WO2004039498A2

WO2004039498A2 - Recovery process

Info

Publication number: WO2004039498A2
Application number: PCT/GB2003/004715
Authority: WO
Inventors: John T. Breckenridge
Original assignee: Breckenridge Associates Limited
Priority date: 2002-10-31
Filing date: 2003-10-31
Publication date: 2004-05-13
Also published as: AU2003301750A1; AU2003301750A8; WO2004039498A3; GB0225429D0

Abstract

This invention relates to a process for converting petrochemical synthesis reaction organic by-products and residues containing spent catalytic materials into substantially stable compounds. In particular, this invention relates to converting by-products containing spent catalytic material from the catalytic formation of terephthalic acid into substantially stable compounds by pyrolysis and catalytic treatment wherein at least part of the converted material may be recycled into fresh catalytic materials.

Description

RECOVERY PROCESS

Field of the Invention

This invention relates to a process for converting, for example, petrochemical synthesis reaction organic byproducts and residues containing spent catalytic materials into substantially stable compounds. In particular, this invention relates to converting byproducts containing spent catalytic material from the catalytic formation of terephthalic acid into substantially stable compounds wherein at least part of the converted material may be recycled into fresh catalytic materials.

Background of the Invention

During a catalytic reaction such as the manufacture of terephthalic acid, the catalyst is gradually deactivated with the result that the catalyst must at some point be renewed. This can be an expensive procedure. Most current techniques also make no attempt to retrieve and recycle all of the reaction by-products containing spent catalytic material unless derived from platinum group metals (i.e. pgms). Furthermore, the byproducts containing spent catalytic material, if not recycled, may have to be disposed of under special conditions, which can be expensive. For example, in the production of ^' terephthalic acid, a significant problem is the formation of a residue which contains spent catalyst. This spent catalyst may be classified as a special waste, attracting high waste disposal costs.

Terephthalic acid (PTA) (C₆H₄ (COOH) ₂) is a precursor molecule of polyester (polyethylene terephthalate) resins used to manufacture polyester fibre and PET resin for soft drink bottles and other applications. PTA is manufactured on an industrial scale by the reaction of p- xylene with oxygen under pressure in the presence of a combination of heterogeneous and homogeneous catalysts.

The most widely used heterogeneous catalyst is 0.5% palladium supported on an activated carbon substrate. This catalyst typically has a life of 2-3 years and is replaced in its entirety at that time. The spent catalyst is sent to specialist re-processors who recover and refine the palladium which is then sent for fresh catalyst manufacture.

The homogenous catalyst system comprises an aqueous solution of cobalt and manganese bromides in acetic acid. This is usually dosed into the p-xylene process stream to give a cobalt concentration of around 1500 ppm with manganese at around 3000 ppm and bromine at around 5000 ppm. However, these levels vary widely from process to process and from plant to plant.

In the terephthalic acid manufacturing process, only a proportion of the primary raw material (i.e. p-xylene) is converted to terephthalic acid on each pass through the reactor and this is accompanied with a significant amount of by-products.

The single-pass conversion efficiency of p-xylene to terephthalic acid varies from plant to plant and process technology to process technology. During the reaction, the process liquor (acetic acid, p-xylene, water, terephthalic acid, cobalt, manganese and bromine) is continuously tapped from the reactor. The terephthalic acid is filtered out and sent for purification. The unreacted p-xylene, acetic acid and dissolved cobalt, manganese and bromine catalyst are returned to the reactor leaving an organic residue.

The organic residues may contain any or all of the following compounds: ortho-phthalic acid; meta-phthalic acid; dimethyl phthalate; benzoic acid; mono, di and trimellitic acids; , ortho, meta and para-touluic acids; their analogues, their equivalent aldehydes and alcohols, together with di ers, trimers and other analogue compounds. An appreciable amount of cobalt, manganese and bromine from the homogeneous catalyst may also be present. The actual make-up of the by-products will depend on any or all of the following:

1. The refinery from which the p-xylene comes.

2. The process technology used to make the terephthalic acid.

3. The design and construction of the manufacturing plant used. 4. How well the plant is running.

The spent heterogeneous catalytic material is sent to specialist re-processors who recover and refine the palladium which is then sent for fresh catalyst manufacture. The by-products containing spent homogeneous catalytic material (i.e. residue) have to undergo controlled disposal which is an expensive process .

The homogeneous catalyst used in the production of terephthalic acid comprises an aqueous solution of cobalt and manganese acetates in the presence of excess acetic acid and bromine. This solution is intimately mixed with the p-xylene in the process circuit using acetic acid as the process solvent. In the prior art, the majority of the by-products containing spent homogeneous catalytic material are not recycled and have to be disposed of.

