REGENERATIVE PROCESS FOR THE REMOVAL AND

RECOVERY OF VOLATILE ORGANIC COMPOUNDS FROM

EFFLUENT GASES

FIELD OF INVENTION

The present invention relates to the removal of volatile organic compounds (VOC's) from gas streams containing the same and recovery of the removed VOC's in liquid form. BACKGROUND TO THE INVENTION

Emissions of VOC's have been identified in various industries, including the automobile paint spraying, printing and publishing, wood product fabrications, pulp and paper, pharmaceuticals, adhesives and electronics industry.

The principal problem with such materials is that many VOC's are hydrocarbons which are photo-chemically active and thereby create urban smog and ozone or ozone precursors. Many VOC's are.unpleasantly odorous as well as present health hazards. Regulations exist in many countries to regulate such emissions, in waste water or soils and in air emissions.

To date, many abatement technologies have been introduced to control VOC emissions. Such technologies include:

♦ absorption and/or adsorption processes

♦ condensation and membrane separation

♦ thermal and catalytic incineration

♦ biological processes All the aforesaid abatement processes can be used to control VOC's in gaseous effluents to achieve various degrees of VOC removal. Except for thermal incineration, the above mentioned processes produce some form of waste which requires further expensive treatments. While producing no such waste, thermal incineration involves the generation of undesirable green-house gases, such as carbon dioxide.

SUMMARY OF INVENTION The present invention relates to a regenerative process for VOC abatement from various sources of effluent gases containing the same. In the prevent invention, the effluent gas stream containing the VOC's is contacted with a spray of fine liquid droplets of a suitable solvent for the VOC's and the VOC's thereby are absorbed into the solvent droplets and removed from the gas stream. The absorbing operation may be carried out at ambient temperature of about 20 to 25 ^β C or a below ambient temperature.

VOC's may comprise alcohols, esters, etones and aromatics of various types or other organic compounds and a solvent or solvents from which the spray is formed is chosen to suit the material comprising the VOC's. The VOC's include condensible and non-condensible VOC's, odorous and non-odorous VOC's and differing boiling point VOC's. The fine spray of liquid solvent droplets is formed in a flowing gas stream containing the VOC's in any convenient manner, such as by using dual-fluid spray nozzles.

The liquid droplets having absorbed VOC's can be coalesced to form a solution of the VOC's. Such solution may be regenerated and the VOC's recovered therefrom as a concentrated liquid. Such regeneration may be effected in any convenient manner, usually by employing a higher temperature than the absorbing step to drive the VOC's out of the absorbing medium. A regeneration temperature close to the boiling point of the VOC in question is particularly convenient. A combination of gas sparging and steam heating may be utilized to effect a highly efficient stripping of the VOC's, which then may be recovered by condensation as liquids for re-use or for sale. This removal and recovery procedure, therefore, enables low concentrations of VOC's contained in the gas stream to be concentrated and recovered in liquid form.

The sorbent or combination of sorbents used as the scrubbing medium recovered from the stripper is cooled and recycled to the absorber via the spray nozzles. The overall simplicity of the process results in a decrease in capital and operating costs while retaining high VOC removal efficiencies. As described earlier and shown in the Examples below, the regenerative process of the present invention is broadly applicable to a wide range of VOC's using suitable organic sorbents. The innovation of the present invention resides with its ability to control VOC's from effluent gases with a sorption process from which no waste is produced. VOC's are removed from the gas stream by a suitable solvent, which then is regenerated and recycled within a closed loop, while the VOC's are provided in a more concentrated liquid form. The cyclic procedure for removal and recovery of VOC's described herein possesses several advantages over known prior art for VOC removal or disposal. With respect to the prior art incineration process, the procedure of the invention has the advantages of:

- lower operating and capital costs

- lower energy consumption

- a non-corroding environment resulting from ambient temperature operation

- longer operating life

- ease of operation

- very low maintenance requirement

- environmentally friendly process Compared to activated carbon absorption, the procedure of the present invention has the advantages of:

- competitive capital and operating costs

- competitive energy consumption

- insensitivity to poisoning, blinding or plugging - does not require sorbent replacement

- stripping of sorbent conducted at relatively low temperatures (as compared to high temperature

stripping of loaded activated carbon and the normal destruction of same by incineration) - handling of a liquid sorbent is much easier than handling a solid sorbent - very low maintenance requirement environmentally-friendly process.

