JPH07265642A - Solvent recovery method - Google Patents

Abstract

PURPOSE:To provide a solvent recovery method capable of sufficiently heating a desorption tower and cooling the tower in various manners after desorption and capable of conducting efficient desorption. CONSTITUTION:A solvent is recovered by an adsorption tower. The adsorption tower after finishing adsorption is successively used in stages (a)...(d) to desorb the solvent. (a) The gas in the adsorption tower 1A is circulated through the circulating lines 5, 18A, 15A, 2A, 1A and 19A including the tower and heated to heat the adsorption bed in the tower. (b) The circulation is stopped, the tower is evacuated 6, air or an inert gas at the flow rate of <=1/5 of that of the gas to be treated is heated 2A and supplied into the tower to desorb the adsorbed solvent, and the solvent is condensed 7 and recovered 8 from the desorbed gas. (c) The heating is stopped, air or an inert gas is passed through the tower still evacuated at the flow rate of <=1/5 of that of the gas to be treated to perform cooling and desorption in the tower. (d) The tower is returned to normal pressure, and the air as such or the cooled air is supplied 20 to the high-temp. adsorption bed to cool the bed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering a solvent using an adsorption tower, and more particularly to a method for recovering a solvent containing a decomposable organic solvent (ketones, esters, halogenated hydrocarbons, etc.) from a dilute solvent-containing gas. The present invention relates to a solvent recovery method.

[0002]

2. Description of the Related Art The conventional technology is, for example, "pollution and measures vo
L.2, No. 12 (1990) P25 to P38 ", and will be explained while pulling out. There are three technologies for removing and recovering organic solvents from exhaust gas: cooling, compression, and adsorption / desorption. The cooling method is divided into 2 minutes depending on whether the cooling temperature is 0 ° C. or higher or 0 ° C. or lower. A device that cools at 5 ° C. or higher, which is commonly used, is relatively cheap, but the recovery rate of low boiling point compounds is low. The equipment that cools below 0 ° C has a high recovery rate, but since water freezes, in order to prevent this, a defrost method that melts ice intermittently or an air shower method that absorbs water with calcium chloride or lithium chloride is used. It is necessary to take measures. The compression method is a method of pressurizing the vapor of the organic solvent and increasing the partial pressure above the saturated vapor pressure of the solvent to liquefy it.However, since the temperature rises due to the compression heat, heat dissipation and cooling are required to increase the recovery rate. Is required. For example, 5 kg / cm ^{2 of} dichloromethane
If compressed to 5 ° C. and cooled to about −30 ° C. at 1 atm, it is the same. That is, the recovery rate of dichloromethane or the like can be increased by using cooling together. However,
Since it is not applicable to low-concentration exhaust gas, there are not many examples of use so far.

The adsorption / desorption method is a method in which an organic solvent is adsorbed on activated carbon or the like, desorbed by steam or hot air, and the desorbed high-concentration organic solvent vapor is liquefied by a cooling method and recovered. The types and characteristics of this adsorption / desorption method are shown below. 1) Preconcentration method using honeycomb-type activated carbon drum In a general adsorption / desorption device, if a large amount of exhaust gas is treated at a low concentration of tens of ppm or less, the device becomes large, which is disadvantageous. Therefore, in such a case, pre-concentration is performed in advance by a simple adsorption / desorption device, and then a full-scale removal / recovery device is used. As a simple preconcentrating device, a device that rotates a honeycomb-type activated carbon drum to repeat adsorption and desorption in a short time is used. 2) Method using granular activated carbon For adsorption and desorption of organic solvent vapor, exhaust gas is sent to a large fixed bed filled with activated carbon molded into a cylindrical shape of 5 to 10 mm or crushed activated carbon to adsorb it, and from several hours A device that sends steam every few days to desorb it and recovers the organic solvent in the desorbed gas by a cooling method has been widely used.

