A process for degassing water

Abstract

The invention relates to a process for degassing water used in steam production, the process comprising: (a) heating a feed water stream (9) to a temperature in a range from 60 to 95 °C in a first heat exchanger (11) and feeding the heated feed water stream (9) into a degassing apparatus (13), which is operated at a pressure of 0.15 to 0.8 bar(abs), so that a part of the feed water stream evaporates due to expansion when entering the degassing apparatus or feeding a feed water stream (9) into a degassing apparatus (13), which is operated at a pressure of 0.15 to 0.8 bar(abs) and in which the feed water is heated to a temperature in a range from 60 to 95 °C, so that a part of the water evaporates; (b) withdrawing a gas stream (15) and a degassed water stream (21) from the degassing apparatus (13); (c) optionally compressing the degassed water stream (21) to a pressure in a range from 0.25 to 30 bar(abs).

Description

[0001] 240138W001

[0002] A process for degassing water

[0003] Description

[0004] The invention relates to a process for degassing water used in steam production and a process for producing steam using the degassed water.

[0005] Steam is a major thermal energy carrier in industry, for example in 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 power compared to electric boilers.

[0006] Besides energy, the main utility for steam production is demineralized and degassed water. Widely used thermal degassing at slightly above 100 °C and 1 bar deploys steam. Using steam from the open loop heat pump for thermal degassing reduces the system's efficiency. Membrane degassing with sweep gas like nitrogen leaves inert gas in the steam, which can affect the functionality of equipment like heat exchangers. The water temperature needs to be below 50 °C because of membrane degrading. Conventional vacuum degassing at temperatures below 25 °C uses steam to reach pressures below 40 mbar(abs). Due to the very low pressure, the equipment choice is limited, its size is large and / or 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. Further degassing processes are described in Hans Gunter Heitmann, "Praxis der Kraftwerk-Chemie”, Vulkan-Verlag Essen, 2ndedition, 1997, pages 462 to 468.

[0008] Therefore, it was an object of the present invention to provide a process for degassing water used in steam production and a process for producing steam, which are more energy efficient and in which an equipment having a smaller size and being less expensive can be used.

[0009] This object is achieved by a process for degassing water used in steam production, the process comprising:

[0010] (a) heating a feed water stream to a temperature in a range from 60 to 95 °C in a first heat exchanger and feeding the feed water stream into a degassing apparatus, which is operated at a pressure of 0.15 to 0.8 bar(abs), so that a part of the feed water stream evaporates due to expansion when entering the degassing apparatus or feeding a feed water stream into a degassing apparatus, which is operated at a pressure of 0.15 to 0.8 bar(abs) and in which the feed water is heated to a temperature in a range from 60 to 95 °C, so that a part of the water evaporates;

[0011] (b) withdrawing a gas stream and a degassed water stream from the degassing apparatus; 240138W001

[0012] 2

[0013] (c) optionally compressing the degassed water stream to a pressure in a range from 0.25 to 30 bar(abs).

[0014] Heating the feed water stream for degassing to a temperature in a range from 60 to 95 °C needs less energy than heating to a temperature above 100 °C. Compared to cold degassing, a pressure in a range from 0.15 to 0.8 bar(abs) is sufficient for degassing at a temperature in this range. Hence, less compression energy and smaller and, thus, less expensive equipment can be used for setting the pressure and degassing. Degassing at a temperature in the range from 60 to 95 °C has the additional advantage that no steam is used which has to be generated with high energy consumption.

[0015] Depending on the origin of the water to be used and to be degassed, gases that may be contained in the water are at least one of nitrogen, oxygen, argon, and ammonia. Nitrogen, oxygen, and argon generally originate from air that is solved in the water. While ammonia is used for conditioning of the feed water to prevent corrosion of the steel equipment. The ammonia will partially remain in the degassed water because it is chemically dissolved. This is beneficial for the steam production.

[0016] In a first embodiment, the water is heated to a temperature in a range from 60 to 95 °C, more preferred to a temperature in a range from 65 to 90 °C and particularly to a temperature in a range from 70 to 85 °C in a first heat exchanger and after heating the feed water stream in the first heat exchanger, the feed water stream is fed into the degassing apparatus, which is operated at a pressure in a range from 0.15 to 0.8 bar(abs), more preferred in a range from 0.2 to 0.6 bar(abs) and particularly in a range from 0.25 to 0.5 bar(abs).

