WO1995003261A1

WO1995003261A1 - Treating unsaturated hydrocarbons

Info

Publication number: WO1995003261A1
Application number: PCT/US1994/007983
Authority: WO
Inventors: James H. Porter; William M. Jackson
Original assignee: Energy & Environmental Engineering, Inc.
Priority date: 1993-07-20
Filing date: 1994-07-15
Publication date: 1995-02-02

Abstract

A method and apparatus for treating an aquaeous waste solution containing hydrocarbons in which the solution is mixed with an OH radical producer, preferably hydrogen peroxide. The mixture is introduced into one end the reaction tube (14) through inlet (11). An ultraviolet laser beam (16) is directed through a quartz window (18) in the other end of the reaction tube. The mixed hydrocarbon-OH radical producer is irradiated as it flows through the tube (14). The irradiated solution exits from the tube (14) through an outlet (20) into a holding tank (22) in which it undergoes degradation reactions. Typically, the hydrocarbon solution is pretreated to remove particulate material, and hydrogen peroxide, in not less than about a stoichiometric amount, is added to the solution. The hydrogen peroxide is subjected to ultraviolet radiation to produce OH radicals, either before or after it is added to the waste solution.

Description

TREATING UNSATURATED HYDROCARBONS

Field of Invention

This invention relates to a process and apparatus for treating unsaturated hydrocarbons, particularly when present in parts-per-million (ppm) concentration in contaminated ground water or industrial aqueous waste solutions.

Background of Invention

Unsaturated hydrocarbons are a significant contaminant in ground water and a variety of industrial waste solutions. There have been a large number of systems and methods provided for reacting or otherwise eliminating various types of contaminants.

For example, laser beams have been directed towards large molecules of toxic organic substances dissolved in water to overcome the bonding energies holding toxic mole¬ cules together and break them into fragments which will then combine with dissolved oxygen injected into the waste water to form molecules of non-toxic substances. Similarly, solu¬ tions containing hydrocarbon wastes and hydrogen peroxide have been irradiated to cause reactions which degrade or destroy the toxic waste. However, the degree of degradation in the first procedure is limited by the amount of oxygen in solution; and the degradation reactions in both procedures typically stop when irradiation ceases.

There remains a need for an efficient, cost-effective way of destroying low-concentration (e.g., about 0.1 to about 5,000 ppm) hydrocarbons in various waste solutions. Su marv of Invention

The invention features a method and apparatus for treating a aqueous waste solution containing hydrocarbons (hereinafter, such a solution is referred to as a "hydrocar¬ bon solution") in which the solution is mixed with an irradiated OH radical producer, preferably hydrogen peroxide. Typically, the hydrocarbon solution is pretreated to remove particulate material, and hydrogen peroxide, preferably in not less than an about stoichiometric amount, is added to the solution. The hydrogen peroxide is subjected to ultraviolet radiation at a high level, preferably after it is added to the waste solution, so that reactions destroying at least a major percentage of the hydrocarbons in the waste solution will continue in the absence of continued applied irradiation.

In preferred practices, pretreating involves filtering to remove particulate any dimension of which is greater than the wavelength of the radiation (e.g., greater than about 0.18 microns) . The ultraviolet radiation has a wavelength in the range of about 184 to about 300 nanometers (and pref¬ erably at the lower end of the range) and an intensity ranging from about 0.1 watts/sq.cm. (10¹⁷ photons/sq.cm.-sec; obtainable from a low intensity ultraviolet lamp) to about 30 watts/sq.cm (3.0 10^9 photons/sq.cm.-sec; obtainable with a laser or high intensity UV lamp) , radiation is applied to produce an irradiation level of at least 0.1 (and preferably at least 1) photon per molecule of hydrocarbon present in the solution, and the degradation procedure is thereafter permitted to proceed for an extended period in the absence of further applied radiation.

Systems for practicing the invention comprise reactor vessels having an annular flow channel, the outer surface of which preferably is coated with a photocatalyst and the inner surface of which is transparent at wave lengths of 184 to 300 (and preferably of about 185) nanometers to ultraviolet light, surrounding a light source.

Description of Drawings

Figure 1 and 2 are schematics of photochemical reactors useful in the practice of the present invention.

