| Gas purification apparatus and process -> Monitor Keywords |
|
|
  Gas purification apparatus and processRelated Patent Categories: Chemical Apparatus And Process Disinfecting, Deodorizing, Preserving, Or Sterilizing, Chemical Reactor, Waste Gas Purifier, With Heat Exchanger For Reaction Chamber Or Reactants Located ThereinGas purification apparatus and process description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060210455, Gas purification apparatus and process. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a division of our copending application Ser. No. 09/930,219, filed Aug. 16, 2001. FIELD OF THE INVENTION [0002] The present invention relates to purification of industrial gases such as carbon dioxide, helium and argon, and especially to removal of hydrocarbon and/or oxygen by catalytically assisted techniques. BACKGROUND OF THE INVENTION [0003] Industrial gases are often required to meet purity specifications. In order to meet these specifications, various impurities must be removed from the gases. Catalytic combustion plays a role in the removal of many impurities. The use of a catalyst allows the combustion to proceed at temperatures lower than would otherwise be necessary, although a temperature of as high as 1000.degree. F. may still be required. [0004] There are numerous examples of catalytic combustion applications in gas purification. It is used to remove hydrocarbon contamination from CO.sub.2. Hydrocarbons removed in this manner include ethane, benzene, methanol, ethanol, and acetaldehyde. Oxygen is added to the CO.sub.2 stream if necessary, and catalytic combustion converts the hydrocarbons into CO.sub.2 and water. The water is easily removed in downstream dryer beds. Oxygen can also be removed from CO.sub.2 by adding hydrogen to the CO.sub.2 stream and passing it over a catalyst to form water. This is employed in point-of-use purifiers where low oxygen levels are required. Hydrogen is removed from helium by this technique because the two gases are difficult to separate by other means such as distillation. Oxygen is added to the helium and catalytic combustion converts the hydrogen to water. The water and excess oxygen are easily removed from the helium. Catalytic combustion is also used in argon purification. Because oxygen and argon are difficult to separate by distillation, hydrogen is added to the argon stream and combusted with the oxygen over a catalyst to form water. The water is easily removed from the argon. [0005] Heat management is very important in all the catalytic combustion applications listed above. Temperatures of 500-1000.degree. F. are required for the catalytic combustion of hydrocarbons in CO.sub.2. The process would not be economical if energy had to be expended to heat the CO.sub.2 to these temperatures. Heat must be recovered from the hot gas and transferred to the incoming gas to reduce the amount of energy required to heat it and to make the process cost-effective. Heat is produced during the combustion of contaminants in all catalytic combustion applications. This heat is used to preheat the incoming gas when the combustion takes place at elevated temperatures, but it must be removed from the gas stream in all cases to allow for further downstream processing of the gas. [0006] Catalytic combustion is a well-known technique for removal of impurities from gases, and many examples of it exist in the prior art. It is normally referred to as catalytic oxidation when hydrocarbons and/or hydrogen are removed from the gas by reaction with oxygen to form CO.sub.2 and water in the case of hydrocarbons and water alone in the case of hydrogen. The combustion process is referred to as "deoxo" when oxygen is removed from the gas by reaction with hydrogen to form water. [0007] A generic catalytic oxidation system is described in the literature (Kohl and Nielsen, 1997) for use in removing volatile organic compounds from an air stream. The catalytic oxidation system consists of three unit operations: a heat exchanger, a burner, and a catalyst bed. The air that is to be purified first passes through one side of the heat exchanger where it is heated by indirect contact with hot gas leaving the catalyst bed. The preheated air then flows to the catalyst bed where its temperature is raised further by mixing it with hot combustion gases from the burner. The hot mixture has a temperature high enough to allow oxidation reactions to occur over the catalyst. The hot air flows across the catalyst where the volatile organic compounds react with oxygen to form CO.sub.2 and water, which are not harmful pollutants. Heat is released by this reaction, and the temperature of the air stream increases somewhat. The hot purified air exits the catalyst bed and flows into the heat exchanger where it is cooled by indirect contact with the incoming air. [0008] The amount of heat that must be added to the air by combusting fuel in the burner is determined by three factors: the amount of combustible contaminants in the air, the efficiency of the heat exchanger, and the amount of heat loss between the unit operations. If the air contains more contaminants, more heat will be generated from their combustion. If enough