Energy-saving VPSA system utilizing direct-drive high-speed centrifugal compressors

Abstract

The present invention relates to a method and system for improving the energy efficiency of a PSA / VPSA plant during periods of reduced production demand and capital efficiency by optimizing the feed compressor, vacuum compressor and centrifugal product compressor to achieve lower energy consumption and lower unit gas product production cost. More specifically, the present invention relates to a novel energy-efficient PSA / VPSA turndown process and system that employs a high-speed direct-driven centrifugal product compressor to achieve desired production. Significantly lower energy consumption can be achieved by employing lower flow rates and lower adsorption top pressures in a lower production range.

Description

Technical Field

[0001] The present invention generally relates to energy efficient gas separation methods and systems that utilize a direct drive variable speed centrifugal compressor to pressurize and / or evacuate an adsorption vessel and further pressurize product gas within an adsorption-type gas separation system. Background Art

[0002] The separation of gases from mixtures with other gases is an important industrial process. In such processes, the objective may be to obtain a product gas that is enhanced in the gas, or to remove undesirable components from the product gas. For example, there are commercial-scale processes for separating air into its component gases to obtain nitrogen, oxygen and argon, as well as air pre-purification processes for pre-treating air before use in other processes such as cryogenic separation of air into its component gases.

[0003] Air separation can be achieved using adsorption methods, specifically pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) type methods. In PSA and VPSA methods, compressed air is pumped through a fixed bed of adsorbents that exhibit an adsorption preference for one of the main components, thereby obtaining an enhanced effluent product stream in the non-adsorbed (or less adsorbed) component. Compared with more traditional cryogenic air separation methods, adsorption methods for air separation require relatively simple equipment and are relatively easy to maintain. However, adsorption methods generally have a lower product recovery rate than many cryogenic methods. For this reason, improvement of the efficiency of the adsorption process remains an important goal. A major improvement method is to discover and develop better adsorbents. Some such adsorbents cause cycle time reduction in a given adsorption process. Therefore, new equipment that can meet the demand for reducing cycle time is needed.

[0004] There is also a continuing need for PSA and VPSA equipment with lower power consumption. The basic process employs a selective adsorbent to remove at least one component of a gas mixture, which employs four basic process steps: (1) adsorption, (2) depressurization, (3) purging, and (4) repressurization. A gas mixture containing a more easily adsorbable component and a less easily adsorbable component is passed through at least one adsorbent bed that is capable of selectively adsorbing the more easily adsorbable component at a predetermined (upper limit) adsorption pressure. The gas stream leaving the bed at this upper limit pressure is now concentrated in the less easily adsorbable component and is removed, for example, as a product. When the bed becomes saturated with the more easily adsorbable component, the bed is then depressurized to a lower desorption pressure to desorb the more easily adsorbable component, and the gas is then discharged from the system. Some processes may include additional steps, such as equalization and product pressurization.

[0005] Conventional PSA and VPSA methods use rotary positive displacement blowers for gas pressurization or evacuation in adsorption beds. These conventional rotary leaf blowers generally have lower efficiency and higher maintenance costs than centrifugal compressors, but they are indeed very well adapted to the oscillating nature of the pressure swing cycle. The attractive feature of a rotary leaf blower suitable for adsorption processes is that the power consumption is proportional to the system pressure requirement. The theoretical power consumption of the blower is proportional to the system pressure difference (i.e., there is a linear relationship between power consumption and pressure). This linear power response to system pressure requirements has made rotary leaf blowers the compression equipment of choice for the PSA and VPSA industries. However, rotary leaf blowers generate strong pressure pulsations in the system. In the absence of proper mitigation, it is known that pressure pulses from rotary leaf blowers cause severe structural damage in downstream process equipment. Although pulsation damping vessels are typically used with rotary leaf blowers, they cannot completely eliminate mismatches, and there is always a considerable level of pressure pulsations in the system.

[0006] In the past, fixed speed centrifugal compressors, with and without inlet guide vanes (IGVs) and with variable frequency drives (VFDs), have been considered for PSA and VPSA approaches because they have higher efficiencies when compared to conventional rotary lobe blowers. Centrifugal compressors typically need to be able to operate at high speeds and follow a highly dynamic variable pressure cycle while maintaining optimal efficiency to take advantage of high adiabatic machine efficiency. Variable speed control and / or IGVs, or a combination of both, must be employed to meet variable pressure conditions. When the PSA or VPSA cycle pressure deviates from the design pressure condition of the fixed speed centrifugal compressor, the stage efficiency drops significantly, especially when operating at a pressure ratio close to 1 (blocking). This results in an increase in average power consumption and a decrease in the overall average compressor efficiency throughout the PSA or VPSA cycle.