[0017] The first heat exchanger may be any heat exchanger suitable for heating water, known to a skilled person. Suitable heat exchangers for example are heat exchangers, in which the water is heated by indirect heat transfer from a heating medium. Preferably, the heat exchanger is a heat exchanger in which the water is heated by indirect heat transfer, for example a tube-and-shell heat exchangers, plate heat exchangers, spiral heat exchangers. Arrangements with counter current flow, which preferably can be archived 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.

[0018] If the water is heated in the heat exchanger by indirect heat transfer from a heating medium, suitable heating media for example are liquid heating media like water or thermal oils, gases like air or flue gases, or gases that condense by heating the water in the heating exchanger, for example benzene, methanol, ethanol, or steam.

[0019] Besides using a liquid heating medium or a gas, it is further possible to use a process stream as heating medium. The process stream may be any process stream that needs to be cooled and that is obtained in a process, for exam- 240138W001

[0020] 3 pie 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.

[0021] The pressure of the feed water stream preferably is in a range from 0.2 to 20 bar(abs), more preferred in a range from 0.5 to 15 bar(abs) and particularly in a range from 1 to 10 bar(abs). Thereby the pressure of the feed water stream is such that the feed water stream does not start to evaporate in the first heat exchanger.

[0022] Due to the pressure in the degassing apparatus, which is below the pressure of the feed water stream, a part of the water evaporates by flash evaporation. By partly evaporating, steam and gaseous components, which were solved in the water, change into the gas phase and are separated from a liquid phase, which contains the degassed water. Preferably, 0.05 to 2 wt- of the feed water stream are evaporated in the degassing apparatus. More preferred, 0.1 to 1 .0 wt% of the feed water stream are evaporated in the degassing apparatus and particularly 0.2 to 0.7 wt% of the feed water stream are evaporated in the degassing apparatus.

[0023] In an alternative embodiment, the feed water stream is fed into a degassing apparatus which is operated at a pressure in a range from 0.15 to 0.8 bar(abs), more preferred in a range from 0.2 to 0.6 bar(abs) and particularly in a range from 0.25 to 0.5 bar(abs). In this embodiment, the feed water is heated in the degassing apparatus to a temperature in a range from 60 to 95 °C, more preferred to a temperature in a range from 65 to 90 °C and particularly to a temperature in a range from 70 to 85 °C, so that the water in the degassing apparatus boils and a part of the water evaporates. The water in the degassing apparatus preferably is heated by indirect heat transfer. For this purpose, it is possible to provide heat exchange means, for example at least one tube or at least one coil in the degassing apparatus or to equip the degassing apparatus with a double jacket or heating coils on the walls of the degassing apparatus, wherein a heating medium flows through the at least one tube, the at least one coil or the double jacket or heating coil on the walls of the degassing apparatus. The at least one tube, the at least one coil or the double jacket or heating coil is arranged at a position of the degassing apparatus at which it is in contact with liquid water.

[0024] The degassing apparatus may be for example a column with internals, or a deaerator. If a column with internals is used, it is preferred that a counter current of liquid water and steam is maintained by dividing the column into chambers which are arranged one above the other, wherein the liquid flows from chamber to chamber by gravity and blowers are used to transport the steam upwards from chamber to chamber. If the degassing apparatus is equipped with the heat exchange means, the heat exchange means are provided below the internals or at the bottom of the deaerator.

[0025] The degassed water and the gas phase are withdrawn from the degassing apparatus as the degassed water stream and the gas stream. To avoid leakages into the degassed water stream by air, particularly oxygen and carbon dioxide, or the liquid or gas used as a heating medium in the first heat exchanger, the degassed water stream preferably is pressurized to a pressure in a range from 0.25 to 30 bar(abs). 240138W001

[0026] 4

[0027] To separate the water from the further gaseous components, it is preferred to cool the gas stream in a condenser, in which water condenses from the gas stream. The condensed water is separated from the gas stream and fed into the feed water stream before being fed into the degassing apparatus. By recycling the condensed water into the feed water stream, the amount of water withdrawn from the process in the gas stream can be minimized. Further, by condensing the water, the amount of gas to be compressed is minimized and hence less energy is needed for compression.