In the production of terephthalic acid, the formed terephthalic acid is filtered out from the process flow. The process flow minus the terephthalic acid is passed to a catalyst recycle unit wherein the residual process flow is distilled at elevated temperatures under low pressure to remove unreacted p-xylene, water, acetic acid and any other volatile compounds. The distillation residue contains most of the Co, Mn and Br used in the process and also comprises a mixed organic acid, alcohol and aldehyde residue. The distillation residue is then neutralised with sodium hydroxide and diluted with water to make a 'gruel-like' 2-phase slurry. The residue would otherwise be a solid at room temperature. Some of the Co, Mn and Br dissolves in the water phase and is returned to the process along with the unreacted p-xylene and acetic acid. The remaining solid organic material contains the remainder of the Co, Mn and Br entrained within it. This organic/inorganic mixed material may be classified as a "special waste" (i.e. residue) for subsequent disposal by incineration or landfill.

The present invention relates to the treatment of this residue in order to destroy the organic compounds and recover the Co, Mn and Br for subsequent reprocessing into fresh catalyst.

It is an object of embodiments of the present invention to obviate or at least mitigate one or more of the aforementioned problems.

It is a further object of embodiments of the present invention to provide a method for converting by-products containing spent catalytic material in a catalytic process into substantially stable compounds that facilitates their ready recycling into fresh homogeneous catalyst . It is a yet further object of embodiments of the present invention to dispose of organic residue formed in catalytic reactions in an environmentally friendly and cost-efficient manner.

Summary of the Invention

According to a first aspect of the present invention there is provided a method for converting by-products obtained from a catalytic process into substantially stable compounds comprising treating the by-products with a combination of a pyrolysis and a catalytic treatment step wherein organic components of the by-product are broken down to carbon dioxide and water, while additionally entrained catalytic materials are converted to forms suitable for directly replenishing any spent catalytic material or indirectly via a reprocessing technique .

The entrained catalytic material is catalytic material which is used up during the initial catalytic process which produces the by-products. The entrained catalytic material includes occluded catalytic material which is, for example, encased in micro-crystals in a matrix form. For example, the catalytic material may be metal based compounds such as cobalt and manganese compounds .

The by-products are formed in the catalytic process (e.g. the formation of terephthalic acid) and may be classed as 'waste material'. The by-products (i.e. 'waste material') may comprise organic components and inorganic components.

The conversion of the by-products (i.e. undesirable) to substantially stable compounds (i.e. less undesirable) and compounds suitable for replenishing the spent catalytic material means that no disposal treatment of any organic and/or inorganic waste may be required. As there is no need for any type of landfill, incineration or biological treatment the said method is therefore extremely efficient, environmentally acceptable and cost- effective.

The by-products may be converted to metals, metal oxides and/or halogenated compounds which facilitates their recycling into fresh homogeneous catalyst.

Conveniently, the catalytic process which produces the by-products containing spent catalytic material may be the catalysed formation of terephthalic acid from p- xylene and oxygen.

Alternatively, the catalytic process which produces the by-products containing spent catalytic material may be selected from any petrochemical manufacturing processes which generate solid organic wastes containing potentially valuable catalyst metals. Other catalytic process which produce by-products containing spent catalytic material are: palladium catalysed carbonylation of methanol to yield acetic acid; catalytic oxidation of benzene to cyclohexane; palladium catalysed manufacture of hydrogen peroxide from anthraquinone; palladium catalysed oxidation of benzene to maleic anhydride; palladium catalysed oxidation of orthoxylene to phthalic acid; and catalytic hydroformulation or 0X0 synthesis of detergent alcohols and acids which use platinum group metal (pgm) catalysts, such as rhodium and platinum.

The catalytic process may also be selected from a range of processes involving catalytic oxidation, reduction or addition to aromatic compounds, to yield their alcohols, aldehydes, acids or other derivatives, which yield unwanted co-products in the form of residues. These residues contain dimers, trimers, homologues and analogues of the target compound and frequently contain entrained valuable catalyst materials. These residues can be treated by the present invention.

In terephthalic acid production the by-products in addition to components of spent catalytic material may include, for example, any of the following: ortho- phthalic acid, meta-phthalic acid, dimethyl phthalate, benzoic acid, mono, di and trimellitic acids, , ortho, meta and para-touluic acids, their analogues, their equivalent aldehydes and alcohols, together with dimers, trimers and other analogue compounds thereof. The by-products may also contain trace impunities such as iron, nickel, chromium, etc. which originate from corrosion of the primary manufacturing plant itself.

The by-products may be converted into Co, Mn and other metal oxides together with Br compounds, in a concentrated form which facilitates their recycling into fresh homogeneous catalyst.

Preferably, the pyrolysis step occurs first and the catalytic treatment step occurs second in the treatment of the by-products. In an alternative, the pyrolysis and catalytic treatment step may occur substantially together and in the same reaction vessel. This may occur by coating at least part of the inside of reaction vessel forming the pyrolyser with catalytic material. Typically, the pyrolysis step heats the by-products up to about 400-600°C which results in molecular fragmentation. The by-products from a terephthalic acid production process may be fragmented into, for example, CO, C0₂, H₂0, Co and Mn oxides, bromine and methyl bromide, methane, ethane and other short chain hydrocarbons and elemental carbon. The pyrolysis step may be optimised by the introduction of controlled amounts of 0₂, N₂, H₂, etc. thereby allowing the pyrolysis reaction to be 'steered' and controlled to modify the composition of the off-gas stream. This provides the off-gas stream with more precisely defined characteristics. This may enable, for example, the amount of carbon in char to be reduced to substantially zero, or to provide a narrower spread of hydrocarbon chain lengths. This therefore allows the process efficiency to be optimised for different residue streams.

