Patent Application
20190366260
This application claims priority under 35 U.S.C. 119 to China Patent Application No. CN201810526206.6, filed May 29, 2018.
The present invention relates to the field of electronic environmental protection for preparing a process ammonia gas (NH3) and recycling NH3 from an exhaust gas in a semiconductor Light Emitting Diode (LED) manufacturing process, and in particular, to methods for extracting and recycling an ammonia gas from a MOCVD (Metal-Organic Chemical Vapor Deposition) process exhaust gas by FTrPSA (Full Temperature Range-Pressure Swing Adsorption).
A MOCVD process (equipment), as a modern method and means for conducting research and production of compound semiconductor materials, in particular as a method and equipment for industrial production of novel light-emitting materials, i.e., LEDs, is irreplaceable by other semiconductor material growing methods and equipment due to its high quality, high stability, high repeatability, and large-scale property, and becomes a main method and means for producing photoelectric device materials and microwave device materials in today's world. In addition to LEDs, lasers, detectors, efficient solar cells, photoelectric cathodes and the like may also be involved, so that the MOCVD process (equipment) is an indispensable method and equipment in the photo-electronic industry. For example, blue and violet LEDs which are widely applied in the market are all produced by using gallium nitride (GaN)-based materials. In an MOCVD epitaxy process, a high-purity Metal Oxide (MO) is used as an MO source, for example, tri-methyl gallium (TMGa), and carried by electronic-level carrier hydrogen (H2, having a purity greater than 99.99999% (7N)) and nitrogen (N2, having a purity greater than 99.99999% (7N)) to enter an MOCVD reaction kettle together with an electronic-level ammonia gas (NH3), and over a sapphire (Al2O3) substrate heated to an appropriate temperature, the gas-state metal oxide TMGa is controlled to be delivered to the surface of the sapphire substrate for growing a semiconductor thin-film epitaxial material GaN having specific components, a specific thickness, and specific electrical and optical parameters. In order to ensure complete reaction in an MOCVD reaction chamber, H2, N2, and NH3 are all provided in an excessive amount, so that an MOCVD tail gas having a large amount of H2, N2, and NH3 is generated. A typical LED GaN MOCVD epitaxial tail gas consists of: N2: 60% (v/v, the same below); H2: 25%; NH3: 14%; and the balance being metal ions, particles, methane (CH4), oxygen (O2), and oxides such as carbon monoxide (CO), carbon dioxide (CO2), and water (H2O).
Because an MOCVD process tail gas in LED preparation contains impurities such as highly corrosive NH3, flammable and explosive H2, metal ions, arsine (AsH3), oxides, and other impurities, it is quite difficult to purify and recycle NH3 and return same to the LED manufacturing process. At present, most LED chip manufacturers or wafer fab. remove or convert corrosive NH3 into ammonia water, ammonia fertilizer or the like for recycling in various ways such as washing, catalytic conversion, adsorption, and rectification, and the NH3 used in the MOCVD process still needs to be supplied by special gas companies. The tail gas in which the ammonia is removed has a low concentration of H2 and contains a large amount of N2, so that the tail gas is further treated, for example, by means of catalytic combustion or acid and alkali treatment to remove harmful and toxic impurity components, and then enter a hydrogen emission system or be directly exhausted to the air.
The main existing methods for recycling NH3 from an ammonia-containing waste gas include, for example, a freezing method, a water washing method (washing), a sulfuric acid absorption method, a method coupling phosphoric acid (ammonium) absorption and rectification, an organic solvent absorption method, an adsorption method (mainly TSA), a method coupling adsorption and rectification, a catalytic combustion method, and a catalytic ammonia decomposition method.
