] (a) heating a feed water stream in a first heat exchanger to a temperature in a range from 40 to 95 °C;

[0011] (b) heating the feed water stream in a second heat exchanger to a temperature in a range from 90 to 115 °C; (c) feeding the heated feed water stream into an apparatus for thermal degassing, which is operated at a boiling temperature in a range from 90 to 115 °C, thereby obtaining a gas stream and a degassed water stream;

[0012] (d) optionally compressing the degassed water stream to a pressure in a range from 0.9 to 20 bar(abs);

[0013] (e) feeding the degassed water stream into a flash apparatus, in which the degassed water stream is expanded to a pressure in a range from 0.04 to 0.8 bar(abs), so that a part of the water flashes to steam, or evaporating the degassed water in an evaporator;

[0014] (f) preferably compressing the steam in at least one compressor with at least one compressor stage.

[0015] For producing steam, generally demineralized water is used. Since gases may be dissolved in the demineralized water, it is necessary to remove the gases prior to the steam generation.

[0016] According to the invention, the water is thermally degassed by using steam as a sweep medium. For this purpose, in a first step (a), a feed water stream is heated in a first heat exchanger to a temperature in a range from 40 to 95 °C, preferably to a temperature in a range from 60 to 85 °C and particularly in a range from 65 to 80 °C.

[0017] In the context of the present invention, the terms “degassing” and “deaerating” may be used interchangeably.

[0018] The first heat exchanger may be any type of heat exchanger suitable for heating water. Suitable heat exchangers may be heat exchangers for indirect heat transfer from a heat transfer medium like shell-and-tube heat exchangers, plate heat exchangers or spiral heat exchangers. Arrangements with counter current flow, which can be achieved preferably in a plate heat exchanger, are best suited to reach the highest temperature of the feed water while exploiting the heat source to lowest temperatures. With this, the volume flow from the heat source for preheating the feed water can be minimized.

[0019] Besides heat exchangers for indirect heat exchange, the first heat exchanger also may be any apparatus suitable for heating, for example an apparatus for electrical heating. Further, it is also possible to use a combination of at least two different types of heat exchangers as first heat exchanger, for example an apparatus for electrical heating and a heat exchanger for indirect heat exchange. However, using a heat exchanger for indirect heat transfer from a heat transfer medium is preferred.

[0020] The heat transfer medium used for heating the water in the first heat exchanger may be a process stream or any suitable heat transfer medium, for example a liquid heating medium or a gas. Suitable liquid heating media for example are heat transfer oils or water.

[0021] Gases that may be used as heat transfer medium for example are air or flue gases. If a gas is used as heat transfer medium, it is particularly preferred to use a gas that condenses by heating the water in the first heat exchanger. It is an advantage of a condensable gas that during heat transfer for heating the water the gas condenses at least partly while the temperature remains constant. For this reason, a larger amount of heat can be transferred than by a heat transfer medium that maintains its phase but is cooled by the heat transfer. Suitable condensable gases for example are benzene, methanol, ethanol, and steam.

[0022] Besides using a liquid heating medium or a gas, it is particularly preferred to use a process stream as heat transfer medium. The process stream may be any process stream that needs to be cooled and that is obtained in a process, for example a chemical reaction, a distillation, a condensation, an absorption, an adsorption, a crystallization or an extraction. The process stream may be a liquid stream or a gas stream.

[0023] After being heated in the first heat exchanger, the feed water stream is fed into a second heat exchanger and further heated to a temperature in a range from 90 to 115 °C, preferably to a temperature in a range from 95 to 105 °C.

[0024] The second heat exchanger is a heat exchanger for indirect heat transfer with a heat transfer medium. Suitable heat exchangers that can be used as the second heat exchanger for example are shell-and-tube heat exchangers, plate heat exchangers or spiral heat exchangers. Arrangements with counter current flow, which can be achieved preferably in a plate heat exchanger, are best suited to reach highest temperature of the feed water while exploiting the heat source to lowest temperatures. With this, the volume flow from the heat source for pre-heating of the feed water can be minimized.

[0025] The heat transfer medium used for heating the feed water stream in the second heat exchanger preferably is degassed water obtained from the apparatus for thermal degassing in step (c), thus reducing the overall necessary amount of specific electric power. Alternatively, it is also possible to use a combination of the second heat exchanger and an electrical heating for heating the feed water stream to the temperature in the range from 90 to 115°C. However, using only the second heat exchanger and the degassed water obtained from the apparatus for thermal degassing as heat transfer medium is particularly preferred.

