WO1988001534A1

WO1988001534A1 - Process and apparatus for recovering impurities, in particular solvents, contained in gases

Info

Publication number: WO1988001534A1
Application number: PCT/FI1987/000113
Authority: WO
Inventors: Veli Sarkkinen; Kosti Mokkila
Original assignee: Seinäjoen Kylmäkone A&T Saikkonen Ky
Priority date: 1986-08-28
Filing date: 1987-08-28
Publication date: 1988-03-10
Also published as: AU7859587A; FI863489A0; FI863489A; FI82200B; FI82200C

Abstract

Process and apparatus for recovering solvent vapours contained in gases. The process comprises adsorbing the solvents onto a layer (2) of fibrous activated carbon and desorbing them therefrom by increasing the temperature of the layer. According to the invention, the layer (2) is heated by conducting a heat releasing medium, e.g. water steam, hot water or hot oil, through a heat exchanger means (1) fitted within the layer. The heat exchanger means (1) has a substantially cylindrical envelope surface and, in a preferred embodiment of the invention, comprises a metal tube spiral. During the regeneration of the activated carbon, a gas flow is conducted through the layer (2) or, alternatively, by the pressure over the bed is decreased to purge the vapourizing solvents. The adsorptive capacity of the activated carbon fibre layer (2) may be improved by conducting water, a cooling mixture of a coolant through the tube spiral.

Description

Process and apparatus for recovering impurities, in particular solvents, contained in gases

The present invention relates to a process according to the preamble of claim 1 for recovering impurities, in particula solvents, contained in gases.

According to such a process the compounds are removed from the gases by adsorbing them onto fibrous activated carbon, and desorbing them therefrom by raising the temperature of the activated carbon layer.

The invention further relates to an apparatus according to the preamble of claim 8 for implementing said process.

New laws and regulations in atmospheric control have made the recovery of solvents used in the industry from exhaust air evermore timely.

Usually, water-unsoluble solvents, such as aromatic, aliphatic and chlorinated hydrocarbons, are removed from air by adsorption onto activated carbon. The adsorbed sol¬ vent is regenerated by steam, the steam-solvent-mixture con- densed and the water removed therefrom.

When solvents that are fully or partly water-soluble are concerned, the condensate(s) obtained, i.e. the aqueous organic phase and the aqueous phase containing organic sol- vent, must, however, be further purified by distillation.

This causes considerable additional costs and makes the re¬ covery thereof economically uninteresting.

The extended adsorption time of granulated activated carbon limits the use thereof for purification of air. Therefore, activated carbon equipment of extremely large capacity is required for recovering solvents present in small amounts (less than 1000 ppm). These are technically practically al¬ most impossible to implement. As mentioned above, a cloth of activated carbon fibres is used in the process according to the invention. Such a pro¬ duct is comercially available, for instance, under the trademark CHARCOAL CLOTH. FI Patent Application 842732 describes the use of fibrous activated carbon for recovering solvents. According to that process, the adsorptive capacity of the activated carbon fibre cloth used is improved by applying a DC voltage across the fibrous layer. The activa¬ ted carbon layer is supported sideways by a separate support gauze. In order to regenerate water soluble solvents, the activated carbon fibre bed is heated with an AC current and the solvent stripped by conducting a hot inert gas flow, preferably deoxygenized flue gas, through the activated car¬ bon.

According to the above-mentioned application, the adsorption bed is configured in the form of a cylinder rendering the gas flow even over the entire surface of the activated car¬ bon laye .

The particular drawback related to that process lies in the fact that, no matter how extensively all possible risk factors may have been considered during installation, the use of electricity in air containing inflammable solvents always involves a risk of explosion. The energy provided by electricity is also rather expensive, which increases the recovery costs. The activated carbon layer supporting gauze increases the gas flow resistance, larger blowers being needed for compensating the pressure drop.

The present invention aims at removing the drawbacks related to the known technique while providing a novel process and apparatus for recovering impurities from gases.

The invention is based on the idea that the activated carbon fibre bed is heated by a heat exchanger means having a substantially cylindrical envelope surface. At the same time, said means is also a support means for the bed. More particulary, the process in accordance with the invention is mainly characterized by what is stated in the characterizing part of claim 1.

in the process in accordance with the invention, the absorp tive capacity of the activated carbon bed may, if desired, be improved by cooling the bed by said heat exchanger means This makes it possible to recover small amounts of impuriti By means of the apparatus, it is also possible to recover solvents, the boiling point of which is low (below room temperature), for instance ethylene chloride.