3) Method using fibrous activated carbon A small fixed bed apparatus using fibrous activated carbon is one in which one or two fibrous activated carbons formed into a hollow cylinder are attached and every 10 to 20 minutes. Adsorption and steam desorption are repeated alternately. Further, two stages of fibrous activated carbon formed into a mat shape are provided, and adsorption and desorption with steam or hot air are repeated every 10 to 20 minutes. 4) Method using spherical activated carbon There is also a system in which an adsorption tower and a desorption tower are vertically connected to each other in a device for continuously adsorbing / desorbing in a fluidized bed using spherical activated carbon. In this apparatus, exhaust gas is passed from the lower part of the adsorption tower divided into several stages, activated carbon is made to flow, and it is adsorbed while being successively dropped to the lower stage, and the adsorbed activated carbon is desorbed by heating in the desorption tower. In this case, the steam is not directly blown in, but the activated carbon is heated through a heat exchanger, expelled with a small amount of air, and cooled and recovered in the condenser. 5) Method using honeycomb-like activated carbon A device that adsorbs on a fixed bed using honeycomb-like activated carbon and desorbs it by heating it electrically while depressurizing it has a small ventilation resistance and a fast adsorption rate, so it can adsorb at a high flow rate. Since steam is not used for separation, there are features such as no water entering the recovered liquid and easy wastewater treatment. However, the adsorption capacity per volume is small, and desorption takes some time. There is also a type of this apparatus in which water vapor is directly introduced for desorption.

As described above, various adsorption / desorption methods have been conventionally known, but when steam desorption commonly used for activated carbon is carried out, cooling and condensation are performed to separate the solvent and water. However, in reality, the solvent slightly dissolves in the separated water, and it is difficult to remove the solvent sufficiently with simple auxiliary equipment such as the aeration method, and the cost of wastewater treatment is high to comply with strict environmental regulations. It is expected to increase more and more. As a countermeasure for that, there is TPSA (temperature pressure swing method) that uses gas for desorption, and has the following features compared to the wet desorption method that uses steam for desorption. Since steam is not used for desorption, the burden of water treatment of the recovered solvent is small. For desorption of the solvent from the adsorption layer, desorption temperature can be lowered by performing depressurization and heating, as compared with the case where depressurization is not performed, and safety is improved. In order to perform desorption efficiently, by setting the desorption gas flow rate to 1/5 or less of the flow rate of the gas to be processed (0 ° C., 1 atm standard), a high concentration desorption gas can be obtained, and recovery is advantageous.

However, the dry TPSA method has a large heat capacity as compared with fibrous activated carbon and the like, and has a large solvent adsorption amount, especially when a granular or spherical adsorbent is used as the adsorbent, so that the desorption gas is removed. It is necessary to raise the temperature sufficiently before passing it through. In addition, when the heating / desorption under reduced pressure shifts to the immediate cooling step, the temperature of the cooling gas rises for a while due to the heat capacity of the adsorbent. In the case of continuous operation using two or more adsorption towers, the temperature-rising gas is usually merged with the upstream of the adsorption tower during the adsorption operation, but after the merging, the gas temperature rises to cause adsorption. There were problems such as adverse effects.

[0007]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, can sufficiently raise the temperature of the adsorption tower, can perform various cooling after desorption, and can perform an efficient desorption operation. It is an object to provide a recovery method of.

[0008]

In order to solve the above problems, in the present invention, in a method for recovering a solvent using an adsorption tower, the adsorption tower after the adsorption operation is
This is a method for recovering a solvent, characterized in that the solvent is desorbed by sequentially treating in the step (d). (A) a step of heating the inside of the adsorption tower while circulating it in a circulation line including the adsorption tower to raise the temperature of the adsorption layer in the tower; (b) stopping the circulation and depressurizing the inside of the adsorption tower. A step of supplying air or an inert gas at a flow rate of ⅕ or less of the gas to be treated into the tower while heating, desorbing the adsorbed solvent, and then condensing and recovering the solvent from the desorbed gas, ( c) A step of cooling and desorbing the adsorption tower while stopping heating and flowing air or an inert gas into the adsorption tower at a flow rate of ⅕ or less of the gas to be treated while keeping the pressure reduced.
After making the pressure inside the adsorption tower normal, in the adsorption layer at high temperature,
The process of cooling by supplying air (ambient air or compressed air).

