Industrial facility for the removal of impurities from a gas

Abstract

The invention relates to an industrial facility for the removal of impurities from a gas by application of a scrubbing technology.

Description

[0001] Description

[0002] The present invention relates to an industrial facility (a plant, an installation, an apparatus) for a removal of impurities from a gas by application of a scrubbing technology.

[0003] The term gas includes gas streams and a combination of gas and liquid streams.

[0004] The gas is a manufactured (industrial) gas such as a flue gas from a power plant (which may comprise up to 20% by vol. CO2), a flue gas from a rotary kiln for producing cement clinker (comprising up to 35% by vol. CO2) or a raw biogas (which may comprise 40-75% by vol. of methane and 25-60% by vol. CO2). All of these gases comprise impurities like CO2, H2S etc., often qualified as acid gas components. Corresponding refining processes are applied to the gases for purifying purposes, meaning the removal of these impurities. While flue gases are purified in particular for environmental reasons, the refining of biogas leaves a high-caloric and pure gas like biomethane, which may then be used, inter alia, as a substitute for fossil fuels.

[0005] A common technology for gas purification is the so-called scrubbing technology. In gas scrubbing processes, impurities like hydrogen sulphide (H2S) and carbon dioxide (CO2) can be removed by application of a solvent, for example an amine based, liquid / aqueous solvent. The corresponding process is called "amine gas scrubbing process" with an impurity removal efficiency of close to 100%. Reference is made to https: / / gsmblog-us.fuiifilm.com / blog / what- is-amine-gas-treatment / with a publication date of 8 December 2024. A common scrubbing technology (facility) comprises

[0006] - an absorber unit, in which the gas is brought into contact with an aqueous absorbent (like amine) to transfer the impurities like CO2 and / or H2S from the gas to the aqueous absorbent by absorption, and

[0007] - a subsequent stripper unit (also called a regenerator), in which the aqueous absorbent, after discharge from the absorber unit and comprising the impurities, is brought into contact with a strip vapour to separate the impurities from the aqueous absorbent and collect the impurities in the strip vapour such that an overhead vapour, rich in said impurities, and an aqueous absorbent, poor in said impurities, may be discharged from the stripper unit. A so- called reboiler for the preparation of the strip vapour is also known as part of the scrubber unit.

[0008] The respective gas and liquid streams (flows) within the absorber and stripper units may be co-current and / or counter-current.

[0009] To increase the contact between the respective gas and liquid streams and correspondingly to increase the exchange of impurities, it is known to install trays and / or packings, characterized by larges surface areas, within the respective units.

[0010] While the absorber unit may be operated without any specific temperature requirements the stripper unit requires high temperatures, in most cases higher than 373K. This requires heating of the liquid absorbent by a heating medium, often steam, produced in external boilers. Accordingly stripping is energy intensive.

[0011] Typically a heat exchanger is integrated in the flow paths of the aqueous absorbent, rich in impurities (along its way from the absorber unit to the stripper unit) and the aqueous absorbent, poor in impurities (along its return from the stripper unit back to the absorber unit). This heat exchanger facilitates the transfer of heat from the latter to the former and thus to reduce the energy demand, but the heat exchanger cannot eliminate the demand for supplementary heat to increase the temperature in the stripper unit to the required level and cannot avoid additional cooling of the absorbent before re-entering the absorber unit. Before this background it is an object of the invention to provide means for reducing the external energy demand in a scrubbing facility of the type mentioned without downgrading the absorption / stripping efficiency and environmental aspects. In particular the demand for fossil fuels to operate boilers for steam production should be minimised.

[0012] The invention is based on the following findings:

[0013] While the gas purification in known facilities has already reached a high degree, a major disadvantage of known scrubbing facilities is the high energy demand, in particular in connection with the provision of (hot) steam for the preparation of the required strip vapor (typically of a temperature between 373-423K) in the stripper unit, cooling the liquid absorbent before re-entering the absorber

[0014] Firstly the necessary and the actual process parameters, in particular temperatures and pressures of the various streams along corresponding conduits (pipe) and apparatuses along the entire treatment process were determined in a systematic series of tests. It was then examined which process streams (flows) could be used at which points for which further or amended process steps, particularly with regard to energy savings. Based on these results, new plant components and process steps were integrated into the process and the overall facility.

