Device and method to dry a damp compressed gas and a compressor installation provided with such a device

Abstract

Device to dry a damp compressed gas, whereby the device (2) is provided with a dryer that is provided with a liquid desiccant and configured to bring compressed gas in contact with the aforementioned desiccant that is capable of absorbing moisture from the compressed gas, characterised in that the dryer is a membrane dryer (11); the device (2) to dry compressed gas contains a circuit (20) in which the aforementioned liquid desiccant is placed and means to allow the circulation of the desiccant in the circuit (20), consecutively through the membrane dryer (11) with a membrane (13) that forms a partition between the compressed, gas on one side and the liquid desiccant on the other side of the membrane (13), whereby the membrane (13) is impermeable or virtually impermeable to the gas in the compressed gas but selectively permeable to the moisture in the compressed gas; a heat exchanger (29} to heat up the liquid desiccant; a regenerator (22) used to remove at least partially the moisture absorbed in the liquid desiccant before this is returned through the membrane dryer (11) for a following cycle, whereby the regenerator (22) is formed by a housing (23) through which the liquid desiccant with the moisture absorbed therein is guided in moisture-transfer contact with a flushing agent that is simultaneously guided through the housing (23) and is capable of absorbing moisture from the liquid desiccant upon contact; and the circuit (20) is provided with a closable bypass (45) between a branching point in the circuit downstream from the regenerator (22) and upstream from the membrane dryer (11) and a confluence point in the circuit downstream from the membrane dryer (11) and upstream from the regenerator (22).

Claims

1. A device to dry a damp compressed gas, whereby the device (2) is provided with a dryer that is provided with a liquid desiccant and configured to bring compressed gas in contact with the desiccant that is capable of absorbing moisture from the compressed gas, wherein: the dryer is a membrane dryer (11); the device (2) to dry compressed gas contains a circuit (20) in which the liquid desiccant is placed and means to allow the circulation of the desiccant in the circuit (20), consecutively through the membrane dryer (11) with a membrane (13) that forms a partition between the compressed gas on one side and the liquid desiccant on the other side of the membrane (13), whereby the membrane (13) is impermeable or virtually impermeable to the gas in the compressed gas but selectively permeable to the moisture in the compressed gas; a heat exchanger (29) to heat up the liquid desiccant; a regenerator (22) used to remove at least partially the moisture absorbed in the liquid desiccant before this is returned through the membrane dryer (11) for a following cycle, whereby the regenerator (22) is formed by a housing (23) through which the liquid desiccant with the moisture absorbed therein is guided in moisture-transfer contact with a flushing agent that is simultaneously guided through the housing (23) and is capable of absorbing moisture from the liquid desiccant upon contact; and the circuit (20) is provided with a closable bypass (45) between a branching point in the circuit downstream from the regenerator (22) and upstream from the membrane dryer (11) and a confluence point in the circuit downstream from the membrane dryer (11) and upstream from the regenerator (22).

2. The device according to claim 1, further comprising adjusting means configured to keep the pressure on the side of the compressed gas equal to or higher than the pressure on the side of the liquid desiccant, whereby the pressure difference between the two sides of the membrane (13) in the membrane dryer (11) is greater than two bar.

3. The device according claim 1, wherein the liquid desiccant contains at least one of the following substances: mono-propylene glycol; dipropylene glycol; tripropylene glycol; mono-ethylene glycol; di-ethylene glycol; tri-ethylene glycol; lithium chloride with chemical formula LiCl; lithium bromide with chemical formula LiBr; calcium chloride with chemical formula CaCl2.

4. The device according claim 1, wherein the material of the membrane (13) in the membrane dryer (11) is a hydrophobic material with virtually no pores; and/or made of a microporous material with pores of a size approximately between fifteen nanometres and one hundred nanometres.

5. The device according to claim 1, wherein the material of the membrane (13) in the membrane dryer (11) is selected from the group consisting of: polypropene; polytetrafluoroethene; polyvinylidene fluoride; polyethersulfone; polyetherimide; polyethene; polydimethylsiloxane; polyimide; titanium dioxide with chemical formula TiO.sub.2.

6. The device according to claim 1, wherein the membrane dryer (11) is made of a housing (12) with an input (15) for the gas to be dried and an output (16) for the dried gas and a separate compartment (17) in the housing (12) with an input (18) and output (19) for the liquid desiccant, whereby the membrane dryer (11) is made of a housing (12) and one or more tubular membranes that extend across the compartment (17) in the housing (12) and which connect the input (15) and the output (16) for the compressed gas to be dried.

