Patent Application
20050217481
The present invention relates to processes and equipment for gas purification, and more particularly to a gas purification method and apparatus using rotary contactors. More particularly, this invention relates to the use of rotary adsorbent contactors to remove impurities from a gas feed stream prior to compression of the gas feed stream.
It is often necessary to remove impurities from a gas stream. There are a variety of gases that require treatment prior to their use or further processing, including air and natural gas. Air plant purification, instrument air drying, and air brakes are a few important examples of processes in which air needs to have one or more impurities removed prior to further processing or use of the air. Natural gas may require removal of water and carbon dioxide. Other gaseous hydrocarbon streams may also require purification. Conventional air separation units for the production of nitrogen and oxygen by the cryogenic separation of air are basically comprised of a two-stage distillation column, which operate at very low temperatures. In addition to the desired products (e.g., nitrogen, oxygen, argon), the air that is used as a starting material for cryogenic processing contains impurities or undesirable components such as water vapor, carbon dioxide and hydrocarbon species. Due to the extremely low temperatures, it is essential that water vapor and carbon dioxide be removed prior to an air stream entering an air separation unit. These impurities must be removed before processing of feed air can be completed because the impurities interfere with continuous and efficient operation of the cryogenic equipment and present operational safety issues. If water and carbon dioxide are not removed, then the low temperature sections of the air separation unit may freeze necessitating a halt to production during which the frozen sections need to be warmed. It is generally recognized that in order to prevent the freezing of the air separation unit, that the content of water vapor and carbon dioxide in the compressed air feed stream must be less than 0.1 ppm and 1.0 ppm respectfully.
The current commercial methods for the purification of gases include reversing heat exchangers, temperature swing adsorption and pressure swing adsorption. In many instances, a chiller precedes the adsorption system to remove much of the moisture and reduce the resulting load on the adsorption system. This system then provides a dried, clean air stream to the plant. The cooled, compressed feed air is passed through an adsorbent material. This cooled air still needs to be cooled further to cryogenic temperatures before it is fed to a cryogenic separation system. Temperature swing adsorption (TSA) pre-purification works by removing impurities at relatively low temperatures, typically about 5° C. and regeneration is carried out at elevated temperatures, typically about 150° to 250° C. The amount of product gas required for regeneration is typically only about 12 to 15%, a significant improvement over reversing heat exchangers. However, TSA processes often require both refrigeration units to chill the feed gas and heating units to heat the regeneration gas. This results in undesirable energy usage as well as high capital costs.
Pressure swing adsorption (PSA) processes are an attractive alternative to TSA processes since in PSA processes both the adsorption and regeneration steps are carried out at ambient temperature. The PSA systems are generally the preferred technology, although the type of product and other considerations usually determine the choice of system. However, PSA processes usually do require substantially more regenerative gas (25 to 40% of the feed) than do TSA processes, which can be disadvantageous when high recovery of cryogenically separated products is desired. This disadvantage can be substantially reduced in a cryogenic plant which has a substantial waste stream comprising typically 40% of the feed. Such waste streams can be ideal for regeneration gas since they are free of water vapor and carbon dioxide and were to be vented in any case. However, there are high capital and energy costs associated with PSA systems. TSA systems are extremely effective at removing the major contaminants, such as water, carbon dioxide and most of the hydrocarbons from an air feed because such adsorbers usually employ strong adsorbents. Therefore, they are preferred for high purity applications. The strong adsorbents used in TSA processes, such as 5A or 13X zeolite require the large thermal driving forces available by TSA to affect adequate desorption. The operating adsorbate loadings and selectivities of the major contaminants on these strong adsorbents is such that carbon dioxide breaks through into the product stream before acetylene and most other hydrocarbons that are harmful to cryogenic air separation plant operation, such as the C3 through C8 hydrocarbons. The feed gas is usually chilled to minimize the water content of the feed, which in turn reduces the amount of adsorbent required. While the TSA process results in a relatively low purge to feed ratio, the inherent heating of the purge and chilling of the feed adds to both the capital and operating cost of the process. PSA prepurifiers use a near-ambient temperature purge to regenerate the adsorption beds. The reduced driving force that is available from pressure swing alone requires a weaker adsorbent (such as alumina), shorter cycles and higher purge to feed ratios compared to TSA processes in order to achieve adequate desorption of water and carbon dioxide contaminants. Typical purge to feed ratios are 40 to 60% in PSA prepurification. Unfortunately, weak adsorbents such as activated alumina are unable to sufficiently retain light hydrocarbons such as acetylene in a reasonable size bed and ethane breaks through into the product stream ahead of carbon dioxide. This leads to a potentially hazardous operating condition in a cryogenic air separation process. While the capital costs associated with a PSA prepurifier are lower than those of a TSA, the overall power requirement can be higher. In particular, blowdown or depressurization losses increase power consumption in the PSA prepurifiers. PSA units cycle much faster than TSA units, resulting in an increase in the frequency of blowdown loss steps. Accordingly, there remains a need for a system for prepurification that requires lower levels of capital, lowered energy costs and lowered use of gas that has been purified as a source of regeneration.