[0015] A key finding during these trials was that different streams within the overall system can be combined or networked with one another, i.e. an energy surplus at one point can be used to compensate for an energy requirement at another point.

[0016] This is especially true for the recovery of thermal energy from the overhead vapor at the top of the stripper unit and the recovery of thermal energy from the mass flow taken from the bottom part of the stripper unit. But even beyond this, energy potentials were found within the process in order to operate (electrify) the entire system (facility) almost completely in- situ in terms of energy. In its most general embodiment the invention relates to an industrial facility for a removal of impurities from a gas (stream) by application of a scrubbing technology, comprising

[0017] - an absorber unit, in which the gas is brought into contact with a liquid absorbent to transfer the impurities from the gas to the liquid absorbent, and

[0018] - a subsequent stripper unit, in which the liquid absorbent, after discharge from the absorber unit and comprising the impurities, is brought into contact with a strip vapour to transfer the impurities from the liquid absorbent to the strip vapour such that an overhead vapour, rich in said impurities, and a liquid absorbent, poor in said impurities, are discharged from the stripper unit, wherein

[0019] 1.1 the overhead vapour, rich in said impurities, is used as a heat source in a heat pump, before the resulting cooled overhead vapour is transferred to atmosphere or further processing,

[0020] 1.2 a first part of the liquid absorbent, poor in said impurities, is used as a heat source in a heat pump, before the resulting cooled liquid absorbent is recycled into the absorber unit,

[0021] 1.3 the heat, transferred in the heat pump, is used as a heat source to heat a second part of said liquid absorbent, poor in said impurities, after being brought into mutual contact, to generate the strip vapour before entering the stripper unit.

[0022] While in many prior art facilities the thermal potential of the stripper overheads remains more or less unutilised, feature 1.1 of the invention allows the transfer of heat (thermal energy) from the stripper overheads via a heat exchanger to a refrigerant of a heat pump, which is part of the overall scrubbing facility. In other words: The heat transferred in 1.1. and 1.2 is further transferred from an evaporator part of the heat pump to a condenser part of the heat pump where additional steam, vapour or both is generated and used to heat the liquid absorbent, poor in said impurities.

[0023] According to feature 1.2 thermal energy within the liquid absorbent extracted from the stripper unit and thus poor in acid components (therefore also called lean absorbent hereinafter) is used in an analogue way as a heat source or as an additional heat source in the heat pump already described and / or in a separate heat pump, and transfers the heat to a refrigerant within the heat pump.

[0024] The refrigerant with a low boiling point evaporates at the evaporator side of the heat pump at temperatures provided by the streams according to 1.1 and / or 1.2. The now gaseous refrigerant is then compressed and transferred (routed) to the condenser side of the heat pump to rise its temperature to the desired value and providing a heat source for a corresponding heat exchanger as part of the heat pump.

[0025] Via this heat exchanger at the condenser side of the heat pump, the generated heat can be extracted from the heat pump and used to support generation of the strip vapour according to feature 1.3.

[0026] Further advantages deriving from 1.1 to 1.3 will be described hereinafter.

[0027] The following embodiments of the invention are options to improve the thermal efficiency of the scrubbing plant and can be realised individually and / or in arbitrary combinations if not otherwise stated or technically nonsensical.

[0028] The heat, transferred at a condenser part of the heat pump, may be used to generate steam, in particular a low pressure steam. Steam is required - inter alia - to produce strip vapour. Accordingly the demand for electrical energy or externally produced thermal energy is at least reduced or even avoided by these means.