7. The device according to claim 1, wherein the membrane (13) of the membrane dryer (11) is provided with an oil-repellent or oil-proof protective layer, and/or a layer of silicone or fluoropolymer.

8. The device according to claim 1, wherein the regenerator (22) is a membrane regenerator with a membrane (31) with on one side the liquid desiccant and with the flushing agent on the other side, whereby the membrane (31) is impermeable or virtually impermeable to the liquid desiccant but selectively permeable to the moisture that is absorbed in the membrane dryer (11) by the liquid desiccant, whereby the membrane (31) in the regenerator (22) is made of a hydrophobic material.

9. The device according to claim 8, wherein the regenerator (22) contains a housing (23) with an input (33) and output (34) for the desiccant and a separate compartment (35) in the housing (23) with an input (36) and output (37) for the flushing gas, whereby the regenerator (22) contains one or more tubular membranes which extend across the separated compartment (35) in the housing (23) and connect the input (33) and the output (34) for the desiccant.

10. The device according claim 1, wherein the regenerator (22) is a vessel connected to the circuit (20) of the liquid desiccant and that is configured to blow air through the desiccant and carry it away.

11. The device according to claim 1, wherein the circuit (20) of the liquid desiccant is provided with a cooler (43) downstream from the regenerator (22) and upstream from the membrane dryer (11), whereby the circuit (20) of the liquid desiccant is provided with a buffer vessel (44) for cooled liquid desiccant originating from the cooler (43) downstream from the regenerator (22); or the circuit (20) of the liquid desiccant is provided with a buffer vessel (44) for liquid desiccant originating from the regenerator (22) downstream from the regenerator (22) and upstream from the cooler (43).

12. The device according to claim 1, wherein the circuit (20) of the liquid desiccant is provided with a buffer vessel (40) for liquid desiccant originating from the membrane dryer (11), whose liquid desiccant is heated in the buffer vessel (40) by allowing it to circulate through the heat exchanger (29).

13. The device according to claim 1, wherein the circuit (20) of the liquid desiccant is provided with a deaerator (48), whereby the deaerator (48) is connected to a buffer vessel (40, 44) for the desiccant, and the bypass (45) is provided between both buffer vessels (40, 44).

14. The device according to claim 1, wherein the means to allow circulation of the desiccant comprise at least one pump (21, 28).

15. A compressor installation with at least one compressor element (3) for the compression of gas, wherein the compressor installation (1) contains a device (2) according to claim 1 to dry compressed gas originating from the compressor element (3), whereby this contains an aftercooler (9) downstream from the compressor element (3) to cool the compressed gas to be dried before being guided through the membrane dryer (11), and the aftercooler (9) contains a condensation separator (10).

16. The compressor installation according to claim 15, wherein the heat exchanger (29) makes use of the residual heat in the compressor element (3) to heat the liquid desiccant upstream from the regenerator (22), whereby the compressor element (3) comprises a cooling jacket and whereby a cooling medium is guided consecutively through the cooling jacket and the heat exchanger (29) in order to heat the liquid desiccant; and/or at least some of the heat removed in the aftercooler (9) is used to heat the liquid desiccant.

17. A method to dry a damp compressed gas, wherein the method comprises the following steps: providing a membrane dryer (11) with a membrane (13) that is impermeable or virtually impermeable to the gas in the compressed gas but selectively permeable to the moisture in the compressed gas; allowing the circulation of a liquid desiccant through the membrane dryer (11) on one side of the membrane (13) that is capable of absorbing moisture from the compressed gas to be dried upon contact; sending the gas to be dried along the other side of the membrane (13) through the membrane dryer (11) at a pressure that is higher than the pressure on the side of the liquid desiccant; heating the liquid desiccant once it has been guided through the membrane dryer (11); regenerating the liquid desiccant to remove at least some of the moisture absorbed therein before it is recirculated in a following cycle through the membrane dryer (11) by bringing it in contact with a flushing agent that is capable of absorbing moisture from the liquid desiccant upon contact, and some of the regenerated liquid desiccant is sent via a closable bypass (45) in an open position and is subsequently further regenerated without passing the membrane dryer (11).

18. The method according to claim 17, wherein the chosen flushing agent for the regeneration of the liquid desiccant is a flushing liquid that contains water and brought into contact with the liquid desiccant to be regenerated; or a flushing gas that contains air and is brought into contact with the liquid desiccant to be regenerated, whereby the flushing gas is ambient air that is brought into contact with the liquid desiccant to be regenerated at atmospheric pressure, the regeneration of the desiccant use is made of a regenerator (22) through which the liquid desiccant to be regenerated is guided and brought into contact with the ambient air which is sucked in or blown through the regenerator (22).

