APPARATUS AND METHOD FOR THE TREATMENT OF VOLATILE ORGANIC COMPOUNDS

The invention relates to an apparatus and method for environmental control, and in particular provides an apparatus and method for the treatment and removal of contaminant materials in contaminated fluids. The invention is particularly applicable to the removal of low/ medium level volatile organic compounds from contaminated air streams, and examples of such use are described herein, but this should not be taken as excluding from the scope of the invention its applicability to the treatment and removal of other organic and inorganic compounds present as contaminant materials in contaminated fluids.

Various existing processes are known which are effective to varying degrees in the removal of organic and inorganic contaminants from fluid volumes, and in particular volatile organic compounds (VOCs) where they have contaminated air volumes.

For example, the volume may be passed through an absorbent material, various materials serving this purpose. Absorption methods can be of varying effectiveness depending on the materials involved, and absorbent capacities can be limited.

In the alternative, it is known to develop adsorption processes using activated carbon or the like. Such processes can be effective at adsorbing quite high volumes of VOCs, but are nevertheless inclined to be relatively expensive. Achieving effective mass transfer is an issue, especially with air streams. The requirement this imposes to keep a pressure drop across the adsorbent material relatively low means that for a given volume large areas are required. The

process can be expensive, in particular in relation to regeneration of the adsorbent material when contaminated.

Various absorption/ adsorption processes are known.

For example UK patent 866383 relates to the selective sorption of organic vapours using sorption towers and fluidised beds. A process which relies on absorption or adsorption can be reversed relatively easily. Any spent material from this type of process will be classified as hazardous waste. This has a significant effect on the cost of disposal of spent material. The spent material can be recycled, by heating it for example, but this requires energy, which can be expensive and the vapours which are emitted still have to be treated. It does not lock away the hazardous waste in such a manner that the resultant sorption product can be treated as inert and non-hazardous.

US patent 5401417 relates to the purification of water rather than contaminated air streams. The clays are used in batch processes, fluidised or expanded beds. The process again works by adsorption, not an irreversible chemical bonding process. Again this has a significant effect on the cost of disposal of spent material. The clays are designed to swell as they adsorb the organic compounds. This will result in the absorption medium becoming more and more viscous. The pressure required to propel the contaminants through the liquid will therefore increase and this will affect the efficiency of the system.

WO83/01205 relates to the removal of organic contaminants from aqueous compositions. In the process described in the patent, organic compounds are adsorbed by the modified clays and therefore the process is reversible and spent material is likely to be classified as hazardous waste with the same

disposal implications. The modified clays will swell as they adsorb the contaminants. The viscosity of the system will increase as time goes by and this will give problems controlling the flow through the process. The invention above utilises batch processes, columns and fluidised beds.

In certain cases, bio-filters can be used. However, bio-filtration of necessity requires a relatively steady ambient load. It can not be suitable to a batch process, or to significantly varying or shock loads. Bio-filtering is really only suited to 24 hour operation, for example in relation to waste water treatment plants.

VOCs in industrial wastes, and in particular in contaminated industrial air streams, may be dealt with by wet scrubbing in which the air stream is passed through scrubbing materials such as gas oil, light spindle oil or the like to dissolve the VOCs. Such a process is particularly suited where there is a desire to recover high value VOCs. However, it raises problems of its own, since emissions from the scrubbing liquid can themselves raise environmental concerns.

It has been well established that modified organo/inorgano clays, such as those used for example in the treatment technology operated under the name Envirotreat (RTM), react with a very wide variety of organic and inorganic compounds which are found in contaminated land. These clays in particular chemically react with VOCs through a number of complex bonding processes. Clays of the type, and operating in the manner described in greater detail in UK patent GB2302685 are particularly relevant in this regard. The various VOCs react in different ways depending upon their structural chemistry, but typically bond to the clays via bonding processes such as Lewis acid/base bonding, Van der Waals bonding, hydrogen bonding, transition metal ligand

bonding, and adsorption due to hydrophobicity. VOCs can be removed from contaminated land in this way in that they are locked irreversibly into the structure of the clay.