[0029] The steam for the production of strip vapour can be processed in at least one compressor unit before being brought into mutual contact with the liquid absorbent, poor in said impurities. Accordingly the compressor is typically installed upstream the preparation of the strip vapour. The saturation temperature of the steam processed is increased to the temperature required in a corresponding reboiler. This compressor unit is an important feature of the new facility and can be split into multiple stages by a so-called Mechanical Vapour Recompression (MVR) unit. MVR as such (but for other applications) is known to the skilled person and for example offered by Piller GmbH, Germany as part of their Vapoline series. It is therefore not described further. This known MVR provides a temperature rise of around 10K per stage.

[0030] The mutual contact between the liquid absorbent, poor in said impurities, and the steam (independently of whether compressed or not compressed previously) may be performed in a reboiler, from which the strip vapour is transferred to the stripper unit. In this embodiment the lean absorbent and strip vapour(s) act as a heat sink at the reboiler. The steam, being the heat source at the reboiler, is extracted from the reboiler as a combined steam condensate stream of a certain temperature Tcon. This temperature Tcon is higher than the temperature Tliq of the liquid absorbent entering the reboiler.

[0031] The combined steam condensate stream mentioned above can be further processed to generate flash steam.

[0032] This can be achieved when the combined steam condensate stream passes a pressure reduction unit. Upon pressure reduction of said condensate (a saturated or subsaturated condensate), the heat energy in the condensate is reduced to a level corresponding to the reduced pressure value, while the total energy is not changed. The reduced pressure stream contains a mixture of steam and steam condensate, which means, that the steam condensate stream has flashed, which coincides with a temperature reduction to the new saturation temperature. In other words: The corresponding enthalpy (energy) liberated by cooling of condensate leads to partial evaporation of the condensate, generating said flash steam. In a subsequent vessel the flash steam and the steam condensate can be separated from each other and finally discharged from the flash vessel.

[0033] The invention is based on the following phenomenon: Reducing the pressure of condensate causes a decrease in saturation temperature according to thermophysical properties of a water substance. If the temperature upstream of said pressure reduction stage exceeds the new saturation temperature downstream of pressure reduction, flashing will occur. Downstream of flashing the stream cools to the new saturation temperature. Liberated sensible heat is converted to latent heat and causes a portion of the condensate to vapourise.

[0034] The flash steam may may be used as an additional energy source for the preparation of strip vapour.

[0035] The overhead vapour, along its way from the stripper unit to the heat pump, may be processed in a heat exchanger, which uses at least part of the condensate, discharged from the flash vessel, as a pre-cooling medium for the overhead vapour, while at the same time steam is discharged from the heat exchanger. This steam is a low pressure steam, a saturated steam (for example with a temperature of 348K at around 39000Pa pressure), which is discharged from said heat exchanger for further use within the facility, which provides another reliable energy optimisation. In known embodiments overhead vapour is just cooled in discrete means.

[0036] The integration of this heat exchanger allows to use the steam, discharged from the heat exchanger, as an additional heating source to further heat at least part of the liquid absorbent, poor in said impurities, for example in the described reboiler.

[0037] Another option is to recycle at least part of the condensate, discharged from the flash vessel, to the condenser part of the heat pump to generate a steam, required to produce the strip vapour.

[0038] The steams provided by the heat pump and the heat exchanger can be provided as saturated steams.

[0039] The steams may also be combined, in particular prior to entering the reboiler.

[0040] According to another embodiment at least part of the condensate, discharged from the flash vessel, is combined with the steams prior to entering the reboiler to provide a so-called desuperheat. A further optimisation is to combine the flash steam and the steams mentioned. This can be realised along corresponding valves before the steam is processed in the compressor (MVR).

[0041] A conduit for the flash steam can be combined with at least some of the conduits for the other steams. This can be realised in a corresponding multi-way valve.

[0042] Further features of the invention derive from the sub-claims and the description hereinafter, including the examples, in which amine is used as a liquid and aqueous absorbent, but without any restriction insofar. Part of the condensate, discharged from the flash vessel, can also be transferred as a heat sink into the heat exchanger used to recover thermal energy from the overhead vapour(s).

[0043] In the attached Figure a scrubbing facility and a corresponding method for the purification of a gas stream 10, rich in CO2 (carbon dioxide), are displayed.