[0026] After being heated, the feed water stream is fed into the apparatus for thermal degassing. For thermal degassing, the water needs to be at its boiling point. For this reason, the apparatus for thermal degassing is operated at the boiling point of water, preferably at a temperature in a range from 90 to 115 °C, preferably in a range from 100 to 110 °C and according pressures.

[0027] Preferably, for thermal degassing, the feed water stream and steam are fed into the apparatus for thermal degassing. In this case, the steam absorbs a part of the gases which are dissolved in the water. In this case, it is not necessary that the water boils. On the other hand, the water must be hot enough that only a small amount of the steam condenses and, preferably, no steam condenses, so that the gases can be removed with the steam.

[0028] By boiling in the apparatus for thermal degassing or by feeding steam into the apparatus for thermal degassing, a liquid phase comprising degassed water, and a gas phase comprising steam and volatile gases that were dissolved in the feed water are formed. From the gas phase in the apparatus for thermal degassing the gas stream is withdrawn and from the liquid phase in the apparatus for thermal degassing the degassed water stream is withdrawn.

[0029] The apparatus for thermal degassing for example is a deaerator, e.g. a Stork spray-type deaerator, or a column with plates.

[0030] To avoid leakage of air or other components into the degassed water, it is preferred that the degassed water stream obtained by thermal degassing in the apparatus for thermal degassing may be compressed to a pressure in a range from 0.9 to 20 bar(abs), preferably to a pressure in a range from 1.1 to 10 bar(abs). If the degassed water stream is compressed, any suitable pump for compressing liquids may be used. Alternatively, compression may be obtained by gravity. For compression by gravity, the apparatus for thermal degassing is located at a higher position than the flash apparatus. In this case, the pressure in the connecting line between the apparatus for thermal degassing and the flash apparatus increases due to the hydrostatic pressure in the connecting line.

[0031] Independently of being compressed or not, in one embodiment of the invention at least a part of the degassed water stream is fed into a flash apparatus. On entry into the flash apparatus the degassed water stream expands, and, due to the expansion a part of the degassed water stream flashes to steam. To be able to flash a part of the water to steam, the pressure in the flash apparatus must be below the boiling pressure of the feed water stream at the temperature the feed water stream is fed into the flash apparatus. According to the invention, the pressure in the flash apparatus is in a range from 0.04 to 0.8 bar(abs), preferably in a range from 0.1 to 0.4 bar(abs).

[0032] Before throttling to the pressure inside the flash apparatus, the degassed water being fed into the flash apparatus preferably has a temperature in a range from 40 to 110 °C, more preferred in a range from 60 to 95 °C, more preferred in a range from 65 to 90 °C, and particularly in a range from 70 to 85 °C and a pressure in a range from 0.9 to 20 bar(abs), more preferred in a range from 1 .05 to 10 bar(abs) and particularly in a range from 1.1 to 2 bar(abs). By flashing a part of the water into steam, preferably 0.1 to 10 wt-% of the water evaporate. More preferred, 0.5 to 5 wt-% of the water evaporate and, particularly, 1 to 3 wt% of the water evaporate.

[0033] The flash apparatus used for flash evaporation of the degassed water for example is a flash column or flash vessel. To expand the stream entering the flash apparatus, generally a throttle is provided at the inlet into the flash apparatus. By partial evaporation of the degassed water stream, a liquid phase and a gas phase form in the flash apparatus. The gas phase contains steam, which is withdrawn from the flash apparatus.

[0034] As an alternative, the degassed water may be evaporated in an evaporator for generating steam. Also in this case, the steam may be used directly or may be compressed, depending on the intended use of the steam. Suitable evaporators for example are falling film evaporators, natural circulation evaporators or shell-and-tube heat exchangers.

[0035] To evaporate the degassed water in the evaporator, it is preferred that the degassed water is transported at a pressure in a range from 0.9 to 5 bar(abs), more preferred in a range from 1 to

[0036] 3 bar(abs) and particularly in a range from 1.1 to 2 bar(abs). Particularly preferably, in this case the pressure of the degassed water is not increased after being withdrawn from the degassing column but fed into the evaporator with the same pressure. Inside of the evaporator, the pressure preferably is throttled to similar pressures as described above for the flash apparatus.

