Process for generating process steam and process steam generation plant

Abstract

The invention relates to a method for generating process steam, comprising the steps of: a) evaporating preheated feedwater to generate process steam in a steam generator (4) by heat exchange with a process medium, wherein the evaporation takes place under negative pressure and the negative pressure is adjusted as a function of the temperature of the process medium, b) superheating the process steam leaving the steam generator (4) under negative pressure in a superheater (2) and / or through a throttle, and c) compressing the superheated process steam under negative pressure in a multi-stage axial compressor (5) having a single housing such that the process steam leaving the axial compressor (5) has excess pressure. The invention further relates to a steam generation plant (1) designed to carry out such a method.

Description

[0001] The invention relates to a method for generating process steam and a process steam generation plant for carrying out such a method.

[0002] Process steam is steam extracted from feed water and used to supply process heat and / or gaseous water to an industrial process. The pressure and temperature of the process steam can vary depending on the industrial process.

[0003] The energy to generate process steam is normally provided by the combustion of fossil fuels. More recently, thermal energy from process media with a comparatively low temperature has been used to generate process steam from preheated feedwater, for example in the form of cooling water heated by the process with temperatures in the range of 60-80°C. One advantage in this case is that the residual energy of the cooling water is utilized. Secondly, the cooling water is recooled so that it can be used again. In order to evaporate feedwater using the thermal energy of process media in the aforementioned temperature range at ambient pressure, refrigerant-driven heat pump processes are used. In such heat pump processes, the refrigerant is evaporated in a heat exchanger using the process medium, then compressed and finally condensed in a condenser.The heat released during condensation is then used in a steam generator to evaporate the feedwater, after which the generated process steam is further compressed. The condensed refrigerant is then expanded and returned to the heat exchanger.

[0004] Both synthetic refrigerants, such as R1233zd(E), and natural refrigerants, such as R600, are used as refrigerants. From an environmental perspective, however, it would be desirable to avoid the use of such refrigerants. Furthermore, even an optimized and safe design results in complex system interconnections. Despite intensive efforts, the use of a heat pump process using a refrigerant remains disadvantageous due to additional temperature fluctuations and the associated loss of process efficiency.

[0005] Based on this prior art, it is an object of the present invention to provide an alternative method for generating process steam and an alternative process steam generation plant for carrying out such a method.

[0006] To achieve this object, the present invention provides a method for generating process steam, comprising the steps of: a) evaporating feed water to generate process steam in a steam generator by heat exchange with a process medium, wherein the evaporation takes place under negative pressure and the negative pressure is adjusted as a function of the temperature of the process medium, b) superheating the process steam leaving the steam generator and under negative pressure by throttling and / or in a superheater, and c) compressing the superheated, under negative pressure process steam in a multi-stage axial compressor having a single housing such that the process steam leaving the axial compressor has overpressure.

[0007] The process according to the invention thus completely dispenses with the use of refrigerants, which leads to significantly improved efficiencies due to the associated reduction in process complexity, which is particularly associated with the elimination of the refrigerant condenser. Instead of using a refrigerant, the feedwater is evaporated directly using the thermal energy of the process medium. In order to achieve evaporation of the feedwater at process medium temperatures that are usually well below 100°C, the steam generator is operated under negative pressure, i.e. at pressures below 1 bar, with the negative pressure required for evaporation being selected depending on the temperature of the process medium. The process steam leaving the steam generator, still under negative pressure, is then superheated in a superheater to ensure dry, slightly superheated steam at the inlet of the downstream axial compressor.Alternatively, or in combination with the superheater, the necessary superheating can also be achieved by throttling the generated saturated steam using throttling devices such as orifices and dampers, or by modifying the piping design with increased pressure drop. The axial compressor is then used to compress the large process steam volume flow caused by the low density from the negative pressure range to the positive pressure range. The axial compressor is characterized by its ability to combine large volume flows with high compression ratios.

[0008] The process medium preferably has a temperature in the range of 30 to 90°C upon entering the steam generator. Within this temperature range, the negative pressure required to evaporate the feedwater can be achieved relatively easily.