The envelope surface of the heat exchanger and support mean may according to the present invention partly, but only partly, coincide with the actual surface of the means.

Structurally the heat exchanger means may be open sideways to permit free gas flow through the activated carbon layer, either from the outside to the inside or in the opposite direction. At least one gable of the heat exchanger means i substantially open to allow air to be conducted through the means.

Preferably the heat exchanger and support means in accordan with the invention comprises a metal tube spiral, through which a heat releasing or heat accumulating medium is con¬ ducted. When the tube spiral is used for heating the bed, preferably steam (e.g. water steam at 80°C to 150°C) or a hot liquid, such as water or oil, is used as said medium. The oil may, e.g., be selected from the group consisting of paraffin, silicone and di-n-butyl-phtalate oils. As media for cooling the bed, water, cooling mixtures, such as ice- water mixtures or mixtures of alkali metal or alkaline eart metal chlorides and water or ice, or coolants, such as ammo nia or chlorofluorocarbons may be employed.

Since the medium conducted through the sprial is normally pressurized, the spiral is preferably manufactured from a metal pipe having a circular cross section. The cross secti- on may, however, be oval or square, if the strenght proper¬ ties required by the medium and the mode of application are fulfilled.

in addition to a tube spiral, other heat exchanger means having a cylindrical envelope surface may also be used. Thus, a pipe bend cofigured in the form of a cylinder having alternately upwards and downwards vertivally extending loops .

The heat exchanger means may further also comprise a cylindrical tank with an annular cross section, having a plurality of apertures expending through the walls of its jacket. In said tank, there should be openings in at least one of its gables.

According to the invention, the adsorption bed is regenera¬ ted by heating it by the means having a cylindrical envelope. The steam pressure of the adsorbed solvents is gradually increased until the solvents eventually desorb. The solvent vapours may subsequently be purged from the adsorption apparatus by means of pressurized gas. If the adsorbed solvent is not water-soluble, e.g. an aromatic hydrocarbon, water steam may be used as flushing gas, otherwise air or inert gas is used.

There is no need for using excessive amounts of regeneration gas. Condensation of the solvent in the condensor creates a pressure difference between the adsorption vessel and the condensor, which causes the desorbed solvent vapours to flow towards the condensor. However, a small regeneration gas flow is still needed for ensuring that no larger solvent concentration are left in the gas space of the vessel.

The above-described adsorption bed structure is steady as well as extremely well pervious to air (the flow resistance is small). The adsorbed solvents may therefore be regenerate according to a preferable mode of application by vacuum, or, to be more precise, by decreased pressure. It is preferable to operate at between 1 and 90 kPa, in particular at between about 10 and 40 kPa absolute pressure. The decreased pressu is provided by some vacuum pump known per se, such as an oi pump or a steam ejector or a diffusion pump. The solvent vapours desorbed are conducted via the vacuum pump to a condensor operating at normal pressure, where they are con¬ densed by a cooling media, usually water. A water separator means, e.g. a liquid nitrogen trap, may be added to the apparatus, if needed.

Vacuum regeneration is particulary well suited for recoveri substances having a low flame point and/or high boiling point at normal pressure as well as substances that polyme¬ rize, decompose etc. under the influence of heat. Fully or. partly water-soluble substances are recovered from the con- densor unmixed with water as no steam is directly used in the desorption. The bed is heated in connection with vacuum regeneration by, e.g. warm water or low pressure steam.

Vacuum regeneration provides yet another advantegeous feature: By stopping the heating of the bed at a suitable point of time (to be empirically determined), adiabatic cooling of the bed is accomplished as the adsorbed substanc takes up its heat of desorption from the heat content of the bed. Thus, the bed is cold after the regeneration and it ma directly and efficiently be used for adsorption without a separate cooling stage. The cycle lenght is therefore significantly shortened.

The activated carbon adsorption process and apparatus according to the invention, in particular in combination with vacuum regeneration, is especially well suited for the recovery of certain gr.oups of compounds, examples of which are given in the following summary:

Chlorinated solvents forming, e.g. corrosive HC1, upon ther¬ mal decomposition: 1,1,1-trichloroethane (= methylchloroform ethylene and ethylene chloride.