[0028] The condenser may be arranged such that fresh water is used as a cooling medium. In this case, the fresh water is preheated in the condenser by heat transfer from the gas stream and the preheated fresh water is mixed with the condensed water upstream the first heat exchanger to obtain the feed water stream. This heat integration recovers the maximum heat for the production of steam.

[0029] As the condensed water has a temperature below the temperature of the water that was heated in the first heat exchanger, and, for this reason mixing the condensed water with the fresh water downstream the first heat exchanger would result in a feed water stream having a temperature below the temperature of the fresh water being heated in the first heat exchanger, it is preferred to feed the condensed water into the feed water stream upstream the first heat exchanger. After mixing the condensed water and the fresh water, the thus obtained feed water stream is heated in the first heat exchanger.

[0030] Depending on the temperature of the fresh water that is preheated in the condenser, the gas stream withdrawn from the degassing apparatus is cooled to a temperature in a range from 20 to 40 °C, more preferred to a temperature in a range from 25 to 35 °C and particularly to a temperature in a range from 27 to 33 °C.

[0031] Alternatively, the condenser may be used for heating water that is evaporated for steam production. In this case, the gas stream is cooled in a condenser to a temperature in a range from 50 to 80 °C to condense water. The condensed water then is combined with the feed water stream downstream the first heat exchanger.

[0032] If the gas stream is cooled to a lower temperature by heating the water for steam production in the condenser, it is alternatively possible that the condensed water is mixed with fresh water upstream the first heat exchanger to obtain the feed water stream.

[0033] To avoid salt deposits on lines or in apparatuses, the water that is degassed for producing steam preferably is demineralized water.

[0034] The invention further relates to a process for producing steam, comprising: 240138W001

[0035] 5

[0036] (i) degassing water in a process as described above;

[0037] (ii) feeding the compressed degassed water stream obtained in step (d) into a flash apparatus, in which the degassed water stream is expanded to a pressure in a range from 0.1 to 0.4 bar(abs), so that a part of the water evaporates to form steam or evaporating the degassed water stream obtained in step (c) in an evaporator to obtain steam;

[0038] (iii) compressing the steam in at least one compressor.

[0039] As only a part of the water fed into the flash apparatus evaporates, it is necessary to withdraw a part of the liquid water from the flash apparatus. The liquid water stream withdrawn from the flash apparatus preferably is compressed to a pressure in a range from 0.25 to 30 bar(abs), more preferred to a pressure in a range from 0.8 to 10 bar(abs) and particularly to a pressure in a range from 1 to 2 bar(abs) and heated to a temperature in a range from 60 to 95 °C, more preferred to a temperature in a range from 65 to 90 °C and particularly to a temperature in a range from 70 to 85 °C. After heating and compressing, the liquid water stream is returned into the flash apparatus and thereby expanded.

[0040] To reduce the feed lines into the flash apparatus, it is preferred to combine the liquid water stream with the degassed water stream before being fed into the flash apparatus.

[0041] By expansion of the degassed water in the flash apparatus to a pressure in a range from 0.05 to 0.45 bar(abs), more preferred to a pressure in a range from 0.1 to 0.4 bar(abs) and particularly to a pressure in a range from 0.15 to 0.35 bar(abs), a part of the degassed water evaporates. The part of the degassed water that evaporates preferably is in a range from 0.1 to 3 wt-%, more preferred in a range from 0.2 to 2 wt-% and particularly in a range from 0.3 to 1 .5 wt- % of the degassed water fed into the flash apparatus.

[0042] As the temperature of the degassed water stream withdrawn from the degassing apparatus preferably has a temperature which differs not more than 40 K, preferably not more than 20 K and particularly not more than 10 K from the temperature to which the liquid water stream is heated, it is preferred that the feed water stream is heated in the first heat exchanger by heat transfer from a heating medium, wherein after having passed the first heat exchanger, the heating medium flows through a second heat exchanger, in which the liquid water stream is heated by heat transfer from the heating medium.

