Pressure switching adsorption process with improved protection and extended operating range

Abstract

The present invention relates to a pressure swing adsorption process wherein a compressed gas is regeneratively purified by means of an adsorbent to produce nitrogen and / or oxygen. More specifically, the present invention relates to a method and associated device for carrying out a pressure swing adsorption process with improved protection and an extended operating range.

Description

DescriptionTitle of Invention: PRESSURE SWITCHINGADSORPTION PROCESS WITH IMPROVED PROTECTION AND EXTENDED OPERATING RANGETechnical Field

[0001] The present invention relates to a pressure swing adsorption process in which a compressed gas is regeneratively purified by means of an adsorbent to produce nitrogen and / or oxygen.

[0002] More specifically, the present invention relates to a method and associated device for carrying out a pressure swing adsorption process with improved protection and an extended operating range.State of the art

[0003] A pressure swing adsorption process, called Pressure Swing Adsorption (PSA) in English, is a process for producing nitrogen and / or oxygen for professional applications. Its use is applied in, among others, the food and beverage industry or for the production of plastics. To this end, high purity levels are required, such as 90 to 99.999% for nitrogen applications, and 85 to 95% for oxygen applications.

[0004] When reference is made in the text to PSA technology, or PSA for short, it refers to a pressure swing adsorption process for producing nitrogen and / or oxygen, as will be explained further.

[0005] Using PSA technology, nitrogen and oxygen are separated from compressed air. This is done by capturing oxygen and nitrogen from a compressed air stream using adsorption. Adsorption occurs under pressure when molecules bind to an adsorbent, such as a carbon molecular sieve or zeolite. The English term for this is Carbon Molecular Sieve (CMS) or Zeolite.

[0006] For this purpose, two separate pressure vessels are used, each filled with a suitable adsorbent, for example CMS. A separation process and a regeneration process are then switched between both pressure vessels. Compressed air is supplied to a first pressure vessel provided in a first tower. Since oxygen molecules are smaller than nitrogen molecules, the oxygen molecules will end up in the pores of the CMS. Nitrogen molecules, on the other hand, can pass through the CMS without being captured in the pores. The result thereof is that nitrogen of a desired purity is obtained. This phase is called the adsorption or separation phase.

[0007] The majority of the nitrogen produced will leave the process via the first tower. However, a portion will flow to a second tower, also comprising a CMS. This flow will then release the oxygen molecules, captured in a previous adsorption and separationphase, in the second tower from the CMS, thereby generating this CMS. In this way, the system frees up space for new oxygen molecules that can attach to the CMS in a subsequent adsorption phase. This process of cleaning or regeneration is also called oxygen saturated tower regeneration.

[0008] Note that a similar operation applies to an oxygen generator.

[0009] In US10589220B 1, a system is disclosed, configured to perform a pressure swing adsorption process. During the process, the pressure dew point of the compressed air at the inlet is controlled and monitored. When the pressure dew point exceeds a fixed predefined value, the process is temporarily interrupted. During this interruption, moisture is extracted from the compressed air to lower the pressure dew point. This step is called flushing. After the value of the pressure dew point is again lower than the fixed predefined value, possibly with a margin, the pressure swing adsorption process can be continued.

[0010] Too high a pressure dew point causes too heavy a load on the adsorbent. Checking and monitoring the pressure dew point is therefore necessary to prevent too heavy a load on the adsorbent, as well as to prevent condensation on it. If the load is too heavy, permanent damage will occur because the adsorbent will absorb too much moisture, which means that this moisture can no longer be regenerated. The adsorbent then loses its capacity to absorb gas.

[0011] In CN210237124U, another system configured to perform a pressure swing adsorption process is disclosed. Here, the pressure dew point is maintained in the range between 2-10 °C. In WO2018077732, another system configured to perform a pressure swing adsorption process is disclosed. Here too, the pressure dew point is maintained at a desired predefined value.

[0012] Furthermore, US9782716B2 discloses a method for removing water from compressed air.

[0013] A system configured to perform a pressure swing adsorption process is thus known, as well as the extraction of water from compressed air to keep the pressure dew point constant.

[0014] A disadvantage of these disclosed known systems is that the operating range remains limited to a limited interval of inlet temperatures of incoming compressed air in the pressure swing adsorption process.