By analogy, it was suggested in UK patent application number 9715863.8 that such clays could be used to remove VOCs from contaminated air or liquid streams. Clay of the type described is retained within a container, and the contaminated fluid passed there through. However, achieving adequate mass transfer is again a problem, and the technique is in practice of limited applicability for high volumes of fluid to be decontaminated, and in particular for decontamination of high volume, relatively low contaminate level air streams.

All of these prior art methods are limited in some respects, and in particular tend to be limited in relation to the removal of VOCs at relatively low concentration from high volume air flows. Examples of such a situation are provided by the general atmospheric levels of VOCs in a paint spray workshop, a printing plant or the like, where the VOCs might be present to a level of, say, 300 to 400 ppm, and a legislative requirement is imposed to reduce this, say, to below 100 ppm.

It is an object of the present invention to provide an alternative system and method for the treatment of VOC contamination that mitigates some or all of the above disadvantages.

It is a particular preferred object of the invention to provide a system and method which lends itself to the removal of VOCs from contaminated fluids, and in particular from contaminated air streams, where the contaminant is present at relatively low levels, and where high volumes need to be treated.

Thus, in accordance with the invention in a first aspect there is provided an apparatus for the treatment of contaminants, and in particular for the removal of volatile organic compounds (VOCs) from a contaminated fluid stream such as a contaminated air stream, the apparatus comprising containment means defining a volume through which fluid to be treated can be passed in use, and for example having an inlet there into and an outlet therefrom; particulate organo/inorgano clay selected to provide a reactive medium for chemically binding with the contaminants; a slurry disperser to cause particulate organo/inorgano clays to form, in use, a fluidised dispersion within the volume the apparatus by forming a suspension of particulate reactive clay in a suitable suspension liquid to form a treatment fluid suspension and passing this into the volume to form, in use, a dispersed mist within the volume.

In accordance with the invention, the reactive clay is provided in fine particulate form, for example in powder or granulated form, within the volume. Further, the clay is fluidised within the volume. Fluidising gives an active surface area that maximises mass transfer rates, and is therefore capable of giving the necessary volume flow for practical decontamination processes. In particular, the reactive clay can then be used at relatively low contamination levels and for high volume flow situations. Fluidising the reactive clay within the container makes use of the clay in a decontamination apparatus much more practical than it was in the apparatus described in UK patent application publication GB2327671.

The clay preferably reacts at least partly via a permanent chemical reaction and not solely by a physical sorption process. Irreversible chemical reactions stabilise the contaminants such that the material created by the bonding processes is stable and inert. Thus, the material created will be classified as stabilised, non-reactive waste, which can be disposed of relatively cheaply. It

can be safely disposed of in normal land-fill sites. It may even improve the quality of the site because the organo/inorgano clays are used in the treatment of contaminated land. This contrasts to the waste from sorption processes, where the process which relies on absorption or adsorption can inherently be reversed relatively easily. Any spent material from this type of process will probably need to be classified as more hazardous waste. This can have a significant effect on the cost of disposal of spent material.

It will of course be understood that reference to an irreversibility of the chemical bonding reaction in the preferred mode of operation of the process underlying the invention is reference to its substantial irreversibility in normal conditions such that the resultant bonded product can be treated as functionally inert in practice. Reference to an irreversibility of the chemical bonding reaction in the preferred mode of operation of the process underlying the invention is reference relative to the inherently reversible nature of sorption processes in many prior art systems that (often as a deliberate feature to allow recovery of the contaminant at a remote site) do not therefore lock up contaminant in a manner that allows the resultant bonded product can be treated as functionally inert in practice. The skilled person will appreciate as the intended meaning that the reaction is substantially irreversible for practical purposes under realistic thermodynamic conditions rather than that the reaction is fundamentally irreversible in all thermodynamic conditions.