[0007] However, this technology has not been successfully employed in the past. The use of conventional fixed speed centrifugal compressors with and without IGVs is not ideal because they cannot follow the faster ramps and ramps required for the cyclic pressure swing adsorption process. Conventional variable speed centrifugal compressors may have an improved operating range and improved energy savings and flow reduction compared to the use of IGVs, but cannot quickly adapt to the transient flow conditions of a PSA or VPSA cycle (due to the large inertia of the gears and the large, slow running conventional induction motors).

[0008] In recent years, PSA and VPSA methods have made several progresses. Some of these progresses include: (a) a significant reduction in the ratio of the top adsorption pressure to the bottom desorption pressure, and (b) a reduction in cycle time (usually less than one minute), resulting in a reduction in adsorbent inventory. An important factor in the total energy demand of a PSA or VPSA process is this ratio of adsorption pressure to desorption pressure. The delivery pressure during bed adsorption by a feed air compression device and the suction pressure during desorption by an evacuation device change continuously with the cycle. In order to achieve the lowest possible total power consumption for a cycle such as this, it is desirable that the feed compression device and the evacuation device operate at peak efficiency over a wide range of pressure ratios.

[0009] VPSA equipment is generally designed and sized to meet maximum production requirements under maximum ambient temperature conditions. However, due to changes in consumer demand and environmental conditions, VPSA equipment is generally operated below design capacity. Typical VPSA turndown operations are achieved by extending the cycle step time, but without changing the feed flow rate and the adsorption and desorption pressure ranges. In addition, a product recycle stream is also used to ensure machine operation. Therefore, VPSA operation is not as energy-efficient in turndown mode as at the design point. The purpose of the present invention is to improve the energy efficiency and capital efficiency of VPSA equipment by optimizing direct-driven variable-speed centrifugal feed compressors, vacuum compressors and product compressors to minimize initial equipment capital investment and achieve low unit production costs during the life of the equipment.

[0010] US Patent No. 4,197,096 proposes a method of delivering a variable product at a desired pressure to a consumer through a recirculation line and a regulating valve via a constant volume compressor.

[0011] U.S. Patent No. 4,197,096 discloses that the energy consumption of the VPSA system can be improved when the production demand is reduced by adjusting the speed of the vacuum pump via a centrifuge driven by a positive displacement machine or a variable speed motor. The inertia loss of operating a variable speed centrifugal compressor after the VPSA cycle is significant and offsets the efficiency gain of the positive displacement machine. U.S. Patent No. 4197096 does not change the VPSA adsorption top pressure to achieve lower production demand. The proposed new lower top adsorption pressure down-regulation method is not applicable at all in the invention scheme disclosed in U.S. Patent No. 4,197,096, because the positive displacement compressor used will not be able to obtain a lower top adsorption pressure from the VPSA and deliver the required final pressure without a recycle line. The lower production is achieved through the recycle line of the product compressor, so for the reduced flow, the product compressor will always consume the same amount of power as the full flow. For production reduction, the VPSA adsorption top pressure remains the same as the full production, and the feed compressor is not expected to save energy. Therefore, it is expected that the down-regulation energy consumption of U.S. 4,197,096 is significantly higher than the new down-regulation method of the current invention.

[0012] U.S. Patent No. 5,258,056 proposes a pressure swing adsorption process and system to generate variable production demand by adjusting the feed flow rate for a given impurity level. The production down-regulation is achieved by simply changing the feed flow rate through a control valve based on feedback to the measured product purity. The adsorption pressure and / or the adsorption pressure to desorption pressure ratio are not changed. This is different from the proposed down-regulation method, in which the feed flow rate, adsorption pressure, and adsorption pressure to desorption pressure ratio are adjusted to achieve the lowest energy separation power when production is reduced while meeting the product purity requirement.

[0013] U.S. Patent No. 6,030,435 proposes a method for regulating a pressure swing adsorption process to achieve optimal performance of PSA. The main invention is to regulate the feed stream temperature so that the adsorption system maintains the optimal temperature for peak performance. One way to regulate the feed temperature is to change the adsorption pressure while maintaining the same production demand. The regulation method in US6030435 shows that regulating the adsorption pressure can have a slight improvement in PSA performance. However, only changing the top adsorption pressure provides limited energy efficiency for reducing production demand while meeting product purity requirements. The proposal to simultaneously regulate the feed flow, adsorption pressure, adsorption pressure and desorption pressure ratio provides significantly more improved energy efficiency.