Figures 3 and 4 illustrate a photochemical reactor useful in the practice of the present invention that includes a plurality of photochemical reactor modules.

Figures 5 and 6 illustrates a photochemical reactor module of the system of Figure 3.

Figures 7 and 8 illustrate a modified photochemical reactor module.

Detailed Description of Preferred Embodiments

According to the present invention, a substance that produces OH radicals upon irradiation (an "OH radical producer") is added to the hydrocarbon solution. Hydrogen peroxide is the preferred OH radical producer, but in some circumstances other OH radical producers may be employed.

The OH radical producer is subjected to radiation, preferably having a wavelength in the range of about 184 to about 300 nanometers. Highest reaction rates are normally obtained with a short wavelength, e.g., about 185 nanometers; at wavelengths below about 184 nanometers, absorption of the radiation in water becomes a significant problem. The intensity of the radiation depends principally on the radiation source used. Radiation intensities of about 0.1 to 30 watts/cm² can be obtained from ultraviolet lamps; although average laser intensities are in the same range, considerably higher instantaneous intensities, e.g., about 10' watts/cm² can be obtained using lasers. Regardless of the radiation source, the radiation procedure is continued for a period sufficient to produce an irradiation level (e.g., the radiation absorbed by the OH radical producer) of not less than about 0.1 photons per molecule of hydrocarbon waste present in the hydrocarbon solution.

Absent a catalyst, a relatively small fraction of the waste hydrocarbons is destroyed during the initial irradia¬ tion period, e.g., the period during which the OH radical producer in the waste hydrocarbon solution is being subjected to radiation. Importantly, it has been found that, if the radiation is applied at a power and for a period of time sufficient to produce a level of irradiation that is more than about 0.1 photon per molecule of hydrocarbon waste, a major fraction of the remaining hydrocarbon waste can be destroyed, after cessation of applied radiation, simply by maintaining the hydrocarbon/irradiated OH radical producer mixture for a period of time (the "propagation period) . This further destruction apparently is obtained in the course of chain free-radical reactions which propagate by repeated reaction after initiation by a sufficient level of ultraviolet radiation. The chain reaction normally will proceed only when the ultraviolet irradiation of the solution is at a level of at least 0.1 photon per molecule of hydrocarbon present, although, as already noted, some initial destruction of hydrocarbons may be realized while the waste is being initially irradiated. As will become apparent hereinafter, initial radiation to produce higher irradiation levels, e.g., 1 to 100 photons per molecule, is preferred.

The specific radicals involved in the chain reactions have not been identified; many are possible, ranging from small OR and RO2 radicals (typically OH* and HO2*) to larger radicals incorporating hydrocarbon fragments. The types of reactions believed to be occurring are shown by the following illustrations, in which hydrogen peroxide is employed as the OH radical producer and OH and HO2 radicals are produced. Similar reactions are believed to occur when other OR and RO2 radicals are produced in the course of the reactions.

I . Initiation

The hydrogen peroxide absorbs radiation and dissociates into two OH radicals: h H₂0 == 20H* (1)

Unsaturated hydrocarbons also absorb ultraviolet ( "uv" ) radiation, activating the double bond in the unsaturated molecule: h R == R** (2)

II. Propagation

The OH radicals react with additional hydrogen peroxide molecules in the solution, producing HO2 radicals:

OH* + H₂θ2 == H0₂* + H2O (3) These HO2 radicals react in a series of steps with the acti¬ vated double bond in hydrocarbon molecules,

(i) producing molecules of carbon dioxide and water,

(ii) leaving behind an unsaturated hydrocarbon that contains one less carbon atom, and

(iii) regenerating OH radicals, e.g.,

R-CH₂-CH=CH-CH2-R' + 3H02* == R-CH₂-CH=CH-R'

(4) + 30H* + C0₂ + H2O

The unsaturated hydrocarbon produced in reaction (4) itself reacts with HO2 radicals, using the same mechanism, and the chain of reactions continues until the unsaturated hydrocarbon has been completely destroyed, e.g., all of the carbon atoms have been reacted to form CO2 • The OH radicals produced in the chain of reactions react with additional hydrogen peroxide molecules (Reaction (3)), thus replacing consumed HO2 radicals and insuring that the chain reaction continues.