In the above recovery method, the steps (a) and (b) can be repeated by adding a step of returning to normal pressure with air or an inert gas after the step (b). For cooling the adsorption layer in d), a cooler may be provided in a line for supplying air, or a cooler may be provided in a circulation line including an adsorption tower for circulation cooling. In the above method, the adsorption tower can be treated by sequentially repeating adsorption and desorption treatments using a plurality of adsorption towers. In this case, cooling of the adsorption layer in step (d) is performed by another adsorption tower instead of air. A part of the processing gas may be used with or without a cooler.

[0010]

In the present invention, the gas in the adsorption tower is heated while being circulated in the circulation line including the adsorption tower to raise the temperature of the adsorption layer in the tower to shorten the time. When the temperature of the adsorption layer decreases during the heating under reduced pressure desorption, the pressure is released, and then the temperature of the adsorption layer is circulated again to perform the heating under reduced pressure desorption, so that the temperature of the adsorption layer can be sufficiently increased. Next, after heating and desorption under reduced pressure, only heating is stopped, cooling and desorption are performed, and after the temperature of the adsorption layer is lowered to a predetermined temperature, the adsorption layer is cooled. ) It has a great effect on the pre-cooling of the adsorbent layer filled with the adsorbent, and can suppress the temperature rise of the gas to be treated when the cooling air joins the gas during the adsorbing operation at the time of cooling the adsorbent layer. As for the cooling method which is the final step of desorption, various cooling methods can be adopted in consideration of plant circumstances.
Further, generally, if the depressurization heating desorption gas flow rate and the cooling desorption gas flow rate are set to a large value, the desorption rate improves, but the solvent concentration in the desorption gas decreases, which is disadvantageous for the condensation recovery, and the amount which cannot be condensed and recovered. Cannot be released into the atmosphere as it is, and, for example, in the case of continuous operation, it is combined with the gas to be treated in the adsorbing tower during operation. Therefore, in the present invention, the desorption gas flow rate is limited to ⅕ or less (0 ° C., 1 atm standard) of the gas flow rate to be treated.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings, but the present invention is not limited thereto. Example 1 FIG. 1 is an explanatory diagram of a solvent recovery flow according to the solvent recovery method of the present invention. This figure shows an example of two adsorption towers, and the same can be done with more adsorption towers. In FIG. 1, 1
A and 1B are adsorption towers, 2A and 2B are heaters, 3 is an exhaust gas cooler, 4 is an exhaust gas blower, 5 is a circulation blower,
6 is a vacuum pump. Further, 7 is a condenser of the desorbed solvent, 8 is a water separator, 9 is a solvent recovery tank, 10 is a solvent transfer pump, 11-13 are filters, and 14A-
Reference numerals 23 and 23 respectively represent valves.

Next, the operation method will be described based on the solvent recovery flow chart of FIG. The gas containing the solvent is cooled to 20 to 25 ° C. by the exhaust gas blower 4 through the exhaust gas cooler 3 and the filter 11 to remove dust, and then the solvent is removed in the adsorption layer of the adsorption tower 1A, and the treated gas is in the atmosphere. Is released to. (Open valves are exhaust gas inlet valve 14A, large flow valve 15A, exhaust gas outlet valve 17
A) After finishing the treatment, the gas passing through the adsorption tower 1B is switched (the valve 14A is closed and the valve 14B is opened). Whether the solvent concentration in the processing gas exceeds the standard value is determined by the switch.
Use a timer or use both. Adsorption tower 1
A is isolated from the ventilation line. (The valve 17A is closed and the valve 17B is opened) Using the circulation blower 5, the heater 2A is utilized and the temperature of the adsorption layer of the adsorption tower 1A is raised. (Circulation heating step, (a)) At this time, there are a method of circulating heating the adsorption layer temperature to a reference value (around 100 ° C) and a method of circulating heating to a value before the reference value (60 to 80 ° C). (The open valve is the circulation blower outlet valve 1
8A, large flow valve 15A, circulating blower inlet valve 19A)