In the initial stages of pyrolysis-induced decomposition of the organic components; a foam may form within the pyrolyser barrel, interfering with the process flow. When hard spheres made of a ceramic or similar corrosion-resistant material (s) are allowed to rotate freely within the pyrolyser barrel, they may break this foam and facilitate the process. Between the pyrolysis step and the catalytic treatment stage of by-products formed in the production of terephthalic acid, there may be a filtering step which traps solids such as Co and Mn oxides and elemental carbon entrained in a gas flow from the pyrolysis step. The filter used in the filtering step may be self- cleaning which is advantageous for problem-free continuous operation of the system. The trapped solids may be sent off for recycling into, for example, fresh homogeneous catalyst . Gas from the filtering step may then be passed through a catalyst bed which performs a catalytic treatment step. Excess steam may also be passed through the catalytic bed. Free hydrogen may also be added to the steam. The catalytic bed in the treatment of by-products formed in the production of terephthalic acid converts substantially all hydrocarbons and CO into C0₂ and H₂0 and any bromine (e.g. methyl bromide obtained directly from the catalyst recycle unit) and bromo-carbon compounds into HBr acid gas .

The catalyst bed may be a metal based catalyst such as a Pd/ZnO/γ-Al₂0₃ catalyst with a surface area of at least 50m²/g (as discussed in US 5,817,896 which is incorporated herein by reference) . Other suitable catalysts comprising palladium, platinum, rhodium, ruthenium, silver, gold, gallium, either alone or in combination with a different metal such as zinc, aluminium, silver, platinum, nickel, gold or gallium which may be present in an oxygenated form. The ratios of palladium to zinc oxide may range from 10:1 to 1:1 or 1:5 to 1:2.

Alternatively, the catalyst bed may be selected from any of the following: palladium on activated carbon (as disclosed in GB 1,578,933 which is incorporated herein by reference) ; palladium on aluminium fluoride (as disclosed in EP 0328127 which is incorporated herein by reference) ; palladium chloride on activated carbon (as disclosed in JP 01,319,442,25 which is incorporated herein by reference) ; and catalysts containing palladium in concentrations by weight ranging from 0.5% w/w to 10% w/w on substrates of alumina, zirconia, zirconia coated alumina or activated carbon.

Gases such as CO, C0₂, H₂0 and HBr exiting from the catalyst bed in the treatment of by-products formed in the production of terephthalic acid may be cooled, passed through a H₂0 scrubber to remove HBr gas which is converted to HBr acid solution suitable for reincorporating into fresh homogeneous catalyst. A caustic scrubber may also be used to ensure that substantially no Br escapes from the treatment method.

According to a second aspect of the present invention there is provided apparatus for converting substantially all by-products from a catalytic process into substantially stable compounds wherein at least part of the converted material is available to replenish spent catalytic material directly or indirectly using a reprocessing technique comprising a pyrolyser for decomposing by-products and a catalyst bed for further treating the decomposed by-products.

The apparatus may also comprise a filter between the pyrolyser and the catalyst bed for separating out solids in a gas flow from the pyrolyser. The apparatus may also comprise a cooling device, and a water and caustic scrubber.

Typically, the pyrolyser and the catalyst bed are separate parts in the apparatus.

In an alternative embodiment, the pyrolyser and the catalyst bed may form a single reaction vessel, with at least part of the inside of the reaction vessel being coated with catalytic material.

Brief Description of the Drawings Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of chemical re-processing of by-products containing spent catalytic material according to the prior art;

Figure 2 is a schematic representation of an incineration re-processing technique of by-products containing spent catalytic material according to the prior art; Figure 3 is a schematic representation of a reprocessing technique for by-products containing spent catalytic material according to the present invention; and Figure 4 is a schematic representation of an alternative re-processing technique for by-products containing spent catalytic material according to the present invention.

Detailed Description

Shown in Figure 1, there is a schematic representation of wet-chemical re-processing apparatus, generally designated 10, for by-products formed in the production of terephthalic acid.