It will be apparent that the above represent only some of the many potential destructive reactions. For example, reaction of the R-CH2-CH=CH-R' compound of reaction (4) with OH* radicals could produce, among other things, OR radicals which then could react to produce RO2 radicals in reactions similar to reaction (3) that, in turn, would take part in a degradation reaction such as that of reaction (4) .

The rate at which reaction (4) proceeds can be increased by use of a photocatalyst, such as titanium dioxide or some other metal oxide in the titanium group.

Termination

The chain of reactions continues until all of the unsat¬ urated hydrocarbons in the solution have been essentially destroyed, or when the HO2 radicals have been lost due to reaction with other species in the solution or have recom- bined on the wall of the reaction vessel.

The invention is exemplified by the following Examples I-IV, in each of which the hydrocarbon destruction was accomplished without use of a catalyst.

EXAMPLE I A series of experiments were conducted to determine the rate of destruction of different hydrocarbon contaminants. Table I sets forth the particular hydrocarbon, the fraction of hydrocarbon destroyed during the period that the waste solution is being subjected to radiation (Initiation Destruc¬ tion and Removal Efficiency, or "DRE, " defined as the change in waste concentration, i.e., initial concentration minus final concentration, divided by the initial concentration) , the length of the post radiation propagation period during which the mixed hydrocarbon/irradiated hydrogen peroxide solution was held quiescent in a vessel (the Propagation Period) , and the total percent of the hydrocarbon that had been destroyed at the end of the propagation period (Propaga¬ tion DRE) . In each instance, the initial concentration of the hydrocarbon in the solution was 50 parts per million (by weight) , approximately stoichiometric amounts of hydrogen peroxide were added to the solution, and the solution containing both the hydrocarbon and the hydrogen peroxide was irradiated with ultraviolet radiation to an level of 10 photons per molecule of hydrocarbon.

Table 1

Initiation Propagation Propagation

Compound DRE Period (Hours) DRE

Benzene 0.29 96.0 0.91

Chlorobenzene 0.31 113.5 0.98

Chlorophenol 0.34 72.0 1.0

Dichloroethene 0.18 624.0 0.88

Benzidine 0.48 288.0 0.88

Phenol 0.35 72.0 1.00

As will be evident from Table 1, although only hydrocarbons with unsaturated double bonds absorb uv radiation and react as described above, the process of the present invention was highly effective with all of the hydrocarbon contaminants tested. It will also be noted that most hydrocarbon destruc¬ tion occurred during the Propagation Period, during which no radiation was applied, and that the length of time required for high level destruction varied for different the hydrocar¬ bons tested.

EXAMPLE II A second series of experiments was conducted to deter¬ mine the extent to which hydrocarbon destruction and length of the Propagation Period required to obtain high level destruction depended on the initial concentration level of the hydrocarbon. Table 2 shows the effect for solutions of chlorobenzene (to which hydrogen peroxide was added in approximately stoichiometric quantities) irradiated to a radiation level of 10 photons per molecule.

Table 2

Initial Propagation Propagation Concentration (ppm) Period (hours) DRE

10 780 0.70 20 700 0.85 50 114 0.98

As shown, the highest concentration produced the highest Propagation DRE, at the lowest Propagation Period. As the hydrocarbon concentration decreased, the Propagation DRE de¬ creased, even at significantly larger Propagation Periods.

Example 3 A third series of experiments was conducted to determine the extent to which the Propagation DRE was effected by the level of irradiation. Table 3 shows the Propagation DRE for solutions of chlorobenzene (initial concentration 50 ppm) to which hydrogen peroxide was added in approximately stoichio¬ metric quantities, irradiated, and maintained for Propagation Periods of 20 and, then 60, hours.

Table 3

Irradiation Propagat .ion Propagation Level DRE DRE (Photons/molecule) After 20 hrs . After 60 hrs.