19. The method according to claim 17, wherein the method is aligned to reach a dew point of the dried compressed gas lying between −20° C. and +10° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With the intention of better showing the characteristics of the invention, some preferred embodiments of a device and a method according to the invention to dry compressed gas as well as a compressor installation provided with such a device are described hereinafter by way of an example without any limiting nature, with reference to the accompanying drawings, in which:

(2) FIG. 1 schematically shows a compressor installation according to the invention with a device according to the invention to dry a compressed gas originating from a compressor;

(3) the FIGS. 2 to 6 each show a different variant of a device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) The device shown in FIG. 1 is a compressor installation 1 which contains a device 2 to dry gas according to the invention.

(5) The compressor installation 1 contains a compressor element 3 with an input 4 connected to an air filter through which a gas, in this case ambient air 5, can be drawn in, and an output 6 to which a pressure conduit 7 is connected to provide the gas compressed by the compressor element 3 at a high pressure to a user network 8 of non-illustrated pneumatic drills, percussion hammers or other tools or devices.

(6) In the pressure conduit 7 an aftercooler 9 is integrated in the known manner to cool the compressed gas before it is supplied to the user network 8. Water present in the compressed gas condenses upon cooling in the aftercooler 9 and is carried off via a condensation separator 10.

(7) The device 2 for drying gas contains a membrane dryer 11 that is included downstream from the aftercooler 9 in the pressure conduit 7 and is composed of a housing 12 which is divided into two compartments or channels using a flat membrane 13, respectively in a first compartment 14 with an input 15 and an output 16 for the compressed gas that is guided from the aftercooler 9 in the direction of arrow A through said first compartment 14 to the user network 8, and in a second compartment 17 with an input 18 and an output 19 for the liquid desiccant.

(8) The device 2 to dry gas also contains a circuit 20 in which the second compartment 17 of the membrane dryer 11 is included and in which the liquid desiccant is circulated using a pump 21 across the second compartment 17 in the direction of arrow B, in the opposite direction to arrow A.

(9) The liquid desiccant must be capable of absorbing moisture from the compressed gas and is preferably highly hygroscopic, in other words attracting moisture, and is characterised by a vapour pressure for water that is lower than the vapour pressure of the moisture in the compressed gas to be dried under the prevailing pressure and temperature in the circuit when the device is in operation.

(10) Examples of suitable liquid desiccants are MPG; DPG; TPG; MEG; DEG; TEG; LiCl; LiBr and CaCl.sub.2, or combinations thereof with the possible addition of water. However, this list is non-exhaustive.

(11) The membrane 13 in the membrane dryer 11 has the characteristic of being impermeable or virtually impermeable to the gas in the compressed gas but selectively permeable to the moisture in the form of water vapour in the compressed gas.

(12) The membrane 13 is preferably made of a hydrophobic, in other words water-repellent, microporous material with pores of between approximately fifteen nanometres and one hundred nanometres.

(13) Examples of such material are summarised below in a non-exhaustive list of materials, namely PP (polypropene); PTFE (polytetrafluoroethene); PVDF (polyvinylidene fluoride); PES (polyethersulfone); porous PEI (polyetherimide); microporous PE (polyethene); PDMS (polydimethylsiloxane) and PI (polyimide).

(14) Ceramic materials, such as titanium dioxide, can also be used as a material for the membrane in the membrane dryer, as well as combinations of ceramic and plastic materials.

(15) Furthermore, the device 2 to dry gas contains a regenerator 22 which is included in the circuit 20 and is made in the form of a housing 23 through which ambient air from the environment 5 is drawn in as a flushing gas using a ventilator or blower 24 or suchlike in the direction of arrow C.

(16) The liquid desiccant is guided by the circuit 20 through the regenerator 22 in an opposite direction to arrow C and atomised at the top using a sprayer 25 in the regenerator 22 on a contactor 26 along which and/or through which the ambient air flows into contact with the atomised desiccant, whereby the moisture absorbed in the desiccant is absorbed by the flushing gas and blown back into the environment 5 together with the absorbed moisture.

(17) The liquid desiccant is collected at the bottom in a tray 27 from which the liquid desiccant is pumped back into the circuit 20 using a second pump 28.

