JPH06226029A - Method for recovering solvent - Google Patents

Abstract

PURPOSE:To provide the method for recovering a decomposable org. solvent by adsorption and desorption which improves the purity, solvent recovery rate and safety of the equipment suitable for treatment of the low-concn. solvent. CONSTITUTION:This method for recovering the solvent consists in subjecting one column of adsorption columns to desorption by the following stage sequence: (a) The stage of executing, >=1 times, the operation of reducing the pressure in the adsorption column 1 to be desorbed and replacing the inside of the column with an inert gas 17 and (b) the stage for heating 8 the replaced inert gas in circulation lines 1 22 6 32 27 8 1 including the adsorption column 1 and heating up the adsorption layer in the column are provided. Further, (c) the stage for stopping the circulation, reducing the pressure in the adsorption column 1 and desorbing the adsorption layer while heating up the inert gas 17 of <=1/10 the flow rate of the gas to be treated, then condensing and recovering 11 the solvent from the desorbing gas, (d) the stage for purging and cooling the adsorption layer while passing the inert gas 17 at a flow rate of <=1/10 the flow rate of the gas to be treated to the adsorption column 1 after the desorption and (e) the stage for introducing the treating gas of the adsorption column 2 under another adsorption operation into the adsorption column 1 with which the stage (d) ends and cooling the adsorption layer, then returning the treating gas used for the cooling to the side of the gas to be treated are provided.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for recovering a solvent,
In particular, the present invention relates to a solvent recovery method for separating a solvent component from a dilute solvent-containing gas containing a decomposable organic solvent (ketones, esters, halogenated hydrocarbons, etc.) using an adsorption tower having an adsorbent.

[0002]

2. Description of the Related Art The conventional technology is, for example, "pollution and measures vo
L.26 NO.12 (1990) P25 to P38 ”, and the description will be given below with reference to this. There are three technologies for removing and recovering organic solvents from exhaust gas: cooling, compression, and adsorption / desorption. The cooling method is divided into two depending on whether the cooling temperature is 0 ° C. or higher or 0 ° C. or lower. The equipment that cools at 5 ℃ or higher, which is commonly used, is relatively cheap,
The recovery rate of low boiling point compounds becomes low. A device that cools below 0 ° C has a high recovery rate, but water freezes. To prevent this, a defrost method that intermittently melts ice or an air shower method that absorbs water with calcium chloride or lithium chloride is used. Need to take.

The compression method is a method of pressurizing the vapor of an organic solvent and increasing the partial pressure above the saturated vapor pressure of the solvent to liquefy.
Since the temperature rises due to the compression heat, heat dissipation and cooling are required to increase the recovery rate. For example, if dichloromethane is compressed to 5 kg / cm ² and cooled to 5 ° C, it will be about -at 1 atm.
It is the same as when cooled to 30 ° C. That is, the recovery rate of dichloromethane or the like can be increased by using cooling together.
However, it is not applicable to low-concentration exhaust gas, so 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 and collected by a cooling method.

The types and characteristics of this adsorption / desorption method are shown below. (1) Preconcentration method using a 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 several 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. The honeycomb-type activated carbon drum is an apparatus used for the above-described preconcentration, and an apparatus that rotates the 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 formed into a cylindrical shape of 5 to 10 mm or crushed activated carbon for adsorption. In many cases, a device has been used in which a steath is sent every few hours to several days to desorb the organic solvent, and the organic solvent in the desorbed gas is recovered by a cooling method. Such a small device is installed in a dry cleaning machine and is also used for treating exhaust gas of dry cleaning.

(3) Method using fibrous activated carbon This is a method using fibrous activated carbon, and a small fixed bed apparatus is known. The apparatus is one in which fibrous activated carbon molded into a hollow cylinder is attached to one or two cylinders, and adsorption and steam desorption are alternately repeated every 10 to 20 minutes. There is also one in which fibrous activated carbon formed in a mat shape is provided in two stages and adsorption and desorption by steam or hot air are repeated every 10 to 20 minutes.

(4) Method using spherical activated carbon Adsorption / desorption is continuously performed in a fluidized bed using spherical activated carbon. This device also has a system in which an adsorption tower and a desorption tower are vertically connected. 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 Adsorption is carried out on a fixed bed using honeycomb-like activated carbon, and desorption is carried out by electrically heating under reduced pressure. Since this device has a low ventilation resistance and a high adsorption speed, it can adsorb at a high flow rate, and because steam is not used for desorption, it has the features that water does not enter the collected liquid and wastewater treatment is easy. . 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.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a solvent recovery method using an adsorption / desorption method, which is suitable for treating low-concentration decomposable organic solvents, such as equipment safety, solvent recovery rate, An object of the present invention is to provide a method for recovering a solvent, which is characterized by a desorption method using reduced pressure and hot air inert gas (nitrogen, etc.) with improved purity.