BRIEF DESCRIPTION OF DRAWINGS FIGURE 1 is a schematic representation of the regenerative process for VOC control in accordance with one embodiment of this invention;

FIGURE 2 is a schematic representation of the nozzle arrangement in two stages in the absorber used in the embodiment of Figure 1;

FIGURE 3 is a schematic representation of an apparatus used as a prescrubber in the embodiment of Figure 1;

FIGURE 4 is a graphical representation of the variation of scrubbing efficiency with time for four VOC's used to represent the paint VOC emission; FIGURE 5 is a graphic representation of the variation of regeneration efficiency of VOC stripping for one embodiment of this invention;

FIGURE 6 is a graphic representation of the variation of stripping efficiency of four paint VOC's at 140 ^β C using a batch operation as described in Example 3 below: and

FIGURE 7 is a graphical representation of the variation of stripping efficiency of four paint VOC's at 140 ^β C using a continuous operation as described in Example 3 below.

GENERAL DESCRIPTION OF INVENTION The process of the present invention is applicable to the removal and recovery of a variety of classes of volatile organic compounds, with the sorbent being chosen to correspond to the VOC present in the gas stream. Such classes of volatile organic compounds include condensible and non-condensible, odorous and non-odorous VOC's and

different boiling point VOC's. Gas streams treated in accordance with the invention may contain a number of different VOC's in which a mixture of sorbents may be used, if required, as the sorbent material. In general, the sorbents employed in the present invention are medium to high boiling stable organic compounds or mixtures of such compounds. The selective absorbent materials have very low vapour pressures, even at the temperatures that may be employed in the stripping step, for example, in the region of about 100 to 150 ^β C.

As an example, water soluble VOC's, such as alcohols, may be absorbed by contact with an aqueous solution of polyethylene glycol or simply water. Other suitable sorbent materials may be used for other VOC materials.

Such suitable sorbent materials are usually regenerable or recoverable simply by applying heat to remove the absorbed VOC's. The sorbent materials are usually used as a spraying medium to generate finely atomized spray droplets by a unique two-phase, atomizing spray nozzle design that has precise gas-liquid mixture control. Such nozzle allows for the flexibility required to control size and number of droplets necessary for efficient removal of solute gases such as VOC's. The two-phase spray nozzle designs are described in U.S. Patent No. 4,893,752 and in published International Patent Application No. WO92/04127, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference. The spray zones generated by such nozzles may be achieved in a horizontal or vertical duct, or in spray towers or packed columns, cr in any housing.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention is described further, by way of illustration, with reference to the accompanying drawings, in which Figure 1 is a schematic flowsheet of

a regenerative procedure for the removal of VOC's from gas streams, in accordance with one embodiment of the invention.

As seen therein, a gas stream 10 containing VOC's may first be passed through a prescrubber 12 to remove particulate materials 14 from the gas stream while cooling the gas stream to its adiabatic dewpoint temperature and then is passed through an absorber 16 in which the gas stream is contacted with very fine droplet sprays of a suitable absorbing solvent for the VOC's, to remove the VOC's from the gas stream and provide a clean gas stream 18 which is sent to a discharge stack.

One or more sprays may be formed in the absorber 16 by employing dual-fluid sprays in which very fine liquid droplet sprays of the solvent are formed by an atomizing gas countercurrent or cocurrent to the flow of the gas stream through the absorber 16. The fine liquid droplets, which may have a droplet size in the range of about 5 to about 200 microns, preferably about 30 to about 200 microns, provide a surface area for absorption up to about 100 times, usually about 30 to 50 times, that of a conventional packed tower. Such fine liquid droplets may be provided by employing an atomizing gas, such as air, at a pressure of about 10 to about 90 psig. The absorber 16, in effect, is an open-ended duct in which one or more dual-fluid spray nozzles produce a spray pattern which is designed to completely fill the duct while minimizing wall effects. Depending on the kinetics of the sorption process, duct velocities of up to about 40 to 50 fps can be employed with a very low pressure drop through the duct, which compares to a maximum of about 10 fps gas velocities for a packed bed contactor conventionally used to remove VOC's.

The gas stream flowing through the absorber 16 contacts fine liquid sprays, which causes rapid mass transfer of fine VOC's contained in the flowing gas stream to the liquid droplets to be absorbed therein.

thereby providing efficient removal of the VOC's from the flowing gas stream.

While less efficient than the spray-contact procedure, there may be employed, as the absorber 16, open spray towers of other constructions, packed bed contactors, tray columns or other conventional gas-liquid contactors. The gas stream leaving the absorber 16 passes through a mist eliminator to coalesce and remove droplets of liquid sorbent from the gas stream before the gas stream passes to the stack 18. The loaded sorbent 20 is passed to a stripper or VOC desorber 22 for recovery of VOC's and regeneration of sorbent.