The most commonly used ammonia gas recycling method at home and aboard is the water washing method (washing), which is applicable to treatment of an ammonia-containing waste gas having relatively simple components, for example, the ammonia-containing waste gas in the MOCVD process. At a certain temperature (usually a low temperature) and pressure, water is used as a detergent (solvent) for absorbing the ammonia in the MOCVD process waste gas, thereby forming the ammonia water having a concentration of 20-25%. Although the absorption can be conducted in stages to achieve a high absorption efficiency, after the waste ammonia gas is absorbed and formed into industrial ammonia water, and the ammonia gas in the waste gas cannot be recycled and purified to return to the MOCVD process. The LED manufacturers or wafer fab. still need to purchase white ammonia or ultra-high ammonia gas on an expensive price. The ammonia recycling by means of water absorption is merely one of the treatment methods for making the LED process waste gas satisfy the emission standards, and the byproduct, i.e., the industrial ammonia water, is comprehensive used for ammonia recycling.
The freezing method is suitable for treatment of an ammonia-containing waste gas having relatively simple components, including the MOCVD process ammonia-containing waste gas. In this method, there is no need to introduce a medium (a solvent, an absorbent, or the like), and the ammonia is recycled by condensing and freezing the ammonia gas component in the waste gas into a liquid at a low temperature based on the physical property that the ammonia component can easily be liquefied, where other non-condensable gas (components having low boiling points) may escape. The method is able to directly recycle the ammonia in the waste gas, and after refining processing, the ammonia can be made into a raw material for an electronic-level ammonia gas; however, because the content of the ammonia gas in the feed gas is relatively low, the power consumption of directly using the freezing method is high, and there are lots of impurity components which have high boiling points or may easily be liquefied in the produced liquid ammonia, so that the ammonia cannot be directly used as a raw material for electronic-level ammonia gas preparation.
The sulfuric acid absorption method is suitable for treatment of an ammonia-containing tail gas having relatively complex components, for example, synthesis ammonia, coke-oven gas, and other tail gases. The equipment investment and maintenance costs are relatively high, and the recycled ammonia gas is made into an ammonium sulfate byproduct as a fertilizer or chemical product, so that the ammonia gas cannot be directly used.
The method coupling phosphoric acid (ammonium) absorption and rectification is applicable to a complex working condition of an ammonia-containing tail gas requiring large throughput, for example, coke oven gas. The USS phosam apparatus (direct or indirect methods) of the U.S. has high absorption selectivity, high desorption and rectification pressure, high energy consumption, and high equipment investment and maintenance costs; 99.98% industrial ammonia can be obtained, the yield is 90%, and the industrial ammonia can be used as a raw material for preparation of electronic-level ammonia gas.
In the organic solvent absorption method, an organic solvent such as liquid hydrocarbon can be used as the absorbent or solvent to selectively absorb ammonia in a waste gas at a certain temperature and pressure, the solubility of the ammonia in the organic solvent is improved, and by recycling the ammonia gas after desorbing and enabling the ammonia gas to escape, the absorbent or solvent can be recycled. The method is applicable to a working condition where VOCs and hydrocarbon components are included and the ammonia content is relatively low, and as for the purification treatment, the investment is high, the purification degree is high, but the purity of the recycled ammonia gas is low and needs further treatment.
The adsorption method, mainly the Temperature Swing Adsorption (TSA) method, is suitable for an ammonia-containing waste gas having simple components, for example, the MOCVD tail gas. The method is simple: activated carbon is usually used as the adsorbent, and the adsorption capacity is greater than 0.5 kg ammonia gas/kg adsorbent. For TSA operations, the yield of the ammonia gas is 90%, but the regeneration temperature is high, operations are complex, energy consumption is high, the service life of the adsorbent is short, and the purity of the ammonia gas is low (Max. 98%). Therefore, the TSA method is usually used in removal and purification of a simple waste gas having a relatively low ammonia concentration. The adsorption method is applied in liquid ammonia dehydration and refining because the polarity of water is higher than that of ammonia, so that the efficiency of removing a trace amount of water from the ammonia by using the adsorption method is relatively high.
For other methods, the catalytic combustion method, for example, uses catalytic combustion to implement high temperature catalytic oxidation on combustible components such as ammonia, hydrogen, and methane in the tail gas, and exhaust the tail gas after the tail gas is further treated to reach the standard, so that the ammonia gas cannot be recycled and reused; the catalytic ammonia decomposition method relates to performing catalytic ammonia decomposition on the tail gas having a relatively high ammonia concentration at a high temperature to obtain H2 and N2, and then the H2 and the N2 are recycled after being treated, so that the ammonia likewise cannot be recycled and reused.
