VOC recovery system in which deep cooling is carried out using air

Abstract

VOC recovery system in which deep cooling is carried out using air, comprising a system for cleaning gaseous air, an air liquefaction system and a VOC recovery cooling box; wherein the VOC recovery cooling box comprises a VOC precooler (9), a VOC condenser (7) and a gas-liquid separator (10);wherein gaseous VOCs first enter the VOCs precooler (9) to remove the moisture contained therein, wherein an outlet of the VOCs precooler (9) is connected to an inlet of the VOCs condenser (7) inside the VOCs recovery cooling box and the outlet of the VOCs condenser (7) is connected to an inlet of the gas-liquid separator (10), wherein the dried gaseous VOCs, after removal of the water vapor, enter the VOCs condenser (7) in the VOCs recovery cooling box to exchange heat with liquid air, wherein the gaseous VOCs condense therein into a liquid, wherein a gas-liquid mixture flows out of the outlet and enters the gas-liquid separator (10) to separate the liquid components contained therein, while gases enter the next process;characterized in that the VOCs precooler (9) is a VOCs finned precooler with three flows for indirect heat transfer, wherein two flows pass over a VOCs channel and one flow passes over a liquefied air channel, wherein the two flows of the VOCs channel are operated alternately, wherein, when precooling one VOCs flow, hot air is simultaneously introduced into the VOCs channel of the other flow for purging to ensure continuous production.

Description

TECHNICAL FIELDThe present invention belongs to the field of volatile organic gas purification and separation, and specifically relates to an efficient and low cost volatile organic gas separation and recovery system and a method thereof.PRIOR ARTVOCs (abbreviation for volatile organic compounds, i.e. volatile organic gases) represent an important cause of smog and PM 2.5. In order to reduce the damage to heavy pollution caused by human life, various VOCs reaction mechanisms have been published in China. The most important requirement concerns the reduction of the emission of gaseous VOCs in industrial production and the enhancement of the recovery and treatment of VOCs.At present, for recovering VOCs in China, the condensation method, the absorption method, the uptake method, the catalytic combustion method and the membrane separation method are mainly used. For example, in recent published papers CN 106 902 617 A "Apparatus and Method for Purifying and Recovering Gaseous VOCs of High Concentration, CN 107 158 945 A "Purification System for Removing VOCs from Industrial Waste Gases and Related Purification Process", and CN 106 731 481 A "Method and Apparatus for Treating VOC waste gases by Cyclone Absorption by Activated Carbon", VOCs are recovered by the absorption process. Despite the simple principle, only limited recovery results are achieved with such methods and it is usually difficult to expect to meet the emission standard. In order to reduce the recovery cost, various methods such as biodegradation, plasma degradation, photocatalyst degradation, etc. have been proposed in recent years. Besides non-mature technology and long treatment cycle, the result of single recovery is not ideal and secondary treatment of gaseous VOCs in cooperation with other agents is necessary, so that they are not yet widely used. For example, in the published document CN 106 871 140 A "Device for treating VOCs by condensation and catalytic combustion", the condensation process and the catalytic combustion are combined. In the publication CN 206 582 866 U "Novel VOCs treatment device", photocatalyst degradation and catalytic combustion are combined. In the publication CN 106 943 829 A "Device for purifying gaseous VOCs by selectable biological treatment and catalytic combustion and methods of use", combustion takes place after biological degradation. In the publication CN 107 202 333 A "Method for making VOCs containing sulphur harmless", burnt exhaust gases are introduced into an alkali solution for uptake after burning. Despite the innovation in such methods, there are still problems in practical application such as complicated process and high energy consumption.In recent years, the VOCs emission standard in China has become more and more severe. By combining conventional methods alone, such as absorption methods, condensation methods, etc., the norm about the emission of gaseous VOCs can no longer be fulfilled. When lowering the condensation temperature or increasing the number of absorption