The circulation is stopped, the pressure inside the adsorption tower 1A is reduced by the vacuum pump 6 (about 0.1 to 0.5 kg / cm ² abs.), And the gas to be treated is flowed at a flow rate of ⅕ or less of the gas to be treated ( The large flow valve 15A is closed and passed through the desorption valve 16A side), and the adsorption layer is desorbed while being heated by the heater 2A. (Desorption process, (b)) The desorption gas is introduced from the vacuum pump 6 through the filter 12 to the condenser 7 (0 to 5 ° C) to condense the solvent component, and then the non-condensable gas (air) is discharged to the upstream target. It is returned to the process gas side (open valves are exhaust gas outlet valve 17A, vacuum pump inlet valve 21A, vacuum pump outlet valve 23). Water in the recovered solvent is removed by the water separator 8 and the solvent content is stored in the solvent recovery tank 9. When the predetermined liquid level is reached, the solvent transfer pump 10 is activated to collect the liquid. The determination of the end of desorption is made by whether the solvent concentration after the condenser 7 falls below a reference value, by a timer, or by using both of them. The heating by the heater 2A is stopped, and other operations are the same. It is performed until the temperature of the adsorption layer reaches about 50 to 80 ° C. (Cooling desorption process, (c))

The vacuum pump 6 is stopped, the circulation blower 5 is utilized, the air (atmosphere air or compressed air) from the outside is used for cooling, and the gas heated by cooling the adsorption layer of the adsorption tower 1A is again covered. Return to the process gas side. (Cooling process,
(D)) Cooling is performed until the temperature of the adsorption layer becomes 40 ° C. or lower. (The open valve is the air introduction valve 20, the circulation blower outlet valve 18
A, large flow valve 15A, exhaust gas inlet valve 14A) After completion, wait until the processing of the adsorption tower 1B is completed. After the treatment of the adsorption tower 1B is completed, ventilation is switched to the adsorption tower 1A side again. It is desirable to place a dehumidifier in the air intake line from the outside. However, when the removal of the water-soluble solvent is the main constituent, the dehumidifier is not necessarily required as in the flow of FIG. For a solvent having a high vapor pressure and a low boiling point (for example, methylene chloride), the condenser 7 having a temperature of 0 to 5 ° C alone is insufficient,
Furthermore, a chiller of about -40 ° C is required in the subsequent stage. Further, for a solvent that is difficult to desorb, the desorption efficiency can be increased by repeating the operation of desorbing under reduced pressure and then returning the pressure inside the adsorption column to normal pressure and again depressurizing and desorbing again (one or more times). The cooling method in the final step of desorption can be arranged as shown in Table 1 depending on the plant circumstances. (Rough standard)

[0015]

[Table 1] Note: The gas in the plant situation is for cooling.

Further, in the case of recovering the decomposable solvent, when desorption using air is carried out, peroxide is generated and the temperature in the adsorption layer rises,
Risk of ignition. As a safety measure, the safety of the device is improved by using an inert gas (nitrogen or the like) during the desorption operation or the cooling desorption operation where the temperature of the adsorption layer becomes high. When an inert gas is used, the inert gas replacement is carried out before starting the step (a) of the present invention, but such an auxiliary operation is included in the present invention even if there is no description.