(18) With the installation of the blower 24, a slight negative pressure prevails in the regenerator 22, but there may also be a slight positive pressure when the blower 24 blows ambient air through the regenerator.

(19) In the circuit 20, downstream from the membrane dryer 11 and upstream from the regenerator 22, a heat exchanger 29 is provided to heat the liquid desiccant originating from the membrane dryer 11.

(20) In the illustrated example of FIG. 1, the heat exchanger 29 contains an electrical resistance 30.

(21) Operation of the compressor installation 1 is very simple and as follows.

(22) When in operation, the compressor element 3 provides a certain flow of damp compressed gas, whereby the gas is guided through the aftercooler 9 where the gas is cooled and where part of the moisture present in the gas is removed from the gas as condensate via the condensation separator 10.

(23) The compressed gas is then guided with a relative humidity of 100% through the membrane dryer 11 where the compressed gas is brought via the membrane 13 into contact with the liquid desiccant on the other side of the membrane 13 for the purposes of transferring moisture.

(24) In doing so, the moisture from the compressed gas diffuses through the membrane 13 and is absorbed there by the liquid desiccant, potentially supported by the pressure difference between the two sides of the membrane 13.

(25) By heating in the heat exchanger 29, the water vapour pressure in the desiccant is increased. After passing through the heat exchanger 29, the liquid desiccant is guided through the regenerator 22 where it is brought into direct contact with the flushing gas, in this case ambient air. The flushing gas extracts moisture absorbed in the desiccant from the desiccant as the vapour pressure in the heated desiccant is now higher than in the flushing gas. Subsequently, the flushing gas is evacuated by the blower 24 together with the moisture extracted from the desiccant from the device 2 for drying gas and blown into the environment 5. The suction effect of the blower 24 also creates a negative pressure in the regenerator 22 which means that the flushing gas can remove moisture even more efficiently from the desiccant.

(26) By heating the desiccant to for example 90° C., the absorption of moisture by the flushing gas is encouraged.

(27) The low-moisture liquid desiccant collected in the tray 27 is then again guided through the membrane dryer 11 to once again extract moisture from the compressed gas.

(28) In this way, the liquid desiccant is circulated continuously in the circuit 20 and moisture is continuously extracted from the compressed gas whereby the gas leaving the membrane dryer 11 is dried compressed gas.

(29) The variation on the compressor installation 1 according to the invention shown in FIG. 2 is comparable to the compressor installation 1 in FIG. 1, however, with the following two differences: in this case the heat exchanger 29 is a heat exchanger with a primary part 29a through which the liquid desiccant flows and a secondary part 29b that is in contact for heat-exchange purposes with the primary part 29a and through which a warm medium is guided during operation; the regenerator contains a membrane 31 that divides the housing 23 into two compartments, these being a first compartment 32 with an input 33 and an output 34 through which the liquid desiccant, after being heated in the heat exchanger 29, is sent in the direction of arrow D, and a second compartment 35 with an input 36 and an output 37 through which the flushing gas is sucked in in the direction of arrow C in a counterflow using the blower 24.

(30) The membrane 31 is impermeable or virtually impermeable to the liquid desiccant but is selectively permeable to the moisture that is absorbed in the membrane dryer 11 by the liquid desiccant.

(31) This membrane is preferably made of a hydrophobic material, for example PP (polypropene); PTFE (polytetrafluoroethene) or PVDF (polyvinylidene fluoride).

(32) Otherwise the operation of this variant device is comparable to that of the device according to FIG. 1 with the difference that in this case the contact between the liquid desiccant and the flushing gas in the regenerator 22 occurs not directly but indirectly via the membrane 31.

(33) FIG. 3 is the improved variant of a compressor installation 1 according to the invention, whereby in this case the residual heat from the compressor element 3 is used to heat the liquid desiccant in the heat exchanger 29.

(34) This can be done for example by allowing a cooling medium to circulate using a circulation pump 38 through a cooling jacket on the compressor element 3 in a closed circuit 39 in which the secondary part 29b of the heat exchanger 29 is also included.

(35) In addition, a buffer vessel 40 is included in the circuit 20 for the liquid desiccant that comes from the membrane dryer 11 and that is charged with moisture.

(36) The liquid desiccant is guided from this buffer vessel 40 using a circulation pump 41 in a closed circuit 42 through the primary part 29a of the heat exchanger 29 whereby the device reacts more stably to changes in operating conditions.