[0043] Alternatively, it is also possible, that the heating medium is divided into a first partial stream and a second partial stream, wherein the first partial stream is used as heating medium in the first heat exchanger for heating the feed water stream in step (a) and the second partial stream is used in the second heat exchanger for heating the liquid 240138W001

[0044] 6 water stream. If the heating 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 heating 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.

[0045] 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.

[0046] 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.

[0047] To evaporate the degassed water in the evaporator, it is preferred that the evaporator is operated at the same pressure as the degassing apparatus. The transport of the water from the degassing apparatus to the evaporator may be carried out at a pressure in a range from 0.1 to 5 bar(abs), more preferred in a range from 0.2 to 2 bar(abs) and particularly in a range from 0.3 to 1 bar(abs). Pressures above 1 bar(abs) have the advantage that no air may leak into the degassed water. However, particularly preferably, the degassed water is not compressed after being withdrawn from the degassing apparatus but fed into the evaporator with the same pressure.

[0048] The condenser may either be used for preheating the fresh water as described above or, alternatively, for preheating the liquid water stream. If the liquid water stream is preheated in the condenser, the liquid water stream after being withdrawn from the flash apparatus first passes the condenser and is preheated in the condenser by heat transfer from the gas stream, and subsequently passes the second heat exchanger.

[0049] The heating medium used for heating the feed water stream in the first heat exchanger and for heating the liquid water stream in the second heat exchanger preferably is water or a condensable gas. Suitable condensable gases are for example benzene, methanol, ethanol, or steam.

[0050] Besides using a liquid heating medium or a gas, it is further possible to use a process stream as heating medium.

[0051] The process stream may be any process stream that needs to be cooled and that is obtained in a process, for exam- 240138W001

[0052] 7 pie 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.

[0053] Before being fed into the flash apparatus, the degassed water stream and the liquid water stream are compressed to a pressure above the pressure in the flash apparatus, so that the degassed water stream and the liquid water stream can expand on entering the flash apparatus. If the degassed water stream and the liquid stream are combined before entering the flash apparatus, it is possible to firstly combine the liquid water stream and the degassed water stream and compress the thus obtained combined water stream. However, as usually the pressure in the degassing apparatus is higher than the pressure in the flash apparatus, it is preferred to compress the degassed water stream and the liquid water stream separately before being combined. In this case, it is preferred to compress the degassed water stream after being withdrawn from the degassing apparatus. The liquid water stream is compressed before passing the second heat exchanger. If the liquid water stream is preheated in the condenser, it is further preferred to compress the liquid water stream upstream the condenser.

[0054] The pressure to which the degassed water stream and the liquid water stream are compressed, preferably is in a range from 0.25 to 30 bar(abs), more preferred in a range from 1 to 2 bar(abs) and particularly in a range from 1.1 to 1.5 bar(abs).

[0055] The flash apparatus for example is a vessel with an expansion valve at the connection with the feed line for the degassed water stream and the liquid water stream. Further, the flash apparatus may be for example a flash column.

[0056] The steam withdrawn from the flash apparatus preferably is compressed to a pressure in a range from 0.25 to 30 bar(abs), more preferred to a pressure in a range from 0.6 to 16 bar(abs) and particularly to a pressure in a range from 1 to 5 bar(abs) in the at least one compressor. The pressure depends on the intended use of the steam.

[0057] The steam obtained after compression may be used in any process in which steam is used for heating, for example as a heat source in a heat exchanger or an evaporator, for example an evaporator of a distillation column, or as a raw material, for example in steam methane reforming. Further, it is also possible to compress the steam to a pressure that is suitable for feeding the steam into a steam grid. The steam grid may be for example a low pressure steam grid, a medium pressure steam grid or a high pressure steam grid.

[0058] In this context, "low pressure steam” means steam having a pressure in a range from 0.9 to 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 160 °C, more preferred in a range from 99 to 140 °C and particularly in a range from 104 to 120 °C. 240138W001

[0059] 8

[0060] The term "medium pressure steam” means steam having a pressure in a range from 4 to 8 bar(abs), more preferred in a range from 4.5 to 7 bar(abs) and particularly in a range from 5 to 6 bar(abs) and a temperature in a range from 143 to 220 °C, more preferred in a range from 147 to 210 °C and particularly in a range from 151 to 200 °C.

