System and method for drying gas

Abstract

The invention relates to a system for drying gas comprising an inlet (1) for the supply of gas to be dried, an outlet (2) for discharging dried gas, a dryer (3) with plural vessels (4A, B) enclosing a regenerable desiccant (5A, B), a variable bypass conduit (18) configured for splitting off a portion of the gas to be dried, and a controller (23). The bypass conduit is provided with a controllable adjustable valve (19) for adjusting the portion of the to be dried gas split off in the variable bypass conduit. The controllable adjustable valve is connected to the controller configured to regulate the portion of the gas to be dried split off by adjusting the controllable adjustable valve such that a predetermined value of a pressure dew point of the gas to be discharged is obtained.

Description

[0001] System and method for drying gas .

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a system for drying gas . More speci fically, it is intended to remove moisture from gas , or compressed gas , such as for example gas originating from a compressor, using a drying agent .

[0004] BACKGROUND OF THE INVENTION

[0005] The gas to be dried is typically a gas mixture such as air or a gas mixture containing, among others , the following components : air / H20, CO2 / H2O, CH4 / H2O, air / 002 , hydrocarbons / air .

[0006] In the case where the gas is compressed air, for example , it is often advisable to reduce the moisture present in the compressed air . This prevents consumers of the compressed air, such as pneumatic tools , or the pipes , valves and the like of a compressed air network, from being damaged by the moisture .

[0007] Dryers for drying gas typically comprise one or more vessels in which a drying agent is placed . The dryers are equipped with an inlet to which the gas to be dried can be supplied, for example compressed gas generated by a compressor, and an outlet from which the dried gas can be discharged for example to a consumer network . The drying agent or desiccant arranged in a vessel of the dryer can take on various forms such as for instance a granular form or another extruded form . It is also known to provide a drying block in the vessel . Known drying blocks may be formed by an open supporting structure which is at least partially coated with a drying agent .

[0008] In use during drying the drying agent present in the vessel of the dryer extracts moisture from the gas to be dried, this drying agent will become increasingly saturated with adsorbed moisture and will in time no longer be ef fective unless replaced or otherwise rej uvenated . Therefore , typically, the drying agent concerned is generally realised in the form of a regenerable drying agent , or in other words a drying agent that can be regenerated after reaching a certain degree of saturation .

[0009] When making use of a regenerable drying agent it is known to use dryers being provided with two or more vessels of regenerable drying agent . The vessels of such dryers can be operated in a drying or in a regenerating modus .

[0010] In case a vessel is being operated in the drying modus , the gas to be dried is supplied via the inlet to the vessel and dried gas exiting the vessel is discharged via the outlet . Hence the vessel is acting as a drying vessel . In case a vessel is being operated in the regenerating modus , the ( saturated) drying agent is exposed to a regeneration gas flow that extracts the moisture from the drying agent . Hence the vessel is acting as a regenerating ves sel . Such a regeneration gas flow can consist of a fraction of the dried gas and / or hot gas for example whose relative humidity is suf ficiently low to be able to realise the regeneration of the drying agent . A dryer with two vessels may be configured such that one vessel is acting as a drying vessel while at the same time the other vessel is acting as a regenerating vessel . Once the drying agent of the drying vessel reaches a certain degree of saturation the flow of the to be dried gas and the regeneration gas can be redirected such that the former drying vessel is acting now as a regenerating vessel and the former regenerating vessel is acting know as a drying vessel . In other words the vessels of the dryer are used cyclically for drying and regenerating as after each cycle the role of a vessel is switched from drying vessel to regeneration vessel or vice versa .

[0011] Although known dryers definitely have their merits they consume a lot of energy . Hence , there is a continuous interest in finding solutions for reducing the amount of energy needed to dry the gas to the desired level . A known way to determine whether the gas to be discharged at the outlet is suf ficiently dry is by measuring the pres sure dew point of the dried gas . The pressure dew point ( PDF ) indicates the temperature at which the water vapour begins to condense out of the compressed air into a liquid form . In other words , the pressure dew point is used to accurately describe the water content in the compressed air . A low pressure dew point indicates a low water content of the compressed air . The achievable pressure dew point varies between dryers and is typically one of the criteria to select a dryer to ensure the dried air meets the consumer or application requirements . The target PDP has typically a value in the range from -70 ° C up to and including 5 ° C .