In a reactor 12, terephthalic acid is made from the reaction of p-xylene and oxygen in the presence of a heterogeneous palladium based catalyst 14 and a homogeneous catalyst. The p-xylene is pumped into the reactor 12 via an inlet pipe 16 and oxygen is pumped into the reactor 12 via an inlet pipe 18. Co, Mn and Br in acetic, which forms the homogeneous catalyst, acid is pumped into the reactor 12 via pipe 17. Via an outlet pipe 20 crude terephthalic acid unreacted p-xylene and by-products are passed into a catalyst recycle unit 22. In the catalyst recycle unit 22, the crude terephthalic acid is stripped from the reaction mix and passed out via pipe 24. The catalyst recycle unit 22 operates at low pressure and elevated temperatures and is used to remove water, acetic acid and any other volatile compounds via a distillation process, leaving a residue which comprises most of the Co, Mn and Br used in the process. The distillation residue is diluted with water and neutralised with a sodium hydroxide solution to make a 'gruel-like' 2-phase slurry which would otherwise be a solid at room temperature. Some of the Co, Mn and Br (e.g. cobalt bromide) dissolves in the water phase and can be recycled to form homogeneous catalyst. The remaining solid organic material contains the remainder of the Co, Mn and Br (e.g. cobalt bromide) occluded within it. This material is a residue and may be classified as "special waste" for subsequent disposal by incineration or landfill.

Via a further outlet pipe 26, from the catalyst recycle unit 22, the catalytic material comprising Co, Mn, and Br dissolved in acetic acid and water is recycled into homogeneous catalyst and is fed back into the reactor 12. Unreacted p-xylene, together with the acetic acid process solvent is also returned to the reactor 12 via a pipe 28, a condenser 30 and a further pipe 32.

The residue which comprises the solid organic material with Co, Mn and Br occluded in it is separated out in the catalyst recycle unit 22 and passed via a pipe 34 to a dilution tank 36. The residue comprises a mixture of any of the following: ortho-phthalic acid, meta-phthalic acid, dimethyl phthalate, benzoic acid, mono, di and trimellitic acids, α, ortho, meta and para- touluic acids, their analogues, their equivalent aldehydes and alcohols, together with dimers, trimers and other analogue compounds thereof; and Co and Mn bromides. In the dilution tank 36, the residue is diluted with water and neutralised with sodium hydroxide to solubilise as much of the entrained Co and Mn as possible.

After diluting the residue in the dilution tank 36, insoluble organics are separated out from the soluble organics in a filter device 38. The insoluble organics exit via pipe 40 and are sent to an effluent treatment plant for incineration, landfill or biological treatment. The soluble organics containing Co and Mn bromides pass via pipe 42 into a vessel 44 wherein the Co and Mn bromides are reacted with sodium carbonate to precipitate the Co and Mn as carbonates. A further filter device 46 then performs a further separation process wherein a dilute organic effluent comprising sodium carbonate, sodium acetate, sodium bromide and soluble organics exit via pipe 48 to an effluent treatment plant for incineration, landfill or biological treatment.

The separated Co/Mn carbonate in a cake-like form is then passed to a reaction vessel 50. Acetic acid is pumped into the reaction vessel 50 via a pipe 52 wherein the mixed Co/Mn carbonate are re-dissolved and returned to the reactor 12 via pipe 54.

The process as set out in Figure 1 loses almost 100% of the bromine component and a significant amount of the Co, Mn and acetic acid. The loss of Co and Mn is between 15 - 100%. Furthermore, the organic waste comprising insoluble Co and Mn compounds formed in the reaction needs to be disposed of resulting in annual waste disposal charges of several hundred thousand dollars per plant. Given that an average terephthalic acid plant has a manufacturing capacity of 400,000 tonnes/annum and that there is an installed global manufacturing base of some 34 million tonnes, this is a loss to the industry of about US$55 to US$110 million per annum.

Shown in Figure 2, there is a schematic representation of an incineration re-processor, generally designated 110, for re-processing by-products in the production of terephthalic acid. Similarly to the process shown in Figure 1, there is a reactor 112 in which terephthalic acid is made from the reaction of p-xylene and oxygen in the presence of a heterogeneous palladium catalyst 114 and a homogeneous catalyst. The p-xylene is pumped into the reactor 112 via an inlet pipe 116 and oxygen is pumped into the reactor 112 via an inlet pipe 118. Co, Mn and Br in acetic acid, which again form the homogeneous catalyst, is pumped into the reactor 112 via pipe 1,17. Via an outlet pipe 120 crude terephthalic acid and by-products are passed into a catalyst recycle unit 122. In the catalyst recycle unit 122, the crude terephthalic acid is stripped from the reaction mix and passed out via pipe 124. Via pipe 126 from the catalyst recycle unit 122, soluble by-products comprising Co and Mn bromide in acetic acid is recycled back into the reactor 112. Via pipe 128, a condenser 128 and pipe 132 unreacted p-xylene and the acetic acid process solvent is returned to the reactor 112.

The undissolved by-products (i.e. the residue which may be referred to as "special waste") are passed out of the catalyst recycle unit 122 and along pipe 134 to an incinerator 136. The undissolved by-products comprise a mixture of any of the following: ortho-phthalic acid, meta-phthalic acid, dimethyl phthalate, benzoic acid, mono, di and trimellitic acids, α, ortho, meta and para-touluic acids, their analogues, their equivalent aldehydes and alcohols, together with dimers, trimers and other analogue compounds thereof; and Co and Mn acetates and bromides.