[0009] The feedwater is advantageously sprayed into the steam generator. The goal is to evaporate the feedwater droplets impinging on the steam generator tubes through which the process medium flows as quickly as possible, thus preventing the formation of a water column within the steam generator and the associated pressure increase. To prevent droplet entrainment, which is quite likely when using a spray process, droplet separation can be performed before or immediately after the process steam exits the steam generator, for example, using a demister.

[0010] According to one embodiment of the present invention, thermal energy is supplied to the superheater via the feed water, thereby achieving a particularly simple and compact design.

[0011] Preferably, the process steam leaving the axial compressor is further compressed in a multi-stage radial compressor to a higher pressure, which is required for a specific industrial process. The use of a radial compressor offers cost and performance advantages.

[0012] The individual stages of the axial compressor and / or the individual stages of the radial compressor are advantageously driven by a motor via a common drive train, with each compressor preferably being assigned its own gearbox. This results in a cost-effective design.

[0013] Advantageously, the process steam leaving the axial compressor is cooled before entering the radial compressor to avoid excessively high inlet or outlet temperatures, which could damage the radial compressor.

[0014] According to one embodiment of the method according to the invention, additional process steam is generated during cooling, which is fed to the process steam leaving the radial compressor, wherein the pressure of the process steam generated during cooling preferably corresponds substantially to the predetermined pressure of the process steam at the outlet from the radial compressor.

[0015] Additionally or alternatively, additional process steam is preferably generated during cooling and fed between individual stages of the radial compressor, especially when the final pressure downstream of the radial compressor is too high.

[0016] Preferably, the process steam is cooled by injecting feed water before entering the radial compressor and / or between individual stages of the radial compressor and / or after leaving the radial compressor, in order to reliably prevent damage to the radial compressor and / or to adjust the temperature of the process steam after compression has taken place.

[0017] Furthermore, to achieve the object mentioned at the outset, the present invention provides a process steam generation plant which is designed in particular to carry out a method according to the invention, comprising a steam generator which is designed to evaporate feed water to generate process steam under negative pressure by heat exchange with a process medium, wherein the negative pressure is adjustable, a superheater which is designed to superheat process steam leaving the steam generator and which is under negative pressure, and an axial compressor having a single housing which is designed to compress superheated process steam leaving the superheater and which is under negative pressure to overpressure.

[0018] The steam generator preferably has a spray device for spraying feed water.

[0019] A feedwater control valve is advantageously arranged in front of the steam generator, via which the required feedwater quantity can be adjusted.

[0020] According to one embodiment of the present invention, the feed water is guided in such a way that it is passed through the superheater as a heat-emitting medium before being fed to the steam generator.

[0021] Advantageously, a multi-stage radial compressor is arranged downstream of the axial compressor.

[0022] The individual stages of the axial compressor and / or the individual stages of the radial compressor are preferably driven via a common drive train, with each compressor being assigned in particular its own gearbox.

[0023] Preferably, a cooler is arranged between the axial compressor and the radial compressor, which cooler is designed to cool the process steam coming from the axial compressor and to generate steam from feed water, wherein at least one line is provided to feed the process steam generated in the cooler downstream of the radial compressor and / or between individual stages of the radial compressor.

[0024] Advantageously, injection devices are provided which are arranged and designed in such a way that feed water can be supplied to the process steam before entering the radial compressor and / or between individual stages of the radial compressor and / or after exiting the radial compressor.

[0025] In one embodiment of the invention, the superheat is additionally or alternatively adjusted by one or more throttling devices. Throttling the generated saturated steam via throttling devices such as orifices and flaps, or by modifying the piping design, creates an increased pressure drop.

[0026] Further features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawing, which shows a schematic view of a process steam generation plant according to an embodiment of the present invention.

[0027] The same reference numbers refer to the same or similar components below.

[0028] The process steam generation plant 1 shown in the drawing according to an embodiment of the present invention comprises a superheater 2, a feedwater control valve 3, a steam generator 4, an axial compressor 5, a cooler 6, and a radial compressor 7, which are connected to one another in said sequence via lines through which feedwater or process steam generated from the feedwater is passed during operation of the process steam generation plant 1. Furthermore, the process steam generation plant 1 comprises additional feedwater control valves 8 and several injection devices 9.