[0037] Depending on the intended use, the steam may be used as withdrawn from the flash apparatus, i.e. having the pressure with which the steam is withdrawn from the flash apparatus, or the pressure with which the steam is withdrawn from the evaporator. However, alternatively and preferably, the steam is compressed to a pressure in a range from 1 to 50 bar(abs), more preferred to a pressure in a range from 2 to 16 bar(abs) and particularly to a pressure in a range from 4 to 7 bar(abs) in the at least one compressor with at least one compressor stage. The pressure depends on the intended use of the steam.

[0038] The steam obtained after compression may be used in any process in which steam is used for heating or as a raw material for a chemical process, e.g., steam methane reforming, or as a heat source in a heat exchanger or an evaporator, for example a reboiler of a distillation column. Furthermore, it is also possible to compress the steam to a pressure suitable for feeding it into a steam grid. The steam grid may be for example a low-pressure steam grid, a mediumpressure steam grid or a high-pressure steam grid.

[0039] In this context, “low pressure steam” means steam having a pressure in a range from 0.9 to

[0040] 4 bar(abs), more preferred in a range from 1 to 2 bar(abs) and particularly in a range from 1 .2 to 1 .5 bar(abs) and a temperature in a range from 96 to 200 °C, more preferred in a range from 99 to 190 °C and particularly in a range from 104 to 180 °C.

[0041] The term “medium pressure steam” means steam having a pressure in a range from 4 to 40 bar(abs), more preferred in a range from 5 to 25 bar(abs) and particularly in a range from 5 to 23 bar(abs) and a temperature in a range from 143 to 350 °C, more preferred in a range from 147 to 300 °C and particularly in a range from 151 to 250 °C. The term “high pressure steam” means steam having a pressure in a range from 41 to 120 bar(abs), more preferred in a range from 41 to 111 bar(abs) and particularly in a range from 49 to 111 bar(abs) and a temperature in a range from 252 to 450 °C, more preferred in a range from 263 to 420 °C and particularly in a range from 280 to 420 °C.

[0042] The compressor used for compressing the steam may be any compressor having at least one compressor stage. Suitable compressors are apparatuses that compress gases. Depending on the required pressure of the steam, compression may be carried out in only one compressor stage or in a cascade of at least two compressor stages. More than one compressor stage can be implemented within one compressor comprising more than one stage or a cascade of more than one compressor comprising one or more than one compressor stages each. Suitable compressors for example are integrally geared turbo compressors, radial compressors, axial compressors, screw compressors, and piston compressors.

[0043] For increasing the energy efficiency and setting the properties of the steam, particularly the temperature, it is possible to inject water before or after at least one compressor or compressor stage. For integrally geared turbo compressors, water is preferably injected after each compressor or compressor stage. The water that is injected is a part of the degassed water stream obtained in step (c). Depending on the pressure of the steam into which the water is injected, the water may need to be compressed to a pressure above the pressure of the steam, before being injected. Besides setting the properties of the steam, it is a further advantage that by injection of water the mass flow of steam increases.

[0044] For reducing the energy consumption and particularly for reducing external energy supply, it is preferred that for heating the water in the apparatus for thermal degassing a part of the compressed steam obtained in step (f) is fed into the apparatus for thermal degassing or that a part of the steam obtained in step (e) or of the compressed steam obtained in step (f) is used as a heat transfer medium for heating the apparatus for thermal degassing. Preferably, the part of the compressed steam used as the heat transfer medium for heating the apparatus for thermal degassing is withdrawn downstream of that compressor or compressor stage, in which the steam is compressed to a pressure in a range from 1 to 5 bar(abs), more preferred to a pressure in a range from 1 ,1 to 4 bar(abs) and particularly to a pressure in a range from 1 ,2 to 3 bar(abs).

[0045] If water is injected into the compressed steam, it is further preferred to withdraw the steam used for heating in the apparatus for thermal degassing before injecting the water.

[0046] If the steam is fed into the apparatus for thermal degassing, the water in the apparatus for thermal degassing is heated by direct heat exchange. In this case, a part of the steam condenses and the remaining steam absorbs gases, which are dissolved in the water. The steam with the gases absorbed in the steam are then withdrawn from the apparatus for thermal degassing. Alternatively, the apparatus for thermal degassing may be heated by indirect heating. For this purpose, an external heat exchanger may be provided, a heating coil may be provided in the apparatus for thermal degassing or the apparatus for thermal degassing may have a double jacket. In this case, the steam obtained in step (e) or the steam obtained in step (f) being used as heat transfer medium flows through the external heat exchanger, the heating coil or the double jacket and the water to be degassed is heated by indirect heat transfer from the steam. Preferably, the steam is fed into the apparatus for thermal degassing and the water in the apparatus for thermal degassing is heated by direct heat transfer.