Another use for dry air is for use by equipment or machinery. The current practice is to first compress the air and then treat it to remove water and other contaminant vapors and gases. In this practice, a dryer containing the appropriate adsorbent is placed downstream from an air compressor and a portion of the compressed air is used as a regeneration medium for the dryer. In thermal swing adsorption (TSA) drying, a portion of the product gas is heated and then used as a regeneration medium. Similarly, in pressure swing adsorption (PSA), a portion of the product gas together with the adsorbent bed are opened to a lower pressure and the expanded gas carries away the contaminants. In both the TSA and PSA systems, the energy used in compressing a portion of the gas has been lost when that portion is used for regeneration instead of remaining with the bulk of the compressed air for use in equipment or machinery.
Rotary adsorbent contactors have been developed for several applications including heating and air conditioning as well as VOC concentration, prior to their destruction. They have been used in dehumidification or desiccant wheels, enthalpy control wheels and open cycle desiccant cooling systems.
Most applications of desiccant wheel technology, particularly in the heating and air conditioning field have focused on bulk drying of air where the humidity of the incoming air is reduced from near saturation by a factor of about 2. A relative humidity of about 80% at ambient temperature would be reduced in such a way that the water content is cut by a factor of 2 to 3. The result in such bulk drying applications is that there is an inevitable gain in sensible heat associated with passing air through an essentially adiabatic drying operation and the temperature rise further lowers the relative humidity of the product stream. However, desiccant wheels have not previously to the present invention been used in applications requiring very dry, pure air, such as air prepurification and instrument air drying.
U.S. Pat. No. 5,632,802 describes one system for drying air at ambient conditions prior to its entry into an air compressing machine. This system comprises desiccant bed adsorption units for removal of water prior to compression of the air.
U.S. Pat. No. 4,769,053 discloses a sensible and latent heat exchange membrane that comprises a gas permeable matrix. An inlet air stream flows in one direction, while the exhaust air stream flows in the opposition direction through different portions of the wheel. The wheel has a corrugated sheet material that contains an adsorbent powder.
In the field of VOC control rotary contactors have been used for concentration of the VOCs prior to their removal. In U.S. Pat. No. 6,080,227, a honeycomb rotor is used in the concentration of VOC. This rotor is described as having disposed thereon a cooling zone, a desorbing zone and an adsorbing zone. The rotor rotates and thus passes through each of the zones in turn.
U.S. Pat. No. 5,788,744 discloses a method of recirculating a portion of the desorption outlet gas in a rotary concentrator to reduce the amount of desorption gas which must be treated at a final processing step.
U.S. Pat. No. 6,051,050 describes a rotor for use in a PSA process for separation of components of a feed gas.
U.S. 2001/0027723 A1 discloses the use of an adsorbent material that is monolithic having a plurality of channels that are aligned in the direction of the flow of a gaseous mixture. This monolithic bed is used to separate the components of the gaseous mixture. This reference does not disclose the use of rotary adsorbent contactors for such applications.