The apparatus may be adapted to process fluid to be decontaminated either as a continuous or as a batch process.

A fluidised dispersion of reactive clays can be achieved in one of two general ways. In the first alternative way, the dispersion is a fluidised solid, and for example makes use of fluidised bed technology. In the second alternative,

particulate reactive clay is suspended in a liquid, for example as a slurry, which is then dispersed, for example as a spray, mist or the like, to form the fluidised dispersion within the volume.

The latter alternative is to be preferred in accordance with the present invention.

Thus, in the mode of operation of the invention, the apparatus makes use of a suspension of reactive clays within a suitable liquid, which is dispersed in use within the volume into which the fluid to be treated is introduced by being dispersed into the form of a mist or the like.

Thus, the apparatus is adapted to be used with a suspension of particulate reactive clay in a suitable suspension liquid, which may for example be water, and includes means in the form of a slurry disperser to form, in use, a dispersed mist within the volume from the suspension. Optionally, the apparatus may comprise a supply of the suspension liquid, a supply of the particulate reactive clay, and a mixing unit to mix these together within the apparatus to prepare a slurry comprising a suspension of particulate reactive clay in the suspension liquid. The particulates are preferably in powder form, for example in that 80% of the particulates have a mean dimension below 50 μm and for example between 2 μm to 20 μm.

Conveniently, to effect this supply of dispersed mist into the volume, the apparatus includes at least one treatment fluid inlet for example comprising a spray nozzle formation to distribute suspension treatment fluid for example as a spray into the contained volume and consequently to create the required fluidised dispersion of particulate clay therein from the said suspension. A

plurality of spray nozzles may be provided, for example having a common supply in the form of a common supply manifold.

In a preferred embodiment, the means to form a dispersed mist/distribute the spray, comprises a venturi apparatus which is to say that at least a spray nozzle directs spray via at least one venturi. Preferably, a contaminated material supply inlet is provided into the venturi downstream of the nozzle outlet through which contaminated material may be supplied. A venturi creates conditions of pressure and turbulence downstream of the nozzle outlet that is found to optimise treatment fluid mist distribution (and hence clay distribution) within the volume and improve mass transfer rates between contaminant and reactive clay. In a particularly preferred embodiment, the apparatus is a low-energy venturi scrubber. Such an apparatus is familiar from other applications in the waste handling field and will offer simplicity of constructional principle when modified to operate in accordance with the principles of the invention.

The fluid to be treated is introduced into the volume either on a batch or continuous basis. If the fluid to be treated is a contaminated liquid, the liquid to be treated is conveniently also dispersed therein as a spray. If the apparatus is being used to treat contaminated gas, such as a contaminated air flow, the gas is introduced directly into, and for example cause to pass through, the volume.

Thus, the invention provides an effective and relatively inexpensive way of treating VOCs both in industrial and domestic situations, and in particular of treating high volume air streams contaminated with low value VOCs.

The reactive clays used in accordance with the invention are organo/inorgano clays which are selected to be reactive with the VOCs to be removed from the fluids to be treated, and are selected in particular to provide an effective medium for an irreversible chemical bonding reaction with the VOCs in question.

At least some of the reactions which occur between the reactive clays and the VOCs are preferably irreversible and the material created by the bonding process is usually stable and inert. The resultant material can thus be safely disposed of as routine waste. Because the clays being used are relatively inexpensive materials, and because the intended contaminants to be treated are likely to be low level and low value, there is no desire or requirement to recover the VOCs and/or to regenerate the absorbent material as would be the case for example with activated carbon systems.

Preferably, the reactive clays are organoclays, and/ or modified to have organophilic properties. In a particular preferred embodiment, the reactive clays and reactions are of the type set out in detail in GB2302685 incorporated herein by reference.