[0047] If the steam is compressed in more than one compressor or more than one compressor stage, the steam used for heating the apparatus for thermal degassing may be withdrawn after any of the compressor stages with sufficient pressure level. Further, it is also possible to withdraw the steam downstream of more than one compressor or compressor stage. In this case the steam streams may be mixed and then used for heating the apparatus for thermal degassing. This increases the overall energy efficiency.

[0048] The gas stream obtained in the apparatus for thermal degassing generally contains steam and gases. The gases dissolved in the feed water stream particularly depend on their origin and usually comprise at least one of nitrogen, oxygen, argon, and ammonia. Nitrogen, oxygen, and argon generally originate from air that is dissolved in the water, while ammonia is used for conditioning the feed water to prevent corrosion of steel equipment. The ammonia will partially remain in the degassed water. If during thermal degassing ammonia is removed from the feed water in such an amount, that the ammonia content is too low and, hence, the pH-value is too low, it is further possible to add ammonia after thermal degassing of the feed water.

[0049] As in the flash apparatus only a part of the water is evaporated and to avoid withdrawing water from the process, which can be used for steam generation, it is preferred to recycle the liquid water from the flash apparatus into the water stream to be evaporated. For this purpose, it is preferred to withdraw a liquid water stream from the flash apparatus, heat the liquid water stream to a temperature in a range from 40 to 110 °C, more preferred in a range from 60 to 95 °C, more preferred in a range from 65 to 90 °C and particularly in a range from 70 to 85 °C. To partly evaporate the liquid water stream on entering into the flash apparatus, it is further necessary to compress the liquid water stream. Preferably, the liquid water stream is compressed to a pressure in a range from 0.9 to 20 bar(abs), more preferred to a pressure in a range from 1 to 10 bar(abs) and particularly to a pressure in a range from 1 to 2 bar(abs). After compression and heating, the liquid water stream is returned into the flash apparatus. To avoid a two phase flow, it is preferred to compress the liquid water stream before heating. For compressing the liquid water stream, it is further preferred to use a pump being placed below the water surface of the flash apparatus to avoid cavitation.

[0050] The liquid water stream preferably is heated in a third heat exchanger. The third heat exchanger may be any type of heat exchanger suitable for heating water, for example heat exchangers for indirect heat transfer from a heat transfer medium like shell-and-tube heat exchangers, plate heat exchangers or spiral heat exchangers. Arrangements with counter current flow, which preferably can be achieved in a plate heat exchanger are best suited to reach highest tempera- ture of the feed water while exploiting the heat source to lowest temperatures. With this, the volume flow from the heat source for pre-heating of the feed water can be minimized.

[0051] Besides heat exchangers for indirect heat exchange, the third heat exchanger also may be any apparatus suitable for heating, for example an apparatus for electrical heating. Further, it is also possible to use a combination of at least two different types of heat exchangers as third heat exchanger, for example an apparatus for electrical heating and a heat exchanger for indirect heat exchange. However, using a heat exchanger for indirect heat transfer from a heat transfer medium is preferred.

[0052] Particularly preferred, the first heat exchanger and the third heat exchanger are heat exchangers for indirect heat transfer from a heat transfer medium and the heat transfer medium used in the third heat exchanger is the same heat transfer medium as used in the first heat exchanger. It may be possible that the heat transfer medium firstly flows through the first heat exchanger and subsequently through the third heat exchanger. However, preferably, the heat transfer medium is divided into a first partial stream and a second partial stream, wherein the first partial stream is used as heat transfer medium in the first heat exchanger for heating the feed water stream in step (a) and the second partial stream is used in the third heat exchanger for heating the liquid water stream. If the heat transfer medium is divided into a first partial stream and a second partial stream, it is further possible, that the second partial stream is used as heat transfer medium in the evaporator for evaporating the degassed water. Independently of evaporating the water in an evaporator or a flash apparatus, it is preferred that the first partial stream comprises 1 to 30 wt-%, more preferred 1 .5 to 25 wt-% and particularly 2 to 20 wt-% of the heating medium.

[0053] The heating medium used for heating the feed water stream, the liquid water stream and / or the evaporator preferably has a temperature in a range from 65 to 100 °C, more preferred in a range from 70 to 95 °C and particularly in a range from 75 to 90 °C.