[0014] U.S. Patent No. 7,641,716 describes a throttling method to maintain a constant feed. This consists of a valve located before the system to keep the flow at an optimal rate to achieve peak performance of the system. The patent proposes a general operating practice in a (V)PSA to control a constant feed flow into the system to maintain optimal performance. Summary of the invention

[0015] The present invention relates to methods and systems for improving VPSA plant energy at reduced production capacity and capital efficiency by optimizing feed compressors, vacuum compressors and centrifugal product compressors to achieve lower energy consumption and lower unit gas product production costs. More specifically, the present invention relates to a novel energy-efficient VPSA turndown process and system that employs high-speed direct-driven centrifugal product compressors to achieve desired production. Significantly lower energy consumption can be achieved by employing lower flow rates and lower adsorption head pressures in the lower production range. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A VPSA process flow diagram is shown.

[0017] Figure 2 A compressor map of an oxygen product compressor is shown. DETAILED DESCRIPTION

[0018] Centrifugal technology is based on dynamic compression. In a dynamic compressor, a gas such as air, oxygen, nitrogen or other gas is drawn between the blades of a rapidly rotating impeller and accelerated to high speed. The air is then discharged through a diffuser, where the kinetic energy is converted into static pressure. Most dynamic compressors are turbo compressors with axial or radial flow patterns and are designed for larger volume flows. The performance of a dynamic compressor is very dependent on environmental conditions such as:

[0019] 1) Inlet temperature

[0020] 2) Inlet pressure

[0021] 3) Relative humidity (RH)

[0022] 4) Cooling water temperature

[0023] Lowering the air inlet temperature increases the air density, resulting in higher free exhaust volume (ACFM), higher compressor power consumption, and a higher available turndown range. Likewise, increasing the air inlet temperature decreases the air density, resulting in lower free exhaust volume (ACFM), lower compressor power consumption, and a lower available turndown range.

[0024] A decrease in inlet pressure will reduce the air density at the compressor inlet. Like higher temperatures, this will result in lower free discharge, lower power consumption, and less available turndown. And an increase in inlet pressure will increase the air density at the compressor inlet, resulting in higher free discharge, higher power consumption, and higher available turndown.

[0025] Adding water vapor to the air makes it humid and reduces the density of the air. This is because the molar mass of water is less than the molar mass of air. Therefore, an increase in relative humidity reduces flow and power, and a decrease in RH will increase flow and power.

[0026] Finally, cooling water temperature affects suction capacity, such that cooler water increases flow and power, and warmer water decreases flow and power.

[0027] PSA / VPSA units are typically designed and sized to meet maximum production requirements at maximum ambient temperature conditions. However, due to changes in consumer demand and environmental conditions, PSA / VPSA units are often operated at less than design capacity, thereby reducing the efficiency of turndown operations.

[0028] In order to improve the energy efficiency of the plant when the consumer demand is below the design capacity, the present invention proposes a novel turndown method, thereby reducing the flow rate and top adsorption pressure to achieve lower specific energy consumption. With the lower flow rate and lower inlet suction pressure of the product compressor, the final desired product pressure can be achieved by increasing the motor speed and increasing the stage pressure ratio, wherein the variable speed controlled centrifugal compressor is driven by a high speed permanent magnet motor or induction motor. The proposed method and system are beneficial to PSA / VPSA plants using Roots compressors or centrifugal feed compressors and / or vacuum compressors, but most of the beneficial effects are expected to come from plants using centrifuges.

[0029] In one embodiment, the claimed invention is directed to a method for improving the energy efficiency of an adsorptive gas separation system during periods of reduced product demand, wherein the system comprises:

[0030] a. at least one container containing at least one adsorption bed comprising at least one adsorption material, and

[0031] b. at least one compressor,

[0032] wherein at least one adsorption bed is configured to be cyclically pressurized and depressurized during operation to separate components of the feed gas into at least a first component and a second component by selectively adsorbing the first component into the adsorption bed comprising at least one adsorbent, the method comprising the following steps cyclically performed:

[0033] I. During the adsorption step of the cycle, the pressure of the feed gas entering the bed is increased to an adsorption pressure to enable the first component to be adsorbed by the adsorbent, which exhibits a mass transfer coefficient of ≥12 / s at 1.5 bar and 300 K and an intrinsic diffusivity of N2 of 3.5×10 -6 m2 / s,

[0034] II. during said desorption step of said cycle, depressurizing said bed to a desorption pressure so as to desorb said first component from said adsorbent, said adsorption pressure to said desorption pressure having a pressure ratio in the range of about 1.5 to 6.0;

[0035] Wherein during periods of reduced product demand, the method includes reducing the top adsorption pressure to a range of about 75% to 90% of the designed maximum operating pressure of the at least one adsorption vessel to reduce production capacity while achieving higher energy efficiency.

[0036] As can be seen from Table 1, reducing the top pressure to 90% of the design operating top pressure reduces production to 80% of normal production. Normal production refers to the contracted oxygen production of the system.

[0037] The design operating top pressure is defined as the maximum pressure achieved by any vessel during the cycle in order to achieve the contracted oxygen production (within 5%) at the contracted power consumption (within 5%) during the contracted environmental conditions (temperature + / - 2C, pressure + / - 0.05 bar, humidity (+ / - 15%, etc.)). This generally corresponds to the lowest cost of producing oxygen with a given set of equipment. In the present invention, the new benefit is that during periods of reduced consumption, the claimed method reduces the relative power consumption of the system by fully utilizing the system capital. The operating cost of operating at reduced production using this new method is now lower than the contract design situation, and is significantly lower than traditional methods.

[0038] The feed unloading time is the time when the feed compressor is venting to the atmosphere. The feed unloading step is the step when the feed compressor is venting to the atmosphere. In common operations, the adsorbent vessels usually participate in an equalization step at this time, where gas from one vessel is transferred to a second vessel or to a tank that can hold gas.

[0039] The pressure ratio of the VPSA is 2.3 to 6, with a typical range of 2.5 to 3.5. The pressure ratio of the PSA is 3 to 8, with a typical range of 4-6.

[0040] In some embodiments, at least one adsorption bed is cyclically pressurized by at least one feed compressor and sometimes evacuated by at least one vacuum compressor, wherein at least one feed compressor or at least one vacuum compressor is a centrifugal compressor driven by an associated high-speed induction motor designed for variable high-speed operation. In such embodiments, the compressor that is not a centrifugal compressor may be a rotary lobe blower driven by an induction motor.

[0041] In some embodiments of the present invention, the adsorbent bed is cyclically pressurized by at least one feed centrifugal compressor driven by an associated high-speed induction variable speed motor, and evacuated by at least one vacuum centrifugal compressor driven by an associated high-speed induction variable speed motor. More specifically and in accordance with the present invention, the centrifugal feed compressor and / or the centrifugal vacuum compressor are driven by a direct drive high-speed induction motor for variable speed operation, so that the compressor and high-speed induction motor combination can accelerate from low speed to high speed and decelerate from high speed to low speed at the faster rates required by the shorter cycle times of current PSA or VPSA systems and methods.

[0042] The use of a variable speed controlled high speed permanent magnet motor or induction motor allows an order of magnitude improvement in the ability to accelerate and slow down a centrifugal compressor in a PSA or VPSA process over a conventional low speed induction motor / gearbox system. The use of a high speed motor allows the elimination of the necessity of a gearbox and thus also the elimination of the necessity of a lubricating oil system. Thus, the centrifugal compressor can be carried on oil-free bearings. Furthermore, in some embodiments, it is expected that the efficiency of the drive system proposed by the present invention will be improved by more than 7 percentage points with the elimination of gear and bearing oil friction losses, lubricating oil system losses, and the ability to improve lead / lag power.

[0043] In order to achieve high efficiency for a centrifugal compressor, it is important to operate the compressor along a peak efficiency line. The peak efficiency line is a line drawn on a compressor performance map. As discussed herein, the compressor performance map (pressure ratio at various speeds relative to mass flow) is generated by the isentropic work coefficient at various speeds for a specific inlet temperature of the compressor. The peak efficiency line represents the locus of points corresponding to all peak efficiency points of the compressor operating curve at different speeds. By operating along the peak efficiency line, the compressor is able to operate in the most efficient mode in terms of its power consumption.

[0044] The performance map may also be in the form of a lookup table or reference table generated from the isentropic work coefficient.The performance map and resulting optimum efficiency line may be stored in a programmable logic controller (PLC) and integrated with the PSA and / or VPSA system.