The loaded sorbent first passes through a heat exchanger 24 wherein it is heated by hot stripped sorbent 26 from the VOC desorber 22. In the VOC desorber 22, the loaded sorbent is contacted with a heated stripping gas stream 30 in such a manner that the VOC's are removed from the solvent into the gaseous phase. Such stripping may be effected by any convenient gas-liquid contact device, such as a packed column.

The stripped solvent 26 exiting the VOC desorber 22 is recycled through the heat exchanger 24 and a cooler 32 to the absorber 16 for reuse in contacting the VOC- containing gas stream. The VOC-containing stripping gas stream 34 is passed through a cooler 36 to a condenser 38, wherein the VOC's are condensed from the stripping gas stream as a liquid product 40, with the clean gas stream 42 being vented to the stack. Alternatively, where the stripping gas is steam, this may be co- condensed, and the VOC product is then decanted. The liquid product 40 is a valuable commodity and may be sold.

In the embodiment shown in Fig. 2, four dual-fluid spray nozzles are illustrated as being employed to effect the spraying of the sorbent solution into the absorber 16. The dual-fluid spray nozzles 50 are arranged in two pairs separated by a demister 52, upstream of the VOC

desorbing zone 54 in which two of the nozzles 50 are located and a downstream VOC-absorbing zone 56 in which the other two of the nozzles 50 are located.

In this embodiment, lean sorbent solution is fed in parallel by line 44 to the two nozzles 50 located in the downstream zone 56. Partially-loaded sorbent solution recovered in the demister 52 is forwarded by line 46 to be fed in parallel by lines 48 to the nozzles 50 located in the upstream zone 54. The above-described embodiment illustrated by Fig. 2 shows a two-stage nozzle arrangement using four nozzles to effect the spraying of the sorbent solution into the absorber 16, with two stages spraying two nozzles per stage. However, any desired number of such spray nozzles may be employed per stage and more than two stages of gas stream contact may be used, depending on the concentration and nature of the VOC's in the gas stream, the affinity and capacity of the sorbent for the VOC's in the gas stream and the degree of desired removal of VOC's from the gas stream.

In addition, a fan may be provided downstream of the conduit to draw the gas stream through the conduit and further to provide further agglomeration of any residual liquid droplets, which may be removed from a demister located downstream of the fan.

The elongate conduit is illustrated as being in a horizontal orientation. However, the elongate conduit may oriented vertically or at an angle. In one embodiment, the elongate conduit may be in a vertical orientation with the inlet end at the lower end of the conduit. The dual-fluid spray nozzles may be arranged to form spray patterns countercurrent and/or co-current to the upward flow of the gas stream through the vertical conduit. In another embodiment shown in Fig. 3, the prescrubber 12 is further depicted with a wash ring, four nozzles and a mist eliminator to remove the particulates

and undesirable materials from the gas stream from which the VOC's are to be removed. The prescrubber also serves to cool the gas stream to its adiabatic dew point. Water is usually used as the spraying medium to the nozzles depicted in Fig. 3. Depending on the amount of particulate and undesirable materials to be removed by the prescrubber, one or more nozzles are used.

EXAMPLES Example 1 This Example illustrates the removal of four typical VOC's generated in a paint spraying operation, in accordance with one embodiment of this invention.

The four VOC's removed from the gas stream were 1- Butanol, Butyl Acetate, 2-Heptanone and Xylene. Three nozzles were employed in an apparatus as illustrated in Fig. 2, in three stages resulting in one spraying nozzle per stage. The duct size used in these runs was 1 ft. O.D. TABLE 1 below presents the scrubber operating conditions and the resulting VOC removal efficiency in a series of tests. The tests were carried out at ambient temperature and pressure.

As shown in TABLE 1, VOC removal efficiencies of 88% to 100% were obtained for the four VOC's under study at a very low liquid/gas ratio of 1.2 to 3.0 US gal. per 1000 cubic ft. of gas treated.