[0054] Particularly when using a liquid heating medium, for increasing the energy efficiency, particularly to reduce the energy consumption for heating the water and compressing the steam, it is preferred to operate the first heat exchanger in counter-current and in such a way that the temperature difference between the heating medium leaving the first heat exchanger and the feed water stream fed into the first heat exchanger is in a range from 1 .5 to 70 K, more preferred in a range from 2.5 to 30 K and particularly in a range from 3 to 10 K. Cooling the heating medium in the first heat exchanger in such a way has the advantage that the first partial stream can be minimized and a larger amount of heating medium remains as second partial stream for heating the liquid water stream in the second heat exchanger and, thus, more heat can be transferred to the liquid water stream and the evaporator or the flash apparatus can be operated at a higher temperature and hence at a higher pressure.

[0055] The liquid water stream may be fed separately into the flash apparatus or may be mixed with the degassed water stream before being fed into the flash apparatus. Particularly preferably, the liquid water stream and the degassed water stream are mixed and then fed into the flash apparatus. Particularly if the liquid water stream and the degassed water stream are mixed, it is preferred that the temperature of the liquid water stream and the degassed water stream differ not more than 50 K, more preferred not more than 25 K and particularly not more than 10 K. Further, it is preferred that the liquid water stream and the degassed water stream have essentially the same pressure. “Essentially the same pressure” here means that the pressure of both streams does not differ more than 10 bar, preferably not more than 5 bar and particularly does not differ.

[0056] As the temperature of the degassed water withdrawn from the apparatus for thermal degassing is above the temperature with which the degassed water stream preferably is fed into the flash apparatus, the degassed water stream cools down to the saturation temperature.

[0057] To use the heat of the degassed water stream, it is preferred that the feed water stream is heated in the second heat exchanger by heat transfer from the degassed water stream. Thereby, the degassed water stream is cooled in the second heat exchanger to a temperature in a range from 60 to 95 °C, more preferred in a range from 65 to 90 °C and particularly in a range from 70 to 85 °C. If a part of the degassed water stream is injected into the discharge of at least one compressor or compressor stage, it is preferred to divide the degassed water stream into the water stream being injected into the steam and the degassed water stream that is fed into the flash evaporator or evaporated in the evaporator downstream the second heat exchanger. However, alternatively, it is also possible to branch off the stream used for injection into the steam downstream of at least one compressor or compressor stage upstream the second heat exchanger. Particularly if a cascade of at least two compressors is used or a compressor comprising at least two compressor stages, it is further possible to withdraw a part of the degassed water to be injected into the stream upstream the second heat exchanger and a part of the degassed water downstream the second heat exchanger. In this case, it is preferred to inject the part of the water withdrawn downstream the second heat exchanger downstream a compressor or compressor stage, in which the steam is compressed to a lower pressure and the part of the water withdrawn upstream the second heat exchanger downstream a following compressor or compressor stage, in which the steam is further compressed to a higher pressure.

[0058] As the gas stream withdrawn from the apparatus for thermal degassing contains steam, it is preferred to separate the steam from the gas stream to maximize energy efficiency and minimize the amount of water withdrawn from the process. For separating the steam from the gas stream, it is preferred to condense the steam. For this purpose, the gas stream is cooled in a condenser, in which water condenses from the gas stream. The pressure of the gas stream to be condensed in the condenser preferably corresponds to the pressure the gas stream is withdrawn from the apparatus for thermal degassing. For condensing the water, the gas stream preferably is cooled to a temperature in a range from 20 to 95 °C, more preferred to a temperature in a range from 30 to 80 °C and particularly to a temperature in a range from 40 to 70 °C. The condensed water separated from the gas stream still may contain gases. For this reason, the condensed water preferably is returned into the apparatus for thermal degassing. The condensed water may be fed into the apparatus for thermal degassing as a separate stream. However, preferably, the condensed water is fed into the feed water stream and then the combined feed water stream and degassed water are fed into the apparatus for thermal degassing. As the temperature of the condensed water is below the temperature with which the feed water stream is fed into the apparatus for thermal degassing, it is preferred to feed the condensed water into the feed water stream upstream the first heat exchanger or upstream the second heat exchanger. Preferably, the condensed water is fed into the feed water stream upstream the first heat exchanger.

[0059] As the feed water stream has a temperature below the temperature of the gas stream, generally a temperature in a range from 0 to 100 °C, more preferred in a range from 10 to 70 °C and particularly in a range from 15 to 50 °C, it is preferred to preheat the feed water stream in the condenser by heat transfer from the gas stream before being heated in the first heat exchanger. By preheating the feed water stream in the condenser, the heat of the gas stream and the heat of condensation of the steam can be used in the process and such the energy to be supplied can be further reduced.