In a series of existing ultra-pure ammonia preparation methods, i.e., preparing the electronic-level ammonia gas having a purity of 99.999% or more by using 99.95% industrial anhydrous ammonia as a raw material, the adsorption method, the rectification method, the method coupling adsorption and rectification, the chemical catalysis method (dehydration), and a metal getter method can be coupled with the ultra-pure ammonia preparation methods, but the ammonia-containing waste gas still cannot be directly recycled and reused.
The present invention provides a method for extracting and recycling ammonia from MOCVD processes exhaust gas by Full Temperature Range-Pressure Swing Adsorption (FTrPSA), that is a method based on Pressure Swing Adsorption (PSA) and capable of being coupled with various separation techniques. On the basis of differences in adsorption separation coefficients and physicochemical properties of different components of a material under different pressures and temperatures, by using an easy-to-match and easy-to-balance cyclic operation of adsorption and desorption in a normal temperature/medium temperature (medium-normal temperature) or shallow condensing/medium temperature (medium-shallow temperature) PSA process, the required effective component NH3 (having a purity greater than or equal to 99.999%) is separated and purified, and on the basis of physicochemical characteristics, relative separation coefficients, corresponding separation and purification methods, and corresponding operating conditions (temperature and pressure) of the multiple components (mainly H2, NH3, N2, methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), water (H2O), silane (SiH4), and the like) in the electronic-level waste gas generated in the LED-MOCVD process, different conventional physical adsorption and chemical adsorption methods are coupled to implement ammonia (NH3) extraction, recycling, and reuse from the MOCVD process waste gas by FTrPSA. For this purpose, the present invention adopts the following technical solution to recycle NH3 from a tail gas and return the NH3 to the MOCVD process for use, thereby implementing recycling and reuse of the NH3:
a method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, including the following steps:
(1) preparing a feed gas, i.e., an exhaust gas in a manufacturing process for an LED based on gallium nitride epitaxial wafer growth which is prepared by MOCVD at atmospheric pressure or low pressure, the feed gas mainly consisting of nitrogen (N2), hydrogen (H2), and ammonia (NH3), as well as a small quantity of metal ions, particles, arsine, methane (CH4), water (H2O), carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2), and other impurity components, the pressure being atmospheric pressure or low pressure, and the temperature being 20-140° C.;
(2) executing pretreatment procedure: introducing the feed gas into a pretreatment unit consisting of a dust remover, a particle removing filter, and an oil mist removing catcher, removing dust, particles, oil mist, and other impurities in sequence under operating conditions that the pressure is 0.2-0.3 MPa and the temperature is 20-140° C., and proceeding to a next procedure, i.e., a medium-shallow temperature PSA concentration procedure;
(3) executing medium-shallow temperature PSA concentration procedure: compressing the feed gas from the pretreatment procedure to 0.3-4.0 MPa, entering a multi-tower PSA concentration procedure consisting of at least four towers, where the operating pressure of the adsorption towers is 0.3-4.0 MPa, the operating temperature is 20-140° C., at least one of the adsorption towers is in an adsorption step, the remaining adsorption towers are in a desorption and regeneration step, and a formed non-adsorbed phase gas is an adsorption exhaust gas, which is introduced into a hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and be directly discharged; a formed adsorbed phase gas is an ammonia-rich concentrated gas, which enters a next procedure, i.e., a condensation and freezing procedure, by means of blowing or pressurization; an adsorbent used in the medium-shallow temperature PSA concentration procedure is one or more combinations of activated aluminum oxide, silica gel, activated carbon, and a molecular sieve; and a regeneration mode of vacuum pumping, or purging, or purging and vacuum pumping is used during desorption;