The Co and Mn acetates and bromides are converted to solid inorganic Co and Mn oxides/ash materials. The solid inorganic Co and Mn oxides/ash materials from the incinerator 136 are passed along pipe 138 and are trapped in a ceramic filter 140 downstream of the incinerator 136 and sent off for recycling along pipe 141.

The incinerator off-gases such as organo-bromides are then passed along pipe 142 to an after-burner 144 which destroys any organo-bromide compounds such as methyl bromide and bromo-dioxins in the off-gas stream.

The off-gases after the after-burner 144 are then passed along pipe 146 to a caustic scrubber 148 which traps any Br and any other acid gases present.

The final emissions from pipe 150 are C0₂, H₂0 and NaBr.

Incineration results in the loss of 100% of the Br and acetic acid component in the catalytic cycle. Incineration typically loses around 2% of input Co and

Mn. However, between 2 and 15% may subsequently be lost when the incinerator fly-ash is reprocessed into fresh homogeneous catalyst. This is because the temperature of the incineration can cause the formation of Co and Mn oxides with a high oxidation state which are difficult to reprocess into their acetates. As a general rule, the lower the oxidation state, the easier the re-dissolution.

Therefore, as incineration tends to favour the formation of higher oxides, a significant proportion of the Co and Mn will not re-dissolve. A further disadvantage of incineration is that the bromine content of the residue is lost.

Figure 3 is a schematic representation of the reprocessing technique according to the present invention, generally designated 210.

Feedstock 212 in Figure 3 is obtained directly from a catalyst recycle unit (not shown) and comprises any of the following: ortho-phthalic acid, meta-phthalic acid, dimethyl phthalate, benzoic acid, mono, di and trimellitic acids, α, ortho, meta and para-touluic acids, their analogues, their equivalent aldehydes and alcohols, together with dimers, trimers and other analogue compounds Co and Mn bromides and acetates and HBr. The solids from the feedstock 212 are fed continuously along pipe 214 into a rotary pyrolyser tube

216 with an external heater (not shown) . The rotary pyrolyser tube 216 is made of titanium, various high chromium stainless steels (e.g. stainless steels with a content of 20% or more such as 254 SMO (Trade Name) (20% Cr) and 654 SMO (Trade Name) (24% Cr) , high nickel alloys of greater than 95% and preferably 99% purity (e.g. Nickel 201 (Trade Mark) which has a minimum nickel content of 99%) ceramics such as alumina/magnesia or silicon carbide or any other similar chemically and thermally resistant material. The pyrolyser 216 operates at between 400 to 600°C, breaking the purged mixture down into any of the following: CO, C0₂, H₂0, Co and Mn oxides, bromine and methyl bromide, methane, ethane and other short chain hydrocarbons, together with elemental carbon.

The pyrolysis off-gas then passes along pipe 218 to a ceramic gas tube filter 220 which traps the solids entrained in the gas flow such as Co and Mn oxides and carbon. The oxidation states of the Co and Mn oxides are low and they are relatively easy to re-dissolve. The collected Co and Mn oxides formed under these conditions and carbon are sent for recycling into fresh homogeneous catalyst. The pyrolysis step may be optimised by the introduction of controlled amounts of 0₂, N₂, H₂, etc. along with the residue in pipe 215.

The filtered gas leaving the ceramic gas filter 220 then passes along pipe 222 into a catalyst bed 224 comprising a Pd/Zn alloy supported on an alumina substrate with a surface area of at least 50m²/g. This type of catalyst is suitable as: it has the ability to handle high concentrations of halogenated volatile organic compounds; it has a high single pass conversion efficiency from halogenated volatile organic compounds to halogen acid gas, water and carbon dioxide; it has a long-term operating stability; and it has resistance to poisoning by sulphur, phosphorous and low concentration of gas-borne contaminants such as lithium, sodium, potassium, cadmium, lead and mercury. The filtered gas is the off-gas from the pyrolyser after it passes through the ceramic gas filter 220. The filtered gas is known to contain any of the following: H₂, CO, C0₂, H₂0, Br₂, Br^", CH₃Br, and a mix of short chain hydrocarbons, generally in the range of Cl to C8. A pipe 225 is also used to directly connect the catalyst recycle unit (not shown) to the catalyst bed 224. The pipe 225 is used to transfer methyl bromide (CH₃Br) which is produced as a by-product of the p-xylene oxidation reaction. Shortly, methyl bromide may not be discharged into the atmosphere without incurring prosecution. Current routes to dealing with the methyl bromide centre round the use of high temperature incineration, followed by caustic scrubbing. This is an expensive and wasteful process and leads to a loss of the bromine contained. Under The Montreal Protocol of 1991, methyl bromide was defined as a chemical that contributes to depletion of the Earth's ozone layer. The definition was based on scientific data. Accordingly, the manufacture and importation of methyl bromide will be phased out in developed countries as follows: 25-percent reduction in 1999, 25-percent reduction in 2001, 20- percent reduction 2003, and complete phase out in 2005. In developing countries, consumption will be frozen in 2002 at 1995-98 average levels, followed by 20-percent reduction in 2005 and complete phase out in 2015.