[0060] Embodiments of the invention are shown in the figures and described in more detail in the following description.

[0061] In the figures:

[0062] Figure 1 shows a process for degassing water and producing steam in a first embodiment;

[0063] Figure 2 shows a process for degassing water and producing steam in a second embodiment.

[0064] A process for degassing water and producing steam in a first embodiment is shown in figure 1 .

[0065] For degassing water, a feed water stream 1 is fed into a first heat exchanger 3, in which the feed water stream 1 is preheated to a temperature in a range from 40 to 95 °C. After being preheated, the feed water stream 1 is heated in a second heat exchanger 5 to a temperature in a range from 90 to 115 °C.

[0066] After being heated in the first heat exchanger 3 and the second heat exchanger 5, the feed water stream 1 is fed into an apparatus for thermal degassing 7. The apparatus for thermal degassing 7 is operated at boiling pressure and a temperature in a range from 90 to 115 °C.

[0067] The temperature in the apparatus for thermal degassing 7 may be maintained by direct heating or by indirect heating. Direct heating here means that a hot medium, particularly steam, is brought into direct contact with the water contained in the apparatus for thermal degassing 7 and indirect heating means heating by using a heating medium that flows through a heat exchanger, e.g., a heating coil or a double jacket without direct contact with the water but heat transfer through a wall. Further, it is also possible to supply heat by electrical heating. If thermal degassing is operated by indirect heating, the feed water is boiling so that a part of the water evaporates and the gases dissolved in the water are partly transferred into the gas phase and removed with the gas phase. By direct heating with steam, a part of the gases dissolved in the feed water is absorbed in the steam and can be withdrawn from the apparatus for thermal degassing 7 with the steam.

[0068] As by boiling of the water gases are transferred into the gas phase, the liquid phase that collects in the apparatus for thermal degassing 7 comprises degassed water. From the liquid phase, a degassed water stream 9 is withdrawn.

[0069] In the embodiment shown here, the degassed water stream 9 is used as a heat transfer medium in the second heat exchanger 5. By heat transfer in the second heat exchanger 5, the degassed water stream 9 is cooled and the feed water stream is heated. After cooling, the thus obtained cooled degassed water stream 11 is fed into a flash apparatus 13.

[0070] Before being fed into the flash apparatus 13, it is preferred to compress the degassed water stream to a pressure in a range from 0.9 to 30 bar(abs), preferably to a pressure in a range from 1 to 20 bar(abs) and particularly to a pressure in a range from 1 to 2 bar(abs). The degassed water stream may be compressed either upstream or downstream the second heat exchanger 5, preferably upstream the second heat exchanger 5.

[0071] On entry into the flash apparatus 13 the degassed water stream expands to a pressure in a range from 0.1 to 0.4 bar(abs) and due to the expansion, a part of the degassed water stream evaporates and forms saturated steam. The part that evaporates usually is in a range from 0.1 to 10 wt-% of the water stream fed into the flash apparatus 13.

[0072] By the partial evaporation, a liquid phase comprising liquid water and a gas phase comprising saturated steam form in the flash apparatus 13. From the liquid phase, a liquid water stream 15 is withdrawn.

[0073] The liquid water stream 15 is heated in a third heat exchanger 17, preferably to a temperature in a range from 40 to 110 °C, more preferred to a temperature in a range from 60 to 95 °C, thereby obtaining a heated liquid water stream 19. To avoid evaporation of water by heating in the third heat exchanger 17, the liquid water stream 15 preferably is compressed before being fed into the third heat exchanger 17. After compressing and before heating, it is preferred that a part of the liquid water stream 15 is withdrawn as blowdown 18 to avoid concentration of impurities in the flash apparatus 13 and only the part that is not withdrawn as blowdown 18 is fed into the third heat exchanger 17. The amount withdrawn as blowdown 18 preferably corresponds to 0.1 to 5 wt.-% of the cooled degassed water stream 11 . After heating in the third heat exchanger 17, the heated liquid water stream 19 and a first part 20 of the cooled degassed water stream 11 are combined and then fed into the flash apparatus 13.