[0010]

In order to solve the above problems, in the present invention, in a method of recovering a solvent using a plurality of adsorption towers, one of the adsorption towers after the adsorption operation is (A) to (e) are desorbed in this order, which is a method for recovering a solvent. (A) A step of reducing the pressure in the adsorption column to be desorbed and replacing the inside of the column with an inert gas at least once. (B) A step of heating the substituted inert gas while circulating it in a circulation line including the adsorption tower to raise the temperature of the adsorption layer in the tower. (C) The circulation is stopped, the pressure inside the adsorption tower is reduced, and the adsorption layer is desorbed while heating the inert gas at a flow rate of 1/10 or less of the gas to be treated, and then the solvent is condensed and recovered from the desorption gas. The process of doing. (D) In the adsorption tower after desorption, an inert gas is added to the gas to be treated 1
A step of purging and cooling the adsorption layer while flowing at a flow rate of / 10 or less. (E) A step of introducing the treatment gas of the adsorption tower in another adsorption operation into the adsorption tower after the step (d) to cool the adsorption layer and returning the treatment gas used for cooling to the gas to be treated side.

In the above method, in order to further improve the desorption rate, the steps (b) and (c) can be repeated by adding a step of returning to normal pressure with an inert gas after the step (c). . In the present invention, the solvent recovery from the desorbed gas in the step (c) is preferably performed using a cooling device including a cooler, a dehumidifier, and a chiller.

[0012]

According to the present invention, by adopting the above-mentioned structure, it has the following effects. Improvement of equipment safety When desorbing the solvent from the adsorption bed, the adsorption tower is heated after the inside of the adsorption tower is replaced with an inert gas, resulting in high safety. Also,
During the subsequent cooling, the inert gas is cooled to some extent until the temperature of the adsorption layer is lowered, and after the purge, a part of the processing gas in operation is cooled, so that the safety is high.

Improvement of solvent recovery rate The solvent is desorbed from the adsorption layer by depressurizing and heating, and the flow rate of the desorption inert gas is set to 1/10 or less (including zero) of the flow rate of the gas to be treated. A high concentration of desorption gas can be obtained. Further, by performing the deep cooling of the desorption gas by setting the deep cooling temperature higher than the melting point of the target solvent but as low as possible, the solvent recovery rate can be improved.

Improvement of purity of processing gas During cooling after desorption of the solvent from the adsorption layer, the inert gas for cooling and the gas to be processed during operation are not circulated but are returned to the gas to be processed again. Very little redeposition of solvent in.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below more specifically with reference to the drawings, but the present invention is not limited thereto. Example 1 FIG. 1 is a process diagram showing an example of the solvent recovery method of the present invention. In FIG. 1, 1 and 2 are adsorption towers, 3 and 4 are heaters or coolers provided in an adsorption layer, 5 is an exhaust gas blower, 6 is a blower, 7 is a vacuum pump, 8 and 9 are gas heaters, 10 is a cooler. , 11 is a condensate tank, 12 is a desuperheater, 13 is a chiller, 14 is an exhaust gas cooler, 15 and 16 are mist separators, and 17 is an inert gas supply device.

Next, the intentional recovery method will be described with reference to the process diagram of FIG. The gas to be treated containing the solvent is passed through the cooler 14 and the mist separator 15 by the exhaust gas blower 5 to 20 to 25.
After cooling to 0 ° C. and removing the mist, the solvent is removed in the adsorption layer of the adsorption tower 1, and the processing gas is released into the atmosphere.

After the treatment in the adsorption tower 1 is completed, the passage of the gas to be treated is switched to the adsorption tower 2. The judgment of switching is made by whether the solvent concentration in the processing gas exceeds a reference value, by a timer, or by using both of them together. The adsorption tower 1 is isolated from the solvent gas recovery line, the inside of the adsorption tower is decompressed by the vacuum pump 7, and then the inert gas is supplied from the dry inert gas supply device 17 to the adsorption tower to be replaced. This replacement operation is performed once or more, and it is confirmed with an oxygen concentration meter that the oxygen concentration is below the reference value, and the possibility of ignition at high temperatures is eliminated. The purge gas of the inert gas is returned to the gas to be processed side through the line of the vacuum pump 7.

The gas heater 8 is operated by operating the fan 6.
A line containing the adsorption tower 1 for the inert gas substituted through
That is, the temperature of the adsorption layer is raised by circulating heating in the line of 1 → valve 22 → blower 6 → valve 32 → valve 27 → heater 8 → 1. At this time, there are a method of raising the temperature of the adsorption layer to a reference value (around 100 ° C.) and a method of circulating and heating the temperature to a value before the reference value (60 to 80 ° C.).

The circulation is stopped, the pressure inside the adsorption tower 1 is reduced by the vacuum pump 7 (about 0.1 to 0.5 kg / cm ² ), and the flow rate of the dry inert gas supply device 17 is 1/10 or less of the gas to be treated. (Although it may be 0), the adsorption layer is desorbed while heating the inert gas through the gas heater 8. When the temperature of the adsorption layer when the circulation is stopped does not reach the reference value, desorption is performed while raising the temperature to the reference value. The desorbed gas is cooled by the cooler 10 (0-5
C.), the dehumidifier 12, and the chiller 13 (the chilling temperature is set higher than the melting point of the target solvent, but set as low as possible) to largely condense and recover, and the recovered gas is the gas to be treated. Return to the side. The completion of desorption is determined by whether the solvent concentration after deep cooling falls below a reference value, a timer, or a combination of both.
As the dehumidifier, a molecular sieve is usually used,
A rotary type or a batch switching type is applied.