1 0.35 0.80

3 0.70 0.95

10 0.92 0.95

As shown, the rate of destruction depends on both the ir¬ radiation level and the length of the Propagation Period. For relatively short Propagation Periods, e.g., 20 hours, the level of destruction achieved increases as the irradiation level rises. For longer Propagation Periods, the increased ti e results in greater destruction at all irradiation levels, but essentially the same total destruction is obtained at irradiation levels of 3 and 10 photons per molecule.

Referring now more particularly to the drawings, Figures 1 through 6 illustrate (somewhat schematically in Figures 1 and 2) systems for practicing the present invention. As will be apparent, the illustrated systems vary, inter alia, in whether the OH radical producer is irradiated while flowing in the same direction ("co-current flow"), opposite direction ("counter-current flow"), or perpendicular ("cross-current flow" ) to the direction of the radiation from the radiation source, and in whether the OH radical producer is irradiated before or after it is mixed with the waste hydrocarbon solution. Counter-current flow allows absorption of irradiated energy in a shorter length of reactor, and more intense radiation near the reactor outlet, than does co- current flow, and thus is normally preferred for systems in which a laser is employed as a radiation source.

In the counter-current flow system, generally designated 8, of Figure 1, a metered amount of the OH radical producer, e.g., hydrogen peroxide, is added to the waste stream 10 through a side inlet 12, before the waste stream enters the reaction tube 14; and the mixed radical producer and waste stream are directed into an inlet 11 at one end of the reaction tube 14. An ultraviolet laser beam 16 is directed through a quartz window 18 in the other end of the reaction tube, generally parallel to the tube axis. The mixed hydrocarbon-OH radical producer is irradiated as it flows through the tube 14. The irradiated solution exits from the tube through an outlet 20 in the side of the reaction tube 14 nearest quartz window 18, and then through outlet line 21 into a holding tank 22 in which it undergoes degradation reactions during the desired Propagation Period. Figure 2 illustrates a counter-current flow treatment system 8' in which ultraviolet radiation 16' is similarly di¬ rected through a quartz window 18' axially into a reaction tube 14'. In the system of Figure 2, a hydrogen peroxide stream 12' is directed into a side inlet 20' near the end of reaction tube 14' axially opposite that through which the radiation is directed into the tube. The peroxide stream is irradiated as it flows through the tube, and the irradiated peroxide exits from outlet 11' at the end of the tube 14' adjacent the laser radiation 16'. The waste stream 10' is added to the irradiated peroxide solution after the latter has exited from the reactor tube 14'. The mixed waste stream/irradiated peroxide solution is then directed through outlet line 21' to a holding tank 221 for propagation.

As indicated above, a principal differences between the systems of Figures 1 and 2, is whether the hydrogen peroxide is added to the waste solution before or after the peroxide is irradiated. Either system may also be designed so that the solution and ultraviolet beam pass in the same direction (co-current flow) through the photochemical reactor, rather than in opposite directions (counter-current flow) direc¬ tions, as shown. For example, system 8' of Figure 2 could be modified to introduce radiation from the laser 16' through a quartz window at the end of the tube adjacent inlet 12', to mix the waste stream 10' with the peroxide solution 12' be¬ fore either enters tube 14, thus providing a co-current flow system in which, as in system 8 of Figure 1, the peroxide is irradiated while mixed with the waste hydrocarbon solution. Similarly, the system 8 of Figure 1 could be modified to mix the waste solution 10 with the irradiated output from outlet 20, rather than mixing it with the non-radiated peroxide input at inlet 11, thus providing a counter-current flow system in which the waste hydrocarbon solution is mixed with the OH radical producer after the latter has been irradiated. However, in each of the two systems, the mixture of irradiated hydrogen peroxide solution and hydrocarbon waste solution is held in a holding tank for the propagation period (ranging from tens to hundreds of hours) required to insure essentially complete destruction of the unsaturated hydrocar¬ bons in the waste solution.

Figures 3 through 6 illustrate a photochemical reactor system, generally designated 8" designed for use in the prac¬ tice of the present invention. System 8" is a cross-current flow system in which the waste hydrocarbon solution and HA radical producer are mixed and the mixture is then introduced into the system and irradiated. Cross-current flow is nor¬ mally is preferred in systems in which an ultraviolet lamp is employed as a radiation source, principally because of con¬ straints imposed by the geometry of available ultraviolet lamps. Ultraviolet lamp-based systems normally also will employ a photocatalyst to increase the rate of hydrocarbon oxidation.