[0074] At the top of the flash apparatus 13 steam 21 is withdrawn and compressed to a required pressure. For compressing the steam at least one compressor with at least one compressor stage is used. In the embodiment shown here, the steam is compressed in a cascade comprising a first compressor stage 23 and a second compressor stage 25. Depending on the required pressure, it also may be necessary to use more than two compressor stages. The compressor stages may be realized by using at least two compressors with each having one compressor stage, or by using only one compressor having at least two compressor stages.

[0075] For setting the properties of the steam, particularly to reduce the superheating of the compressed steam, it is preferred to inject water into at least one of the compressors or compressor stages, or downstream of at least one compressor or compressor stage. Preferably, water is injected into each compressor or compressor stage or downstream of each compressor or compressor stage. If the compression is carried out in at least two compressor stages, injection of water downstream each compressor stage but the last one means that the injection simultaneously is upstream the following compressor stage. This has the additional advantage that due to the cooling of the steam by injecting the water the volume flow is reduced which results in a more energy efficient compression.

[0076] For this purpose, it is particularly preferred to inject a second part 26 of the cooled degassed water stream 11 at respective injection points 27, 29.

[0077] To minimize the energy supply to the process, it is preferred to branch off a part 31 of the steam and use the part 31 of the steam for heating the apparatus for thermal degassing 7. For this purpose, it is particularly preferred to feed the part 31 of the steam into the apparatus for thermal degassing 7, so that the water in the apparatus for thermal degassing 7 is heated by mixture with the part 31 of the steam. Besides branching off the part 31 of the steam downstream of the injection point 27 as shown here, it is also possible to branch off the part 31 of the steam upstream any of the injection points 27, 29. Particularly preferably, the part 31 of the steam is branched off between two compressor stages upstream the injection point 27.

[0078] Due to the boiling of the water, the gas phase obtained in the apparatus for thermal degassing 7 comprises saturated steam and gases. To maximize energy efficiency and minimize the amount of water withdrawn from the process with an off-gas stream 33, a gas stream 35 withdrawn from the apparatus for thermal degassing 7 is fed into a condenser 37.

[0079] In the condenser 37, the gas stream 35 is cooled to a temperature below the boiling point of the water so that steam condenses from the gas stream 35. The thus obtained condensed water 39 is fed into the feed water stream 1. To maximize energy efficiency and minimize the amount of a separate cooling medium, particularly for avoiding the use of a separate cooling medium, it is preferred to use the feed water stream 1 as a cooling medium for cooling the gas stream 35 in the condenser 37. The condensed water 39 obtained in the condenser 37 may be fed into the feed water stream 1 , for example upstream the heat exchanger 3 as shown here, or downstream the heat exchanger 3. To design the condenser 37 as small as possible, a bypass 40 is provided, which allows to pass only a part of the feed water stream 1 as cooling medium through the condenser 37.

[0080] For heating the feed water stream 1 in the first heat exchanger 3 and the liquid water stream 15 in the third heat exchanger 17, a heat transfer medium 41 is used. In the embodiment shown here, the heat transfer medium 41 is separated into a first partial stream 43, which passes the first heat exchanger 3 thereby heating the feed water stream 1 , and a second partial stream 45, which is fed into the third heat exchanger 17 for heating the liquid water stream 15. The heat transfer medium may be a process stream or for example a liquid heat transfer medium like a heat transfer oil or water, a gas like air or a flue gas, or a condensable gas like benzene, methanol, ethanol, or steam.

[0081] Figure 2 shows a process for degassing water and producing steam in a second embodiment.

[0082] The process shown in figure 2 differs from the process of figure 1 in the manner of steam generation. Instead of using a flash apparatus 13 for generating steam, in the embodiment shown in figure 2, the steam 21 is generated by evaporation of the first part 20 of the cooled degassed water stream 11 in an evaporator 47.

[0083] The second partial stream 45 of the heat transfer medium 41 is used as a heat transfer medium in the evaporator 47. In the evaporator 47, the cooled degassed water stream 11 is evaporated, thereby obtaining the steam 21. By evaporating the cooled degassed water stream 11 in the evaporator 47, it is not necessary to flash the cooled degassed water stream 11 to obtain steam and, for this reason, no flash apparatus for producing steam is needed.

[0084] Degassing of the feed water stream 1 and compression of the steam 21 is carried out as in the process shown in figure 1.