(4) executing condensation and freezing procedure: enabling, by means of blowing or pressurization, the ammonia-rich concentrated gas from the medium-shallow temperature PSA concentration procedure to enter a condensation and freezing procedure consisting of a condenser, an evaporator condenser, and a freezer to form liquid ammonia having an ammonia concentration greater than or equal to 98-99%, and proceeding to a next procedure, i.e., ammonia vaporization, where a non-condensable gas produced in the condensation and freezing procedure is mixed with the feed gas for further ammonia recycling, or is mixed with the adsorption exhaust gas in the medium-shallow temperature PSA concentration procedure, or is introduced to the hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and directly discharged;
(5) executing liquid ammonia vaporization procedure: directly introducing the liquid ammonia from the condensation and freezing procedure to a liquid ammonia vaporization procedure for vaporization, the liquid ammonia vaporization procedure consisting of a liquid ammonia evaporator and an ammonia gas buffer tank, so as to form an ammonia gas having an ammonia concentration greater than or equal to 98-99% (industrial ammonia), and proceeding to a next procedure, i.e., an ammonia gas purification procedure;
(6) executing PSA ammonia extraction procedure: directly introducing industrial ammonia from the liquid ammonia vaporization procedure to a multi-tower PSA ammonia gas purification procedure consisting of at least four adsorption towers, where the operating pressure of the adsorption towers is 0.6-2.0 MPa, the operating temperature is 60-120° C., at least one of the adsorption towers is in the adsorption step, the remaining adsorption towers are in the desorption and regeneration step, and a formed non-adsorbed phase gas is an ultra-pure ammonia gas with a purity greater than or equal to 99.995%, and proceeding to a next procedure, i.e., ammonia gas purification, where vacuum pumping and purging are adopted in the desorption and regeneration mode, the formed desorbed gas is directly exhausted to a waste steam treatment system outside the area for treatment, and an adsorbent used in the PSA ammonia extraction procedure is one or more combinations of activated aluminum oxide, silica gel, activated carbon, and a molecular sieve; and
(7) executing ammonia gas purification procedure: enabling the ultra-pure ammonia gas from the PSA ammonia extraction procedure to pass through an intermediate product storage tank, or directly be subjected to heat exchange, under a temperature of 60-500° C., decompressing the gas directly or by a reducing valve to a pressure required by the ammonia gas used in an LED-MOCVD process, entering an ammonia gas purification procedure consisting of a metal getter purifier, or an adsorbent purifier carrying a metal oxide active component, or an ammonia gas purifier coupling an adsorbent and a metal getter, purifying at the operating temperature of 60-500° C. and the operating pressure ranging from the atmospheric pressure to a pressure condition required by the ammonia gas in the MOCVD process, removing trace impurities to obtain a final electronic-level ammonia gas product, the purity of which satisfies an electronic-level ammonia gas (white ammonia) product standard specified by the state or the Semiconductor Equipment and Materials International (SEMI), the purity of the ammonia gas being greater than or equal to level 7-8N, enabling the final product to be subjected to temperature reduction by means of heat exchange, or decompression, or introducing the final product to an electronic-level ammonia gas product storage tank for storage, or enabling the final product to pass through an ammonia gas product buffer tank to directly return to a section requiring ammonia gas in the MOCVD process, where the operating temperature of the ammonia gas purification procedure is determined by the used metal getter or adsorbent, the service life of the metal getter or adsorbent is at least greater than two years, and no regeneration is needed, so that the yield of the obtained electronic-level ammonia gas product is greater than 70-86%.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, the feed gas includes a waste gas or a tail gas containing main components such as hydrogen, nitrogen, and ammonia gas, and other impurity components generated in other semiconductor processes.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, for the pretreatment, in a working condition that the feed gas is a waste gas or a tail gas containing high concentrations of other impurity components, in addition to the dust remover, the particle removing filter, and the oil mist removing catcher, a caustic scrubber, a neutralizing tower, a dryer, and the like are additionally provided to remove acidic and volatile organic matters (VOCs) and other impurity components that may have a greater impact on operations in the medium-shallow temperature PSA concentration procedure.