As shown, system 8" includes a plurality of reactor mod¬ ules, generally designated 50 and one of which is shown in more detail in Figures 5 and 6, connected in parallel between a main fluid inlet header 52 and a main fluid return header 54. Main inlet header 52, like the inlet 11 of the system of Figure 1, receives a solution 10 that includes both the hydrocarbon contaminant and hydrogen peroxide. Main outlet header 54, similar to the outlets of the systems of Figures 1 and 2, provides for flow to a holding tank 56 where the degradation reaction initiated by the irradiation continues until the hydrocarbons in the waste solution have been substantially destroyed. As will be evident, the volume of tank 56 is many times greater than that of the modules 50.

In the system shown, four inlet risers 55 and four outlet risers 57 are connected, respectively, to the main inlet and outlet headers; and a plurality of reactor modules 50 in turn are connected to, and extend in parallel between, a respective inlet riser 55 and outlet riser 57.

Referring to Figures 5 and 6, each module 50 includes a cylindrical housing 60 coaxially surrounding an ultraviolet lamp 62 and providing an annular flow channel 64 between outer cylindrical surface 66 of lamp 62 and the outer wall of the surrounding housing 60. A tangential flow inlet 68 is provided at one end of the housing; an tangential flow outlet 70 is provided at the housing's other end. In the embodi¬ ments of Figures 5 and 6, the inner surface 72 of the wall of housing 60 is coated with a photocatalyst such as titanium dioxide. The tangential feed arrangement induces flow patterns in the flow channel 64 that assure that lamp 62 will cause reasonably uniform irradiation of all solution flowing through the module, and that the solution will come into con¬ tact with the photocatalyst. As previously indicated, no catalyst was employed in the tests that are the subjects of the foregoing examples. The use of such a catalyst (as in the system of Figures 3-6 or with the modified module, discussed below, of Figures 7-8) appears significantly to in¬ crease degradation during the period that the waste solution is passing through the reactor vessel and being subjected to radiation, and thus may significantly reduce the length of any otherwise necessary propagation period.

As indicated in Figure 3, the modules 50 of system 10" are connected to their respective headers through individual shut-off valves 80 so that, for example, individual modules may be replaced without requiring shut-down of the entire system. The positive and negative electrodes 74 and 76 for connecting the ultraviolet lamp 62 to a power source (now shown) are shown in Figures 4 and 5.

Figures 7 and 8 illustrate a modified module, generally designated 150. As shown most clearly in the cross-section of Figure 8, module includes a cylindrical housing 160 (inner diameter = 3.022 cm; axial length of about 26 cm.) coaxially surrounding an ultraviolet lamp 162 the quartz outer shell

161 of which has an o.d. of 2.5 cm. A second quartz tube 163

(o.d. = 2.852 cm; i.d. 2.552 cm) is mounted coaxially with housing 160 and lamp 162. Tube 163 is made by Xenon Corp.

Both it and outer shell 161 are preferably Suprasil quartz of the type manufactured by, among others, Amerasil, Inc., and is transparent to radiation having a wavelength of about

185 nanometers. An air gap 165 separates the outer quartz tube 163 from the quartz outer shell 161 of lamp 162. An annular flow channel 164 (i.d. = 2.852 cm; o.d. = 3.022 cm) is provided between the outer cylindrical surface of tube 163 and the inner wall of the surrounding housing 160. Typically housing 160 is a stainless steel tube. Preferably, and as shown, its inside wall is coated with a titanium dioxide catalyst 170. Air gap 165 and tube 163 are sized so that the outer surface of the lamp may be maintained at a temperature o of about 600 C, and the outer surface of quartz tube may be maintained at about 16 C, during operation of the system. It will be appreciated that the heat from the lamp will be convected away in the reacting fluid in flow channel 164.

Because the quartz walls of shell 161 and tube 163 are transparent to ultraviolet radiation, the light from lamp 162 reacts with fluid in flow channel 164. The narrow radial width of the flow channel (about 0.17 cm) insures that es¬ sentially all of the uv radiation will be absorbed by the flowing fluid, and the turbulence in the flow stream carries the irradiated fluid into contact with the titanium dioxide catalyst on the inside surface of housing 172.