[0029] In the illustrated embodiment, the steam generator 4 comprises a spray device 10, via which the feed water supplied to the steam generator 4 is sprayed into a steam generation chamber of the steam generator 4. Furthermore, a process medium is supplied to the steam generator 4 via a process medium line 11, which transfers heat to the feed water in the steam generator 4 in order to evaporate it, as will be explained in more detail below.

[0030] The axial compressor 5 has a single housing and is designed to compress superheated and negatively pressurized process steam leaving the superheater 2 to overpressure, as will be described in more detail below.

[0031] The radial compressor 7 is designed in several stages.

[0032] In the present case, the individual stages of the axial compressor 5 and the individual stages of the radial compressor 7 are driven via a common drive train 12 and a common motor 13, with each compressor being assigned its own gearbox 14, 15.

[0033] A possible operation of the process steam generation plant 1 is explained below using an example. To generate steam, preheated feedwater is continuously supplied to the process steam generation plant 1 via a first line 16. In this case, it has a pressure of 10 bar, a temperature of 95°C, and a mass flow of 21.0 kg / s. It should be noted that these and the parameter values ​​stated below are only approximate values ​​and serve only to better understand the functioning of the process steam generation plant 1. A partial mass flow of the feedwater is branched off via a feedwater branch line 17, in this case 5.8 kg / s, while the larger partial mass flow of 15.2 kg / s is passed to the superheater 2, where it is used as a heat-dissipating medium.From the superheater 2, the feedwater flows via a line 18 towards the steam generator 4, into which it enters after passing the feedwater control valve 3 at a pressure of 0.13 bar, which is also the pressure prevailing in the steam generation chamber of the steam generator 4. The process medium, which serves as the heat-emitting medium in the steam generator 4, is supplied to the steam generator via the process medium line 11 at a pressure of 6 bar, a temperature of 63°C, and a mass flow of 995 kg / s. The process medium can, for example, be cooling water used in an industrial process that must be recooled in order to be reused for cooling in the industrial process. Within the steam generator 4, the feedwater in this case is sprayed in the form of small droplets via the spray device 10 onto pipes through which the process medium flows, and is evaporated.The negative pressure prevailing in steam generator 4 is adjusted accordingly to the temperature of the process medium to effect this evaporation. Feedwater droplets entrained in the generated process steam are preferably separated by a water separator, for example in the form of a demister, before the process steam leaves steam generator 4 and is fed to superheater 2 via a line 19. The process medium has a temperature of 55°C upon exiting steam generator 4. In superheater 2, the process steam is superheated by the feedwater and leaves superheater 2 at a pressure of 0.12 bar and a temperature of 54°C, with the enthalpy having increased from 400 to 2600 kJ / kg after passing through steam generator 4 and superheater 2. From the superheater 2, the process steam is led via a line 20 to the axial compressor 5, which leaves it at a pressure of 1.5 bar, a temperature of 390°C and an enthalpy of 3250 kJ / kg.The process steam is then fed to the cooler 6 via a line 21 and cooled by feedwater, which is fed to the cooler 6 via the feedwater branch line 17 at a mass flow rate of 2.7 kg / s. Process steam with a pressure of 7.2 bar and a temperature of 166°C is generated from the feedwater, which is then fed out of the cooler 6 via a line 22. The process steam cooled in the cooler 6 leaves the cooler 6 via a line 23, where it is further cooled by feedwater being injected at a mass flow rate of 0.77 kg / s through an injection device 9 fed via the feedwater branch line 17. Downstream of the injection device 9, the temperature of the process steam at which it enters the radial compressor 7 is 119°C. After passing through the individual compressor stages, the process steam leaves the radial compressor 7 via a line 24 with a pressure of 7.2 bar and a temperature of 226°C.Within the radial compressor 7, feedwater, also supplied via the feedwater branch line 17, is injected between the individual compressor stages via corresponding injection devices 9 to cool the process steam before it enters the next compressor stage. Downstream of the radial compressor 7, the process steam is then mixed with the process steam generated in the cooler 6 and finally cooled to a predetermined temperature required for a downstream industrial process by further injection of feedwater using an injection device 9. The process steam leaves the process steam generation plant 1 at a pressure of 7 bar, a temperature of 170°C, and a mass flow of 21.0 kg / s.