The use of high surface area materials for use as adsorbents is well known in the art. High surface area activated carbon is one well known type of adsorbent, and it has extensive commercial application as an adsorbent. Among these high surface area materials which have gained considerable commercial use are the inorganic oxides. In particular silica gel, activated alumina, and zeolites are used as adsorbents.
Zeolites are crystalline aluminosilicates with complex three dimensional infinite lattices. While some commercially used zeolites are natural minerals, most commercial zeolite adsorbents are produced synthetically. They are normally synthesized containing cations from group IA or IIA of the Periodic Table, in particular sodium, potassium, magnesium, and calcium. Chemically, zeolites are often represented by the empirical formula:
M2/nOAl2O3.ySiO2.wH2O
whereby y is 2 or greater, n is the valence of the cation M, and w represents the water contained in the voids of the zeolite.
Zeolites are often classified by their crystal structure. The International Zeolite Association maintains a listing of known zeolite structure, and assigns the well known three letter designation for the structure. Commercially important zeolites include, zeolite A, described in U.S. Pat. No. 2,882,243, and given the designation LTA, and zeolite X described in U.S. Pat. No. 2,882,244, and zeolite Y, described in U.S. Pat. No. 3,130,007, both of which have the structure of the mineral faujasite, and have the designation, FAU, but with different ratios of silicon and aluminum in the framework lattice.
It is well known that the cations in the zeolite can be replaced by other cations by an ion exchange process. The affinity of a zeolite for a particular cation is known to vary with the structure, and the ratio of silicon and aluminum in the framework. The affinity of the zeolite for the cation determines the conditions needed to obtain the amount of exchange desired in the zeolite.
Many of these ion exchanged forms of zeolites are used as commercially. The potassium form of LTA, known as 3A because the pore opening of the zeolite is reduced to approximately 3 angstroms, is often used as an adsorbent. It has gained favor over the sodium form of LTA, known as 4A, in drying the air space between dual pane windows because unlike 4A, its reduced pore size will not allow 3A to adsorb air at low temperature. The calcium exchanged form of LTA, 5A, is favored in iso-normal paraffin separations where a slightly larger pore size improves performance.
Many ion exchanged forms of FAU are also known. DDZ-70 is a rare earth exchanged form of FAU available from UOP LLC, Des Plaines, Ill.
In the present invention, a gas is dried and otherwise treated at ambient pressure with rotary adsorbent contactors before it enters a compressor. Energy consuming components downstream of the compression stages are thereby eliminated. Also, in the case of air being treated, by lowering the dew point of the air before compression, the compressor produces air at dew points which meet or exceed the capabilities of current compressed air drying equipment.
The components that can be eliminated in connection with air drying operations include aftercoolers, moisture separators, compressed air dryers and oil/water separators. Aftercoolers, moisture separators and air dryers create pressure drops in a compressed air system. These pressure drops require energy to overcome losses. Energy is saved both through the elimination of components such as air cooled aftercoolers and air dryers and by the more efficient operation of air compressors through the use of dry compressing air.
Environmental benefits are also realized in elimination of refrigerated type compressed air dryers that use chlorofluorocarbon refrigerants which are damaging to the earth's ozone layer. A further advantage of the drying of the air prior to compression is that oil lubricated compressors may contaminate the gas flow with compressor oil. Moisture separators and refrigerated air dryers then produce condensate which is contaminated with this compressor oil. If this condensate is not eliminated properly or if an oil/water separator is not used to scrub the condensate of oil, the condensate can cause contamination such as to the ground and groundwater supplies. Also, the condensate can produce a burden on wastewater treatment facilities if the condensate is introduced into a sewage system. Since the air is dry when it leaves the compressor, no condensate is formed.
The present invention comprises at least one rotary adsorbent contactor to purify a gas stream. The number of rotary adsorbent contactors is dependent upon the particular application which determines the purity of gas that is necessary. At least one rotary adsorbent contactor is used with additional contactors added in certain applications.