[0015] It is therefore an object of the present invention to provide a system and method that overcomes one or more of the described disadvantages of state of the art solutions. More specifically, it is an object of the present invention to provide a system and method for operating a pressure swing adsorption process with an extended operating range of inlet temperatures of incoming compressed air.Summary of the invention

[0016] According to the present invention, the above-identified object is achieved by providing, according to a first aspect of the invention, a method for controlling a pressure swing adsorption process, comprising the separation of nitrogen and / or oxygen from a gas stream by adsorption onto an adsorbent, the gas stream comprising a compressed gas at a predefined pressure, originating from a compressor configured to compress ambient air, the method comprising the steps of:

[0017] - supplying the compressed gas to the pressure swing adsorption process;

[0018] - measuring the pressure dew point of the compressed gas; and

[0019] wherein, when the pressure dew point exceeds a predefined value, the method further comprises the steps of:

[0020] - interrupting the step of supplying;

[0021] - flushing the compressed gas to lower the pressure dew point below the predefined value;

[0022] CHARACTERIZED IN THAT the method further comprises the step of:

[0023] - measuring the inlet temperature of the compressed gas;

[0024] and wherein the predefined value is a function of the measured inlet temperature.

[0025] The pressure swing adsorption process, further referred to as PSA, is carried out using a PSA nitrogen generator as known in the state of the art. The PSA generator comprises two separate pressure vessels and separates nitrogen and oxygen by means of adsorption by allowing the oxygen molecules in the compressed gas, also called compressed air, to adhere to a molecular carbon sieve. One of the pressure vessels filters out the oxygen, while the other pressure vessel is regenerated. Regenerating means that the oxygen attached to the sieve is removed.

[0026] For the process, a compressed gas or compressed air is supplied. The pressure dew point of this compressed air is measured. Measuring also means that the pressure dew point is determined, monitored and guarded. Determining the pressure dew point is done by known measuring techniques and therefore does not constitute a limitation of the invention. The pressure dew point is the dew point of the gas under pressure.

[0027] When the pressure dew point exceeds a predefined value, the method further comprises the steps of interrupting the supply of compressed air to the adsorbent, and flushing the compressed air tank, or more generally the upstream compressed air network between the dryer and the generator. The process is therefore temporarily interrupted when the measured pressure dew point exceeds the predefined value. Subsequently, the compressed gas will then be flushed in order to lower the pressure dew point below the predefined value. According to a practical embodiment, a margin will be taken into account, whereby the pressure dew point is lowered below thismargin, starting from the predefined value. Flushing is carried out using a known technique.

[0028] Flushing can also be understood as purifying compressed gas or compressed air in a compressed air tank from excess moisture by allowing the gas in the compressed air tank to escape to the atmosphere and doing so by means of a controlled reduced flow rate, such that the drying installation has the ability to offer a lower dew point.

[0029] Measuring, monitoring, and guarding the pressure dew point serves to prevent damage to the adsorbent. If the pressure dew point is too high, water vapor present in the compressed air can condense on the adsorbent. If the adsorbent absorbs too much moisture, this moisture can no longer be extracted from the adsorbent and the adsorbent loses its capacity to absorb gas. Therefore, it has to be prevented that the value of the pressure dew point exceeds a certain value.

[0030] In the state of the art, the limit or threshold value is set at a fixed predefined value. In this case, the PSA generator will operate in accordance with a predefined protocol in all circumstances, namely with a fixed threshold value for an allowed pressure dew point. This also means that the environmental conditions in which the PSA generator is installed, such as inside a company building or in the open air, are not taken into account. In addition, the location where the PSA generator is installed is also not taken into account, such as, for example, on a location with a Mediterranean climate, where the summers are warm and dry and the winters mild and wet, or a subarctic climate with cold winters and mild summers.

[0031] According to a new and innovative aspect of the invention, the method further comprises the step of measuring the inlet temperature of the compressed gas, and the predefined value, unlike known methods in the state of the art, is not a fixed value or is within a fixed range, but is dependent on the measured inlet temperature of the compressed gas. In other words, the predefined value is a function of the measured inlet temperature.

[0032] As will be explained further, the term function-dependent means that the predefined value is not a fixed value, nor a fixed range, but is dependent on the measured inlet temperature according to this first embodiment. In the function, the argument or variable is the measured inlet temperature, while the function value is the predefined limit value. The function value remains a predefined limit value, which can however be variable. The term predefined therefore means that at a certain measured inlet temperature, it is possible to determine the limit value of the pressure dew point before the supply of the compressed gas is interrupted, and / or restarted.

[0033] Several advantages are identified.

[0034] The location and therefore the environmental conditions in which the PSA generator is installed are taken into account. As mentioned above, this is a disadvantage for theknown method in the state of the art. For example, the PSA generator can be installed in a company building, but just as well outside a company building and therefore in the open air. By making the limit value, up to which the pressure dew point may rise, dependent on the measured inlet temperature, the operating range of inlet temperatures of the compressed gas within which the PSA generator can operate in a safe and reliable manner is expanded.