processes alone to increase the recovery rate, the recovery cost is inevitably increased remarkably. The present invention aims to reduce the recovery cost and simultaneously increase the recovery efficiency in order to overcome the above disadvantages and problems and to meet the demand for continuous industrial series production. In comparison with the above patents, the present invention has great differences therefrom in principle, process, plant and operation, particularly also considerably from the applicant's earlier allowed patent CN 104 606 915 B "High efficiency and low cost VOC recovery system and related method". In the applicant's earlier allowed patent specification (CN 104 606 915 B "High Efficiency and Low Cost VOC Recovery System and Method thereof"), liquid nitrogen is used as the heat exchange medium and the process does not comprise a gas purification system and liquefaction system. Further, heat exchange between the liquid nitrogen and gaseous VOCs is by direct contact to achieve deep condensation and separation of VOCs. In addition, in the patent specification CN 104 606 915 B "High-efficiency and cost-effective VOC recovery system and associated method"), the microchannel heat exchanger used is designed as a plate module. In the present specification, on the other hand, a spiral microchannels heat exchanger for indirect heat transfer is used as the heat exchanger.CN 102 441 290 A describes an oil-gas condensation and recycling method based on cooling by a turbine expansion machine. A low temperature environment is obtained by combining expansion cooling with a two stage heat exchanger.Further relevant prior art are CN 1 04 606 915 A, EP 3 026 322 A1 and US 2009 / 0 293 502 A1.CONTENT OF THE PRESENT INVENTIONThe object of the present invention is to provide a cryogenic cooling and recovery system for recycling VOCs by means of air, in which a deep condensation of VOCs and finally a separation and recovery are realized by means of air. The principle is as follows: Purified air is compressed first and then expanded to be converted into liquid air, heat exchange being performed between liquid air and gaseous VOCs to cool the gaseous VOCs to -130°C to -140°C, thus lowering the nonmethane hydrocarbon content to 70 to 120 mg / m3. The treated gaseous VOCs meet entirely the governmental emissions standard and can be discharged directly to the atmosphere. As compared with the conventional condensation method, the method can effectively shorten the recovery process, simplify the recovery process, and well realize norm-compliant emission of gaseous VOCs. The device is distinguished by a compact construction, increased flexibility, good ease of assembly and a broader range of application and is not subject to any restriction by the concentration of the starting gases.According to the invention, the object is achieved by the following configuration:A VOCs recovery system in which air cryogenic cooling is performed includes a gaseous air purification system, an air liquefaction system, and a VOCs recovery cooling box. The gaseous air purification system includes, among other things, an air filter, a refrigerant dryer, an air cleaner and associated line valves, and a gauge. Within the air filter, a molecular sieve, zeolite or other adsorbent is filled for preliminary removal of minute particles and contaminants from the air. The refrigerant dryer serves to remove moisture contained in the air. The purifier is for further purifying the gases to remove sulfur dioxide, hydrogen sulfide and some NOxcontained in a small amount in the air. The air liquefaction system includes a primary compressor, a turbo expander, a microchannel air precooler, and a gas bearing, conduit, valve, temperature measurement device, frequency converter, and other accessory devices. The air compressor serves primarily for pre-compressing the air. The microchannel precooler serves to cool the high temperature air at an outlet of the compressor. The turboexpander serves primarily for further compressing and expanding the pre-compressed air in order to liquify the air. The temperature measuring device serves to measure the gas temperature. The conduit and the valve serve for conveying the gas. The VOCs recovery cooling box includes, among other things, primarily a VOCs precooler, a VOCs spiral microchannels condenser, a gas-liquid separator, and a liquid level indicator. The air precooler is disposed inside the VOCs recovery cooling box. The recovery cooling box is an important equipment for recovering VOCs. In the cooling