[0061] The term "high pressure steam” means steam having a pressure in a range from 8 to 40 bar(abs), more preferred in a range from 10 to 30 bar(abs) and particularly in a range from 16 to 20 bar(abs) and a temperature in a range from 170 to 280 °C, more preferred in a range from 180 to 260 °C and particularly in a range from 201 to 240 °C.

[0062] The compressors used for compressing the steam may be any compressors known to a skilled person that can be used for compressing steam. Depending on the pressure to which the steam is compressed, it is possible to use only one compressor or a cascade of at least two compressors. Further, if only one compressor is used, the compressor may be a compressor comprising at least two compressor stages.

[0063] For setting the properties of the steam, particularly the temperature, it is possible to inject water upstream or downstream at least one compressor or compressor stage. Preferably, water is injected upstream or downstream each compressor or compressor stage. The water that is injected upstream or downstream the at least one compressor or compressor stage preferably is a part of the degassed water obtained in step (d). Depending on the pressure of the steam into which the water is injected, the water may be compressed to a pressure above the pressure of the steam, before being injected. Using hot water, particularly the degassed water obtained in step (d), improves the thermodynamic efficiency of the process, because more steam evaporates before or after the compression stages. The specific power demand of steam compression and / or production is reduced and the first compression stages can be designed smaller, hence less cost intensive.

[0064] Embodiments of the invention are shown in the figures and described in more detail in the following description.

[0065] In the figures:

[0066] Figure 1 shows a process for degassing water and producing steam in a first embodiment;

[0067] Figure 2 shows a process for degassing water and producing steam in a second embodiment;

[0068] Figure 3 shows a process for degassing water and producing steam in a third embodiment;

[0069] Figure 4 shows a process for degassing water and producing steam in a fourth embodiment.

[0070] A process for degassing water and producing steam in a first embodiment is shown in figure 1 . 240138W001

[0071] 9

[0072] For degassing water, fresh water 1 passes a condenser 3, in which the fresh water 1 is preheated. After being preheated, the preheated fresh water 5 is mixed with condensed water 7 to obtain a feed water stream 9. The feed water stream 9 is fed into a first heat exchanger 11 . In the first heat exchanger 11 , the feed water steam 9 is heated to a temperature in a range from 65 to 90 °C.

[0073] After being heated, the feed water stream 9 is fed into a degassing apparatus 13. The pressure in the degassing apparatus is below the boiling pressure of the feed stream so that the feed stream expands on entry into the degassing apparatus 13 and a part of the feed stream evaporates. The feed stream being fed into the degassing apparatus preferably has a pressure in a range from 1 to 2 bar(abs) and the pressure in the degassing apparatus preferably is in a range from 0.2 to 0.6 bar(abs). Due to the expansion on entering the degassing apparatus, preferably 0.05 to 2 wt-% of the feed water stream evaporate.

[0074] By the partial evaporation in the degassing apparatus 13 a gas phase and a liquid phase are obtained. The gas phase contains saturated steam and gases that were solved in the water. The liquid phase is degassed water.

[0075] From the gas phase in the degassing apparatus 13 a gas stream 15 is withdrawn. The gas stream 15 is fed into the condenser 3, in which the gas stream 15 is cooled. By cooling, at least a part of the saturated steam contained in the gas stream 15 condenses and is withdrawn from the condenser 3 as the condensed water 7. The remaining gas phase 17 is removed from the process.

[0076] For withdrawing the gas stream 15 from the degassing apparatus 13, a compressor 19 may be used. The compressor 19 preferably is arranged downstream the condenser 3. In the compressor, the remaining gas phase 17 preferably is compressed to ambient pressure.

[0077] A degassed water stream 21 is withdrawn from the liquid phase of the degassing apparatus 13. The degassed water stream 21 is compressed to a pressure in a range from 1 to 2 bar(abs) and then fed into a flash apparatus 23. In the flash apparatus, the degassed water 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 evaporates forming saturated steam. The part that evaporates usually is in a range from 0.5 to 5 wt-% of the water stream fed into the flash apparatus 23.