Steam is also fed into the catalytic bed 224 via inlet 223. As the filtered gas passes through the catalyst bed 224 in the presence of excess steam, substantially all of the residual hydrocarbons and the CO are converted to C0₂ and H₂0. The catalyst in the catalyst bed 224 also converts the Br and any bromo-carbon compounds (e.g. methyl bromide from the catalyst recycle unit) present in the gas stream into HBr acid gas.

The C0₂, H₂0 and HBr exiting the catalyst bed 224, flows along pipe 226 to a cooling tower 228 which cools the gas, then along pipe 230 to a water scrubber 232 where the HBr gas is recovered as HBr solution, exiting via pipe 234 which is suitable for re-incorporation into fresh homogeneous catalyst.

The remaining gas then passes along pipe 236 to a caustic scrubber 238 which ensures that no Br escapes from the system, reducing the environmental impact of the process effectively to zero. All of the Br is available therefore to be recycled into the homogeneous catalyst.

Exiting the caustic scrubber 238 in pipe 240 is H₂0 and C0₂. The procedure set out in Figure 3 therefore combines pyrolysis with a catalytic step which effects almost complete recovery of the spent Co, Mn and Br and eliminates the need for subsequent treatment and disposal of any organic waste. There is therefore no need and no incurring of the costs involved in incineration, landfill or biological treatment. The method therefore has minimal environmental impact when compared with all existing recovery processes. Figure 4 is a schematic representation of the reprocessing technique according to the present invention, generally designated 310. The process shown in Figure 4 is very similar to that in Figure 3. However, a major difference is that in Figure 4, at least part of the inner reactive surface of the pyrolyser 316 is coated with a catalyst coating 324. The pyrolysis and catalytic step therefore occurs within the same vessel. This has advantages in that it simplifies the apparatus required for re-processing the by-products.

Example 1 A pilot plant was constructed to replicate the plant shown in Figure 3 but in a smaller scale. The pyrolyser tube (216) is fed with residue from a catalyst recycle unit at the rate of 30g/minute via a peristaltic pump (not shown) . The first step in downstream off-gas treatment is filtration to remove the entrained metal oxides in a ceramic gas filter (220) . The next stage is to pass the gases through a catalyst bed (224) which converts the entrained bromine and bromides to HBr. The same catalyst bed (224) also oxidises the hydrocarbon gases to C0₂ and H₂0.

The gas stream exiting the catalyst bed is cooled and fed through a water cooling tower (228) to collect the HBr and finally exhausted to atmosphere via a caustic scrubber (238) . The residue from the catalyst recycle unit is a two- phase system, a finely-divided solid organic slurry dispersed in an acidic water phase. Analysis of the sample showed that most of the cobalt, manganese and bromine was present in the water phase. This is shown in Table 1 below. (It should be noted that in a production scale unit, dry residue will be directly obtained from the catalyst recycle unit overcoming the need to separate the acidic water phase. There will be no acidic water phase in a production scale unit due to the excess heating energy required to evaporate the water. A production scale unit will therefore be fed directly from the catalyst recovery unit with a hot, molten residue which is easily treated by the pyrolyser) .

Table 1

However, it was found that pyrolysis of the two- phase wet slurry is extremely inefficient and the results are unreliable. Accordingly, and in the light of this experience, the decision was taken to decant most of the aqueous phase and pyrolyse only the solid phase organic dispersion.

The organic dispersion was fed to the pyrolyser (216) via a peristaltic pump (not shown) at the rate of 30 g/min and processed at 600°C. The off-gas from the pyrolyser (216) was fed, first through a sintered ceramic filter (220) to trap all solid particulate matter and then passed through a proprietary catalyst bed (224) at 500°C. The catalyst bed (224) is a Pd/ZnO catalyst using a ceria-doped zirconia substrate as described in US 5,899,678. The gas stream from the catalyst bed (224) is fed in sequence through a cooler (228), water scrubber (232) and caustic scrubber (238) before being exhausted to atmosphere.

• The weight of material pyrolysed was 3,600g.

• The weight of char (i.e. material comprising Co and Mn oxides) collected was 16.420g.

Both the input residue and output char (i.e. material comprising Co and Mn oxides) were analysed. The results are shown in Table 2.

Table 2

The output gases and residue were characterised as: water vapour, hydrobromic acid, carbon dioxide, and char (~ 0.05% w/w of the original input residue) .

The char contained approx 44% w/w cobalt, and manganese oxides (magnetic) , sodium and bromine, together with other trace metal impurities, equivalent to a cobalt metal concentration of approx. 0.07% in the input residue.