[0012] SUMMARY OF THE INVENTION

[0013] It is an obj ective of the present invention to provide an alternative solution for drying gas which is ef fective and more ef ficient in that less energy is required for drying a same load of gas .

[0014] The inventor has monitored the PDF of the gas to be discharged at the outlet of the dryer during operation of the dryer . He has observed that during a cycle of the dryer the PDP of the dried gas is not stable and only approaches the target PDP near the end of the cycle and that during the remainder of the cycle the PDP of the dried gas is signi ficantly lower than the target PDP . Thus during a substantial part of the cycle the PDP of the dried gas is signi ficantly lower than required for the target application .

[0015] One more obj ective of the present invention is to provide a solution for drying gas which brings the pressure dew point of the gas closer to the target pressure dew point for the application during the whole cycle .

[0016] To at least partially address one or more of the mentioned or other challenges and / or to at least partially meet one or more of the mentioned or other obj ectives , the present invention concerns a system for drying gas .

[0017] The system comprises an inlet for the supply of gas to be dried, an outlet for discharging dried gas , and a dryer operatively connected to the inlet and the outlet allowing the gas to flow from the inlet through the dryer to the outlet .

[0018] Preferably the dryer is provided with plural ves sels each enclosing a regenerable desiccant , each vessel comprising an entry and an exit , wherein the dryer is configured such that at least one of the vessels is operated in a drying mode while at least one other of the vessels is operated in a regenerating modus for regenerating the desiccant enclosed in the at least one other of the vessels , and wherein the at least one vessel operated in drying mode is fluidly connected with its entry to the inlet and with its exit to the outlet while drying gas passing from the entry to the exit .

[0019] The system according to the invention comprises a variable bypass conduit configured for splitting of f a portion of the gas to be dried supplied at the inlet and for mixing the split of f portion with the dried gas exiting the dryer upstream the outlet , the by-pass conduit being provided with a controllable adj ustable valve for adj usting the portion of the to be dried gas split of f in the variable bypass conduit by adj usting the controllable adj ustable valve ; and a controller, connected to the controllable adj ustable valve , configured to regulate the portion of the gas to be dried split of f in the variable bypass conduit by adj usting the controllable adj ustable valve such that a predetermined value of a pressure dew point of the gas to be discharged is obtained , This predetermined value is typically chosen slightly, for example a few degrees C, or up to ten degrees C, below the target pressure dew point of the application envisaged .

[0020] The bypass conduit is typically a pipe or hose extending between the inlet and the outlet while bypassing the dryer . The hose or pipe can be made of metal or plastic . The controllable adj ustable valve is typically configured such that by adj usting the valve a cross-sectional area of conduit is locally changed hence af fecting the flow rate of the gas through the conduit . The valve can be adj usted such that the flow of gas through the bypass conduit is completely cut-of f .

[0021] This configuration allows to control the system such that the gas to be discharged has a value of the pressure dew point close to the predetermined value of the pressure dew point by controlling the portion of the to be dried gas which is split of f and bypasses the dryer . The bypass portion lowers the load of to be dried gas to the dryer without af fecting the amount of gas discharged from the system resulting in signi ficantly lower energy consumption . The amount of energy saving depends on the si ze of the bypass portion which depends among others on the target pressure dew point of the application . For instance , for a target pressure dew point of -40 ° C, the bypass portion is typically a few percent of the gas to be dried supplied, while for a target pressure dew point of 5 ° C, the bypass portion typically has a value in the range from 20% to 25% of the gas to be dried supplied . In an embodiment of the invention the system comprises a pressure dew point sensor , connected to the controller, for determining a value of the pressure dew point of the gas to be discharged at the outlet . The controller may be configured to iteratively adj ust the controllable adj ustable valve to a set value and to determine the value of the pressure dew point of the gas to be discharged using the pressure dew point sensor, wherein for each iteration the set value is adj usted in order to reduce the di f ference between the determined and predetermined value of the pressure dew point . This iteration is continued till the di f ference between the determined and predetermined value of the pressure dew point reaches a stable minimum . Preferably this iteration is performed in a smooth way, for example by changing the set value in an incremental way wherein each increment is chosen dependent on the observed di f ference between the determined and predetermined value of the pressure dew point and / or on the rate of change of that temperature di f ference .