[0034] A key advantage of the process described above is that it does not require a conventional refrigerant circuit, which leads to the advantages already mentioned at the beginning.

[0035] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

[1] A process for generating process steam, comprising the steps of: a) evaporation of feed water to produce process steam in a steam generator (4) by heat exchange with a process medium, wherein the evaporation takes place under negative pressure and the negative pressure is adjusted depending on the temperature of the process medium, b) superheating the process steam leaving the steam generator (4) under negative pressure by throttling and / or in a superheater (2) and c) compressing the superheated process steam under negative pressure in a multi-stage axial compressor (5) having a single housing such that the process steam leaving the axial compressor (5) has an overpressure. [2] Method according to claim 1, characterized by that the process medium has a temperature in the range of 30 to 90°C when entering the steam generator (4). [3] Method according to claim 1 or 2, characterized by that the feed water is sprayed into the steam generator (4). [4] Method according to one of the preceding claims, characterized by that heat energy is supplied to the superheater (2) via the feed water. [5] Method according to one of the preceding claims, characterized by that the process steam leaving the axial compressor (5) is further compressed to a higher pressure in a multi-stage radial compressor (7). [6] Method according to one of the preceding claims, characterized by that the individual stages of the axial compressor (5) and / or the individual stages of the radial compressor (7) are motor-driven via a common drive train (12), each compressor (5, 7) preferably being assigned its own gear (14, 15). [7] Method according to one of claims 5 or 6, characterized bythat the process steam leaving the axial compressor (5) is cooled before entering the radial compressor (7). [8] Method according to claim 7, characterized by that during cooling, additional process steam is generated which is fed to the process steam leaving the radial compressor (7), wherein the pressure of the process steam generated during cooling preferably corresponds substantially to the predetermined pressure of the process steam at the outlet from the radial compressor (7). [9] Method according to claim 7 or 8, characterized by that additional process steam is generated during cooling, which is fed between individual stages of the radial compressor (7). [10] Method according to one of claims 5 to 9, characterized by that the process steam is cooled by injecting feed water before entering the radial compressor (7) and / or between individual stages of the radial compressor (7) and / or after leaving the radial compressor (7). [11] Process steam generation plant (1), which is designed in particular for carrying out a method according to one of the preceding claims, comprising a steam generator (4) which is designed to evaporate feed water to generate process steam under negative pressure by heat exchange with a process medium, wherein the negative pressure is adjustable, a superheater (2) and / or by a throttle which is designed to superheat process steam leaving the steam generator (4) and which is under negative pressure, and an axial compressor (5) having a single housing which is designed to compress superheated and negatively pressurised process steam leaving the superheater (2) to overpressure. [12] Process steam generation plant (1) according to claim 11, characterized by that the steam generator (4) has a spray device (10) for spraying feed water. [13] Process steam generation plant (1) according to claim 11 or 12, characterized by that a feedwater control valve (3) is arranged upstream of the steam generator (4). [14] Process steam generation plant (1) according to one of claims 11 to 13, characterized by that the feed water is guided in such a way that it is passed through the superheater (2) as a heat-emitting medium before being fed to the steam generator (4). [15] Process steam generation plant (1) according to one of claims 11 to 14, characterized by that a multi-stage radial compressor (7) is arranged downstream of the axial compressor (5). [16] Process steam generation plant (1) according to one of claims 11 to 15, characterized by that the individual stages of the axial compressor (5) and / or the individual stages of the radial compressor (7) are driven via a common drive train (12), wherein each compressor (5, 7) is preferably assigned its own gear (14, 15). [17] Process steam generation plant (1) according to one of claims 11 to 16, characterized by that a cooler (6) is arranged between the axial compressor (5) and the radial compressor (7), which cooler is designed to cool the process steam coming from the axial compressor (5) and to generate steam from feed water, wherein at least one line (22) is provided to feed the process steam generated in the cooler (6) downstream of the radial compressor (7) and / or between individual stages of the radial compressor (7). [18] Process steam generation plant (1) according to one of claims 11 to 17, characterized by that injection devices (9) are provided which are arranged and designed in such a way that feed water can be supplied to the process steam before entering the radial compressor (7) and / or between individual stages of the radial compressor (7) and / or after exiting the radial compressor (7).

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