Method and apparatus for purifying ammonia

Abstract: A method of producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the method comprising introducing the feed stream into a membrane selected to form a permeate stream enriched in the ammonia and the moisture; forming an intermediate product stream by adsorbing moisture from the permeate stream; partially condensing the intermediate product stream to produce liquid ammonia lean in light components of the other impurities; and introducing a stream of the liquid into a product pressure vessel; partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; and forming the purified, pressurised ammonia stream from the pressurised ammonia vapour.

Description: This invention relates to a method and apparatus for purifying ammonia in which an ammonia-containing stream is passed through an adsorption unit to remove moisture and a membrane unit to concentrate the ammonia. More particularly, the present invention relates to such a method and apparatus in which a stream, formed by processing the ammonia-containing stream in the adsorption and membrane units, is partially condensed and then partially vaporised to a delivery pressure to form a pressurised product stream lean in heavy and light impurities. There are many industrial processes which require ammonia of ultra-high purity. Typically, an ammonia feed is purified to produce a product having about 1 ppm moisture, heavy impurities at about a 0.5 ppm level, and light impurities at about 0.5 ppb. Heavy impurities comprise mobile ions such as sodium and potassium, light metals such as aluminium or calcium, such heavy metals as iron, nickel or chromium, and heavy hydrocarbons. Light impurities can include nitrogen and oxygen. Purification of the ammonia feed is effected by distillation. However, the problem inherent in distillation is that it is conducted at low pressure and thus, the product stream must be recompressed. The recompression presents a risk of contamination from lubricants and the like that are used in connection with the compressor: Distillation also requires a significant amount of energy input as it is operated at low-temperature. The invention generally provides a process of purifying an ammonia-containing stream in which compressors and the like are not used after the ammonia is purified. In accordance with the invention, there is provided a method of producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the method comprising introducing the feed stream into a membrane selected to form a permeate stream enriched in the ammonia and the moisture; forming an intermediate product stream by adsorbing moisture from the permeate stream; partially condensing the intermediate product stream to produce liquid ammonia lean in light components of the other impurities; and introducing a stream of the liquid into a product pressure vessel; partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; and forming the purified, pressurised ammonia stream from the pressurised ammonia vapour. In another aspect of the invention, a method may be conducted in which moisture may be adsorbed prior to the use of the membrane system. In this aspect, the moisture is adsorbed from the feed stream to produce an intermediate product stream and the intermediate product stream is introduced in the membrane to form a permeate stream enriched in the ammonia. Thereafter, the permeate stream is partially condensed to produce liquid ammonia lean in light impurities. A stream of the liquid ammonia is introduced into a product pressure vessel and partially vaporised to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy impurities. The purified, pressurised ammonia stream is formed from the pressurised ammonia vapour. In another aspect, the invention provides an apparatus for producing a purified, pressurised ammonia stream from the feed stream comprising ammonia, moisture and other impurities. In accordance with this apparatus, a membrane unit is provided. The membrane unit has at least one membrane selected to form a permeate stream enriched in the ammonia and the moisture. An adsorption unit is provided to adsorb moisture from the permeate stream. A product condensation vessel is used to partially condense the intermediate product stream to produce liquid ammonia lean in light impurities. A product pressure vessel is connected to the product condensation vessel to receive a stream of the liquid and partially to vaporise the liquid to a delivery pressure, thereby to form pressurised ammonia lean in the heavy impurities. The product pressure vessel has an outlet for forming the purified, pressurised ammonia stream from the pressurised ammonia vapour. In yet another aspect, the invention provides an apparatus in which the adsorption unit first adsorbs the moisture from the feed stream to produce an intermediate product stream. A membrane unit is provided. The membrane unit has at least one membrane and is selected to form a permeate stream enriched from the ammonia in the intermediate products stream. A condensation unit is provided for partially condensing the permeate stream to produce a liquid lean in light impurities. A