[0022] To facilitate stabili zation of the pressure dew point of the gas to be discharged one may opt to provide the system with a fixed bypass conduit configured for splitting of f a fixed portion of the gas to be dried supplied at the inlet and for mixing it with the dried gas exiting the dryer upstream the outlet , the fixed bypass conduit being provided with a controllable valve for opening or cutting of f the fixed bypass conduit . This fixed bypass conduit is arranged parallel to the variable bypass conduit . Using an additional fixed bypass is particularly advantageous when the bypass portion exceeds a certain threshold value .

[0023] In a further embodiment of the invention the system comprises a filter positioned upstream the outlet for filtering the mixture of dried gas exiting the dryer and the to be dried gas bypassing the dryer prior to discharging . Preferably the pressure dewpoint sensor is positioned downstream the filter which positively impacts the accuracy of the pressure dew point determination .

[0024] In one more embodiment of the invention the controller is configured to cyclically switch the vessels of the dryer operated in the drying mode to the regeneration mode and vice versa, and wherein during each switch the controller is configured to adj ust the controllable adj ustable valve to a state wherein flow of the gas through the variable bypass conduit is blocked . Although the switching is almost instantaneously by doing so the discharge of gas with a pressure dew point exceeding the target pressure dew point is prevented and / or the stabili zation of the pressure dew point of the gas to be discharged in the next cycle is facilitated .

[0025] The invention also concerns a method for drying gas comprising the steps of :

[0026] - splitting the to be dried gas in a portion to be dried and a bypass portion;

[0027] - drying the portion of the gas to be dried using a dryer ; preferably the dryer is provided with plural vessels each enclosing a regenerable desiccant , wherein the dryer is configured such that at least one of the vessels is operated in a drying mode for drying the gas supplied while at least one other of the vessels is operated in a regenerating modus for regenerating the desiccant enclosed in the at least one other of the vessels ;

[0028] - mixing the dried gas exciting the dryer with the bypass portion of the to be dried gas bypassing the dryer ;

[0029] - regulating a ratio of the splitting of the gas between the portion to be dried and the bypass portion such that a predetermined value of a pressure dew point of the gas mixture is obtained .

[0030] The method according to the invention may be executed using a system as described in one or more of the previous system embodiments or in a combination of these embodiments .

[0031] In an embodiment according to the invention, the method comprises the steps of measuring the pressure dew point of the gas mixture and determining a di f ference between the measured and the predetermined pressure dew point , and wherein, during the regulating step, the ratio of the splitting of the gas between the portion to be dried and the bypass portion is regulated such that the di f ference between the measured and the predetermined pressure dew point is reduced .

[0032] In one more embodiment according to the invention, the step of drying the portion of the gas to be dried comprises the sub-steps of cyclically switching the vessels of the dryer operated in the drying mode to the regeneration mode and vice versa, and interrupting, during switching, splitting of f the bypass portion of the to be dried gas .

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] With the intention of better showing the characteri stics of the invention, hereafter, as an example without any limitative character, a preferred embodiment is described of a system for drying gas , with reference to the accompanying drawings , wherein : figure 1 schematically depicts a system for drying gas according to an embodiment of the invention; figure 2 shows a graph of a comparative measurement of the pressure dew point of gas to be discharged with and without us ing the system according to the invention as depicted in fig . l .

[0035] DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] Figure 1 depicts an example of a system for drying gas according to the invention . The system has an inlet 1 to which gas , in this example compressed air , to be dried is supplied, and an outlet 2 for discharging gas .

[0037] The system comprises a dryer with two vessels 4A, 4B wherein a desiccant 5A, B is enclosed . The desiccant in this example is a drying block . The vessels 4A, 4B have a general cylindrical shape and are shown in an upright orientation, although the invention is clearly not limited thereto but applicable also to other shapes and orientations of the vessels .