[0045] Although it should not be construed as limiting, the present invention is implemented in a PSA or VPSA adsorption process for separating a gas (e.g., air) into oxygen and nitrogen. Other gas separation processes that may benefit from the use of the present invention include, but are not limited to, PSA and / or VPSA separations involving the recovery of gases such as O2, N2, CO2, H2, or He.

[0046] VPSA equipment usually has three rotating equipment, such as Figure 1As shown. The feeder (22) provides air to the device. The air is separated to produce oxygen. The vacuum machine (50) removes waste nitrogen from the device. The product compressor (73) raises the oxygen pressure from the device to the supply pressure desired by the consumer. Typically, the feeder and vacuum machine are Roots-type rotary blowers. These machines operate at a fixed speed and the flow rate cannot be changed once installed. Therefore, once the VPSA is designed and built, the amount of oxygen produced cannot be increased. Typically, for single-stage compression, the product compressor is a reciprocating compressor or a fixed-speed centrifugal compressor. Similar to conventional feeders and vacuum machines, conventional product compressors, once selected and installed, cannot be adjusted to increase oxygen capacity. If less oxygen product is required, the compressor will operate in a recirculation mode through a recirculation line (77) together with a recirculation control valve (75, 76), and the machine will consume the same amount of power as if it were producing 100% of its capacity. In other words, capacity reduction will not generate any energy savings by the product compressor.

[0047] VPSA plants are typically designed to meet maximum production requirements, even though consumers typically require lesser amounts of oxygen production most of the time. Therefore, VPSA plants are typically operated in a turn-down mode. Having an energy-efficient turn-down method enables VPSA plants to achieve high energy efficiency and reduce energy costs within the operating window of the plant.

[0048] Typical VPSA plant turndown operation is achieved by running the plant intermittently by extending specific cycle step times without changing feed flow, adsorption pressure, and desorption pressure ranges. This mode of operation inherently has higher specific power because the motor and compressor are still running when the system is not producing product. An alternative is to shut down the motor and compressor when the plant is not running, however this is neither feasible nor preferred due to wear and tear on the machine controls and the frequent startup and shutdown caused by the shorter VPSA cycle steps.

[0049] Conventional VPSA turndown is achieved by feeding the same amount of flow and extending the cycle time. If the goal is to produce 80% oxygen product, the conventional VPSA turndown method can be used by simply extending the cycle time while keeping the feed flow and top pressure the same, the energy consumption and oxygen recovery from the turndown are less than normal production. In other words, the turndown performance is not as effective as if the equipment were producing 100% of the output.

[0050] For example, an improved, more energy-efficient method of producing an 80% oxygen product is to lower the top adsorption pressure. One skilled in the art would normally expect that low top pressures would produce adverse VPSA performance results because adsorption capacity decreases at lower top adsorption pressures. In order to compensate for the lower adsorption capacity due to lower top pressures, the feed flow must be increased and the cycle shortened, both of which will adversely affect recovery. However, the energy-saving benefits due to lower top pressures surprisingly offset the adverse effects of decreased recovery due to increased feed flow and shorter cycles.

[0051] A further improvement to the turndown method is to provide lower feed flow and lower overhead pressure to maximize recovery using the reduced feed rate into the adsorption vessel. The lower overhead pressure from the adsorption vessel translates into a lower suction pressure for the oxygen product compressor. By employing a direct drive variable high speed centrifugal product compressor, the motor can be sped up to increase the pressure ratio from the compressor, thereby achieving the same product pressure as if the suction pressure were maintained at the same level.

[0052] In a VPSA system, the top pressure of the adsorption vessel can be reduced while maintaining control of the product flow by reducing the pressure restriction in the system. This can be achieved by further increasing the opening of the product control valve 68 (always), valve 62 / 64 (during product production of the corresponding bed), or valve 60 (during product production). This will inherently increase the flow that can be balanced by increasing the feed unloading time. This can be done by reducing the total adsorption / product preparation step time or increasing the feed unloading step time. Increasing the feed unloading step time has the side effect of reducing the vacuum pressure, which may be desirable in some cases where the system can operate more efficiently. This is equipment-specific and is evaluated on a case-by-case basis because it can be affected by adsorbent characteristics, loading procedures, vessel configuration, equipment piping layout, etc. If a product centrifugal compressor is used, the product machine speed can be increased to achieve a lower suction pressure. This can be used in conjunction with changing the pressure restriction in valve 68, 62 / 64 or 60 or before changing the pressure restriction. It will still be necessary to achieve flow balance by adjusting the feed unloading time.