As shown most clearly in Figure 8, the reactor module 150 includes ceramic end caps 172 having a pair of axially facing, annular grooves 100 which hold the quartz tube 163 and housing 160 in place and a central base 102 through which lamp 162 passes. Outer housing 160 provides a tangential inlet 176 into, and a tangential outlet 174 from, the opposite ends of flow channel 164. Other embodiments will be within the scope of the fol¬ lowing claims.

Claims

hat is claimed is :

1. The method of treating an aqueous solution including aromatic or aliphatic unsaturated hydrocarbon waste or contaminants, said method including the steps of: providing a mixture of an irradiated OH radical source and said hydrocarbon solution, said mixture having an irradiation level of not less than about 0.1 photons per molecule of unsaturated hydrocarbons in said mixture, and thereafter retaining said mixture for a period of time such that continuing reactions therein resulting from said irradiation level destroy said hydrocarbons.

2. The method of claim 1 including the steps of mixing said OH radical source with said solution and irradiating said mixture with ultraviolet light having a wavelength in the range of about 184 to about 300 nanometers to produce said irradiation level, and wherein said mixture is retained for said period without substantial additional irradiation.

3. The method of claim 1 wherein said level is not less than 1 photon per molecule.

4. The method of claim 1 including the step of irradiating said OH radical source with ultraviolet light having a wave length of about 185 nanometers.

5. The method of claim 1 wherein said OH radical source is irradiated after mixing with said hydrocarbon solution.

6. The method of claim 1 including the step of pretreating said hydrocarbon solution to remove therefrom particulates having any dimension that is greater than the wavelength at which said OH radical source is irradiated.

7. The method of claim 1 wherein said hydrocarbon solution includes unsaturated hydrocarbons at a concentration of not less than about 0.10 parts per million.

8. The method of claim 7 wherein said concentration is not more than about 5000 parts per million.

9. The method of claim 2 wherein said OH radical source is mixed with said hydrocarbon solution in a concentration such that said OH radical source is present in said mixture in at least stoichiometric amounts.

10. The method of any of claim 2 wherein said OH radical source is hydrogen peroxide.

11. The method of claim 1 wherein said mixture is retained for a period of time sufficient to permit reactions therein resulting from said irradiation level to destroy a major fraction of said hydrocarbons.

12. A photochemical reactor comprising: a longitudinally extending source of ultraviolet radia¬ tion having a wavelength in the range of about 184 to about 300 nanometers; and a reactor vessel defining a flow path for fluid to be treated in said system, said reactor vessel including an annular flow channel coaxially surrounding said ultraviolet radiation source, the flow channel having an inner cylindrical wall that is transparent to said radiation and is arranged to permit said radiation to irradiate fluid in said annular flow channel, and an outer cylindrical wall having a photocatalyst coated on the inner surface thereof.

13. The reactor of claim 12 wherein said vessel includes a tangential inlet into said flow channel adjacent one end thereof and a tangential outlet from said flow channel adjacent the other end thereof.

14. The reactor of claim 12 wherein said vessel includes a quartz tube mounted coaxially around said radiation source and defining the inner wall of said flow channel, said tube being transparent to radiation having a wavelength in the range of about 184 to 300 nanometers.

15. The reactor of claim 14 wherein the inner wall of said tube is spaced from the outer wall of said radiation source to provide an annular air gap therebetween.

16. The reactor of claim 14 wherein said source produces radiation at a wavelength, and said tube is transparent to radiation having a wavelength, of about 185 nanometers.

17. The reactor of claim 14 including a titanium dioxide catalyst on an interior wall of said flow channel.

18. A waste treatment system including, in combination: an inlet header, an outlet header, a plurality of said reactors of claim 13 mounted in par¬ allel with the inlet of each connected to said inlet header and the outlet of each connected to said outlet header, and a treatment tank having a volume at least an order of magnitude greater than the sum of the volumes of the flow channels of said plurality of reactors connected to an outlet of said outlet header.