[0038] The vessels 4A, B, which are generally made from metal , are provided at their bottom with an entry 6A, B . The entries of the vessels are connected in parallel to the inlet 1 through inlet pipes 7A, B . Each o f the inlet pipes 7A, B is provided with a controllable inlet valve 8A, B which when enabled shut of f the corresponding inlet pipe such that no gas can flow from the inlet 1 to the vessel 4A, B concerned . When the controllable inlet valve 8A, B is disabled gas to be dried can freely flow from the inlet 1 to the entry 6A, B of the vessel 4A, B concerned .

[0039] The vessels 4A, B, are provided at their top with an exit 9A, B . The exits of the vessels are connected in parallel to the outlet 2 through outlet pipes 10A, B . Each of the outlet pipes 10A, B is provided with a controllable outlet valve 11A, B which when enabled shuts of f the corresponding outlet pipe such that no gas can flow from the exit 9A, B of the vessel 4A, B concerned to the outlet 2 . The entry 6A, B and the exit 9A, B of each vessel 4A, B are fluidly connected allowing the gas to flow in a general vertical flow direction from the bottom entry to the top exit .

[0040] The dryer 3 comprises a regeneration input 12 through which a regeneration gas can be supplied, for example by blowing in heated ambient air . The invention is not limited to regeneration gas supplied by blowing in heated ambient air, but regeneration gas may be supplied in any alternative manner or by any alternative means , such as for instance by splitting of part of the dried gas for use as regeneration gas . The exits 10A, B of the vessels 4A, B are connected in parallel to the regeneration input 12 through input pipes 13A, B . Each of the input pipes 13A, B is provided with a controllable input valve 14A, B which when enabled shut of f the corresponding input pipe such that no regeneration gas can flow from the regeneration input 12 to the exit 9A, B of the vessel 4A, B concerned . When the controllable input valve 14A, B is disabled regeneration gas can flow from the input 12 to the exit 9A, B of the vessel 4A, B concerned and subsequently through the vessel while taking up moisture from the desiccant 5A, B enclosed to the corresponding input 6A. The entries 6A, B of the vessels are connected in parallel to a blow-of f output 15 through output pipes 16A, B . Each of the output pipes 16A, B is provided with a controllable output valve 17A, B which when enabled shut of f the corresponding output pipe such that no gas can flow from the entry 6A, B of the ves sel 4A, B concerned to the blow-of f output 15 .

[0041] The system comprises a variable bypass pipe 18 connecting the inlet 1 to the outlet 2 while bypassing the dryer . This allows to split o ff a portion of the gas to be dried at the inlet 1 and to mix this portion with the dried gas , exiting the dryer 3 through the exit pipes 10A, B , in the discharge pipe 24 upstream of the outlet 2 . The variable bypass pipe 18 is provided with a controllable adj ustable valve 19 for adj usting the portion of the to be dried gas split of f in the variable bypass conduit . When adj usting the valve 19 a cross-sectional area of the variable bypass pipe 18 is locally changed hence af fecting the flow rate of the gas through this pipe . The valve can be adj usted such that the flow of gas through the variable bypass pipe 19 is completely cut-of f .

[0042] Optionally a fixed bypass pipe 20 may be provided parallel to the variable bypass pipe 18 , the fixed bypass pipe being provided with a controllable valve 28 for opening or cutting of f the fixed bypass pipe . This controllable valve 28 allows to cut of the fixed bypass pipe when no bypass is needed or desired .

[0043] The system comprises a filter 21 positioned in the discharge pipe 24 upstream the outlet 2 for filtering the gas to be discharged . Between the filter 21 and the outlet 2 a pressure dew point sensor is provided for determining the pressure dew point of the gas to be discharged . Alternatively, when no filter is provided, a mixing vessel can be used .

[0044] A controller 23 is provided which is operatively connected to the pressure dew point sensor 22 , the controllable adj ustable valve 19 , the controllable inlet valves 8A, B, the controllable outlet valves 11A, B, the controllable input valves 14A, B, and the controllable output valves 17A, B either via control cables or wirelessly (not shown in figure 1 ) .