That is to say, the clay is preferably a modified organoclay having organophilic properties and treated by a pillaring agent and a salt of a transition metal in quantities corresponding to no more than 100% stoichiometric exchange of the exchangeable cations of the clay.

As described in the reference, the pillaring agent creates interlamellar spacing sufficient to provide access for larger molecules such as polychlorinated biphenyls and polyaromatic hydrocarbons. This is preferably at least 15A (15 xlO ^"10 m).

The pillaring agent may comprises an aluminium or other metal hydroxide polymer, or transition metal clusters, or appropriate silicon or titanium oxides.

In accordance with this feature, the modified clays are pillared and therefore have a set structure. This means that the viscosity of the system will remain constant, as contaminant is locked away reactively. By contrast in many prior existing systems the clays are designed to swell as they adsorb organic compounds. This will result in the absorption medium becoming more and more viscous. The pressure required to propel the contaminants through the liquid will therefore increase and this will affect the efficiency of the system.

In particular the modified organoclay may be further provided with intercalation additives comprising one or more in combination of an aluminium hydroxide polymer, a quaternary ammonium salt and a salt of a transition metal, such as a ferric salt and for example ferric chloride.

The amount of clay to be used, and the absorption rate which is achievable by the apparatus, will be dependent in part upon the type of pollutants to be removed from the air streams, and on input and desired output contaminant levels. However, clays of the type described in GB2302685 are known to offer exceptionally high absorption rates in theory, for example as high as 1 to 20, and the fluidising apparatus of the present invention allows this theoretically high adsorption rate to be exploited in particularly effective manner.

In accordance with the invention in a further aspect there is provided a method of treatment of contaminated fluid to remove contaminants such as volatile organic compounds therefrom, which method comprises providing a fluidised dispersion of particulate clays selected to bind chemically with the

contaminant within a suitable containment volume by first suspending the particulate clay within a suitable suspension fluid to form a slurry/ suspension and then dispersing the slurry/ suspension as a spray, mist or the like within the volume; causing the fluid to be decontaminated to pass into the volume; removing the fluid with reduced contamination from the volume.

The process may be operated as a batch process, whereby a quantity of fluid to be treated is passed into the containment volume, and simultaneously or sequentially therewith the reactive clay therein is caused to pass into a fluidised state to remove VOCs from the fluid to be treated, and the decontaminated fluid is then removed from the volume. However, more preferably, the process is operated continuously or semi-continuously, in that the clay within the volume is caused to go into a fluidised state, and fluid to be decontaminated is passed on a continuous or semi-continuous basis into the volume by suitable inlet means, and extracted at a reduced contamination level via suitable outlet means.

The fluidised dispersion of reactive clays is provided within the volume by first suspending the particulate clay within a suitable suspension fluid, such as an aqueous suspension fluid largely or completely based on water, to form a slurry/ suspension and then dispersing the slurry/ suspension as a spray, mist or the like within the volume.

In particular, the particulate clay is suspended in a suitable suspension fluid and the resultant suspension is dispersed within the volume via at least one suitable inlet nozzle formation and for example is dispersed within the volume via at least one venturi system downstream of the inlet nozzle as above described.

Conveniently, contaminated material to be treated is also injected into the venturi, downstream of the inlet nozzle. Conditions created via the venturi maximise mass transfer and optimise efficiency of the process.

The invention will now be described by way of example only with reference to Figure 1 of the accompanying drawings which illustrates an apparatus embodying the principles of the invention.

Figure 1 is a low-energy venturi scrubber designed to operate using a slurry comprising approximately 300 litres of water and up to 50 kilograms of organo/inorgano clay.

There are three modifications which are carried out on the basic Smectite clay

(usually Bentonite) to make it reactive.

The clay is pillared using Hydroxy Aluminium pillars. This gives the clay a set structure, so that it does not swell when it comes into contact with water.