(37) The buffer vessel 40 can be provided with an automatic deaerator 48, or air vent, which is responsible for venting the circuit 20 in the event that the membrane 13 in the membrane dryer 11 also allows gas from the compressed gas to flow through it, whereby the pressure in the circuit 20 may increase which may negatively affect the drying capacity of the device and whereby gas might even accumulate in certain parts of the circuit 20 and may cause hindrance or even blockage of the flow of the desiccant, which can lead to poor or even non-performance of the device 1.

(38) Another possibility to recover the residual heat from the compressor element 3 is in the case of an oil-injected compressor element with an oil circuit used to inject oil in the compressor element for the lubrication and cooling of the compressor element 3.

(39) In this case the warm oil can be used in the circuit 39 to heat up the liquid desiccant.

(40) In oil-injected compressor elements, there is typically a very small amount of oil present in the compressed gas.

(41) Depending on the material from which the membrane 13 in the membrane dryer 11 is made, it may be necessary to provide this membrane 13 with an oil-repellent or oil-proof protective layer.

(42) If the membrane 13 is made from microporous plastic, such as for example PP, PE, PEI, PES, PI, PDMS, PTFE or PVDF a protective layer made for example of silicone may be applied or an amorphous protective layer made of a fluoropolymer, such as for example Teflon™, which for example is particularly recommended in the case of a membrane made of microporous PTFE.

(43) In FIG. 4, a further variant is shown of a compressor installation 1 according to the invention which differs from the compressor installation in FIG. 3 in the fact that a cooler 43 is included in the circuit 20 downstream from the regenerator 22 and upstream from the membrane dryer 11 and that a buffer vessel 44 is present to buffer the low-moisture cooled liquid desiccant originating from the cooler 43.

(44) Between the buffer vessel 40 of the moisture-rich liquid desiccant and the buffer vessel 44 of the low-moisture liquid desiccant a closable bypass 45 is provided that can be used when starting up the device to dry gas 2 to become operational more quickly.

(45) In doing so, it is not precluded that the buffer vessel 44 for buffering the low-moisture liquid desiccant is set up upstream from the cooler 43 and downstream from the regenerator 22. The cooler 43 is then located upstream directly before the membrane dryer 11. In this way, the liquid desiccant remains in the buffer vessels 40 and 44 at a higher temperature, causing the vapour pressure of the liquid desiccant to be higher which results in a more efficient regeneration of the liquid desiccant.

(46) In FIG. 5, a further variant is shown that differs from the device in FIG. 4 in the fact that the membrane dryer 11 in the case of FIG. 5 has no flat membrane 13, but rather a membrane 13 that is composed of various tubular membranes, for example in the form of hollow fibres 46.

(47) Hereto the housing 12 is provided with two partitions 47 which delimit a second compartment 17 with an input 18 and an output 19 for the liquid desiccant and two sub-compartments 14a and 14b, respectively with an input 15 and an output 16 for the compressed gas, whereby the two sub-compartments 14a and 14b are fluidly connected by the aforementioned fibres 46 which extend across the compartment 17 and through which the compressed gas is led from the sub-compartment 14a to the sub-compartment 14b and in doing so passes its moisture to the liquid desiccant through the walls of the hollow fibres 46.

(48) Analogously, the regenerator can be provided with tubular or fibre-shaped membranes as opposed to a flat membrane 31.

(49) In FIG. 6, a further variant is shown that differs from the device in FIG. 4 in the fact that no separate regenerator 22 is included in the circuit 20. In this case regeneration of the desiccant is done by blowing air through the desiccant. In FIG. 6, it is shown how this can be achieved by blowing air in the buffer vessel 40, but another or additional vessel may also be used to this purpose. Using a blower or ventilator, air is blown through a filter 50 and guided via a conduit 51 in the vessel 40. The filter 50 is not essential and can also be integrated in a different way. The filter 50 prevents dust or other contaminants from being blown in the desiccant. At the point where the conduit 51 enters the vessel 40, all kinds of known means, for example fine bubbles, can be provided to achieve the desired distribution, for example fine bubbles, of the air blown into the desiccant in the buffer vessel 40, for example diffusers made from tubes or membranes provided with fine openings, or made from porous materials. The air bubbles will extract some of the moisture from the desiccant and carry this off as water vapour. In this way, the desiccant can be regenerated in a very simple manner.

(50) The present invention is by no means limited to the embodiments described as example and shown in the drawings, but a device and a method according to the invention to dry compressed gas and a compressor installation provided with such a device can be realised in all kinds of embodiments without departing from the scope of the invention.