[0035] In addition, the adsorbent is better protected against possible damage because, unlike the method known in the state of the art, the limit value can also be adjusted downwards, as will be explained further.

[0036] A third advantage is that the method is more energy efficient, compared to the method as known in the state of the art. This is because, at a higher inlet temperature, a higher pressure dew point is allowed, which means that the required power for a dryer, present in the process, can be limited.

[0037] According to an embodiment, the method further comprises the step of measuring the ambient air temperature; and wherein the predefined value is further a function of the measured ambient air temperature.

[0038] In addition to taking into account the inlet temperature, the ambient air temperature can also be taken into account, either alone or in combination therewith.

[0039] For example, an average can be made between the measured ambient air temperature and the measured inlet temperature. Furthermore, a weight can be assigned to one of the two temperatures, such that the measured value thereof weighs more heavily in the function dependency.

[0040] According to an embodiment, the function is a monotonically increasing function.

[0041] A monotonically increasing function with respect to the disclosed invention means that the predefined value for the allowed pressure dew point does not decrease or fall with increasing measured inlet temperature. This further means that there is or can be an interval where the predefined value remains the same with increasing inlet temperature.

[0042] A monotonically increasing function is further understood to mean that its derivative is always positive or equal to zero. Furthermore, the function is continuous, in other words there are no discontinuities present. Note that this does not constitute a limitation of the invention, but that other functions are also possible.

[0043] For example, the function is then a linearly increasing function, hence the limit value is linearly dependent on the measured inlet temperature.

[0044] Preferably, the function comprises three segments, being a first segment comprising a first strictly increasing function defined on a first interval between a left and right limit, a second segment comprising a second strictly increasing function defined on a second interval between a left and right limit, and an intermediate segment comprisinga strictly increasing linear function defined on a third interval between the first and the second interval, comprising a left limit connected to the right limit of the first interval and a right limit connected to the left limit of the second interval. Being connected means that the function is continuous over the three segments, but that a break point in the function at the transition between the segments is possible.

[0045] The combination of the three segments then forms the function over a range of measured inlet temperatures. In this case, there is a lower limit which is the left limit within the first segment, and an upper limit which is the right limit within the second segment.

[0046] The lower limit corresponds to the lowest limit value of the pressure dew point that is allowed to operate the pressure swing adsorption process. The upper limit is then the highest limit value of the pressure dew point that is allowed to operate the pressure swing adsorption process. In other words, this determines the two extreme limit values of the pressure dew point between which the pressure swing adsorption process is allowed to operate.

[0047] Furthermore, there is an intermediate segment that connects the two segments. Connecting means that the function value of the right limit of the first segment matches the function value of the left limit of the intermediate segment. It also means that the function value of the left limit of the second segment matches the function value of the right limit of the intermediate segment. As a result, the function is completely defined over the interval between the lower limit and the upper limit. As already mentioned, a break point is possible. Two break points are then possible, namely on the right limit of the first segment, which is equal to the left limit of the intermediate segment, on the one hand, and on the left limit of the second segment, which is equal to the right limit of the intermediate segment. In a break point, the left and right derivatives are not equal to each other, where the function cannot be derived in that point.

[0048] Referring to the identified advantages as discussed above, the three segments are discussed in further detail.

[0049] The first segment encompasses the lower operating range of the pressure swing adsorption process in terms of allowable pressure dew point and inlet temperature. When the inlet temperature of the compressed gas is lower than the lower limit, the pressure swing adsorption process is completely interrupted. Note further that this lower limit can also be used to interrupt not only the pressure swing adsorption process, but also previous steps such as the compression of ambient air. Furthermore, according to an embodiment and as already explained above, the temperature of the ambient air can also be measured and this measured value can also determine the lower limit.

[0050] In addition, there is an operating range between the lower limit and the right limit of the first segment between which the pressure swing adsorption process operates. When a comparison is made with the method as known in the state of the art, this operating range corresponds to values of the pressure dew point that are lower than the fixed predefined value as used in the method of the state of the art.

[0051] The function between the left and right limits in the first segment is a strictly increasing function. In other words, with an increasing value of the measured inlet temperature of the compressed gas, the allowed value of the pressure dew point increases, whereby the right limit then converges to the above-mentioned limit value as used in the state of the art method.

[0052] Formulated differently and this time viewed from the right limit of the first segment with further decreasing values of the measured inlet temperature, the permitted values of the pressure dew point will decrease with a decreasing inlet temperature. While with the method as known in the state of the art, a pressure swing adsorption process will be interrupted when the temperature of the ambient air is too cold, according to this new and innovative method, the pressure swing adsorption process will continue to operate, whereby the permitted value of the pressure dew point will also decrease. Thereby, the operating range is also extended, compared to the method as known in the state of the art.