product pressure vessel is connected to the condensation unit to receive a stream of the liquid for partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean and heavy impurities. The product pressure vessel has an outlet to form the pressurised ammonia stream from the pressurised ammonia vapour. The condensation followed by purification constitutes both a purification step and a repressurisation step without the use of a mechanical compressor and the potential contamination that could be introduced thereby. As used herein and in the claims, light impurities include, nitrogen, oxygen, carbon monoxide, carbon dioxide, and light hydrocarbons such as methane and heavy impurities include the metals and heavy hydrocarbons. DD-A-159 259 (Veb Leuna-Werke) discloses the elimination of low-boiling point hydrocarbons and water by two adsorption steps, followed by removal of high-boiling point hydrocarbons by fractional condensation. Chemical Abstracts Accession number 123:117586 (Xu et al) discloses the concentration of ammonia by the separation of water therefrom by means of a membrane and subsequently the drying of the ammonia by absorption with a liquid absorbent, for example sulphuric acid, or obtained as a product stream b y condensation. The present invention, however, defines steps of adsorption and then partial evaporation. Other impurities may also be treated. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings in which: Figure 1 is a schematic view of an apparatus for carrying out a method in accordance with the invention; Figure 2 is an alternative embodiment of Figure 1 for carrying out a method in accordance with the invention; Figure 3 is an alternative embodiment as applicable to both Figures 1 and 2. The drawings show alternative embodiments of the invention. In order to avoid duplication, reference numbers for the same elements have been repeated in the various Figures. With reference to Figures 1 and 2, an apparatus 1 and an apparatus 2 are illustrated for carrying out methods in accordance with the invention. Generally, in both apparatus 1 and 2, a feed stream 10 of ammonia is compressed by a compressor 12 and filtered by a filter 14. The feed stream 10 has a typical impurity content of no greater than about 100 ppm of gaseous impurities and also, up to about 100 ppm of metallic impurities. Ammonia, and/or ammonia and moisture (depending on the particular embodiment set forth in Figures 1 and 2) in the feed stream 10 is concentrated in a membrane unit 16 and moisture is removed in an adsorption unit 18 to purify the ammonia. The stream resulting from actions of the membrane unit 16 and the adsorption unit 18 is partially condensed in a product condensation vessel 20 and is then partially vaporised in a product pressure vessel 22 to a delivery pressure. The condensation produces ammonia liquid lean in light impurities and the vaporisation produces ammonia vapour lean in heavy and light impurities. The vaporisation within a fixed volume of the product pressure vessel 22, raises the pressure of the ammonia to a delivery pressure without subsequent compression by a compressor and thereby produces a product stream 34. With specific reference to Figure 1, the feed stream 10 after having been compressed by the compressor 12 is introduced in to the filter 14, which is preferably designed to remove particles of above about 0.1 µm in size. It is possible however that filter 14 could be positioned upstream of the pump 12. Thereafter, the feed stream 10 and a recirculation stream 24, which will be discussed in more detail below, is introduced into the membrane unit 16. The membrane unit 16 is designed to produce a permeate stream 26 in which moisture and ammonia have been concentrated and a rejected stream 28 concentrated in carbon dioxide, carbon monoxide, oxygen, argon, hydrogen, nitrogen, methane and the like. The membrane unit 16 can consist of one or more membranes such as those formed from a copolymer of tetrafluoroethylene and perfluro 3, 6-dioxa-4-methyl-7-octene-sulphonic acid. Preferably, such a membrane can be prepared from NAFION (Trade Mark) polymer obtained from E.I. du Pont de Nemours and Company of Wilmington, Delaware, United States. The membrane unit 16 can be a single stage or a known multiple stage unit of the type in which a feed is further compressed and recirculated so that a permeate or retentate stream passes through several of cascades of membranes. The permeate stream 26 passes through the adsorption unit 18 which, for example, can be designed to operate in accordance with a temperature-swing adsorption cycle. In this regard, the adsorption unit 18 can consist of multiple beds of adsorbent which can contain either alumina or zeolites such as the 5A or 13X to remove moisture and any residual carbon dioxide. In a temperature-swing adsorption process, beds of adsorbent are regenerated and operated so that each adsorbent bed is subjected to adsorption phase in which the bed is on-line and adsorbing moisture and residual carbon dioxide. While a previously regenerated bed is brought on-line, the current on-line bed is then brought off-line for regeneration. Although