[0017]

According to the present invention, the following effects can be obtained. Since steam is not used for desorption, the burden of water treatment of the recovered solvent is small. Desorption of the solvent from the adsorption layer can be performed by depressurizing and heating, so that the desorption temperature can be lowered as compared with the case where the depressurization is not performed (TSA method), and the safety against the solvent is enhanced. Since desorption can be efficiently performed by the TPSA method, a desorption gas having a high concentration can be obtained by setting the desorption air device at 1/5 or less of the flow rate of the gas to be treated, and recovery is advantageous. Especially in the case of an adsorption layer filled with a granular (spherical) adsorbent, the desorption efficiency is improved. Since the gas in the adsorption tower is heated while being circulated in the circulation line including the adsorption tower, the time for raising the temperature of the adsorption layer in the tower can be shortened. After heating under reduced pressure and desorption, only heating is stopped and cooling desorption is performed, which is also effective for pre-cooling the adsorption layer filled with a granular (spherical) adsorbent having a large heat capacity and the heater in the cooling line.

[Brief description of drawings]

FIG. 1 is an explanatory view of a solvent recovery flow of the present invention.

[Explanation of symbols]

1A, 1B ... Adsorption tower, 2A, 2B ... Heater, 3 ... Exhaust gas cooler, 4 ... Exhaust gas blower, 5 ... Circulation blower, 6 ...
Vacuum pump, 7 ... Condenser, 8 ... Water separator, 9 ... Solvent recovery tank, 10 ... Solvent transfer pump, 11 ... Filter, 12
... filter, 13 ... filter, 14A, 14B ... exhaust gas inlet valve, 15A, 15B ... large flow valve, 16A, 16B ...
Desorption valve (small flow rate, normally open), 17A, 17B ... Exhaust gas outlet valve, 18A, 18B ... Circulation blower outlet valve, 19A, 1
9B ... Circulation blower inlet valve, 20 ... Air introduction valve, 21A,
21B ... vacuum pump inlet valve, 22 ... vacuum pump circulation valve,
23 ... Vacuum pump outlet valve

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Ishikawa 11-1 Haneda Asahi-cho, Ota-ku, Tokyo Inside the EBARA CORPORATION (72) Inventor Hirofumi Inagawa 11-11 Haneda-Asahi-cho, Ota-ku, Tokyo Inside the EBARA CORPORATION

Claims (5)

[Claims] 1. A method for recovering a solvent using an adsorption tower, wherein the adsorption tower after the adsorption operation is
A method for recovering a solvent, characterized in that the solvent is desorbed by sequentially performing the treatment in the step (d). (A) a step of heating the inside of the adsorption tower while circulating it in a circulation line including the adsorption tower to raise the temperature of the adsorption layer in the tower; (b) stopping the circulation and depressurizing the inside of the adsorption tower. A step of supplying air or an inert gas at a flow rate of ⅕ or less of the gas to be treated into the tower while heating, desorbing the adsorbed solvent, and then condensing and recovering the solvent from the desorbed gas, ( c) a step of cooling and desorbing the adsorption tower while stopping heating and flowing air or an inert gas into the adsorption tower at a flow rate of 1/5 or less of the gas to be treated while keeping the pressure reduced, (d) atmospheric pressure in the adsorption tower After that, a step of supplying air to the adsorption layer having a high temperature to cool it. 2. The solvent according to claim 1, wherein the step (a) and the step (b) are repeated by adding a step of returning to normal pressure with air or an inert gas after the step (b). Recovery method. 3. The cooling of the adsorption layer in the step (d) is characterized in that a cooler is provided in a line for supplying air, or a cooler is provided in a circulation line including an adsorption tower for circulation cooling. The method for recovering a solvent according to claim 1 or 2. 4. The adsorption tower is characterized in that a plurality of towers are used to sequentially repeat the adsorption and desorption processes.
Alternatively, the method for recovering a solvent according to 3 above. 5. The cooling of the adsorption layer in the step (d) is characterized in that instead of air, a part of the processing gas in another adsorption tower is used with or without a cooler. The method for recovering a solvent according to claim 4.