Easily decomposing solvents: Esters such as ethylene acetate butyl acetate, methyl formate and ethyl formate; Ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone_? Aldehydes: such as formaldehyde (formalin) and benzaldehyde; Acetals.

Solvents having a low boiling point (low point of self-ignition): ethylene chloride, diethyl ether, ethylene oxide, and carbon disulphide.

Polymerizing solvents: styrene monomer, methyl methacrylate, ethylene oxide and propylene oxide.

High boiling solvents (boiling point over 180°C): aliphatic hydrocarbons, aromatic hydrocarbons, such as toluene and pyridine and derivatives thereof.

Considerable advantages are achieved by means of the solution according to the invention. Thus, the risk of explosion posed by electric equipments may now be avoided. Since the bed is not directly heated by means of water steam blown-in, drying of the bed at the end of the desorption stage is unnecessary. The cycle lenght is shorter than that for water steam de¬ sorption. The energy costs are decreased in comparison with electric heating: the costs of the medium conducted through the tube spiral are considerably less than those for elect- ric energy. Thus, energy costs for steam are only about 25 % of the corresponding cost for electric energy.

Since the heat exchanger spiral also forms a support means for the activated carbon bed, no separate support construc- tion is needed. The air flow through the adsorption bed is facilitated.

There are several further benefits to be gained from the vacuum desorption in accordance with the invention.

Provided the pressure is sufficiently decreased, the concentration of oxygen in the apparatus is low enough not to create a risk for ignition of the solvent vapours. This increases the safety, especially during recovery of substan- ces having a low flame point. By lowering the operational pressure the desorption temperature is lowered and the re¬ quired amount of desorption energy is decreased. The use of expensive hot oil systems may be avoided. In comparison wit other desorption processes, the lower desorption temperatur diminishes thermal decomposition or polymerization or other similar, undesirable reactions of the adsorbed substances. In this way, there may be avoided investment and operationa costs caused by the separation of reaction products from th recovered solvents or by the elimination of the detrimental effects of said reaction products, e.g. by adding neutrali¬ zing or stabilizing substances to the recovered solvent.

Pumped by the vacuum pump, most of the desorbed substances are discharged from the system in the form of clean, con¬ centrated gas streams. In comparison with inert gas desorp¬ tion, the absence of non-condenseable gases gives the bene¬ fit of smaller heat exchanger means and lower operational costs, partly compensating the additional investment costs caused by the vacuum pump.

Further scope of applicability of the present invention wil become apparent from the detailed description given herein¬ after. However, it should be understood that the detailed description and specific examples, while indicating prefer¬ red embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the are from this detailed description.

The figur.e is a sectional view of the apparatus according t the invention.

The apparatus in accordance with the invention comprises a heat exchanger tube spiral 1, and a layer of activated carb 2 wound around said tube spiral. The spiral may be made out of some suitable heat conducting metal not susceptible to corrosion caused by the impurities in the contaminated air.

Thus, different kinds of stainless steel tubes and acid proof steel tubes as well as copper and copper alloy tubes may be used.

In order to achive a large heat transfer surface (and a good support surface), there should be enough worms in the tube spiral. The amount of worms in vertical direction is prefe¬ rably 5 to 30, in particular 10 - 15 per metre.

One end of the tube spiral 1 is attachable on one hand to a source of a heat releasing medium and on the other to a source of a heat accumulating medium, not depicted in the figure. The heat releasing medium used generally comprises steam (pressurized water steam) and the heat accumulating medium water or mixtures of salt and water and/or ice. The medium is conducted either once through or_r preferably, circulated several times through the tube spiral.

The fibrous activated carbon 2 is preferably in the form of a woven cloth or mat, easily handled. One preferable product is the woven cloth supplied by Charcoal Cloth Limited under the name "Charcoal Cloth", having fibres of good adsorptive capacity and consisting to substantially 100 % by weight of activated carbon. At least one, preferably 5 to 15 layers of activated carbon are fitted around the tube spiral 1. The strength of the layers of activated carbon fibres may of course, if desired, be increased by bonding a cloth of natural or synthetic fibres. This measure is not very often necessary in the solution in accordance with the invention, and as it obstructs the gas flow it may, in fact, even be detrimental when it comes to the operation of the gas scrubber.