[0078] In the flash apparatus 23, a liquid phase comprising liquid water and a gas phase comprising saturated steam form. From the liquid phase, a liquid water stream 25 is withdrawn.

[0079] The liquid water stream 25 preferably is compressed to a pressure which corresponds to the pressure of the degassed water stream 21 . After compressing, the liquid water stream 25 is heated in a second heat exchanger 27 to a temperature which preferably is similar to the temperature of the degassed water stream 21, preferably to a temperature in a range from 60 to 95 °C, thereby obtaining a heated liquid water stream 29. After heating in the second heat 240138W001

[0080] 10 exchanger 27, the heated liquid water stream 29 and the degassed water stream 21 are combined and then fed into the flash apparatus 23.

[0081] At the top of the flash apparatus 23 steam 31 is withdrawn and compressed to a required pressure. For compressing the steam at least one compressor is used. In the embodiment shown here, the steam is compressed in a cascade comprising a first compressor 33 and a second compressor 35. Depending on the required pressure, it also may be necessary to use more than two compressors. Further, besides using individual compressors for compressing the steam, it is also possible to use only one compressor having at least two compressor stages.

[0082] For setting the properties of the steam, particularly to cool the compressed steam and increase steam mass flow, it is preferred to inject water upstream or downstream of at least one compressor or compressor stage. Preferably, water is injected between each compressor or compressor stage and upstream or downstream of the first or last compressor or compressor stage, respectively.

[0083] For this purpose, it is particularly preferred to inject degassed water obtained in the degassing apparatus 13 at respective injection points 37, 39.

[0084] For heating the feed water stream 9 in the first heat exchanger 11 and the liquid water stream 25 in the second heat exchanger, a heating medium 41 is used. In the embodiment shown in figure 1, the heating medium 1 passes the first heat exchanger 11 thereby heating the feed water stream 9. After having heated the feed water stream 9, the heating medium is fed into the second heat exchanger 27 for heating the liquid water stream 25. The heating medium may be for example water, a thermal oil, a process stream or a condensable gas like benzene, methanol, ethanol or steam.

[0085] Figure 2 shows a process for degassing water and producing steam in a second embodiment, which differs from the embodiment shown in figure 1 in the arrangement of the condenser 3.

[0086] In the embodiment shown in figure 2, the fresh water 1 is mixed with the condensed water 7 to obtain the feed water stream 9. In difference to the embodiment shown in figure 1, here the fresh water 1 is not preheated before being mixed with the condensed water 7 and the obtained feed water stream 9 is directly heated in the first heat exchanger 11 without preheating.

[0087] The condenser 3 for condensing water from the gas stream 15 is arranged in the recycling line for the liquid water stream 25 upstream the second heat exchanger 27. By this arrangement, the gas stream 15 is cooled to a temperature in a range from 50 to 80 °C at which water condenses from the gas stream 15. The condensed water 7 is fed into the fresh water 1 and the remaining gas phase 17 is removed from the process as off-gas. 240138W001

[0088] 11

[0089] By cooling the gas stream 15 and condensing water from the gas stream 15, the liquid water stream 25 withdrawn from the flash apparatus 23 is preheated. After preheating, the liquid water stream 25 is heated in the second heat exchanger 27.

[0090] A third embodiment of a process for degassing water and producing steam is shown in figure 3.

[0091] Preheating the fresh water 1 and heating the feed water stream 9 corresponds to preheating and heating the feed water stream 9 of the embodiment shown in figure 1 . However, in contrast to the embodiment shown in figure 1, here the heating medium 41 is divided into a first partial stream 43 and a second partial stream 45. The first partial stream 43 is used for heating the feed water stream 9 in the first heat exchanger 11 .

[0092] The second partial stream 45 is used as a heating medium in an evaporator 47. In the evaporator, the degassed water stream 21 obtained in the degassing apparatus 13 is evaporated, thereby obtaining the steam 31. By evaporating the degassed water stream 21 in the evaporator 47, it is not necessary to circulate the degassed water stream between a flash apparatus and a heat exchanger to obtain steam and, for this reason, in the process shown in figure 3, no flash apparatus for producing steam is needed. To prevent solid deposit in the evaporator 47, a part of the water, preferably 0.1 to 1 % of the water, is blown down from the evaporator and withdrawn from the process.