[0053] The intermediate segment that forms the connection between the first and the second segment corresponds to values of the method as known in the state of the art. In other words, there is a segment or range that partly corresponds to the method as known in the state of the art, however with essential differences, as explained further.

[0054] In the intermediate segment, there is a strictly increasing function, connecting the right limit of the first segment to the left limit of the second segment. A strictly increasing function implies that the limit value of the pressure dew point at a measured inlet temperature at the right limit of the first segment is lower than the limit value of the pressure dew point at a measured inlet temperature at the left limit of the second segment.

[0055] According to the method as known in the state of the art, there is a constant function over the entire operating range. With reference to the segments, this means, for this known method, a fixed value for the pressure dew point from the left limit of the first segment to the right limit of the second segment. According to this disclosed method, there is, on the one hand, no constant function but a strictly increasing linear function, and furthermore this strictly increasing function is limited to the operating range determined by the intermediate segment. Furthermore, the strictly increasing functions in the two other segments preferably have a different slope than those within the intermediate segment.

[0056] Because this intermediate segment is bounded between the aforementioned left and right limits, the adsorbent is better protected, compared to the method known in the state of the art. This is due, on the one hand, to the fact that there is a strictly increasing function instead of a constant function, and, on the other hand, to the fact that this strictly increasing function is bounded between the right limit of the first segment and the left limit of the second segment. The slope of this strictly increasing function can then be adapted to the location where the pressure swing adsorption process takes place, as well as to the size and / or capacity of the adsorbent.

[0057] Finally, the second segment is adjacent to the intermediate segment, wherein again the function between the measured inlet temperature and the limit value of the pressure dew point is a strictly increasing function. The advantage of this second segment is that there is lower energy consumption of the dryer at higher temperatures because a higher limit value of the pressure dew point is permitted. Furthermore, it follows that the required power of a dryer in the process is smaller than a dryer as used in methods according to the state of the art.

[0058] The strictly increasing functions in the first and second segments respectively are preferably quadratic functions.

[0059] A quadratic function is meant to be a flat quadratic function that can be graphed in a plane by a parabola. The Cartesian equation of a quadratic function isf(x) = ax2 + bx + x. For the first segment, there is a downward opening parabola, which means that in the equation a < 0, and for the second segment, there is an upward opening parabola, which means that a > 0, as will be explained further.

[0060] The quadratic function as a downward opening parabola in the first segment means that only an interval to the left of the symmetry axis and thus of the maximum of the parabola of the function is considered. The part to the right of the symmetry axis and thus the maximum is a strictly decreasing function, while to the left of this the function is strictly increasing.

[0061] The quadratic function as an upward opening parabola in the second segment means that only an interval to the right of the symmetry axis and thus of the minimum of the parabola of the function is considered. The part to the left of the symmetry axis and thus the minimum is a strictly decreasing function, while to the right thereof, the function is strictly increasing.Short description of the drawings

[0062] The invention will be further illustrated with reference to the figures, wherein

[0063] Fig. 1 schematically shows a process for producing nitrogen;

[0064] Fig. 2 schematically illustrates the switching between normal operation and flushing based on set setpoints;

[0065] Fig. 3 graphically illustrates the difference between a pressure dew point with a fixed and variable set value respectively;

[0066] Fig. 4 shows a function between the inlet and / or ambient temperature on the one hand and the value of the pressure dew point on the other hand; and

[0067] Fig. 5 illustrates two areas with an increased operating area, on the one hand, and an area in which energy is saved, on the other hand.Detailed description of the embodiments

[0068] The present invention will be described with reference to certain embodiments and with reference to certain drawings, but the invention is not limited thereto and is defined solely by the claims. The drawings described are only schematic and non- restrictive. In the drawings, the size of certain elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and relative sizes do not necessarily correspond to actual practical embodiments of the invention.

[0069] Furthermore, the terms first, second, third and the like are used in the description and in the claims to distinguish between similar elements and not necessarily to describe a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention may be practiced in sequences other than those described or illustrated herein.

[0070] Furthermore, the terms above, below, over, under and the like are used in the description and claims for illustrative purposes and not necessarily to describe relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein may be practiced in orientations other than those described or illustrated herein.

[0071] Furthermore, the various embodiments, although referred to as “preferred forms”, should be construed as exemplary of how the invention may be practiced rather than as a limitation on the scope of the invention.