The hydrobromic acid content of the water scrubber was measured by sodium hydroxide titration at 0.245g (as bromine) . Taken together with the bromine trapped in the char, this gives an overall recovery of 0.366g or a recovery efficiency of 93.42% for the Br. Recycling of the Collected Cobalt, Manganese and Bromine

The recycling procedure was as follows: 1. The recovered char was mixed with 100 ml of de- mineralised water, brought to the boil, refluxed for 5 minutes, cooled and filtered. The filtrate was collected and reserved.

2. The process was repeated twice more, yielding 290 ml of a dilute sodium bromide solution and a char filter cake which, on drying, weighed 13.317g.

3. The char filter cake was then refluxed in 150 ml of 50% acetic acid for 6 hours, after which it was cooled and filtered. 4. The filter cake after drying weighed 9.133g.

Following drying, it was calcined in a muffler furnace for 60 minutes, cooled to room temperature and weighed.

The residue weighed 0.005 g, suggesting that the char was almost pure carbon. 5. The acetic acid filtrate from step 3 was diluted to

200ml with demineralised water and cooled.

6. The cobalt and manganese content were measured by means of atomic absorption spectrophotometry . The solution was measured at Co 1.27% w/w, Mn 0.79% w/w. equating to a re-solutioning efficiency of 97.8% on the recovered metal oxides.

7. The filtrate arising from the washing process (steps 1 & 2 above) was evaporated with boiling until the volume was reduced to approximately 25ml. It was then cooled. 8. 25ml 50% sulphuric acid was added to the sodium bromide solution obtained at step 7 and the mixture brought to the boil. 9. The vapour from the boiling solution was led, via a condenser and mist-trap into a bubbler jar containing 2M sodium hydroxide solution.

10. Boiling was continued until the temperature started rising, indicating that almost all of the water had boiled off.

11. The sodium hydroxide in the bubbler jar was back- titrated with 2M sulphuring acid, using a methylene blue indicator. The result showed that 0.112g bromine had been evolved as hydrobromic acid, a recovery efficiency of 92.6%.

Discussion

Cobalt and Manganese The system recovery efficiencies for cobalt and manganese were 98.82% and 97.71%, respectively. The presence of the char means that both metals are recovered in a low oxidation state, suitable for recycling into fresh cobalt and manganese acetates as was demonstrated. The efficiency with which the collected oxides could be re-dissolved in acetic acid was high at 97.78%, equivalent to an overall system efficiency of 96.6% for cobalt and 95.5% for manganese. This is well in excess of the efficiencies reported for competing technologies (i.e. as shown in Figures 1 and 2) . This is due to low oxidation states for the Co and Mn.

Sodium

Sodium recovery efficiency was of the same order at 91.52%.

Sodium is present in the residue as a result of the use of caustic solutions to clear plant blockages. Its presence in concentrations >100 ppm in the gas stream entering the catalyst bed (224) can be expected to have a negative effect on catalyst life. Great attention must be paid to controlling the temperature of the gas stream entering the catalyst bed (224) at the design stage to ensure that only the minimum amount of sodium transits the system. A "sodium absorber" may be incorporated between the filter (220) and catalyst bed (224) in the production equipment.

Bromine

Bromine retention in the char at 30.56% is higher than expected. A balance of 61.86% (giving a -total recovery of efficiency of 92.42%), was recovered from the water scrubber. There are a number of possible explanations for this result, such as the formation of NaBr with the high amount of sodium in the sample.

It proved possible to extract the bromine in the char as sodium bromide by washing. Subsequent treatment with 50% sulphuric acid released the bromine as hydrogen bromide which was collected in sodium hydroxide.

Where the char from the filter is to be recycled directly into a cobalt manganese bromide catalyst solution, then the bromine retention issue becomes relevant .

Claims

1. A method for converting by-products obtained from a catalytic process into substantially stable compounds comprising treating the by-products with a combination of a pyrolysis and a catalytic treatment step wherein organic components of the by-products are broken down to carbon dioxide and water, while additionally entrained catalytic materials are converted to forms suitable for directly replenishing any spent catalytic material or indirectly via a reprocessing technique.

2. A method according to claim 1 wherein the conversion of the by—products to substantially stable compounds and compounds suitable for replenishing the spent catalytic material means that no disposal treatment of any organic and/or inorganic waste is required.

3. A method according to any of claims 1 or 2 wherein the by—products are converted to metals, metal oxides and/or halogenated compounds which facilitates their recyling into fresh homogeneous catalyst.

4. A method according to any preceding claim wherein the catalytic process which produces the byproducts containing spent catalytic material is the catalysed formation of terephthalic acid from p— ylene and oxygen.

5. A method according to any of claims 1 to 3 wherein the catalytic process which produces the byproducts containing spent catalytic material is selected from any petrochemical manufacturing process which generate solid organic wastes containing potentially valuable catalyst metals, or catalytic processes which produce by-products containing spent catalytic material such as any of the following: palladium catalysed carbonylation of methanol to yield acetic acid; catalytic oxidation of benzene to cyclohexane; palladium catalysed manufacture of hydrogen peroxide from anthraquinone; palladium catalysed oxidation of benzene to maleic anhydride; palladium catalysed oxidation of orthoxylene to phthalic acid; and catalytic hydroformulation or OXO synthesis of detergent alcohols and acids which use platinum group metal (pgm) catalysts, such as rhodium and platinum.