TABLE 1 Removal of VOC's from Paint Spraying Operation

VOC Scrubber Conditions

4 7 8 RUN NUMBER

Number of Serial Stages 3 3 3

Number of Nozzles 3 3 3

Nozzle diameter (mm) 3 30*1 30*1

Nozzle pressure (psig) 50 46 65

Flow per nozzle (Usgph) 9 48 84

Mean Sauter Diameter (microns) 47 36 36

Gas flow in (cfm) 373 470 469

L/G (USgal/1000 ft ³ ) 1.2 1.7 3.0

VOC GAS INLET

Concentrations (ppmv)

Butyl Acetate 267 454 559

1-Butanol 503 305 241

2-Heptanone 258 438 525

Xylenes 203 326 556

VOC REMOVAL EFFICIENCY (%)

Butyl Acetate 88 87 91

1-Butanol 93 96 94

| 2-Heptanone 98 96 98

Xylenes 100 90 96 1

Example 2 This Example illustrates the stripping of the four VOC's absorbed in Example 1 by a regeneration operation in accordance with one embodiment of this invention. In this Example, the loaded sorbent obtained from the absorber in Example 1 was continuously regenerated in a 4.25 inch diameter packed column. The effect of air sparging, liquid flow, temperature and increased column packing length were studied. The results presented in TABLE 2 below show that VOC removal efficiencies improve substantially with increased air sparging rate, temperature close to the boiling point of the VOC's under study and increased packing length.

Example 3 This Example illustrates the regeneration efficiency of VOC stripping in a continuous flow packed column as well as from a batch operation.

The sorbent loaded with four VOC's generated in a paint spray booth was used in both tests. Fig. 6 and 7 present the resulting VOC removal efficiency with respect to sampling time. The results indicate higher and faster stripping in the continuous flow packed column than in the batch operation at a regeneration temperature of 140 ^β C. From the results presented in Examples 1 to 3, it is therefore feasible to design a continuous absorption and stripping process for the removal of these four VOC's.

SUMMARY OF DISCLOSURE In summary of this disclosure, the present invention provides a novel method of removing and recovering VOC components from gas streams by a regenerative solvent absorbent procedure in which the VOC components are removed and collected as a product stream and a VOC-free gas stream is vented, while the solvent absorbent is regenerated for recycle. Modifications are possible within the scope of this invention.

TABLE Z

Regeneration of Loaded Sorbant and Paint VOC's in a Continuous Flow Packed Column (4.25" ID x 48"L)

Effect of Air Flow Rate for Sparging

1 Air Sorbant Temp VOC Concentration (ppm) % VOC Removal

L/min (mL/min) (°C) BuOII BuOAc MA xylene BuOII BuOAc MAK xylene

0 - - 19633 19543 24028 18998 0 0 0 0

8 ^% 225 143 3505 3209 9345 5576 82 84 69 71

12 225 144 3236 2800 8453 5040 83 85 65 73

15 225 144 2152 1794 6257 3650 89 91 74 81

16 200 150 335 225 1642 687 98 99 93 96

Effect of Liquid Flow and Temperature

Air Sorbant Temp VO C Concentration (ppm) % VOC Removal L/min (mL/min) (°C) BuOII BuOAc MAK xylene BuOII BuOAc MAK xylene

0 - - 19633 19543 24028 18998 0 0 0 0

16 200 150 335 225 1642 687 98 99 93 96

16 300 150 788 842 3230 1870 96 96 87 90

16 400 150 2989 1883 7481 4412 85 90 69 77

16 400 170 0 0 1065 490 100 100 96 97

Regeneration of Loaded Sorbant and Paint VOC's in a Continuous Flow Packed Column (4.25" ID x 48"L)

Effect of Increased Column Packing Length

Run Gas Sorbant Temp VOC Concentration (1000's ppm) % VOC Removal | L/min (mL/min) (°C) BuOII BuOAc xylenes MAK BuOII BuOAc xylene MAK 1

I 0 - - 8740 9827 4100 7355 0 0 0 0 la 16 250 160 495 280 440 1340 94 97 90 82 lb 16 250 160 0 0 210 575 100 100 95 92

2 0 - - 5980 5220 3220 5490 0 0 0 0

2a 16 250 145 960 365 580 1645 84 93 82 70

2b 16 250 155 0 0 190 600 100 100 94 89

3 22 250 160 0 100 195 1 605 | 100 98 | 94 89

Runs 1 and 2 were two stage runs.

Example 3 This Example illustrates the regeneration efficiency of VOC stripping in a continuous flow packed column as well as from a batch operation. The sorbent loaded with four VOC's generated in a paint spray booth was used in both tests. Fig. 6 and 7 present the resulting VOC removal efficiency with respect to sampling time. The results indicate higher and faster stripping in the continuous flow packed column than in the batch operation at a regeneration temperature of 140°C.

From the results presented in Examples 1 to 3, it is therefore feasible to design a continuous absorption and stripping process for the removal of these four VOC's. SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novel method of removing and recovering VOC components from gas streams by a regenerative solvent absorbent procedure in which the VOC components are removed and collected as a product stream and a VOC-free gas stream is vented, while the solvent absorbent is regenerated for recycle. Modifications are possible within the scope of this invention.