box, vacuum and a powder are used for thermal insulation to secure an ultra-low temperature environment inside the cooling box. The VOCs fin precooler serves to remove a portion of the water vapor from gaseous VOCs to prevent freezing of the gases after entering the VOCs microchannel condenser and thus plugging of the channel. The VOCs spiral microchannels condenser serves primarily for condensation and recovery of gaseous VOCs, whereby damage to the plant as a result of thermal loading can simultaneously be prevented. The gas-liquid separator serves primarily to separate condensed VOCs liquid and to carry the liquid into a container for storage while gases return to the VOCs microchannel precooler and exchange heat with gaseous VOCs at room temperature.It is further provided that the VOCs lamella precooler is a precooler with three flows for indirect heat transfer, wherein two flows flow via a VOCs channel and one flow flows via a channel of liquefied air. The two flows of the VOCs channel are operated alternately. In precooling a VOCs flow, hot air is simultaneously introduced into the VOCs duct of the other flow for rinsing in order to remove ice and prevent the duct from being clogged. After completing the purging of the channel of one VOCs flow, the channel of the other VOCs flow is then purged. Gaseous VOCs are further introduced into the flushed channel for condensation. This is done alternately to ensure continuous production.It is further provided that the VOCs inlet and the VOCs outlet of the VOCs fin precooler are arranged symmetrically on the left and the right side of the precooler, respectively, while the inlet and the outlet of liquefied air are arranged symmetrically on the upper end and the lower end of the precooler, respectively.It is also provided that the gaseous VOCs in the VOCs precooler and the liquefied air flow in mutually crossing directions.Compared with existing patents and the prior art, the present invention is characterized by the following advantageous effects: 1. air which does not cause a cost is used as the condensing medium, whereby the running cost of the system can be substantially reduced. 2. a deep condensation of gaseous VOCs is realized by means of the ultra-low temperature of liquefied air, whereby gaseous VOCs can be cooled to -130° C. to -140° C. The treated gaseous VOCs can meet the emission standard well. This cannot be achieved solely by the conventional condensation process. 3. the use of the process is not subject to any restriction by the concentration, the constituents and the flow of the starting gases, which provides a broader range of application. Thus, the technical bottleneck is also overcome that the conventional condensation method is applicable only to gaseous VOCs of high concentration. 4. in the plant, simultaneous operation of a plurality of compressors of large capacity is not necessary, thereby greatly reducing the construction cost. Further, the recovery operation is shortened and the operation stability of the plant is increased.BRIEF DESCRIPTION OF THE DRAWINGSShown therein are FIG. 1 shows a first exemplary embodiment of the present invention in a flow chart, FIG. 2 shows a lamella precooler of the present invention in a schematic structural illustration, FIG. 3 shows a second exemplary embodiment of the present invention in a flow chart.DETAILED DESCRIPTIONFirst EmbodimentThe present invention is described in more detail below with reference to the drawings of the description on the basis of specific exemplary embodiments. It is to be understood that the described embodiments constitute a part of the embodiments of the present invention instead of all the embodiments. All other embodiments obtained by persons skilled in the art from the embodiments of the invention without inventive activity also fall within the scope of protection of the invention.As is apparent from FIG. 1, a VOCs recovery system includes a gaseous air purification system, an air liquefying system, and a VOCs recovery cooling box. The gaseous air purifying system comprises an air filter 1, a refrigerant dryer 4 and an air cleaner 5. the air liquefying system comprises an air compressor 2, an air container 3, a turbo expander 6 and an air precooler 8. the VOCs recovery cooling box comprises a VOCs condenser 7, a VOCs precooler 9 and a gas-liquid separator 10.In operation of the system, air at atmospheric pressure and room temperature enters the air filter 1 through a conduit for prefiltration to remove most of the dust and particles from the air. The pre-purified