6. A method according to any of claims 1 to 4 wherein, in terephthalic acid production the byproducts in addition to components of spent catalytic material include any combination of the following: orthophthalic acid; meta-phthalic acid; dimethyl phthalate; benzoic acid; mono, di and trimellitic acids; , ortho, meta and para-touluic acids; their analogues, their equivalent aldehydes and alcohols, together with dimers, trimers and other analogue compounds thereof.

7. A method according to any preceding claim wherein the by-products are converted into Co, Mn and other metal oxides together with Br compounds, in a concentrated form which facilitates their recycling into fresh homogeneous catalyst .

8. A method according to any preceding claim wherein the pyrolysis step occurs first and the catalytic treatment step occurs second in the treatment of the by- products .

9. A method according to any of claims 1 to 7 wherein the pyrolysis step and the catalytic treatment step occur substantially simultaneously.

10. A method according to any preceding claim wherein the pyrolysis step heats the by-products up to about 400-600°C which results in molecular fragmentation.

11. A method according to any preceding claim wherein the by-products from a terephthalic acid production process are fragmented into CO, C0₂, H₂0, Co and Mn oxides, bromine and methyl bromide, methane, ethane and other short chain hydrocarbons and elemental carbon.

12. A method according to any preceding claim wherein the pyrolysis step is controlled by feeding in controlled amounts of 0₂, H₂ and N₂ in the pyrolyser.

13. A method according to any of claims 4 to 12 wherein between the pyrolysis step and the catalytic treatment stage of by-products formed in the production of terephthalic acid, there is a filtering step which traps solids such as Co and Mn oxides and elemental carbon entrained in a gas flow from the pyrolysis step.

14. A method according to claim 13 wherein the trapped solids are sent off for recycling into fresh homogeneous catalyst.

15. A method according to any of claims 13 or 14 wherein gas from the filtering step is passed through a catalyst bed which performs a catalytic treatment step.

16. A method according to claim 15 wherein excess steam is also passed through the catalytic bed.

17. A method according to claim 16 wherein free hydrogen is also added to the steam.

18. A method according to any of claims 15 to 17 wherein the catalytic bed in the treatment of by-products formed in the production of terephthalic acid converts substantially all hydrocarbons and CO into C0₂ and H₂0 and any bromine (e.g. methyl bromide) and bromo-carbon compounds into HBr acid gas .

19. A method according to any of claims 15 to 18 wherein the catalyst bed is a metal based catalyst such as a Pd/Zn0/γ-Al₂0₃ catalyst with a surface area of at least 50m²/g and a Pd/ZnO catalyst on a ceria doped zirconia substrate.

20. A method according to claim 19 wherein other catalysts comprising palladium, platinum, rhodium, ruthenium, silver, gold, gallium, either alone or in combination with a different metal such as zinc, aluminium, silver, platinum, nickel, gold or gallium are present in an oxygenated form.

21. A method according to any of claims 19 or 20 wherein the ratios of palladium to zinc oxide range from 10:1 to 1:1 or 1:5 to 1:2.

22. A method according to any of claims 15 to 18 wherein the catalyst bed is selected from any of the following: palladium on activated carbon; palladium on aluminium fluoride; palladium chloride on activated carbon; and catalysts containing palladium in concentrations by weight ranging from 0.5% w/w to 10% w/w on substrates of alumina, zirconia, zirconia coated alumina or activated carbon.

23. A method according to any of claims 15 to 22 wherein gases such as C0₂, H₂0 and HBr exiting from the catalyst bed in the treatment of by-products formed in the production of terephthalic acid are cooled, passed through a H₂0 scrubber to remove HBr gas which is converted to HBr acid solution suitable for reincorporating into fresh homogeneous catalyst.

24. A method according to claim 23 wherein a caustic scrubber is also used to ensure that substantially no Br escapes from the treatment method.

25. Apparatus for converting substantially all byproducts from a catalytic process into substantially stable compounds wherein at least part of the converted material is available to replenish spent catalytic material directly or indirectly using a reprocessing technique comprising a pyrolyser for decomposing byproducts and a catalyst bed for further treating the decomposed by-products.

26. Apparatus according to claim 25 wherein the apparatus also comprises a filter between the pyrolyser and the catalyst bed for separating out solids in a gas flow from the pyrolyser.

27. Apparatus according to any of claims 25 or 26 wherein the apparatus also comprises a cooling device, and a water and caustic scrubber.

28. Apparatus according to claim 25 wherein the pyrolyser and the catalytic bed form a single reaction vessel with the catalytic bed formed onto at least part of the inside surface of the reaction vessel.