Untreated Bentonite will swell to infinity when mixed with water. The treated clay has a honeycomb structure containing millions of reactive sites. The Hydroxy Aluminium pillars react very readily with Chlorine atoms , making the treated clay very effective in treating chlorinated hydrocarbons, dioxins and similar compounds.

Quaternary Ammonium Salts (QAS) are added to the clay. These compounds are very organophilic and attract organic molecules into the clay.

Transition metal ions are added to the clay. These react with Benzene rings by a process called Transition Metal Ligand Bonding.

The apparatus comprises a venturi (10) and main body (1), a fan (6), a recirculation pump (3), and an additional recirculation pump (4) acting from the bottom of the main body to its top in order to recirculate the slurry as it tends to become solid when left immobile. The fan (6) operates via a mist eliminator (8).

Treatment fluid is introduced via the inlet (9) for example as a spray and passed to the columnar venturi (10) which generates conditions of pressure and turbulence in the spray dispersion so as to optimise the dispersion for operation of the invention. Contaminated air flow is also introduced to the venturi via the ductwork (12), and the spray dispersion of fine particulate clay binds to and effects removal of the volatile organic compounds from the air stream.

The columnar venturi structure (10) is of particular importance to operation of the invention. Treatment fluid is injected at the top of the column via the inlet (9) for example in the form of a spray. Contaminated fluid to be treated is also introduced in the venturi, immediately downstream of the inlet (9) via the ductwork (12). The waste material to be treated is conveniently in liquid form, for example comprising inherently liquid waste, or some liquid colloid incorporating waste. The liquid may also be conveniently introduced as a spray.

In any event, conditions created by the venturi, in particular in relation to pressure and turbulence in a manner which will be familiar, optimise dispersal of both materials, optimise mixing and mass transfer, and create optimum conditions for efficient operation of the reactive processes on which the invention relies.

The venturi is fluidly connected to and forms a part of the treatment volume with the main body (1), the overall apparatus being familiar generally within the waste disposal industry as a venturi scrubber. It is found to be a particularly effective way of creating a dispersed mist of the reactive clays, in conjunction with a dispersed mist of fluid waste to be treated, which maximises mixing of the contaminate material with the reactive clay, and maximises reaction rates. The venturi scrubber apparatus requires little adaptation from its more conventional application, and offers an effective apparatus solution to make use to maximum effect of the reactive nature of the clay in removing, and in particular preferably permanently binding with, the contaminants in the fluid to be treated, so as to produce an inert and readily disposed waste product.

The result is improved reaction rate when compared with conventional solid fluidisation techniques. For example, a conventional approach might be considered to fluidise reactive clays of the type used in accordance with the invention using a fluidised bed or like technology. However, fluidisation in accordance with the invention by mixing the clay with a suitable suspension liquid to form a suspension, and distributing this suspension in the reactive volume as a mist, for example as a spray, is found to be particularly and surprisingly effective when compared with such solid fluidisation techniques. This effect is particularly marked using a venturi, such as the venturi scrubber of the preferred embodiment.

The potential reactions which can occur between a modified clay, as described above, and organic molecules are as follows.

1. Lewis acid/base reactions between any Chlorine atoms present and the Hydroxy Aluminium pillars.

2. The organic compounds will be attracted by the QAS and will react with it.

3. Hydrogen bonding will occur between the organic compounds and the silicate plates which are present in the clay.

4. Van der WaIs reaction will occur. 5. Transition Metal Ligand bonding will occur between the Ferric ions and any Benzene rings which are present.

Thus the clay reacts at least partly via a permanent chemical bonding reaction and not solely by a physical sorption process. Irreversible chemical reactions stabilise the contaminants to produce relatively stable and inert relatively non- reactive waste, which can be disposed of relatively cheaply. It can often be safely disposed of in normal land-fill sites for example. The venturi system optimises mass transfer rates and optimises the reactive processing rate of the system. A compact, convenient and efficient system for the safe treatment of the waste is offered.