[0072] Fig. 1 schematically illustrates a process for producing nitrogen. In a compressor 100 compressed gas is produced which is led to a filter and dryer 101. Furthermore, there is a storage tank 102, a nitrogen generator 103 and a nitrogen storage vessel 104. With this, nitrogen can be produced with a concentration of 99.999%, wherein oxygen is present in a concentration of 10 parts per million, ppm.

[0073] Fig. 2 schematically illustrates switching between normal operation 200 and flushing 202 on the basis of set setpoints 201, 203 as will be explained further. When the dew point of the compressed gas at the outlet of the storage vessel 102 is less than a predefined value 203, then, there is a switch to normal operation 200. When the dew point of the compressed gas at the outlet of the storage vessel 102 is greater than a predefined value 201, possibly supplemented with an offset, then there will be a switch from normal operation to flushing 202.

[0074] Referring to Fig. 3, the difference between a pressure dew point with a fixed and variable set value is graphically illustrated. Both graphs illustrate the average daily temperature 304 over the various months of a year. Furthermore, the average values of the dew point per month are shown by hatched bars. Two situations are distinguished. In a first situation, the limit value for the value of the dew point is a fixed value 302. Here, it can be seen that the average dew point in the months of June, July, and August is higher than this fixed set value. In a second situation and this according to the invention, the limit value for the value of the dew point is not a fixed value but a variable value 303. These values 303 in turn depend on the inlet temperature, measured at the storage tank 102, which in itself has a dependency on the ambient temperature. Hence, instead of working with a fixed value 302 as shown in graph 300, it is possible to switch to a method wherein a variable value 303 as shown in graph 301, is used.

[0075] Furthermore, Fig. 4 illustrates a function between the inlet or ambient temperature and the limit value for the dew point. As already explained, according to the state of the art, there is a fixed value 401 for the limit value for the dew point. According to the method of the invention, this value becomes a variable value 404. Here, the limit value for the dew point exhibits a decreasing function 403 for decreasing inlet or ambient temperatures at a lower inlet or ambient temperature, and an increasing function 402 for increasing inlet or ambient temperatures at a higher inlet or ambient temperature, which decreasing and increasing functions are combined over the entire operating range into a resulting variable value 404 for the limit value for the dew point as a function of the inlet or ambient temperature.

[0076] As illustrated in Fig. 5, there is an extended operating range 510 at low temperatures, compared to the state of the art, and energy will be saved in the range 511 at high temperatures. This is because the flushing is done at a variable value 504 for the limit value for the dew point instead of a fixed value 503 at higher inlet or ambient temperatures for higher values for the limit value for the dew point, and for low inlet or ambient temperatures at lower values for the limit value for the dew point.

[0077] The term “comprising”, as used in the claims, shall not be construed as being limited to the means or steps listed thereafter; the term shall not exclude other elements or steps. The term shall be construed as specifying the presence of the named features, elements, steps or components referred to, but shall not exclude the presence or addition of one or more other features, elements, steps or components, or groups thereof. The scope of the expression “a device comprising means A and B” shall thus not be limited to devices consisting only of components A and B. The meaning is that, with respect to the present invention, only components A and B of the device are listed, and the claim shall further be construed as including equivalents of these components.

Claims

Claims

1. - A method for controlling a pressure swing adsorption process, comprising the separation of nitrogen and / or oxygen from a gas stream by adsorption onto an adsorbent, the gas stream comprising a compressed gas at a predefined pressure, originating from a compressor configured to compress ambient air, the method comprising the steps of:- supplying the compressed gas to the pressure swing adsorption process;- measuring the pressure dew point of the compressed gas; and wherein, when the pressure dew point exceeds a predefined value, the method further comprises the steps of:- interrupting the step of supplying;- flushing the compressed gas to reduce the pressure dew point below the predefined value;CHARACTERIZED IN THAT the method further comprises the step of:- measuring the inlet temperature of the compressed gas; and wherein the predefined value is a function of the measured inlet temperature.

2. - The method according to claim 1, further comprising the step of:- measuring the temperature of the ambient air; and wherein the predefined value is further a function of the measured ambient air temperature.

3. - The method according to any one of the preceding claims, wherein the function is a monotonically increasing function.

4. - The method according to any one of claims 1 or 2, the function comprising:- a first segment comprising a first strictly increasing function defined on a first interval between a left and right limit;- a second segment comprising a second strictly increasing function defined on a second interval between a left and right limit; and- an intermediate segment comprising a strictly increasing linear function defined on a third interval between the first and the second interval, comprising a left limit connected to the right limitof the first interval and a right limit connected to the left limit of the second interval.

5. - The method according to claim 4, wherein the first and / or second strictly increasing function comprises a quadratic function.

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