The fibrous activated carbon bed 2 is mounted into a tank or vessel 3, provided with a supply duct 4 for the contaminated solvent-rich air, a withdrawal duct 5 for the purified air and supply and withdrawal ducts 6 and 7, respectively, for the regeneration gas. One gable of the fibrous activated carbon 2 configured in the form of a cylinder, i.e. the top gable in the figure, is closed by an air-tight sheet 8, whereas the other gable, i.e. the lower gable, is open and .surrounds the opening of the air withdrawal duct 5. The lenght of the activated carbon cylinder 2 is at least some¬ what smaller than the inner height of the vessel 3, whereby the air feed from the supply duct 4 is allowed to pass the air-tight gable of the cylinder. The contamined air is therefore conducted from the supply duct 4 to the vessel 3 and further through the bed such that the air flows through the torus formed between the outer face of the fibrous acti vated carbon cylinder and the inner face of the vessel 3. Having reached the inner space of the cylinder, the air is discharged from the adsorber through the withdrawal duct 5. The air may, within the scope of the invention, also flow i the opposite direction, the air being conducted via duct 5 to the cylinder 2, through the wall of the activated carbon layer and discharged from the vessel via duct 4. This direction of flow is chosen especially when the tube spiral 1 is used for cooling the bed 2 in order to improve its ad¬ sorptive capacity. The tube spiral 1 cools the air as it reaches the activated carbon, whereby the total amount of cooling effect needed is diminished.

Air is preferably conducted once through only, but, if needed, it may of course be circulated to improve the adsorp tion process.

During regeneration, the activated carbon bed is heated by the tube spiral 1 at the same time as air, inert gas (nitrogen, argon, deoxygenated flue gas) or steam is fed through supply duct 6. According to the solution according to the invention, it is not necessary to use warm regenera- tion gas. The medium which is conducted through the heat exchanger tupe spiral heats not only the bed 2 but also the gas outwards flowing gas. The solvent containing regeneration gas is conducted to a condensor where the solvent and the other possible impurities are separated. In an alternative embodiment of the invention, the supply and withdrawal ducts 4 ja 5 are closed by means of valves (not shown), and the pressure inside the vessel 3 is reduced by a vacuum pump connected to pipe 7. A separate supply duct 6 for the regeneration gas is not actually needed in this embodiment. However, if such a duct has been fitted to the vessel to permit the use of the other alternative regene¬ ration processes, it is also closed by a valve in connection with the vacuum regeneration.

At a gas recovery plant, there are preferably at least two adsorption units having the above-described construction in accordance with the invention. Thus, it is possible to rege¬ nerate one unit while the other is operated.

In an exemplifying embodiment, the tupe spiral 1 was made of acid proof steel, having a diameter of 40 cm, height 80 cm and a screw pitch of 12 cm. 10 layers of a woven activated fibre cloth ("Charcoal Cloth") were wound around the tube spiral 1, the total amount of activated carbon being 1.1 kg.

Example 1:

Isopropanol (IPA) was recovered from industrial exhaust air by the apparatus described above. The adsorption was performe by cooling the adsorption bed as well as without cooling. In the following, the test results of the air purification are summarized:

a) Adsorption stage:

1. The inlet flow was 480 m*Vh, the air-flow through the bed (in perpendicular direction to the bed) 0.30 m/s. The gas contained 450 mg/m3 of isopropanol. The activated carbon bed was not cooled. The isopropanol concentration in the outlet air was 360 mg/m^, yield = 20 %. The dynamic capacity of the activated carbon fibres was 4.3 %. 2. The flow rate and the composition of the inlet flow were the same as above. Potassium chloride brine, the temperature of which was +5°C, was conducted through the tube spiral. The solvent concentration in the outlet air was 54 mg/m^, yield = 88 %. The dynamic capacity of the activated carbon fibres was 19 %.

b. Regeneration stage:

The isopropanol containing activated carbon bed of the above alternative 2 was regenerated in the following manner: Steam of about 130°C was conducted through the tupe spiral to heat the activated carbon fibre bed up to the boiling point of IPA. Warm air was fed to the vessel, the isopropanol was distilled off and flushed from the vessel, and the air-IPA-mixture was condensed in a condensor and reused. 178 g of isopropanol was obtained.