One embodiment of the invention comprises a process of producing compressed gases comprising first removing impurities from a gas by passing said gas through at least one rotary adsorbent contactor in a direction parallel to an axis of rotation of said rotary adsorbent contactor wherein said rotary adsorbent contactor comprises an adsorbent material and then compressing said first gas.
Another embodiment of the invention comprises a system for drying and compressing gases, such as air or natural gas, comprising at least one rotary adsorbent contactor comprising at least one adsorbent material, connections to allow purified gas to flow from the rotary adsorbent contactor to a compressor.
In yet another embodiment, the invention comprises a method of purifying a gas comprising passing said gas through a plurality of rotary adsorbent contactors wherein said method comprises passing said gas through at least one rotary adsorbent contactor to remove moisture and to cool said stream, and a rotary adsorbent contactor to remove other impurities from said gas. Additional rotary adsorbent contactors may be used to achieve higher levels of purity.
Another embodiment of the invention comprises a process for ultra-purification of a feed stream containing one or more contaminant species, the process comprising passing a feed stream containing one or more contaminant species across a first continuously rotating rotary adsorbent contactor. The first rotary adsorbent contactor is regenerated by passing a suitable regeneration gas stream through a sector of the first rotary adsorbent after which the rotary adsorbent contactor is prepared for passage of said the stream by passing a suitable gas stream through a sector of the rotary adsorbent contactor, and then the feed stream is partially purified after passage through the rotary adsorbent contactor. Then the partially purified feed stream is passed to at least one feed sector of a second rotary adsorbent contactor to further purify the feed stream, with regeneration of the second rotary adsorbent contactor by passing a suitable regeneration gas stream through a sector of said second rotary adsorbent contactor. Following the regeneration of the second rotary adsorbent contactor, the second rotary adsorbent contactor is prepared for passage of the feed stream by passing a suitable gas stream through a sector of the second rotary adsorbent contactor, and the feed stream is further purified after passage through said rotary adsorbent contactor. The further purified feed stream is then passed to at least one feed sector of a third rotary adsorbent contactor and the third rotary adsorbent contactor is regenerated by passing a suitable regeneration gas stream through a sector of the third rotary adsorbent contactor and following regeneration of the third rotary adsorbent contactor, the third rotary adsorbent contactor is prepared for passage of the feed stream by passing a suitable gas stream through a sector of the third rotary adsorbent contactor.
In another embodiment of the invention is provided a process for producing a purified gas stream. In this process first the contaminants are adsorbed by passing a feed gas stream through a sector of a continuously rotating rotary adsorbent contactor, followed by regenerating the rotary adsorbent contactor by passing a regeneration gas stream through a second sector of the continuously rotating rotary adsorbent contactor then by preparing the rotary adsorbent contactor for adsorption of said contaminants from said feed gas stream, wherein said preparation is done by passing a preparation gas stream through a third sector of said continuously rotating rotary adsorbent contactor.
In accordance with the present invention, a rotary adsorbent contactor (also known as an adsorbent wheel or desiccant wheel in some applications) is employed to dry, purify or separate components from a gas stream. A continuous system is thereby provided for the purification of a gas stream that is to be compressed or used in other applications.
The integration of the upstream dryer and the compressor offers an opportunity to reduce energy consumption associated with the compression and purification of gas. In many applications including air plant prepurification and instrument air drying, two or more stages of compression are normally used to achieve the final product pressure required by the application. Gas inter-coolers are typically employed to manage the temperature of the product gas. Use of the energy, ordinarily released to the ambient or to cooling water, as a heat source for regeneration is a means of saving substantial quantities of energy. Prepurification of air prior to air separation by cryogenic means is a particularly useful application of the present invention.