After the desorption is completed, the adsorption layer of the adsorption tower 1 is purged and cooled while flowing the inert gas from the dry inert gas supply device 17 at a flow rate of 1/10 or less of the gas to be treated. In this step, in order to supplement the desorption and recovery step of, the vacuum pump is subsequently started while cooling, and at the end, the inside of the adsorption tower 1 is returned to normal pressure with a dry inert gas, up to a certain temperature in the middle of this step. When the adsorption temperature is lowered, the inside of the adsorption tower 1 is returned to normal pressure with dry inert gas, and then cooled again. From the beginning of this step, the inside of the adsorption tower 1 is returned to normal pressure with dry inert gas, and then cooled. There is a way to do it.
During cooling under reduced pressure when the vacuum pump is activated, it is desirable to utilize the recovery device (cooler 10, dehumidifier 12, chiller 13) to return the gas to the side of the gas to be treated. The cooling under normal pressure can be returned to the gas to be processed side by bypassing the recovery device by the original pressure of the dry inert gas supply device 17.

When the temperature of the adsorption layer of the adsorption tower 1 reaches a temperature at which it is safe to cool with the processing gas, for example, 50 to 80 ° C., the line of the valve 30 is used to change the temperature from the valve 27 to the heater 8.
The processing gas is introduced into the adsorption tower 1 by the blower 6 through the line (heater is off) to cool the adsorption layer, and after cooling, it is returned from the valve 31 to the target gas side. Cooling has an adsorption layer temperature of 3
Perform until 0 ° C or lower. By the above operation, the adsorption tower 1 is desorbed and regenerated, and when the adsorption treatment of the adsorption tower 2 is completed, the adsorption tower 1 is used for adsorption and is recycled.

[0022]

EFFECTS OF THE INVENTION According to the present invention, a decompression operation is performed by adding a decompression, heating, and chilling device to a gas capable of safely adsorbing a decomposable organic solvent and containing a low concentration solvent. As a result, the recovery rate of the solvent can be improved. Also, even when the adsorption layer is cooled after desorption, when the adsorption layer is cooled by initially purging with an inert gas and cooling the adsorption layer to some extent by cooling, the processing gas is partially cooled. Since the amount of solvent adhering to the column can be considerably reduced, the treatment performance of the adsorption tower could be improved.

[Brief description of drawings]

FIG. 1 is a process diagram showing an example of a solvent recovery method of the present invention.

[Explanation of symbols]

1, 2 ... Adsorption tower, 3, 4 ... Heater or cooler, 5 ... Exhaust gas blower, 6 ... Blower, 7 ... Vacuum pump, 8, 9 ... Gas heater, 10 ... Cooler, 11 ... Condensate tank, 12 ...
Dehumidifier, 13 ... Chiller, 14 ... Exhaust gas cooler, 15, 1
6 ... Mist separator, 17 ... Dry inert gas supply device, 18-37 ... Valve,

Continuation of front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location // F25J 3/08 8925-4D (72) Inventor Kaoi Murakoshi 4-2-1 Motofujisawa, Fujisawa-shi, Kanagawa No. Stock Company Ebara Research Institute

Claims (3)

[Claims] 1. A method for recovering a solvent using a plurality of adsorption towers, wherein one of the adsorption towers after the adsorption operation is completed is desorbed in the order of steps (a) to (e) below. Solvent recovery method. (A) A step of reducing the pressure in the adsorption column to be desorbed and replacing the inside of the column with an inert gas at least once. (B) A step of heating the substituted inert gas while circulating it in a circulation line including the adsorption tower to raise the temperature of the adsorption layer in the tower. (C) The circulation is stopped, the pressure inside the adsorption tower is reduced, and the adsorption layer is desorbed while heating the inert gas at a flow rate of 1/10 or less of the gas to be treated, and then the solvent is condensed and recovered from the desorption gas. The process of doing. (D) In the adsorption tower after desorption, an inert gas is added to the gas to be treated 1
A step of purging and cooling the adsorption layer while flowing at a flow rate of / 10 or less. (E) A step of introducing the treatment gas of the adsorption tower during another adsorption operation into the adsorption tower after the step (d) to cool the adsorption layer, and returning the treatment gas used for cooling to the gas to be treated side. 2. The recovery of the solvent according to claim 1, wherein the step (b) and the step (c) are repeated after the step (c) by adding a step of returning to normal pressure with an inert gas. Method. 3. The solvent recovery from the desorbed gas in the step (c) is performed by using a cooling device including a cooler, a dehumidifier, and a chiller. Method.