[0045] The controller 23 is configured to regulate the portion of the gas to be dried split of f in the variable bypass conduit 18 by adj usting the controllable adj ustable valve depending on the value of the pressure dew point of gas to be discharged determined with the pressure dew point sensor 22 . The controller 23 is configured to switch the controllable inlet valves 8A, B, the controllable outlet valves 11A, B, the controllable input valves 14A, B, and the controllable output valves to switch from an enabled state to a disabled state and vice versa depending on the operation of the dryer 3 .

[0046] The operation of the example system for drying gas according to the invention as depicted in fig . l is explained as follows .

[0047] The system in this example is operated cyclically such that during each cycle one of the vessels 4A, B of the dryer 3 is operated in the drying mode and is hence acting as a drying vessel while the other vessel 4A, B is operated as a regenerating vessel .

[0048] At the start of a first drying cycle the drying vessel is vessel 4A, hence the controller 23 has disabled inlet valve 8A, and outlet valve 11A, and enabled input valve 14A and output valve 17A. The regenerating vessel is vessel 4B, hence the controller 23 has disabled input valve 13B and output valve 17B, and enabled inlet valve 8B, and outlet valve 11B . As a consequence a portion of the to be dried gas supplied at the inlet 1 flows through pipe 7A, enters the drying vessel 4A through entry 6A, and is guided to the exit 9A through the desiccant 5A. The desiccant extracts moisture from the gas during the flow o f the gas through the vessel 4A. The gas that leaves the vessel 4A through exit 9A will hence be drier than the gas that entered the vessel 4A via the entry 6A. The dried gas then flows from the exit 9A through the exit pipe 10A to the discharge pipe 24 .

[0049] The controllable adj ustable valve 19 is adj usted by the controller 23 to a set value such that a small portion of the gas to be dried flows from the inlet through the bypass pipe 18 to the discharge pipe 24 while bypassing the dryer 3 . At the discharge pipe this bypass portion of the gas is mixed with the dried gas exiting the dryer 3 via exit pipe 10A. The resulting gas mixture is guided through a filter 21 to the outlet 2 , which can be connected to a consumer network and / or application of dried gas . At the outlet 2 , as shown in fig . 2 , the pressure dew point of the gas mixture is determined by means of a pressure dew point sensor 22 . In this example , the target pressure dew point for the application 25 is -40 ° C . The controller 23 compares the determined value of the pressure dew point with a predetermined value of the pressure dew point . In this example this predetermined value is -45 ° C . Based on the di f ference determined through this comparison a new set value for the controllable adj ustable valve 19 is determined and the controllable adj ustable valve 19 is adj usted in order to reduce said di f ference . This is done iteratively . As a result by varying the bypass portion of the gas to be dried a stable pressure dew point is obtained as shown in the curve 26 of fig . 2 . For comparison, also the PDF curve 27 without any bypass gas is shown, showing an unstable PDP curve with an average PDP value signi ficantly below the target PDF 25 .

[0050] At the start of a first drying cycle the regenerating vessel is vessel 4B and the controller 23 has disabled ( opened) input valve 14B and output valve 17B . As a consequence in case saturated desiccant 5A is present in vessel 4B the generation gas is supplied to the regeneration input 12 and guided through input pipe 13B . The regeneration gas enters the regenerating vessel 4B through exit 9B, and is guided to the entry 6B through the desiccant 5A. The regeneration gas has a suf ficiently low relative humidity to extract moisture from the desiccant 5B present in the regenerating vessel 4B . In other words the desiccant is dried in the regenerating vessel 4B . The hot moist gas that leaves the vessel 4B through entry 6B flows from the entry 6B through the output pipe 16B to the blowof f output 15 where it is blown-of f .

[0051] When during the drying cycle the desiccant 5B is suf ficiently regenerated ( dried) , the regenerating vessel 4B goes into standby and the flow of regeneration gas through the vessel 4B is interrupted .

[0052] Once the desiccant 5A of the drying vessel 4A reaches a certain degree of saturation, for instance when the pressure dew point determined of the gas mixture exceeds the predetermined value of -45 ° C, the flow of the to be dried gas and the regeneration gas can be redirected by switching the corresponding valves such that the former drying vessel is acting now as a regenerating vessel 4A and the former regenerating vessel is acting now as a drying vessel 4B . In other words the vessels 4A, 4B of the dryer 3 are used cyclically for drying and regenerating as after each cycle the role of a vessel is switched from drying vessel to regeneration vessel or vice versa . The present invention is in no way limited to the form of embodiment described by way of an example and represented in the figures , however, such an improved invention for an adsorption device and an adsorption element can be reali zed in various forms without leaving the scope of the invention .