[0085] As required, before entering the evaporator 47, the pressure of the first part 20 of the cooled degassed water stream 11 may be reduced to the intended evaporation pressure inside the evaporator 47. The steam 21 obtained by evaporation of the first part 20 of the cooled degassed water stream 11 in the evaporator 47 may be saturated or slightly superheated. To avoid concentration of impurities within the evaporator 47, a liquid stream corresponding to e.g. 0.1 to 5 wt.-% of the first part 20 of the cooled degassed water stream 11 is withdrawn from the evaporator 47 as blowdown 18.

[0086] Besides separating the heat transfer medium 41 in a first partial stream 43 and a second partial stream 45 as shown in figures 1 and 2, it is also possible that the heat transfer medium 41 flows sequentially through the first heat exchanger 3 and the third heat exchanger 17 or the first heat exchanger 3 and the evaporator 47, respectively. In this case, the heat transfer medium 41 may flow firstly through the first heat exchanger 3 and then through the third heat exchanger 17 or the evaporator 47 or vice versa. Particularly if the degassed water stream is evaporated in the evaporator 47 it is preferred that the heat transfer medium 41 firstly flows through the evaporator 47 and then through the first heat exchanger 3, if the heat transfer medium passes them sequentially.

Claims

Claims1 . A process for producing steam, comprising:(a) heating a feed water stream (1 ) in a first heat exchanger (3) to a temperature in a range from 40 to 95 °C;(b) heating the feed water stream (1 ) in a second heat exchanger (5) to a temperature in a range from 90 to 115 °C;(c) feeding the heated feed water stream into an apparatus for thermal degassing (7), which is operated at a boiling temperature in a range from 90 to 115 °C, thereby obtaining a gas stream (35) and a degassed water stream (9);(d) optionally compressing the degassed water stream (9) to a pressure in a range from 0.9 to 20 bar(abs);(e) feeding the degassed water stream into a flash apparatus (13), in which the degassed water stream is expanded to a pressure in a range from 0.04 to 0.8 bar(abs), so that a part of the water flashes to steam (21 ), or evaporating the degassed water in an evaporator (47);(f) preferably compressing the steam (21 ) in at least one compressor with at least one compressor stage (23, 25).

2. The process according to claim 1 , wherein a part of the compressed steam obtained in step (f) is fed into the apparatus for thermal degassing (7) or wherein a part of the steam (21) obtained in step (e) or of the compressed steam obtained in step (f) is used as a heat transfer medium for heating the apparatus for thermal degassing (7).

3. The process according to claim 2, wherein the part of the compressed steam (21 ) used as the heat transfer medium for heating the apparatus for thermal degassing (7) is withdrawn downstream of that compressor or compressor stage (23, 25), in which the steam (21) is compressed to a pressure in a range from 1 to 5 bar(abs).

4. The process according to any of claims 1 to 3, wherein the gas stream (35) comprises steam and gasses.

5. The process according to claim 4, wherein the gasses comprise at least one of nitrogen, oxygen, carbon dioxide, argon, and ammonia.

6. The process according to any of claims 1 to 5, wherein a liquid water stream (15) is withdrawn from the flash apparatus (13), compressed to a pressure in a range from 0.9 to 20bar(a), heated to a temperature in a range from 40 to 110 °C and returned into the flash apparatus (13).

7. The process according to claim 6, wherein the liquid water stream (15) is mixed with the degassed water stream before being fed into the flash apparatus (13).

8. The process according to any of claims 1 to 7, wherein the feed water stream (1 ) is heated in the second heat exchanger (5) by heat transfer from the degassed water stream (9), which is cooled in the second heat exchanger (5) to a temperature in a range from 65 to 90 °C.

9. The process according to any of claims 1 to 8, wherein the gas stream (35) is cooled in a condenser (37), in which water condenses from the gas stream (35).

10. The process according to claim 9, wherein the condensed water (39) is fed into the feed water stream (1).11 . The process according to claim 9 or 10, wherein the feed water stream (1) is preheated in the condenser (37) by heat transfer from the gas stream (35) before being heated in the first heat exchanger (3).

12. The process according to any of claims 1 to 11 , wherein the steam (21 ) is compressed in a cascade of at least two compressors each having at least one compressor stage (23, 25) or a compressor comprising at least two compressor stages (23, 25).

13. The process according to any of claims 1 to 12, wherein a part of the degassed water stream is injected into the steam (21) downstream of at least one compressor stage (23, 25).

14. The process according to any of claims 1 to 13, wherein a liquid heating medium or a condensable gas is used as a heat transfer medium (41) in the first heat exchanger (3) and for heating the liquid water stream (15).

15. The process according to any of claims 1 to 14, wherein the feed water stream (1) comprises demineralized water.

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