air flows out of an outlet of the air filter 1 and enters the air compressor 2 for pre-compression. The compressed air is conveyed into the air container 3 and buffered and stored there. An outlet of the air container 3 is connected to the refrigerant dryer 4, and the compressed air having a certain pressure and a certain temperature enters the refrigerant dryer 4 via the outlet of the air container 3 to remove the moisture contained therein and prevent the channel from being clogged due to freezing of the water vapor in the air in subsequent operations and thereby impairing the operation of the system. The dried compressed air enters the air cleaner 5 along the conduit to remove a part of sulfur dioxide, hydrogen sulfide and other gaseous impurities contained in the air, thus completing the final purification. A part of the purified air enters a gas bearing of the turbo expander 6, while the other part enters the air precooler 8 for precool, the precooled high pressure air enters the turbo expander 6 and is converted into liquefied air by expansion. The liquefied air enters the VOCs condenser 7 and exchanges heat with precooled gaseous VOCs to liquify the gaseous VOCs. After the heat exchange, the air is still at a low temperature, causing waste of cooling performance when immediately discharged into the atmosphere. Therefore, the low-temperature air is introduced into a cold liquid passage of the air precooler 8 to allow heat exchange with the high-temperature, high-pressure air flowing out of the air cleaner 5 and thus precool the same. Thus, cascade utilization of the cooling performance in the liquefied air is realized. After heat exchange, the air may be discharged directly to the atmosphere or may be sent back to the compressor for repackaged and liquification for recycle.Gaseous VOCs originating from production steps first enter the VOCs precooler 9 and is precooled to 3° C. to 4° C. in the VOCs precooler 9 in order to remove the water vapor contained therein. The dried gaseous VOCs enter the VOCs condenser 7 along the conduit and exchange heat with the liquefied air from the turbo expander 6. In the VOCs condenser 7, VOCs are cooled by the liquefied air to -130°C to -140°C to reduce the total content of nonmethane hydrocarbons to 70 to 120 mg / m3, and then enter the gas-liquid separator 10 to separate out the VOCs condensate and carry it into a container for storage. The purified low temperature exhaust gas is low temperature and therefore enters the precooler via a cold liquid inlet of the VOCs precooler 9 to allow heat exchange with high temperature VOCs, followed by exhaust to the atmosphere via the outlet of the VOCs precooler 9.FIG. 2 shows the VOCs lamella precooler 9 in a schematic structural illustration. In the three-flow vane precooler, the liquefied air channel is formed as a microchannel having a diameter of less than 1 mm and the VOCs channel is formed as a wide channel. Here, 9-1 and 9-3 are inlets of VOCs in the gas, 9-2 and 9-4 are outlets of VOCs, 9-5 is inlet of purified air, and 9-6 is outlet of air. In operation, gaseous VOCs and purified low temperature air enter the precooler via 9-1 and 9-5, respectively, and after heat exchange in the precooler, the air flows out via the outlet 9-6 at the top of the precooler and enters the next process, while the gaseous VOCs from which water vapor is removed flow out via the outlet 9-2 at the other end. After operation for a period of time, the channel between 9-1 and 9-2 is to be flushed with hot air to prevent the channel of the precooler from being clogged due to the freezing of the water vapor. Now gaseous VOCs are introduced via 9-3 into the precooler and after heat exchange flow out via outlet 9-4. Thus, the two channels operate alternately by switching to ensure continuous production.Second EmbodimentIn addition to the flow according to the first embodiment, in the present invention, the gases separated by the gas-liquid separator may be further treated to reduce the VOCs content. As shown in FIG. 3, the gases discharged from the gas-liquid separator 10 enter the microchannel mixer 11 and are directly mixed with liquid air, whereby the separated liquid components enter the container for storage while the low temperature gases enter the next process. In the present embodiment, the VOCs precooler and the air precooler may be formed the same as or different from each other. However, various functions are realized thereby. The other devices are similar to those according to the first embodiment, and therefore repetition is omitted.