[0053] Additional overhead pressure and flow regulation can be achieved if the VPSA feed compressor / vacuum compressor is variable, an example is a centrifugal compressor with a variable frequency drive. In these cases, the compressor can be slowed down to produce a lower pressure ratio while still maintaining a constant flow. To balance the lower overhead pressure, valves 68, 62 / 64 or 60 or the centrifugal product compressor or some combination thereof must still be adjusted. But when the machine is regulating flow, the cycle timing does not need to be changed to balance the flow. The regulation in the PSA system is basically the same, except that the PSA system does not have a vacuum compressor.

[0054] Additional top pressure and flow regulation can be achieved if the VPSA feed compressor / vacuum compressor is variable, an example is a centrifugal compressor with a variable frequency drive. In these cases, the compressor can be slowed down to produce a lower pressure ratio while still maintaining a constant flow.

[0055] If a product centrifugal compressor is used, the product machine speed can be increased to achieve a lower suction pressure. Figure 1 The illustrated method is used in conjunction with or prior to changing the pressure restriction in valve 68, 62 / 64 or 60. It is still necessary to achieve flow balance by adjusting the feed unloading time.

[0056] The newly proposed turndown method achieves significant energy consumption reduction over the conventional constant feed flow turndown method, as shown in the simulation results in Table 1.

[0057] Table 1 Summary of VPSA performance of down-regulation methods :

[0058]

[0059] The present invention can be implemented in a (vacuum) pressure swing adsorption system having various PSA / VSA / VPSA cycles. A pressure swing adsorption system separates the components of a feed gas mixture into at least a first component and a second component by selectively adsorbing the first component into an adsorbent bed. A pressure swing adsorption cycle consists of the following basic steps: during the adsorption step, the pressure of the feed gas is increased to the adsorption top pressure in order to be able to adsorb the first component, and during the desorption step, the bed is depressurized to the desorption pressure.

[0060] The desired low pressure is > about 0.1 atm and the desired high pressure is < about 100 atm. In another embodiment, the low pressure in the cycle is ≥ 0.3 atm and the high pressure in the cycle is ≤ 6 atm.

[0061] The pressure ratio of the adsorption pressure to the desorption pressure of the oxygen VPSA mentioned in the present invention is generally less than 6.0, in one embodiment in the range of about 1.5 to about 6, and in another embodiment in the range of about 1.5 to about 3.5.

[0062] The PSA / VPSA system for which the mentioned turndown method is advantageous may include a single adsorption bed, two adsorption beds in a single series with one adsorption bed in feed mode and the other adsorption bed in regeneration mode, or a plurality of adsorption beds exceeding two beds.

[0063] Various adsorbents may be used in the process of the present invention. In one embodiment, a LiX adsorbent may be used. In another embodiment, any faujasite type with any cation or mixed cation configuration of Li, Ca, Na, K may be employed. The present invention works with a high-speed adsorbent defined as having a mass transfer coefficient of >= 12 / s and an intrinsic diffusivity of N2 of ≥ 3.5×10 -6 m2 / s.

[0064] The energy improvement percentage will be affected by the base system configuration and the reference adsorbent as well as the compression machine efficiency. The above simulation results illustrate the present invention using a LiX adsorbent for obtaining oxygen product from air feed, which exhibits a mass transfer coefficient of ≥12 / s at 1.5 bar and 300 K and an intrinsic diffusivity of N2 of ≥3.5×10 -6 m 2 / s.

[0065] Various alternatives and modifications may be devised by those skilled in the art without departing from the invention. Therefore, the present invention is intended to cover all such alternatives, modifications and differences that fall within the scope of the appended claims.

[0066] At lower overhead pressures from the adsorption vessel, the suction pressure of the product compressor will be lower than when the VPSA is operated using the conventional constant feed and constant overhead adsorption pressure approach.

[0067] With low flow and inlet suction pressure into the product compressor, the final desired pressure can be achieved by increasing the motor speed and increasing the stage pressure ratio. For a centrifugal product compressor with a high speed motor and VFD, the compressor is operated at the compressor's design point (pressure, flow, and speed). The flow from the machine can be increased by increasing the speed while maintaining the same outlet pressure.