[0093] As in the processes shown in figures 1 and 2, after evaporation, the steam 31 is compressed in the compressors 33, 35.

[0094] To completely evaporate the degassed water in the evaporator 47, it is preferred that the evaporator is operated at a pressure that corresponds to the pressure in the degassing apparatus. Preferably, in this case the degassed water 21 is not compressed after being withdrawn from the degassing apparatus 13 but fed into the evaporator at the same pressure as it is withdrawn from the degassing apparatus 13.

[0095] Besides arranging the first heat exchanger 11 and the second heat exchanger 27 such that the heating medium flows first through the first heat exchanger 11 and then through the second heat exchanger 27 as shown in figures 1 and 2, it is also possible to reverse the flow direction of the heating medium 41 so that it firstly flows through the second heat exchanger 27 and then through the first heat exchanger 11 . However, as the pressure in the flash apparatus 23 generally is lower than the pressure in the degassing apparatus 13 it is preferred that the heating medium firstly flows through the first heat exchanger 11 and then through the second heat exchanger 27. In a further alternative, heating of the first heat exchanger 11 and the second heat exchanger 27 in the embodiments shown in figures 1 and 2 may be operated as in the process shown in figure 3 by dividing the heating medium 41 into a first partial stream and a second partial stream and to heat the first heat exchanger 11 with the first partial stream and the second heat exchanger 27 with the second partial stream. 240138W001

[0096] 12

[0097] It is further possible that the first heat exchanger 11 and the evaporator 47 of the embodiment shown in figure 3 are arranged as in the embodiments shown in figures 1 and 2 in such a way, that the heating medium 41 firstly flows through the first heat exchanger 11 for heating the feed water stream 9 and then through the evaporator 47 for evaporating the degassed water stream 21 . Alternatively, if the heating medium 41 flows through the first heat exchanger 11 and the evaporator 47 in series, it may also be possible that the heating medium firstly flows through the evaporator 47 and subsequently through the first heat exchanger 11 .

[0098] Besides heating the water in the first heat exchanger 11 and feed the heated water into the degassing apparatus 13, it is further possible to heat and degas the water in the same apparatus as shown in the embodiment of figure 4.

[0099] In difference to the embodiment shown in figure 1, no external first heat exchanger 11 is provided but the feed water stream 9 after being preheated in the condenser 3 is fed into the degassing apparatus 13, which includes heat exchange means 49. The condensed water 7 may be fed into the degassing apparatus 13 as a separate stream as shown here or may be mixed with the preheated fresh water as shown in figure 1 .

[0100] By providing the heat exchange means 49 in the degassing apparatus 13, the water is heated in the degassing apparatus 13. The degassing apparatus is operated at a pressure in a range from 0.15 to 0.8 bar(abs) and the water is heated to a temperature in a range from 60 to 95 °C by heat transfer from the heat exchange means 49. By heating the water, the water starts to boil and at least a part of the water evaporates so that a gas phase and a liquid phase are obtained in the degassing apparatus 13. As described above, the gas phase contains saturated steam and gases that were solved in the water and the liquid phase contains the degassed water.

[0101] The heat exchange means 49 for heating the water in the degassing apparatus 13 may be for example at least one tube, particularly a tube bundle or at least one coil arranged in the degassing apparatus 13, wherein for indirect heat transfer to the water, a heating medium 41 flows through the at least one tube or the at least one coil. Alternatively or additionally, the heat exchange means 49 may comprise a heating coil on the wall of the degassing apparatus or a double jacket, wherein the heating medium 41 flows through the heating coil or the double jacket.

[0102] The degassed water stream 21 obtained in the degassing apparatus is used for producing steam, which may be carried out as described in context with the embodiments shown in figures 1 or 2 and as shown here or by using an evaporator 47 as shown in figure 3.