Claims

A VOCs recovery system in which air supercooling is performed, comprising a gaseous air purifying system, an air liquefying system and a VOCs recovery cooling box; wherein the VOCs recovery cooling box comprises a VOCs precooler (9), a VOCs condenser (7) and a gas-liquid separator (10); wherein gaseous VOCs first enter the VOCs precooler (9) to remove the moisture contained therein, wherein an outlet of the VOCs precooler (9) is connected to an inlet of the VOCs condenser (7) within the VOCs recovery cooling box and the outlet of the VOCs condenser (7) is connected to an inlet of the gas-liquid separator (10), the dried gaseous VOCs entering the VOCs condenser (7) in the VOCs recovery cooling box after removing the water vapor to exchange heat with liquid air, the gaseous VOCs condensing therein into a liquid, a gas-liquid mixture flowing out of the outlet and entering the gas-liquid separator (10) to separate the liquid components contained therein while gases enter the next process; characterized in that the VOCs precooler (9) is a VOCs lamella precooler with three flows for indirect heat transfer, wherein two flows flow via a VOCs channel and one flow flows via a channel of liquefied air, wherein the two flows of the VOCs channel are operated alternately, wherein during precooling of one VOCs flow hot air is introduced simultaneously into the VOCs channel of the other flow for flushing in order to ensure continuous production.The VOCs recovery system according to claim 1, characterized in that the gaseous air purification system comprises an air filter (1), a refrigerant dryer (4) and an air cleaner (5), wherein the air liquefaction system comprises an air compressor (2), an air container (3), a turbo expander (6) and an air precooler (8).The VOCs recovery system according to claim 2, characterized in that inside the VOCs recovery cooling box, the air precooler (8) and the turbo expander (6) are disposed, and wherein an intermediate layer of the housing of the VOCs recovery cooling box is filled with a powder and vacuum sucked to be thermally insulating.The VOCs recovery system according to claim 1, 2 or 3, characterized in that, using liquefied air as a cooling medium for VOCs, supercooling and recovery of gaseous VOCs are realized, wherein air first enters the air filter (1) to remove the minute particles and contaminants contained therein, wherein an outlet of the air filter (1) is connected to an inlet of the air compressor (2) and an outlet of the air compressor (2) is connected to an inlet of the air container (3), and wherein the pre-purified air after being compressed by the air compressor (2) enters the air container (3) for storage.The VOCs recovery system according to claim 4, characterized in that an outlet of the air container (3) is connected to an inlet of the refrigerant dryer (4) and an outlet of the refrigerant dryer (4) is connected to a hot air inlet of the air precooler (8) inside the VOCs recovery cooling box, wherein the precompressed air is further purified by the refrigerant dryer (4) and the air cleaner (5) to remove water vapor and gaseous contaminants.The VOCs recovery system according to claim 5, characterized in that a hot air outlet of the air precooler (8) is connected to an inlet of the turbo expander (6), an outlet of the turbo expander (6) is connected to an air inlet of the VOCs condenser (7) and an air outlet of the VOCs condenser (7) is connected to a cold air inlet of the air precooler (9), wherein the precooled air after liquefying via the turbo expander (6) enters the VOCs condenser (7) to exchange heat with gaseous VOCs, then it enters the air precooler (8) to exchange heat with the hot air coming from the air cleaner (5), thus realizing cascade utilization of the cooling performance.The VOCs recovery system according to claim 1, characterized in that a gas outlet of the gas-liquid separator (10) is connected to an inlet of the VOCs precooler (9), wherein a heat exchange is performed again between the standard-compliant low-temperature exhaust gas flowing out of the VOCs recovery cooling box and high-concentration gaseous VOCs to realize a highly efficient use of the energy and thus further reduce the recovery cost.The VOCs recovery system of claim 1, characterized in that the liquefied air channel is a microchannel having a diameter of less than 1 mm and the VOCs channel is a wide channel.The VOCs recovery system according to claim 1, characterized in that the respective VOCs inlet (9-1; 9-3) and VOCs outlet (9-2; 9-4) are symmetrically disposed at the left and right sides of the precooler (9), respectively, while the liquefied air inlet (9-5) and outlet (9-6) are symmetrically disposed at the upper end and the lower end of the precooler (9), the gaseous VOCs in the VOCs precooler (9) and the liquefied air flowing in directions crossing each other.The VOCs recovery system according to any one of claims 1 to 9, characterized by further comprising a microchannel mixer (11), wherein the gases flowing out of the gas-liquid separator (10) enter the microchannel mixer (11) and are directly mixed with liquid air, wherein the separated liquid components enter the container for storage while the low temperature gases enter the next process.

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