contaminants are present, no heat will need to be added. Because of inefficiencies in gas-to-gas heat transfer, it is impossible to recover all the heat from the purified air leaving the catalyst bed into the incoming contaminated air. The amount of heat recovery is increased by using a larger and more costly heat exchanger or more than one heat exchanger. Any heat lost between the catalyst bed and the heat exchanger, between the heat exchanger and the heater, and between the heater and the catalyst bed will increase the amount of energy required from the heater. The cost of heating the air is reduced by minimizing the heat loss. [0009] The catalyst in the system described by Kohl and Nielsen often consists of a platinum group metal deposited on an alumina support. The support is either in the form of pellets that are arranged in a packed reactor bed or in the form of a monolithic structure whose passages are coated with the catalyst material. Older designs used the pellet catalysts exclusively, but more modern systems often employ monoliths. [0010] FIG. 1 shows one kind of previously known system for CO.sub.2 purification. Hydrocarbons in the impure CO.sub.2 feed 101 are oxidized to CO.sub.2 and water over a catalyst. The catalytic oxidation system again consists of three unit operations: a heat exchanger 102, a heater 103, and a catalyst vessel 104, all of which are distinct units connected by suitable piping. The CO.sub.2 flows through the shell side of a shell and tube heat exchanger 102 for preheating, through a supplemental heater 103 for heating to reaction temperature, through the catalyst vessel 104 for reaction of the hydrocarbons, and through the tube side of the heat exchanger 102 for cooling. The purified CO.sub.2 stream 107 continues on for further processing in the CO.sub.2 plant. [0011] If the CO.sub.2 contains enough combustible contaminants, its temperature will rise to such a degree that the incoming gas is preheated to too great a temperature. In this case some of the incoming gas is bypassed around the heat exchanger via bypass line 105 controlled by bypass valve 106. The cool bypass gas is mixed with the preheated gas before entering the reactor to establish the correct initial combustion temperature. The supplemental heater 103 is normally an electric heater. The catalyst comprises a platinum group metal deposited on alumina in pellet form. Operation of a catalytic oxidation unit in a CO.sub.2 plant differs from that of the system described in Kohl and Nielsen in two major ways. Oxygen must be added to the CO.sub.2 so that combustion can take place over the catalyst, while sufficient oxygen is always present during air purification. CO.sub.2 purification takes place at a pressure of approximately 300 psig, while air is processed at atmospheric pressure. [0012] The use of catalytic oxidation for hydrocarbon removal from CO.sub.2 is described in U.S. Pat. Nos. 3,317,278 and 4,460,395. In both patents the catalytic oxidation system consists of separate vessels for the heat exchanger, heater, and catalyst vessel. [0013] Catalytic oxidation has previously been used in helium plants for removal of hydrogen from helium. Hydrogen content of approximately 2% is removed by adding oxygen to the helium and using it to oxidize the hydrogen. The catalytic oxidation arrangement for helium purification is slightly different than that described for CO.sub.2 and air. Hydrogen reacts with oxygen at a much lower temperature than do hydrocarbons, so heat recovery is not an important issue. Heat must be rejected from the purified helium instead so that it can be processed further. The helium stream containing hydrogen and oxygen is preheated slightly to approximately 150.degree. F. It passes into a catalyst vessel where a portion of the hydrogen reacts with oxygen. Complete oxygen removal is not possible over this catalyst because the temperature rise due to combustion would be too great and the catalyst would be destroyed. The hot partially purified helium stream flows through a heat exchanger where it exchanges heat with a cooling fluid. It then passes to another catalyst bed where the remainder of the hydrogen reacts. Later processing steps cool the helium further and remove the water formed during hydrogen oxidation. The catalytic oxidation portion of the helium plant consists of five separate vessels: a heater, two catalyst vessels, and two heat exchangers. The catalyst consists of pellets of a platinum group metal deposited on alumina. [0014] Catalytic combustion has also been used in the removal of oxygen from argon. It is referred to as "deoxo" in this case. Liquid argon from the crude argon column containing approximately 1.5% oxygen is vaporized and hydrogen is added to the gas stream. The mixture is compressed and passed to a catalyst bed where the hydrogen and oxygen react. The gas is heated by this reaction and it is cooled in a heat exchanger downstream of the reactor. The water formed in the reaction is removed from the argon stream downstream of this heat exchanger. The deoxo system consists of two separate vessels: the catalyst vessel and the heat exchanger. The catalyst consists of alumina pellets with a platinum group