not specifically illustrated, but in a known manner and through the use of well-known valve trees, during the regeneration, the bed is first vented to atmosphere, from its inlet, to initiate desorption of the previously adsorbed ammonia. Thereafter, the bed is purged with a purge stream to remove ammonia from the void spaces within the adsorbent bed. A hot purge stream follows next to drive out moisture that has previously been adsorbed by the adsorbent. A product purge can then be used to drive out residual constituents of the cold and hot purge streams. The final step is to close off the bottom of the adsorbent bed and repressurise the bed with product. In the illustrated embodiment, a cold nitrogen stream 30 is then introduced into the bed in the purge step discussed above. This cold nitrogen is recovered as a purge stream 32. Thereafter, the cold nitrogen stream 30 is heated and introduced as a hot purge stream. The hot purge stream drives moisture out of the adsorbent and, although not illustrated, may be sent to a scrubber to remove any residual ammonia before being vented. Part of the ammonia product stream 34 can be recirculated as a product purge stream 36 and introduced into the temperature-swing adsorption unit to drive out nitrogen. This produces an ammonia product purge stream that is also recovered as the purge stream 32. This action is controlled by a valve 38. The inlet valve to the bed being regenerated is then closed so that product purge stream 36 repressurises the bed to operational pressure and the bed can be subsequently brought on-line. A valve 40 can be opened to introduce purge stream 32 into a the purge stream condensation vessel 42 . The purge stream condensation vessel 42 can be a container or vessel sitting within a carbon dioxide bath or any other low-temperature source. The purge stream 32 partially liquefies within the purge stream condensation vessel 42 to separate out the nitrogen as a vapour. The residual of liquid ammonia (designated by reference number 44) contained within the purge stream condensation vessel 42 is also lean in light components of the impurities. A liquid stream 46 formed from such residual liquid, under control of a valve 48, is then introduced into the product condensation chamber 20 which again can be a container or pressure vessel located within a carbon dioxide bath or other suitable low-temperature source. An intermediate product stream 19 is introduced into the product condensation vessel 20 along with liquid ammonia stream 46. Liquid ammonia is then introduced as a liquid stream 50, controlled by a valve 52, into product pressurisation vessel 22 where it partially vaporises to produce ammonia vapour. Part of the ammonia vapour can be recirculated as a recirculation stream 54 (by opening a valve 56) back to condensation vessel 20 for further purification by partial condensation. The recycle stream 24 (previously mentioned) is composed of ammonia liquid that has been formed within product vaporisation vessel 22. Upon the opening of a valve 58, the recycle stream 24 is vaporised via a vaporiser 60 and then recirculated back to membrane unit 16 for further purification. With additional reference to Figure 2, the intermediate product stream 19 is produced from the adsorption unit 18 and hence has had moisture removed down to levels of approximately below 1 ppm. Recovered liquid ammonia 44 from the purge stream 32 can be vaporised within a vaporiser 47 and combined with the intermediate product stream 19 The combined stream is then introduced into the membrane unit 16. For either of the embodiments disclosed above that the recovery of ammonia from the purge stream 32 is optional. The resultant permeate stream 26 is introduced into the condensation vessel 20 by opening a valve 62. Although not illustrated, a recycle stream, such as the recycle stream 24 illustrated in Figure 1, can be recycled back to the membrane unit 16. However, the use of a recycle stream is optional for either of the embodiments described herein. The product purge stream 34 can come either from the final product 34 (as illustrated), or from the intermediate product stream 19. In all cases, one or more product and feed tanks may be used for feed and product storage during the purification process. With additional reference to Figure 3, the embodiment shown in Figure 2 (or in Figure 1) can be modified by provision of a valve 66. The valve 66 can be opened so that a liquid stream 64 is recirculated back to the product condensation vessel 20 or sent to drain. Valves 68 and 70 control the flow direction for such purposes. The advantage of such an embodiment is that the recirculated liquid partially condenses and then partially vaporises again to increase product purity. In all of the embodiments discussed above, a packing material may be installed in the product condensation vessel 20 and/or the product pressure vessel 22 to enhance purification efficiency. Another point is that the pressure generation mechanism can also be used to replace feed compressors such as compressor 12 to produce a feed with the required pressure for temperature-swing adsorption or membrane operation.