Claims

Claims .

1. A system for drying gas, the system comprising an inlet (1) for the supply of gas to be dried; an outlet (2) for discharging dried gas; a dryer (3) being provided with plural vessels (4A,B) each enclosing a regenerable desiccant (5A,B) , each vessel comprising an entry (6A,B) and an exit (9A,B) , wherein the dryer (3) is configured such that at least one of the vessels is operated in a drying mode while at least one other of the vessels is operated in a regenerating modus for regenerating the desiccant enclosed in the at least one other of the vessels, and wherein the at least one vessel operated in drying mode is fluidly connected with its entry to the inlet and with its exit to the outlet while drying gas passing from the entry to the exit; a variable bypass conduit (18) configured for splitting off a portion of the gas to be dried supplied at the inlet and for mixing the split off portion with the dried gas exiting the dryer upstream the outlet, the bypass conduit being provided with a controllable adjustable valve (19) for adjusting the portion of the to be dried gas split off in the variable bypass conduit by adjusting the controllable adjustable valve; and a controller (23) , connected to the controllable adjustable valve, configured to regulate the portion of the gas to be dried split off in the variable bypass conduit by adjusting the controllable adjustable valve such that a predetermined value of a pressure dew pointof the gas to be discharged is obtained, and to cyclically switch the vessels (4A,B) of the dryer (3) operated in the drying mode to the regeneration mode and vice versa, wherein during each switch the controller is configured to adjust the controllable adjustable valve (19) to a state wherein flow of the gas through the variable bypass conduit (18) is blocked.

2. The system according to claim 1, characterized in that the system comprises a pressure dew point sensor (22) , connected to the controller (23) , for determining a value of the pressure dew point of the gas to be discharged at the outlet (2) .

3. The system according to claim 2, characterized in that the controller (23) is configured to iteratively adjust the controllable adjustable valve (19) to a set value and to determine the value of the pressure dew point of the gas to be discharged using the pressure dew point sensor (22) , wherein for each iteration the set value is adjusted in order to reduce the difference between the determined and predetermined value of the pressure dew point .

4. The system according to any of the preceding claims, characterized in that the system comprises a fixed bypass conduit (20) configured for splitting off a fixed portion of the gas to be dried supplied at the inlet (1) and for mixing it with the dried gas exiting the dryer (3) upstream the outlet (2) , the fixed bypass conduit being provided with a controllable valve (28) foropening or cutting off the fixed bypass conduit.

5. The system according to any of the preceding claims, characterized in that the system comprises a filter (21) positioned upstream the outlet (2) for filtering the mixture of dried gas exiting the dryer (3) and the to be dried gas bypassing the dryer prior to discharging.

6. The system according to claim 5, characterized in that the pressure dewpoint sensor (22) is positioned downstream the filter (21) .

7. A method for drying gas using a system according to any of the claims 1 to 6, the method comprising the steps of :- splitting to be dried gas in a portion to be dried and a bypass portion;- drying the portion of the gas to be dried using the dryer ( 3 ) ;- mixing the dried gas exciting the dryer with the bypass portion of the to be dried gas bypassing the dryer; and- regulating a ratio of the splitting of the gas between the portion to be dried and the bypass portion such that a predetermined value of a pressure dew point of the gas mixture is obtained,- wherein the step of drying comprises the sub-steps of cyclically switching the vessels (4A, B) of the dryer (3) operated in the drying mode to the regeneration mode and vice versa, and interrupting, during switching, splitting off the bypass portion of the tobe dried gas .8 . The method according to claim 7 , characteri zed in that the method comprising the steps of : measuring the pressure dew point of the gas mixture and determining a di f ference between the measured and the predetermined pressure dew point , and wherein, during the regulating step, the ratio of the splitting of the gas between the portion to be dried and the bypass portion is regulated such that the di f ference between the measured and the predetermined pressure dew point is reduced .

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