[0068] Figure 2 A compressor map for an oxygen product compressor is shown. The design point of the compressor is shown on the map. The design point includes the flow, pressure ratio and speed at which the wheel is designed to operate to meet the process requirements. For a typical compressor using a fixed speed motor and gearbox, the compressor operates at or near the design point. For a compressor driven by a high speed motor and VFD as described in the patent, the compressor can operate within the entire map range. The compressor can also be operated at a higher flow at the same head by increasing the speed of the motor. This provides flexibility for compressors that are designed for specific flows and pressures, but operate at higher flows if production requirements change.

[0069] For centrifugal compressors with high-speed motors and VFDs, the compressor can be accelerated at lower flows to increase the pressure ratio and achieve the desired higher final pressure. Figure 2 This concept is shown in Figure 1. At 80% flow, a compressor with 90% speed can deliver a pressure ratio of 1.85. If the compressor speed is increased to 110%, the pressure ratio can be increased to 2.22. By increasing the speed of the motor to increase the pressure ratio, the final product pressure can be achieved. This method of turning down the increased pressure ratio at lower flow is fundamentally more energy efficient than the traditional turndown method of recycling the product portion.

[0070] For a single stage direct drive high speed centrifugal compressor, the range of turndown can be extended by a combination of varying compressor speed and varying recirculation flow to account for reduced feed flow and lower suction pressure.

[0071] The turndown range can be extended by using inlet guide vanes and varying the angle of the inlet guide vanes.

[0072] The described adjustment method of increasing motor speed and increasing compressor stage pressure ratio can be extended to a multi-stage compressor with a further extended operating range.

[0073] At lower feed flow and lower suction pressure, the first stage pressure ratio can be reduced and the second and / or third stage pressure ratios increased to achieve the desired final pressure. Pressure ratio variation can be achieved by varying the compressor speed via a direct drive high speed motor.

Claims

1. A method for improving the energy efficiency of an adsorptive gas separation system during periods of reduced product demand, wherein the system comprises: a. at least one container containing at least one adsorption bed comprising at least one adsorbent, and b. at least one compressor, wherein the at least one adsorption bed is configured to be cyclically pressurized and depressurized during operation to separate components of a feed gas mixture into at least the first component and a second component by selectively adsorbing the first component into the adsorption bed comprising at least one adsorbent, the method comprising the following steps which are cyclically performed: I. During the adsorption step of the cycle, the pressure of the feed gas entering the adsorption bed is increased to an adsorption pressure to enable the first component to be adsorbed by the adsorbent, the adsorbent exhibiting a mass transfer coefficient of ≥12 / s at 1.5 bar and 300 K and an intrinsic diffusivity of N2 of ≥3.5×10 -6 m 2 / s, II. during the desorption step of the cycle, depressurizing the adsorption bed to a desorption pressure so as to desorb the first component from the adsorbent, the pressure ratio of the adsorption pressure to the desorption pressure being in the range of 1.5 to 6.0; wherein during periods of reduced product demand, the method includes reducing the top adsorption pressure to a range of 75% to 90% of the designed maximum operating pressure of the at least one vessel to reduce production capacity while achieving greater energy efficiency, wherein the at least one compressor is a positive displacement compressor, And wherein the top adsorption pressure of the at least one vessel is reduced while reducing pressure restrictions in the system by increasing the opening of product control valves, product making valves and / or equalization valves to maintain control of product flow rate.

2. The method of claim 1, wherein the system is a PSA or VPSA system configured to recover at least one gas selected from the group consisting of: O2, N2, CO2, H2, Ar or helium.

3. The method according to claim 2, wherein the intrinsic diffusivity of the adsorbent for the first component is equal to or greater than 4.0×10 -6 m 2 / sec, and the pressure ratio of the adsorption pressure to the desorption pressure is in the range of 1.5 to 3.5 for a VPSA system, and in the range of 3.0 to 6.0 for a PSA system.

4. The method of claim 2, wherein the feed gas is air, the first component is nitrogen and the second component is oxygen.

5. The method according to claim 4, wherein the adsorbent is an X-type zeolite having a SiO2 / Al2O3 ratio less than or equal to 2.5 and exchanged with Li, and / or a mixed cation zeolite.

6. The method of claim 1, comprising one or more adsorption beds connected in series or in parallel, wherein at least one adsorption bed is in a feed mode and the other adsorption beds are in a regeneration mode.

7. The method of claim 1, wherein the at least one compressor is driven by an associated motor, wherein the at least one compressor is a rotary lobe blower and the associated motor is an induction motor.