Example 2:

The apparatus in accordance with the invention was used for recovering 1,1,1-trichloroethylene. The following test re¬ sults were obtained:

a) Adsorption stage:

The inlet flow was 600 m h, the air-flow through the be (in perpendicular direction to the bed) was 0.20 m/s . The amount of 1,1,1-trichloroethylene was 20 g/m-3. The acti- vated carbon fibre bed was not cooled. The trichloro- ethylene concentration of the outlet air was 0.5 g/rπ.3. The dynamic capacity of the fibrous activated carbon was 20 %.

b) Regeneration stage:

The bed was regenerated by creating a decreased pressure in the apparatus . The pressure level was about 30 kPa absolute pressure. The bed was heated by conducting water at 90°C through the tube spiral. The 1,1,.1-trichloro- ethylene was discharged from the system through the vacuu pump to a condensor operating at normal pressure. At the condensor it was condensed by the indirect effect of the cooling water.

No increase in the acidity of the recovered 1,1,1-tri- chloroethylene or other indices of thermal decomposition could be detected.

The adsorption-desorption cycle was repeated with the same batch of solvent several times. However, unlike the recovery processes based on e.g. water steam regeneration, not even during repeated operation did this process result in any traceable decomposition of the solvent.

Claims

WHAT IS CLAIMED IS

1. A process for recovering impurities, in particular solvents, from gases, which comprises - adsorbing the impurities onto a layer of fibrous activated carbon (2), and - desorbing them therefrom by increasing the temperature of said layer of activated carbon (2), c h a r a c t e r i z e d in that the layer (2) is heated by conducting a heat releasing medium through a heat exchanger means (1) fitted within the layer, said means (1) having a substantially cylindrical envelope surface, and forming a support means for said layer (2).

2. The process according to claim 1, c h a r a c t e r ¬ i z e d in that water steam, hot water or hot oil, the temperature of which is between 80 and 150°C, is conducted through said heat exchanger means.

3. The process according to claim 1 or 2, in particular for recovering solvents having a low boiling point, c h a r a c t e r i z e d in that the active carbon layer

(2) is cooled during the adsorption stage by conducting heat accumulating medium through said heat exchanger means.

4. The process according to claim 3, c h a r a c t e r ¬ i z e d in that water; a cooling mixture, such as a mixtur of ice and water or a mixture of an alkali metal or earth alkaline metal chloride and water or ice; or a coolant, suc as ammonia or a chlorofluorocarbon are conducted through said heat exchanger means (1).

5. The process according to any previous claim, c h a r a c t e r i z e d in that gas, such as air or iner gas, is conducted through the layer (2) in connection with the heating thereof to desorb the adsorbed compounds.

6. The process according to any of claims 1 - 4, c h a r a c t e r i z e d in that the pressure over the bed is decreased to a value of 1 to 90 kPa, preferably of 10 to 40 kPa absolute pressure, to desorb the adsorbed com¬ pounds .

7. The process according to claim 6, c h a r a c t e r ¬ i z e d in that the heating of the bed is stopped at least some lenght of time before all of the substances adsorbed have been desorbed to cool the bed at the end of the desorp¬ tion.

8. An apparatus for implementing the process according to claims 1 - 7, comprising

- a tank (3) ,

- a layer (2) of fibrous activated carbon cloth fitted within the tank (3),

- ducts (4, 5) fitted in the tank for supplying and withdrawing the air which is to be purified,

- means (6, 7) connected to the tank (3) for regeneration of the layer (2), and - means (1) for heating the layer (2) of fibrous activated carbon, c h a r a c t e r i z e d in that said heating means (1) comprises a tupe spiral (1) around which the fibrous activa¬ ted carbon cloth is wound to form a cylindrical adsorption bed (2) .

9. The apparatus according to claim 8, c h a r a c t e r ¬ i z e d in that the tube spiral comprises in vertical direction 5 to 30, preferably 10 to 15 worm per metre.

10. The apparatus according to claim 8 or 9, c h a r a c t e r i z e d in that the tube spiral is made of a metal selected from the group comprising stainless steel, acid-proof steel, copper and copper alloys.