Claims: A method of producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the method comprising: introducing the feed stream into a membrane selected to form a permeate stream enriched in the ammonia and the moisture; forming an intermediate product stream by adsorbing moisture from the permeate stream; partially condensing the intermediate product stream to produce liquid ammonia lean in light components of the other impurities; and introducing a stream of the liquid into a product pressure vessel; partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; and forming the purified, pressurised ammonia stream from the pressurised ammonia vapour. A method of producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the method comprising: adsorbing the moisture from the feed stream to produce an intermediate product stream; introducing the intermediate product stream into a membrane selected to form a permeate stream enriched in the ammonia; partially condensing the permeate stream to produce liquid ammonia lean in light components of the other impurities; and introducing a stream of the liquid into a product pressure vessel; partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; and forming the purified, pressurised ammonia stream from the pressurised ammonia vapour. A method according to Claim 1 or Claim 2 in which the moisture is adsorbed in adsorbent beds subjected to an out-of-phase cycle so that while each of the adsorbent beds is adsorbing the moisture, at least one other bed is off-line and being regenerated by subjecting such other bed to the steps of nitrogen purging to help remove ammonia from void spaces in adsorbent contained within the adsorbent beds, thereby to form a purge product stream comprising nitrogen and ammonia, heated nitrogen purging to cause desorption of the moisture, thereby to regenerate the adsorbent, a product purge with a product purge stream to purge nitrogen introduced by the nitrogen and heated nitrogen purging steps, and a repressurisation step to bring the at least one other bed up to operating pressure so that it can be brought on-line to adsorb moisture; and the product purge stream is formed from part of the purified, pressurised ammonia stream. A method according to Claim 3 in which the purge product stream is cooled so that the ammonia contained therein is liquefied and separated from the nitrogen; and a liquid ammonia stream is introduced into the liquid ammonia condensed from the intermediate product stream. A method according to Claim 3 in which the purge product stream is cooled so that the ammonia contained therein is liquefied and separated from the nitrogen; and a liquid ammonia stream is vaporised and introduced into the membrane. A method according to any preceding claim in which the part of the liquid ammonia condensed from the intermediate product stream is recycled as a recycle stream back to the membrane. A method according to any preceding claim in which part of the pressurised ammonia vapour is reliquefied along with the intermediate product stream. Apparatus for producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the apparatus comprising: a membrane unit having at least one membrane selected to form a permeate stream enriched in the ammonia and the moisture; an adsorption unit to adsorb moisture from the permeate stream; a product condensation vessel to partially condense the intermediate product stream to produce a liquid lean in light components of the other impurities; and a product pressure vessel connected to the product condensation vessel to receive a stream of the liquid for partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; the product pressure vessel having an outlet of for forming the purified, pressurised ammonia stream from the pressurised ammonia vapour. Apparatus for producing a purified, pressurised ammonia stream from a feed stream comprising ammonia, moisture and other impurities, the apparatus comprising: an adsorption unit having an adsorbent to adsorb the moisture from the feed stream to produce an intermediate product stream; a membrane unit having at least one membrane selected to form a permeate stream enriched in the ammonia from the intermediate product stream; a condensation unit for partially condensing the permeate stream to produce a liquid lean in light components of the other impurities; and a product pressure vessel connected to the condensation unit to receive a stream of the liquid for partially vaporising the liquid to a delivery pressure, thereby to form pressurised ammonia vapour lean in heavy components of the other impurities; the product pressure vessel having an outlet to form the purified, pressurised ammonia stream from the pressurised ammonia vapour. Apparatus according to Claim 8 in which the adsorption unit has adsorbent beds subjected to an out-of-phase cycle so that while each of the adsorbent beds is adsorbing the moisture, at least one other bed is off-line and being regenerated by subjecting such other bed to the steps of nitrogen purging to help remove ammonia from void spaces in adsorbent contained within the adsorbent beds, thereby to form a purge product stream comprising nitrogen and ammonia, heated nitrogen purging to cause desorption of the moisture, thereby to regenerate the adsorbent, a product purge with a product purge stream to purge nitrogen introduced by the nitrogen and heated nitrogen purging steps, and a repressurisation step to bring the at least one other bed up to operating pressure so that it can be brought on-line to adsorb moisture; and the adsorption unit is connected to the outlet of the product pressure vessel so that the product purge stream is formed from part of the purified, pressurised ammonia stream.

Inventors: ATHALYE ATUL M, JI WENCHANG, SADKOWSKI PIOTR J, SHIRLEY ARTHUR I, ATHALYE, ATUL M., JI, WENCHANG, SADKOWSKI, PIOTR J., SHIRLEY, ARTHUR I
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1999
Main classification: C01C1
IPC codes: C01C1