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, in the medium-shallow temperature PSA concentration and adsorption tower desorption steps, after the adsorption tower desorption step ends and before a pressure equalizing dropping or normal flowing step starts, the ultra-pure ammonia gas from the PSA ammonia extraction procedure is used for replacement, for improving the yield of the ammonia gas in this procedure to be 80-90% or higher.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, in the medium-shallow temperature PSA concentration, the feed gas from the pretreatment procedure is fed to the medium-shallow temperature PSA concentration by the blower without being compressed; the procedure is composed of two stages of PSA systems; that is, the feed gas from the pretreatment procedure is pressurized to 0.2-0.3 MPa by the blower and enters from the bottom of a first PSA adsorption tower (first-stage PSA), the non-adsorbed phase adsorption exhaust gas flows out of a first-stage PSA tower top, and the exhaust gas is introduced into the hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and be directly discharged; the desorbed gas desorbed (reverse flowing, purging, or vacuum pumping) at and flowing out of a first-stage PSA tower bottom is fed to the bottom of a second PSA adsorption tower (second-stage PSA) by means of the blower, a non-adsorbed phase mixed intermediate gas flows out of a second-stage PSA tower top and returns as a first-stage PSA feed gas for further recycling ammonia, and the ammonia-rich concentrated gas flowing out of a second-stage PSA tower bottom enters the next procedure, i.e., the condensation and freezing procedure, by means of blowing or pressurization, where after the adsorption step ends and before the pressure equalizing dropping or normal flowing step starts, a replacement step is added in the second PSA adsorption tower (second-stage PSA), in which the ultra-pure ammonia gas from the PSA ammonia extraction procedure is used as a replacement gas, for improving the yield of the ammonia gas in this procedure to be 80-90% or higher.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, in the medium-shallow temperature PSA concentration, the feed gas from the pretreatment procedure is pressurized to 0.3-4.0 MPa and then fed to the medium-shallow temperature PSA concentration procedure constituted by two stages of PSA; the feed gas enters from the bottom of the first PSA adsorption tower (first-stage PSA), and a non-adsorbed phase intermediate gas flows out of the first-stage PSA tower top; a part of the non-adsorbed phase intermediate gas serves as a feed gas to the bottom of the second PSA adsorption tower (second-stage PSA), and the other part serves as the adsorption waste gas, which is introduced into a hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and be directly discharged; the desorbed gas (the ammonia-rich concentrated gas) desorbed (reverse flowing, purging, or vacuum pumping) at and flowing out of the first-stage PSA tower bottom is fed to the subsequent procedure, i.e., the condensation and freezing procedure, by means of the blower; the feed gas from the pretreatment procedure is pressurized to 0.3-4.0 MPa and then mixed with a part of the intermediate gas flowing out of the first-stage PSA tower top, serves as a feed gas to the second PSA adsorption tower (second-stage PSA), and then is fed from the tower bottom, and the non-adsorbed phase intermediate gas flows out of the second-stage PSA tower top; a part of the non-adsorbed phase intermediate gas serves as the feed gas to the bottom of the first PSA adsorption tower (first-stage PSA), and the other part serves as the adsorption waste gas, which is introduced into a hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and be directly discharged; the desorbed gas (the ammonia-rich concentrated gas) desorbed (reverse flowing, purging, or vacuum pumping) at and flowing out of the second-stage PSA tower bottom is mixed with the desorbed gas (the ammonia-rich concentrated gas) desorbed at and flowing out of the first-stage PSA tower bottom, and is then fed to the subsequent procedure, i.e., the condensation and freezing procedure, by means of the blower, where after the adsorption step ends and before the pressure equalizing dropping or normal flowing step starts, a replacement step is added in the first and the second PSA adsorption towers, in which the ultra-pure ammonia gas from the PSA ammonia extraction procedure is used as a replacement gas, for improving the yield of the ammonia gas in this procedure to be 80-90% or higher, or the desorbed gas (the ammonia-rich concentrated gas) flowing from this procedure is used as the replacement gas after being compressed, for improving the yield of the ammonia gas in this procedure to be 80-90% or higher.