[0044] The gas stream 10 is introduced in an absorber unit 12 at its lower end 121, while a liquid stream in the form of a CCh-poor (lean) aqueous amine stream 14 is introduced in the absorber unit at its upper end 12u. The absorber unit 12 is of conventional design and includes a packing to allow intensive contact between the amine stream 14 and the gas stream 10 along their counter-current paths through said absorber 12. Gas phase to liquid phase mass transfer occurs, during which the aqueous amine stream 14 absorbs CO2 from the gas stream 10 such that a CO2 rich liquid amine stream 18 is extracted at the lower end 121 of absorber 12 and a gas 16 substantially void of carbon dioxide leaves the absorber 12 at its upper end 12u.

[0045] Amine stream 18 is transferred to a heat exchanger 42 and further via conduit (pipe) 22 to an upper end 20u of a stripper unit 20 such that it flows through the interior of said stripper unit 20 downwardly, before being discharged at a lower end 201 of stripper unit 20 and extracted via conduit 26. A substantially CO2 lean strip vapour is transported via a conduit 24 into stripper unit 20 at its lower end 201 and forced to flow upwardly towards the upper end 20u of stripper unit 20. During their respective ways through said stripper 20 in a counter-current flow, strip vapour and amine absorbent are brought into intensive contact such that liquid to gas phase mass transfer occurs and the strip vapour strips carbon dioxide from the liquid amine absorbent.

[0046] An amine absorbent, substantially devoid of CO2, is finally obtained at the lower end 201 of stripper unit 20. This lean absorbent can be reused in the absorber unit 12. For this purpose part of this amine stream in conduit 26 is further transported via conduit 40, heat exchanger 42 and a further conduit 44 to the upper end 12u of absorber unit 12.

[0047] In the heat exchanger 42 the CO2 rich amine absorbent is pre-heated on its way to stripper unit 20, while the CO2 poor amine absorbent, extracted from stripper unit 20, is pre-cooled before released from heat exchanger 42 via conduit 44 and reused in the absorber unit 12. Accordingly the absorber unit 12 operates at a lower temperature than the stripper unit 20.

[0048] This is common design of a scrubbing unit.

[0049] However, this heat integration using heat exchanger 42 alone cannot eliminate - inter alia - the need for supplementary heating (to produce the necessary strip vapours) as well as the need for an external cooling source (like cooling water) for the overhead vapours and / or for the CO2 poor absorbent.

[0050] To avoid external installations like boilers to provide the required energy, the new facility (according to its embodiment displayed in the Figure and described hereinafter) comprises a heat exchanger 64. The overhead vapour, discharged at the upper end 20u of stripper unit 20 at a temperature of ca. 378K and a pressure of ca. 210000Pa, is transported into the heat exchanger by a conduit 28 to allow a saturated steam to be discharged from heat exchanger 64 via conduit 72. This steam has a temperature T72 of about 348K at a pressure of about 39000Pa. T72 is lower than T28, the temperature of the overhead vapour. Correspondingly the overhead vapour is cooled during its way through heat exchanger 64, leaving it as a CO2 rich overhead vapour (steam and condensate) stream via conduit 70 at a temperature T70 of about 353K before flowing through a further heat exchanger 76 as part of the evaporator side of a heat pump 74 and a subsequent conduit 78 (along which the overheads temperature T78 is about 318K at a pressure of still around 210000Pa to a final processing unit 32. This can be an installation to store the CO2 or to use the CO2 (for example as gas in beverages). Other applications are possible.

[0051] A second heat source for said heat pump 74 is provided by the liquid lean amine absorbent with a temperature T44 of still about 328K. For this purpose corresponding conduit 44 forms part of a further heat exchanger 80 as part of the heat pump 74. The lean amine liquid leaves heat exchanger 80 at about 318K (T82) via conduit 82, followed by conduit 14, before being redirected into absorber unit 12.

[0052] If required the liquid absorbent may pass a further cooling stage (behind heat exchanger 80) to lower its temperature before re-entering the absorber unit.