In those applications in which air is the gas to be purified and compressed, at least one and preferably at least two rotary adsorbent contactors may be employed with different adsorbent materials and different process schemes for operation. There are several different types of rotary adsorbent contactors that may be used in the present invention. One type is a rotary heat and mass exchanger that treats incoming air continuously using a countercurrent stream of gas that is a cleaner, dryer and/or cooler gas than the incoming air to be treated. Such a device has application in managing the load on a downstream dryer or purifier. Reduction of either contaminant level or temperature relative to the incoming air stream can provide a benefit. Reductions in water content by a factor of two or more can be achieved by use of this heat and mass exchange device. Such devices are referred to in the HVAC industry as “enthalpy control wheels”. A second type of device is a deep dehumidification wheel employing an adsorbent with an isotherm shape that is ideally suited for water removal. One or more wheels of this nature can be used in series to dry air to extremely low dew points. When extremely dry air is needed, it may be advantageous to use two dehumidification wheels with a heat exchange device interposed in between the two wheels in order to remove a portion of the sensible heat gain caused by drying of air in the first wheel. If the load reduction by the first wheel is sufficient, there is generally no need to cool the product of the second rotary adsorbent contactor. A third type of device used in the present invention is a rotary adsorbent contactor where the adsorbent is chosen for its affinity towards some contaminant other than water. An example is a rotary adsorber using an adsorbent that is useful in removing various inorganic or organic contaminants. Such a rotary adsorbent contactor may be used to take the product from the dehumidification rotary adsorber and further remove traces of water and carbon dioxide or other contaminants. While some of the adsorbents may be quite selective for water, it is also desirable to select adsorbents that have a pronounced affinity and capacity for inorganic contaminants when the water content of the stream is low. N2O, NO2, SO2, H2S, HCl are among the contaminants that can be trapped by rotary adsorbent contactors.
In one embodiment of the invention, a gas separation system is used for obtaining oxygen and nitrogen products employing a rotary adsorbent contactor system for air prepurification. A series of three rotary contactors is preferred, including an enthalpy rotary adsorbent contactor, a deep desiccant rotary adsorbent contactor and a 13X rotary adsorbent contactor. The enthalpy rotary adsorbent contactor removes most of the moisture and cools the air stream into the rotary adsorbent contactor. The deep desiccant rotary adsorbent contactor serves to further lower the dew point of the air flow and the 13X rotary adsorbent contactor removes carbon dioxide as well as virtually all of the remaining moisture. The feed air is passed through an enthalpy rotary adsorbent contactor, deep desiccant rotary adsorbent contactor and 13X rotary adsorbent contactor in series. The feed air then passes through a series of heat exchangers and coolers to be brought to the appropriate temperature for separation. Nitrogen and oxygen product is separated cryogenically at this point. A portion of the nitrogen product returns to cool the rotary adsorbers to operating temperatures. A regenerative gas flow passes countercurrently through a sector of the rotary adsorbent contactors to desorb carbon dioxide and water.
The adsorbent used in the present invention is at least one adsorbent selected from the group consisting of rare earth exchanged faujasite, calcium exchanged faujasite, H+ exchanged faujasite, silica gel, alumina and mixtures thereof.
The drawings illustrate a number of embodiments of the invention.
In
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A single rotary adsorbent contactor was assembled. It had an outside diameter of 250 mm with a 64 mm hub. The depth of the wheel in the flow direction is 200 mm. The adsorbent media contained UOP MOLSIV DDZ-70. The adsorbent media is nominally 70 wt-% of the DDZ-70 adsorbent, the balance being fibrillated polyaramid fibers and a small amount of organic binder. The adsorbent media is a corrugated structure having cells running parallel to the axis of rotation. The structure has an open face area fraction of about 72%. The adsorbent media density as flat stock has a characteristic density when activated of about 0.83 grams of media per cubic centimeter. The apparent density of the adsorbent portion of the rotary contactor is about 0.224 gram/cubic centimeter.
A laboratory test facility was constructed. A blower capable of supplying approximately 4248 standard liters per minute (SLPM) (150 standard cubic feet per minute (SCFM)) at approximately 5 inches of water column head pressure was provided.
A variable damper was used to control the flow. The outlet of the blower and its damper was directed into a humidistat that was used to introduce moisture into the air stream.
The flow rate, temperature, static and dynamic pressure and moisture content of the air stream from the humidistat were measured and controlled.