[0103] For heating the water in the degassing apparatus 13, it is possible to arrange the heating means 49 and the second heat exchanger 27 in series like the first heat exchanger 11 and the second heat exchanger 27 as shown with the embodiments of figures 1 and 2. Alternatively the heating means 49 and the second heat exchanger 27 may be arranged in parallel like the first heat exchanger 11 and the evaporator 47 of the embodiment shown in figure 3 and shown here.

Claims

240138W00113Claims1 . A process for degassing water used in steam production, the process comprising:(a) heating a feed water stream (9) to a temperature in a range from 60 to 95 °C in a first heat exchanger (11) and feeding the heated feed water stream (9) into a degassing apparatus (13), which is operated at a pressure of 0.15 to 0.8 bar(abs), so that a part of the feed water stream evaporates due to expansion when entering the degassing apparatus or feeding a feed water stream (9) into a degassing apparatus (13), which is operated at a pressure of 0.15 to 0.8 bar(abs) and in which the feed water is heated to a temperature in a range from 60 to 95 °C, so that a part of the water evaporates;(b) withdrawing a gas stream (15) and a degassed water stream (21) from the degassing apparatus (13);(c) optionally compressing the degassed water stream (21) to a pressure in a range from 0.25 to 30 bar(abs).

2. The process according to claim 1 , wherein the gas stream (15) is cooled in a condenser (3), in which water condenses from the gas stream (15), the condensed water (7) is separated from the gas stream (15) and mixed with fresh water (1) to obtain the feed water stream (9).

3. The process according to claim 2, wherein the fresh water (1) is preheated in the condenser (3) by heat transfer from the gas stream (15) and the preheated fresh water (5) is mixed with the condensed water (7) upstream the first heat exchanger (11) to obtain the feed water stream (9).

4. The process according to claim 3, wherein the gas stream (15) is cooled to a temperature in a range from 20 to 40 °C by heat exchange with the fresh water (1).

5. The process according to claim 1 , wherein the gas stream (15) is cooled in a condenser (3) to a temperature in a range from 50 to 80 °C to condense water, and the condensed water (7) is combined with the feed water stream (9) downstream the first heat exchanger (11).

6. The process according to any of claims 1 to 5, wherein 0.05 to 2.0 wt% of the feed water stream (9) are evaporated in the degassing apparatus (13).

7. The process according to any of claims 1 to 6, wherein gases being removed from the feed water stream (9) comprise at least one of nitrogen, oxygen, carbon dioxide, argon, and ammonia.240138W001148. The process according to any of claims 1 to 7, wherein the feed water stream (9) comprises demineralized water.

9. A process for producing steam, comprising:(I) degassing water in a process according to any of claims 1 to 8;(ii) feeding the compressed degassed water stream (21) obtained in step (d) into a flash apparatus (23), in which the degassed water stream (21) is expanded to a pressure in a range from 0.1 to 0.4 bar(abs), so that a part of the water evaporates to form steam (31) or evaporating the degassed water stream obtained in step (c) in an evaporator (47) to obtain steam (31);(ill) compressing the steam (31) in at least one compressor (33, 35).

10. The process according to claim 9 wherein a liquid water stream (25) is withdrawn from the flash apparatus (23), heated to a temperature in a range from 60 to 95 °C and compressed to a pressure in a range from 0.25 to 30 bar(abs) and the thus obtained heated liquid water stream (29) is returned into the flash apparatus (23).11 . The process according to claim 10, wherein the heated liquid water stream (29) is combined with the degassed water stream (21) before being fed into the flash apparatus (23).

12. The process according to any of claims 9 to 11, wherein the feed water stream (9) is heated in the first heat exchanger (11) by heat transfer from a heating medium (41), wherein after having passed the first heat exchanger (11), the heating medium (41) flows through a second heat exchanger (27), in which the liquid water stream (25) is heated by heat transfer from the heating medium (41).

13. The process according to claim 11 or 12, wherein the liquid water stream (25) is preheated in the condenser (3) by heat transfer from the gas stream (15), before being fed into the second heat exchanger (27).

14. The process according to any of claims 11 to 13, wherein the heating medium (41) is water or a condensable gas.

15. The process according to any of claims 9 to 14, wherein the degassed water stream (21) and the liquid water stream (25) are compressed to a pressure in a range from 0.25 to 30 bar(abs) before being combined.

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