[0053] Another part of the lean amine stream 26 (with a temperature T34 of about 390K) is recycled via conduit 34 and guided into a reboiler 36 to support generation of strip vapour, which is discharged from said reboiler 36 via conduit 24 which enters stripper unit 20 at its lower end 201. A further stream of steam is transported via conduit 38 into the reboiler 36 to support the generation of strip vapour, necessary for regeneration of the amine absorbent in stripper unit 20. Reboilers are known and used as heat exchangers that provide heat to boil a liquid and to generate vapour. Because of their known construction they are not further described here.

[0054] The invention allows to produce the thermal energy necessary to operate the reboiler 36 in a reliable manner without any external boiler systems and in particular without any fossil based combustion installations, as at least part of the required heat (thermal energy) is now provided by heat exchanger 64 and heat pump 74. Saturated steam is transferred via conduits 72 and 75 to a compressor unit 60, designed as a multi stage Mechanical Vapour Recompression unit, which increases the steam pressure to about 270000Pa before entering the reboiler 36 via conduit 38.

[0055] The invention further provides for a steam condensate stream, which is discharged from reboiler 36 via conduit 48 at a temperature T48 of about 390K and a pressure of about 270000Pa, before being processed in a pressure reduction device 50, which lowers the steam condensate pressure to about 39000Pa at a temperature of about 348K (= saturation temperature). The pressure reduction device 50 is a control valve. The corresponding steam / steam condensate stream is then transported via a conduit 52 into a steam / steam condensate separation vessel (also called a flash vessel) 54, to separate the steam (flash steam) from the liquid condensate stream. While the flash steam is discharged via an upper end 54u of flash vessel 54, the condensate stream leaves flash vessel 54 at its lower end 541 via conduit 56.

[0056] This treatment step allows to recycle the generated flash steam via a conduit 58, which is combined in a valve 77 with the steam of conduit 72, before its common compression in unit 60. The recycling of flash steam increases the steam volume and thus the thermal energy for the production of strip vapour in reboiler 36.

[0057] A portion of the condensate, discharged from vessel 54 via conduit 56 is transferred via conduit 68 at a temperature of about 348K (i.e. below T70) to heat exchanger 64, thus forming a heat sink for said heat exchanger 64 and allowing additional heat recovery.

[0058] Heat exchanger 64, as heat exchangers 76, 80, 86, are plate heat exchangers, although other types may be used likewise.

[0059] A further portion of the condensate in conduit 56 is transferred via conduit 84 to a heat exchanger 86 at the condenser side of heat pump 74. Heat exchanger 86 allows to transfer heat from the compressed refrigerant to the steam condensate stream of lower temperature, which then is transported via conduit 88 and combined with conduits 58 and 72 in valve 77 before entering the compressor 60. Heating of the steam condensate by heat exchanger 86 gives rise to a saturated steam stream (due to a temperature of ca. 348K at a pressure of ca 39000Pa) in conduit 88. At a temperature of 358K the pressure should be ca. 58000Pa to achieve the desired saturated steam.

[0060] Heat pump 74 is employed to upgrade low-grade heat available at various positions in the overall scrubbing process and thus to minimize the need for external energy (heat / energy) sources.

[0061] The stripper overheads (overhead vapour), the lean amine streams and / or the steam deriving from the reboiler serve as such in-situ heat sources in heat exchangers 76,80.

[0062] In this context even a third conduit 62 can be provided at the outlet of the flash vessel 54 to allow a third portion of the condensate stream in conduit 56 to be redirected and combined with conduits 88, 72 and 58 to increase the heat transfer efficiency.

[0063] The temperatures and pressure values disclosed are examples with respect to the facility displayed in the figure and were calculated.

[0064] The features disclosed in connection with the facility apply analogously to the method, performed with this facility.

[0065] The new facility does not generate contaminated wastewater, due to a recycling of pure steam condensate that does not come into contact with contaminated process streams.