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, an ammonia gas rectification procedure is used to replace the condensation and freezing procedure, i.e., the ammonia-rich concentrated gas from the medium-shallow temperature PSA concentration procedure enters the condenser by means of blowing or pressurization to form the liquid ammonia, and then enters the ammonia liquid rectification procedure; the ammonia gas flows out of the top of a rectification tower, and a part of the ammonia gas returns to the condenser for reflux; the non-condensable gas escapes from the condenser, and returns to the raw gas, or is mixed with the adsorption waste gas flowing out of the medium-shallow temperature PSA concentration procedure, or is introduced into a hydrogen extraction process, or is treated by means of catalytic combustion, spraying, or other methods to meet the national emission standards for atmospheric control and be directly discharged; a part of the ammonia has a concentration greater than or equal to 99%, and directly enters the subsequent procedure, i.e., the PSA ammonia extraction procedure, omitting the liquid ammonia vaporization procedure, where the operating temperature of the ammonia liquid rectification procedure is 70-140° C. and the operating pressure is 0.3-2.0 MPa; an impurity component having a boiling point higher than that of the ammonia, for example, water, hydrocarbon and the like, flows out of the bottom of the rectification tower and is fed outside the area for treatment.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, the feed gas of the PSA ammonia extraction procedure, i.e., the ammonia gas from the liquid ammonia vaporization or the ammonia gas from rectification procedure, is added with metal palladium as a catalytic de-aerator for removing oxygen in a working condition containing a small amount of oxygen, where the operating temperature is 70-90° C., and the depth reaches 0.1 ppm or lower; and the oxygen-removed ammonia gas enters the next procedure, i.e., PSA ammonia extraction.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, the liquid ammonia formed in the condensation and freezing procedure and having an ammonia concentration greater than or equal to 98-99% directly enters a liquid phase adsorption procedure; under a temperature ranging from −40° C. to 40° C. and at pressure ranging from the atmospheric pressure to 4.0 MPa, the liquid ammonia enters, from the tower top, a Temperature-PSA (TPSA) tower consisting of two or three towers, and is subjected to liquid phase adsorption at the adsorption temperature ranging from −40° C. to 40° C. and at the adsorption pressure ranging from the atmospheric pressure to 4.0 MPa; a small amount of water and other adsorption impurity components are adsorbed by the adsorbent with which the adsorption tower is filled; the ammonia, as the non-adsorbed phase which is non-absorbable, is made into the liquid ammonia having a purity of 99.999%, flows out of the bottom of the adsorption tower as a product output, which is subjected to bottled or canned pressurized vaporization and then fed to ammonia gas purification; the adsorbed small amount of water and other adsorption impurity components serve as the adsorbed phase, are subjected to regeneration of a thermal regeneration carrier gas (steam or nitrogen) and desorption by rinsing at the atmospheric pressure or by vacuum pumping and rinsing, and then exhausted from the top of the adsorption tower; when one adsorption tower allows the liquid ammonia having the purity of 99.999% to flow out after the adsorption ends, another adsorption tower enters the liquid phase adsorption procedure again after thermal regeneration and desorption, thereby achieving a continuous cyclic adsorption operation; when three towers are working, one tower performs adsorption, one performs regeneration, and the remaining one stands by or performs regeneration.
Furthermore, according to the method for extracting and recycling ammonia from an MOCVD process exhaust gas by FTrPSA, in the medium-shallow temperature PSA concentration procedure and the PSA ammonia extraction procedure, under an operating condition that the adsorption pressure is greater than or equal to 0.6 MPa, slow and uniform control is implemented on pressure changes in a cyclic operation process of adsorption and desorption by means of a program control valve and a regulating valve on pipelines connected between the adsorption towers, so as to prevent beds of the adsorption towers from being scoured by airflow caused by an excessive pressure change of the system, or prevent the occurrence of adsorbent pulverization, thereby keeping system operations in this procedure stable and secure.