The rotary contactor of Example 1 was mounted inside a cassette that encloses the contactor, the drive motor, and provides for ducts that direct flows to and away from the faces of the wheel. On the feed air supply side of the wheel one partition separates the feed air from the combined regeneration and cooling waste products. The face areas allotted to these parts of the face are approximately equal.
On the product side of the contactor, the face of the wheel is divided into three chambers. Approximately half the product face of the wheel directs the gross product of the wheel to a cooler and the remaining half is divided equally between a regeneration portion and a cooling portion.
The cassette was mounted immediately downstream of the humidistat and its associated instrumentation.
An air cooler was introduced immediately downstream of the cassette.
Flow meters, temperature sensors and humidity meters were introduced into the test facility to enable us to close mass and heat balances on the adsorbent contactor. Further heaters and control valves were introduced to allow for great flexibility in directing and controlling air flows to and from the wheel.
The test facility of Example 2 with the rotary adsorbent contactor of Example 1 was run with the conditions shown in Table I. In the row labeled Observation number 39, an air flow of 2832 SLPM (100 SCFM) was introduced into the humidistat and subsequently into the adsorber section of the contactor. In this example the regeneration flow was ambient air with approximately the same conditions as the stated feed air with the exception that the regeneration air was heated to 151° C. (304° F.). This air was flowing in a direction counter-current to the feed air. The cooling air was taken as a minor portion of the gross product of the adsorbing sector of the contactor. The rotation rate of the adsorbent contactor was 37.89 revolutions per hour. In this example, the contactor removed a major portion of the water contained in the feed air. The final product contained 748 parts of water by volume per million parts of water containing air (ppm(v/v)). Water content of the air was reduced by a factor of 13.25. A net product of 1965 SLPM (69.4 SCFM) was obtained.
Example 4 shown in Table I in the row labeled observation 43 also used fresh airs but at an increased flow rate. The contactor's rotation rate was reduced to about 19 revolutions per hour and the temperature of the regeneration air was reduced to 141° C. (286° F.). In this example, the contactor produced about the same net product flow rate but now the moisture content was reduced to 345 ppm(v/v). This represents a reduction of the water content of the air by a factor of 25.79.
Examples 5-7, shown in Table I in row labeled observation 2, 1, and 9 respectively, used varying feed flows, each at increased moisture content relative to Examples 3 and 4. The regeneration and cooling flows for Example 5 were both taken as minor portions of the gross product of the adsorber. With the use of partial product for regeneration and cooling we were able to reduce the regeneration temperatures to about 139° C. (282 F). Final products varied as the regeneration and cooling flows were varied.
In these examples we added a second contactor, also constructed using MOLSIV™ DDZ-70 with all properties the same as the contactor of Example 1 with the exception that the depth in the flow direction was 400 mm.
Examples 8-10 are provided as demonstrations of the use of two rotary adsorbent contactors in series. In each of these examples we ran the first contactor at 19 revolutions per hour with the indicated feeds shown in Table 2 in rows labeled 62, 53, and 52. The regeneration air was fresh air at conditions essentially the same as the stated feed air. The cooling air for the first contactor was a minor portion of the product from the first adsorber. The gross product of the contactor is the feed air minus the cooling air.
The product air was recorded as 631, 1150 and 1723 ppm (v/v) for Examples 8-10. In each example the product air was cooled back to a condition close to the feed air.
The cooled gross product of the first contactor as shown in Table 2 was fed to the adsorber section of the second contactor. The conditions of operation are stated in Table 3 in the rows labeled 62, 53 and 52 respectively. The second contactor used minor portions of the product air of the second contactor for both cooling and regeneration.
The driers operated according to these examples produced net product flows of 1359 to 1776 SLPM (48 to 62.7 SCFM) and yielded water contents of 1, 5, and 10 ppm (v/v) respectively.
These examples demonstrate that two rotary adsorbent contactors operated in series can achieve extremely low water contents.
Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, but that various changes and modifications can be effected therein by those skilled in the art without departing from the scope or spirit of this invention.