8. The method of claim 1, wherein the at least one compressor is a centrifugal feed compressor optionally including inlet guide vanes, wherein the top adsorption pressure of the at least one vessel is reduced by reducing the feed flow rate entering the vessel, wherein the reduced feed flow rate is achieved by reducing the compressor speed of the centrifugal feed compressor, or adjusting the angle of the inlet guide vanes of the centrifugal feed compressor, or a combination of both.

9. A method of operating an adsorption system for gas separation, the system comprising: a. at least one container containing at least one adsorption bed comprising at least one adsorbent, wherein the at least one adsorption bed is configured to be cyclically pressurized and depressurized during operation; b. at least one compressor configured to be driven by an associated motor, the associated motor comprising a direct drive high speed permanent magnet motor or a high speed induction motor designed for variable speed operation; and c. means for receiving a data signal of a condition in the system and for communicating with the high-speed induction motor driving the at least one compressor in response to the condition so that the at least one compressor can operate at a specified speed; and d. a product compressor configured to deliver a desired product pressure at an average capacity and a peak capacity, wherein during periods of reduced product demand, the method includes reducing the top adsorption pressure to a range of 75% to 90% of the design operating pressure of the at least one vessel to reduce production capacity while achieving greater energy efficiency, wherein the at least one compressor is a positive displacement compressor, And wherein the top adsorption pressure of the at least one vessel is reduced while reducing pressure restrictions in the system by increasing the opening of product control valves, product making valves and / or equalization valves to maintain control of product flow rate.

10. The method of claim 9, wherein the product compressor is a centrifugal compressor driven by a direct drive high speed permanent magnet motor or a high speed induction motor, and the speed and pressure ratio of the centrifugal compressor can be increased to achieve the desired product pressure and flow.

11. The method of claim 10, wherein the at least one compressor is a centrifugal feed compressor optionally including inlet guide vanes, wherein when product demand is lower than the average production demand of the system, the top adsorption pressure of the at least one vessel is reduced by reducing the feed flow rate entering the vessel, wherein the reduced feed flow rate is achieved by reducing the compressor speed of the centrifugal feed compressor, or adjusting the angle of the inlet guide vanes of the centrifugal feed compressor, or a combination of both.

12. The method of claim 11, wherein the centrifugal feed compressor and / or the product compressor is driven by a direct drive high speed permanent magnet motor or a high speed induction motor.

13. A method according to claim 12, wherein the associated motor of the centrifugal feed compressor and / or the product compressor communicates with at least one associated variable frequency drive, and the at least one associated variable frequency drive communicates with the device for receiving data signals, and wherein the associated motor of the centrifugal feed compressor and / or the product compressor communicates with at least one associated variable frequency drive, and the at least one associated variable frequency drive communicates with the device for receiving data signals.

14. The method of claim 13, wherein the means for receiving a data signal is configured to determine an outlet pressure / inlet pressure of the centrifugal feed compressor, and / or an outlet pressure / inlet pressure of the product compressor.

15. A method according to claim 14, wherein the device for receiving data signals transmits the speed of operation in response to the pressure ratio and inlet temperature of the centrifugal feed compressor and / or the product compressor to the associated high-speed permanent magnet motor or high-speed induction motor of the centrifugal feed compressor and / or the product compressor during operation.

16. The method of claim 15, wherein the associated high speed permanent magnet motor or induction motor is directly coupled to the centrifugal feed compressor such that the centrifugal feed compressor is capable of operating at or near a predetermined efficiency line.

17. The method of claim 16, wherein the predetermined efficiency line is a best efficiency line represented by a locus of points corresponding to peak efficiency points of a compressor operating curve at different speeds and process conditions of the centrifugal feed compressor.

18. The method of claim 9, wherein the system comprises a PSA or VPSA system.

19. The method of claim 18, wherein the PSA or VPSA system comprises a system for recovering at least one gas selected from the group consisting of: O2, N2, CO2, H2, Ar or helium.

20. The method of claim 19, wherein the system is a VPSA system, and the turndown energy efficiency of the VPSA process is improved by reducing the top adsorption pressure in the system, increasing the feed flow rate into the system, and / or reducing the cycle time.

21. The method of claim 20, wherein the turndown range is extended by directly driving a high speed direct drive VFD controlled multi-stage centrifugal compressor, employing inlet guide vanes, controlling the angle of the inlet guide vanes, varying the stage pressure ratio via different stage speeds, or any combination of the foregoing.

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