The present invention includes the following beneficial effects:
(1) according to the present invention, NH3 can be extracted from the MOCVD process waste gas and returned to the MOCVD process for use, solving the problems such as high energy consumption, low purity of a recycled material, or the recycled material being other products that cannot be returned to the MOCVD process in existing removal, purification, or ammonia and ammoniate recycling methods such as washing, freezing, sulfuric acid absorption, a method coupling phosphoric acid (ammonium) absorption and rectification, catalytic combustion, and catalytic decomposition, thereby realizing the recycling and reuse of all components of the waste gas, reducing the emission of the waste gas, and making up for the gap of the LED process exhaust gas treatment technology;
(2) according to the present invention, on the basis of physicochemical and relative separation coefficient characteristics at the medium-low temperature (20-140° C.) and under the medium-lower pressure (0.3-4.0 MPa), the adsorbed phase NH3 is selectively separated and recycled, thereby avoiding the technical difficulties that NH3 with strong polarity is deeply adsorbed during the adsorption cyclic operation and is difficult to be regenerated and it is difficult to use conventional TSA and PSA techniques to process highly corrosive NH3 components, which makes it difficult to realize the adsorption and regeneration cyclic operations of the medium-low temperature FTrPSA system based on coupling of adsorption, condensation and freezing, vaporization or rectification, absorption, separation techniques in the present invention; finally, an electronic-level ammonia gas product is obtained, and the technical bottleneck of a difficulty in recycling NH3 and returning same to the MOCVD process for reuse in conventional adsorption and separation processes is overcome;
(3) according to the present invention, in the process of realizing recycling and reuse of NH3, the MOCVD process and no oxygen-containing compound to which the NH3 is sensitive, especially H2O, is brought to the system, so that the entire recycling and reuse process is stable and the impact on the quality of LED chips is reduced to zero;
(4) according to the present invention, in the process of purifying, recycling, and reusing waste gas at atmospheric pressure or low pressure, pressurized treatment or non-pressurized treatment is used according to use conditions of the process (electronic-level) ammonia gas to obtain an electronic-level ammonia gas product;
(5) according to the present invention, a medium-shallow temperature PSA concentration procedure is used, which can not only avoid deep adsorption of the adsorbed phase NH3, but also relieve the strong corrosive problem of high-concentration ammonia in the adsorption towers due to excessively long adsorption or desorption time;
(6) according to the present invention, on the basis of differences in operating temperature between different procedures, by arranging a reasonable heat exchange system, the heat of the entire operating system can be fully used;
(7) according to the present invention, different procedures, including process simplification, switching between ammonia product forms (liquid and gas), and the like, can be flexibly recombined according to actual working conditions of a user; and
(8) the present invention solves the greatest difficulty in recycling ammonia-containing MOCVD process waste gas: the NH3 purification, recycling and reusing process is not only limited by different original front-end pretreatment processes of the MOCVD tail gas and but also subject to different fixed rear-end purification procedures, so that the requirements on the NH3 purification, recycling and reusing process for the tail gas between the front-end pretreatment and the rear-end purification are more strict.
In order to make a person skilled in the art better understand the present invention, the technical solutions in the embodiments of the present invention are clearly and fully described below with reference to the accompanying drawings in the embodiments of the present invention.
As shown in
(1) a raw gas, i.e., an exhaust gas in a manufacturing process for an LED based on gallium nitride epitaxial wafer growth which is prepared by MOCVD at atmospheric pressure or low pressure, the feed gas mainly consisting of 46% (v/v, the same below) of nitrogen (N2), 34% of hydrogen (H2), and 19% of ammonia (NH3), and the remaining 1% being a small quantity of metal ions, particles, arsine, methane (CH4), water (H2O), carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2), and other impurity components, the pressure being atmospheric pressure or low pressure, and the temperature being 50-70° C.;
(2) pretreatment: introducing the feed gas into a pretreatment unit consisting of a dust remover, a particle removing filter, and an oil mist removing catcher, removing dust, particles, oil mist, and other impurities in sequence under operating conditions that the pressure is 0.2-0.3 MPa and the temperature is 50-70° C., and proceeding to a next procedure, i.e., a medium-shallow temperature PSA concentration procedure;