metal deposited on them. [0015] The use of a deoxo unit for removal of oxygen from argon is described many places in the literature. An article by Latimer (February 1967) is one example. U.S. Pat. No. 6,168,774 describes a compact deoxo system for removal of oxygen from nitrogen at a small scale. [0016] Because the cost of heating the inlet gas for a catalytic combustion system can be so expensive, heat integration of these systems has been reported in the literature. Eigenberger and Nieken (January 1994) discuss the advantages to be gained by better integration of the catalyst bed with the heat recovery mechanisms. They concentrate on regenerative heat recovery for the most part. Regenerative heat recovery involves direct contact of the fluids being cooled and heated with a large thermal mass. One example of regenerative heat recovery is the flow of hot gas over refractory material to heat it. The gas flow path is then switched and cool gas flows over the hot refractory material and is heated by it. This invention is not concerned with regenerative heat recovery. Recuperative heat recovery involves the indirect contact of two fluids in a heat exchanger, and this invention does concern recuperative heat recovery. When Eigenberger and Nieken do consider recuperative heat recovery, they propose filling the tubes of the heat recovery heat exchanger with catalyst. While effective for heat recovery, this method is likely to add additional expense to the system. The present invention is not concerned with loading catalyst into the tubes of a shell and tube heat exchanger. [0017] U.S. Pat. No. 5,914,091 proposes a point-of-use catalytic oxidation unit for volatile organic compound removal. This compact unit is designed to sit in a small well-insulated cabinet to minimize heat loss. The heat exchanger and the catalyst vessel are in close proximity, but they are separate vessels. The differentiation of vessels is not as important in this application as in those at higher pressures because this application is for gas at essentially atmospheric pressure. [0018] However, there has remained a need for a system that provides the combination of features and freedom from drawbacks that the present invention provides. BRIEF SUMMARY OF THE INVENTION [0019] One aspect of the present invention is apparatus useful for purifying a gas stream, comprising [0020] (a) a shell-and-tube heat exchanger comprising a shell inlet and a shell outlet in fluid communication with the shell inlet, and further comprising a plurality of tubes each having an inlet and an outlet; [0021] (b) a catalyst system comprising a catalyst supported on a monolithic unitary support having passages therethrough, the support having a length and upstream and downstream ends at opposite ends of the length, wherein the diameter of said support is from one-half to two times the diameter of the shell of the heat exchanger, and wherein the downstream end of said support is connected in fluid communication with the inlets of said tubes by a passageway whose length does not exceed the length of the support and whose diameter is at no point less than the smaller of the diameter of said support and the diameter of said shell; and [0022] (c) a source of gas to be purified in fluid communication with said upstream end of said support. [0023] Another aspect of the present invention is a method for purifying a gas stream, comprising passing the gas through said apparatus under conditions effective to remove one or more contaminants from said gas stream. [0024] As used herein, a "shell-and-tube heat exchanger" is a heat exchanger comprising a plurality of tubes aligned generally parallel to each other and whose outer surfaces are spaces apart from each other, a pair of plates (often termed "tube sheets") through which the tubes pass and to which the outer surfaces of the tubes are sealed at or near each end of the tubes, and a shell surrounding all of the tubes and sealed to the outer edges of the tube sheets thereby defining an enclosed space which contains all of the tubes. The shell also extends past the tube sheet at both ends of the heat exchanger and each end normally terminates in a flange connection. The volume between the flange connection and the tube sheet-is termed a "heat exchanger bonnet". Continue reading about Gas purification apparatus and process... Full patent description for Gas purification apparatus and process Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Gas purification apparatus and process patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Gas purification apparatus and process or other areas of interest. ### Previous Patent Application: Production of high purity and ultra-high purity gas Next Patent Application: Arrangement for separating coarse ash out of a flue gas stream Industry Class: Chemical apparatus and process disinfecting, deodorizing, preserving, or sterilizing ### FreshPatents.com Support Thank you for viewing the Gas purification apparatus and process patent info. IP-related news and info Results in 0.55638 seconds Other interesting Feshpatents.com categories: Accenture , Agouron Pharmaceuticals , Amgen , AT&T , Bausch & Lomb , Callaway Golf |
||