[0066] Compared with a prior art scrubbing plant the various amendments and additional installations of the facility described lead to substantial improvements according to the following calculations: Starting point is a scrubbing facility designed to capture 4700kg carbon dioxide per hour. In a common facility the specific reboiler duty is about 3 GJ steam per tonne carbon dioxide. The proposed amendments lead to a reduction of the external energy demand of about 75%, i.e. to a reboiler duty of just 0.7 GJ electricity per tonne carbon dioxide. This corresponds to about 950kW electric energy. If only some of the disclosed improvements are realized the reduction is correspondingly lower but still considerable and leads to a dramatic reduction in energy costs.

[0067] The realisation of one or more of the proposed embodiments enables a considerable electrification of a scrubbing plant and its scrubbing process and can even be retrofitted to existing facilities.

Claims

Claims1. An industrial facility for a removal of impurities from a gas (10) by application of a scrubbing technology, comprising: a) an absorber unit (12), in which the gas (10) is brought into contact with a liquid absorbent (14) to transfer the impurities from the gas to the liquid absorbent, and b) a subsequent stripper unit (20), in which the liquid absorbent, after discharge from the absorber unit (12) and comprising the impurities, is brought into contact with a strip vapour to transfer the impurities from the liquid absorbent to the strip vapour such that an overhead vapour, rich in said impurities, and a liquid absorbent, poor in said impurities, are discharged from the stripper unit (20), wherein1.1 the overhead vapour, rich in said impurities, is used as a heat source in a heat pump (74), before the resulting cooled overhead vapour is transferred to atmosphere or further processing,1.2 a first part of the liquid absorbent, poor in said impurities, is used as a heat source in a heat pump (74), before the resulting cooled liquid absorbent is recycled into the absorber unit (12),1.3 the heat, transferred in the heat pump (74), is used as a heat source to heat a second part of said liquid absorbent, poor in said impurities, after being brought into mutual contact, to generate the strip vapour before entering the stripper unit (20).

2. The facility of claim 1, wherein the heat, transferred at a condenser part of the heat pump (74), is used to generate steam.

3. The facility of claim 2, wherein the steam is processed in at least one compressor unit (60) before being brought into mutual contact with the liquid absorbent, poor in said impurities.

4. The facility of claim 3, wherein the mutual contact between the liquid absorbent, poor in said impurities, and the steam is performed in a reboiler (36), from which the strip vapour is transferred to the stripper unit (29) while the steam is extracted from the reboiler (36) as a combined steam condensate stream.

5. The facility of claim 4, wherein the combined steam condensate stream is further processed to generate flash steam.

6. The facility of claim 4, wherein the combined steam condensate stream passes a pressure reduction unit (50) before separating the steam from the condensate in a flash vessel (54) and discharging steam and condensate from the flash vessel (54) .

7. The facility of claim 6, wherein the overhead vapour, along its way from the stripper unit (20) to the heat pump (74), is processed in a heat exchanger (64), which uses at least part of the condensate, discharged from the flash vessel (54), as a pre-cooling medium for the overhead vapour, while steam is discharged from said heat exchanger (64).

8. The facility of claim 7, wherein the steam, discharged from the heat exchanger (64), is used as an additional heating source to further heat at least part of the liquid absorbent, poor in said impurities.

9. The facility of claims 2 and 6, wherein at least part of the condensate, discharged from the flash vessel (54), is recycled to the condenser part of the heat pump (74) to generate steam.

10. The facility of claim 2 and 7, wherein the steams provided by the heat pump (74) and the heat exchanger (64) are provided as saturated steams.

11. The facility of claim 10, wherein the steams are combined prior to entering the reboiler (36).

12. The facility of claim 11, wherein at least part of the condensate, discharged from the flash vessel (54) is combined with the steams prior to entering the reboiler.

13. The facility of claim 5 and 11, wherein a conduit (58) for the flash steam is combined with at least some of the conduits (62, 88, 72) for other steams.

14. The facility of claim 1, wherein the absorber (12) and stripper (20) are equipped with internal packings.

Images