(3) medium-shallow temperature PSA concentration: compressing the feed gas from the pretreatment procedure to 1.6 MPa, entering a multi-tower PSA concentration procedure consisting of six towers, where the operating pressure of the adsorption towers is 1.6 MPa, the operating temperature is 50-70° C., one-tower adsorption, secondary pressure equalizing, a slow and uniform mode, and desorption and regeneration by vacuum pumping and purging are used, and a formed non-adsorbed phase gas is an adsorption exhaust gas, which has an NH3 concentration controlled to be below or equal to 0.5% and is treated by means of catalytic combustion and spraying to meet the national emission standards for atmospheric control and be directly discharged; a formed adsorbed phase gas is an ammonia-rich concentrated gas, which has an ammonia concentration of 65% and is compressed to 0.6 MPa and then enters a next procedure, i.e., a condensation and freezing procedure; an adsorbent used in the medium-shallow temperature PSA concentration procedure is a composite combination of activated aluminum oxide, silica gel, activated carbon, and a molecular sieve;
(4) condensation and freezing: compressing the ammonia-rich concentrated gas from the medium-shallow temperature PSA concentration procedure to 0.5-0.6 MPa, enabling the gas to enter a condensation and freezing procedure consisting of a condenser, an evaporator condenser, and a freezer to form liquid ammonia having an ammonia concentration greater than or equal to 98-99%, and proceeding to a next procedure, i.e., ammonia vaporization, where a non-condensable gas produced in the condensation and freezing procedure is mixed with the adsorption exhaust gas in the medium-shallow temperature PSA concentration procedure, and then is treated by means of catalytic combustion and spraying to meet the national emission standards for atmospheric control and directly discharged;
(5) liquid ammonia vaporization: directly introducing the liquid ammonia from the condensation and freezing procedure to a liquid ammonia vaporization procedure for vaporization, the liquid ammonia vaporization procedure consisting of a liquid ammonia evaporator and an ammonia gas buffer tank, so as to form an ammonia gas having an ammonia concentration greater than or equal to 98-99% (industrial ammonia), and proceeding to a next procedure, i.e., an ammonia gas purification procedure;
(6) PSA ammonia extraction: directly introducing industrial ammonia from the liquid ammonia vaporization procedure to a multi-tower PSA ammonia gas purification procedure consisting of six adsorption towers, where the operating pressure of the adsorption towers is 0.5-0.6 MPa, the operating temperature is 50-70° C., one-tower adsorption, secondary pressure equalizing, a slow and uniform mode, and desorption and regeneration by vacuum pumping and purging are used, and a formed non-adsorbed phase gas is an ultra-pure ammonia gas with a purity greater than or equal to 99.995%, and proceeding to a next procedure, i.e., ammonia gas purification, where the formed desorbed gas is directly exhausted to a waste steam treatment system outside the area for treatment, and an adsorbent used in the PSA ammonia extraction procedure is a composite combination of activated aluminum oxide, silica gel, activated carbon, and a molecular sieve; and
(7) ammonia gas purification: enabling the ultra-pure ammonia gas from the PSA ammonia extraction procedure to pass through an intermediate product storage tank, then heating the ultra-pure ammonia gas to 300-400° C. by means of heat exchange, adjusting the pressure to a pressure required by the ammonia gas used in an LED MOCVD process, which is 0.3 MPa in the present embodiment, entering an ammonia gas purification procedure consisting of a metal getter purifier, purifying at the operating temperature of 300-400° C. and under the operating pressure of 0.3 MPa, removing trace impurities to obtain a final electronic-level ammonia gas product, the purity of which satisfies an electronic-level ammonia gas (white ammonia) product standard specified by the state or the SEMI, the purity of the ammonia gas being greater than or equal to 99.99999-99.999999% (level 7-8N), reducing the temperature to the normal temperature by means of heat exchange, and introducing the final product to an electronic-level ammonia gas product storage tank for storage, and making the final product enter the MOCVD process according to the requirements on the ammonia gas in use during the process, where the operating temperature of the ammonia gas purification procedure is determined by the used metal getter, a high-temperature metal getter is used in the present embodiment, the temperature is 300-400° C., the service life of the getter is at least longer than two years, and no regeneration is needed; in this way, the yield of the obtained electronic-level ammonia gas product is greater than 70-86%.
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Obviously, the above-mentioned embodiment is only part of the embodiment in the present invention rather than the whole embodiment. Based on the embodiment recorded in the present invention, with respect to all other embodiments obtained by those skilled in the art without paying creative work, or the structural changes made under the inspiration of the present invention, all technical solution that are identical or similar to the present invention fall into the protection scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| CN201810526206.6 | May 2018 | CN | national |
