WO1994012432A1

WO1994012432A1 - Gas purification

Info

Publication number: WO1994012432A1
Application number: PCT/AU1993/000616
Authority: WO
Inventors: Jacek Antoni Lapszewicz; Narendra Dave; Hans Joachim Loeh; Ashit Mohan Maitra; Ralph James Tyler; Jann Rise Chipperfield
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1992-12-02
Filing date: 1993-12-02
Publication date: 1994-06-09

Abstract

A process of converting hydrocarbon material in a gas to carbon dioxide and water, a process of selectively converting hydrocarbon material in a gas to carbon dioxide and water, a process of purifying a gas containing hydrocarbon impurities, a process of selectively purifying a gas containing hydrocarbon impurities, a system for converting hydrocarbon material in a gas to carbon dioxide and water and a system for converting selectively selected hydrocarbons of a hydrocarbon material in a gas to carbon dioxide and water are disclosed. The process of converting hydrocarbon material in a gas to carbon dioxide and water includes exposing the gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon material to carbon dioxide and water. The process of selectively converting hydrocarbon material in a gas to carbon dioxide and water includes exposing the gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively the selected hydrocarbons to carbon dioxide and water.

Description

GAS PURIFICATION TECHNICAL FIELD

The present invention relates to a process of converting hydrocarbon material in a gas to carbon dioxide and water, a process of selectively converting hydrocarbon material in a gas 5 to carbon dioxide and water, a process of purifying a gas containing hydrocarbon impurities, a process of selectively purifying a gas containing hydrocarbon impurities, a system for converting hydrocarbon material in a gas to carbon dioxide and water and a system for converting selectively selected hydrocarbons of a hydrocarbon material in a gas to carbon dioxide and water. 0 BACKGROUND OF THE INVENTION

The majority of gases, in their natural state, have some level of impurities. For a variety of reasons it is often necessary to purify the gases before they are used for particular applications. The basic aim of the purification is to remove substantially all of the impurities from the gas with only minor losses of the gas to be used. 5 An example of one application where it is important to purify a gas before use, is the use of carbon dioxide in the food and beverage industry. For the ease of reference the present invention will be described with reference to the purification of carbon dioxide. However, there are many other applications where it is important to purify gases and it will be understood that the process of the present invention is not limited solely to the purification of o carbon dioxide. The food industry requires high purity carbon dioxide for beer, soft drinks etc. High quality food grade carbon dioxide should not contain more than 10 ppm of hydrocarbons. Oxygen content is also important and should not exceed 50 ppm. Higher oxygen content adversely affects the quality of food products.

There are many potential sources of carbon dioxide. It can be from natural sources (e.g. 5 from natural underground reservoirs) or as a by product from various industrial processes (e.g. fermentation or ethylene oxide production).

Carbon dioxide from these sources contains various amounts of hydrocarbons and other impurities. The impurities typically consist of low molecular weight aliphatic hydrocarbons (Cl - C7 chain length) and olefins (ethylene, propylene) or their mixtures. In many instances 0 inert gases such as nitrogen may be present. For example, in a typical analysis of carbon dioxide, in its natural state, the level of impurities would generally include 1.5% methane, 0.1 % ethane, 200ppm propane, lOOppm butane, and 50ppm of higher hydrocarbons. The available technologies for carbon dioxide purification can be broadly divided into three categories: Distillation

Traditionally, carbon dioxide has been purified by a distillation process. The use of a distillation process is effective for the removal of methane and nitrogen from natural carbon dioxide. It works well when the difference in boiling point between the hydrocarbon and the CO₂ is high and the hydrocarbons do not form an azeotrope with carbon dioxide. In practice only methane, nitrogen and carbon monoxide (see next paragraph) can be removed by this method. However, it is not an effective method for the removal of ethane and other higher hydrocarbons which are present in carbon dioxide. More particularly, when boiling points of CO₂ and hydrocarbon are close (e.g. ethylene, ethane, propane) distillation becomes unattractive as either more efficient distillation column is required or greater loss of CO₂ to purge is incurred. The traditional process of purifying gases by distillation is also energy consuming and expensive. Catalytic oxidation

Several catalysts can be used for oxidation of hydrocarbons to CO₂ and water. They are usually supported noble metals. The nature of activity of these catalysts is such that they have relatively high iniation point (i.e. the reaction is initiated at about 350°C for methane and 300°C for ethylene). They also require an excess oxygen (typically about 600 ppm above the stoichiometry of combustion) to avoid the formation of carbon monoxide and catalyst deactivation by coking. Their performance is satisfactory if the hydrocarbons content in the CO₂ stream is relatively low (about 1 mol% carbon). At this hydrocarbon content the adiabatic temperature rise due to the heat of combustion) along the reactor is approximately 150°C. This means that in case of 1 % methane in the feed the temperature inside the reactor will rise from initial 350°C to about 500°C. Such conditions promote the formation of carbon monoxide which has to be removed (max. acceptable level is 10 ppm). Both the carbon monoxide and the excess oxygen are usually removed by subsequent distillation. If hydrocarbon content in CO₂ is higher than 1 mol%, the adiabatic temperature rise is too high for the process to be viable. Removal of Impurities using adsorbents

In some cases the impurities are removed from gases using suitable adsorbents. For example, carbon dioxide derived from fermentation is usually purified using this technique. In such case the gas contains impurities consisting of alcohols, aldehydes and other volatile compounds which give it a characteristic odour. These compounds are highly polar and are easily removed from carbon dioxide by passing it over a high surface area adsorbents (e.g. activated carbon). The adsorbent has to be replaced and/or regenerated when it becomes saturated with adsorbates. This technology is not suitable for the removal of light hydrocarbons which are adsorbed less strongly than carbon dioxide.

OBJECTS OF INVENTION Accordingly, objects of the present invention include providing a process of converting hydrocarbon material in a gas to carbon dioxide and water, a process of selectively converting hydrocarbon material in a gas to carbon dioxide and water, a process of purifying a gas containing hydrocarbon impurities, a process of selectively purifying a gas containing hydrocarbon impurities, a system for converting hydrocarbon material in a gas to carbon dioxide and water and a system for converting selectively selected hydrocarbons of a hydrocarbon material in a gas to carbon dioxide and water.

DISCLOSURE OF THE INVENTION In one broad embodiment of the invention there is provided a process for the purification of a gas wherein said process comprises exposing the gas to a metal oxide at an elevated temperature.

In particular, the process may be used to remove hydrocarbons from the gas by oxidation of the hydrocarbons to carbon dioxide and water when said gas is passed over the metal oxide. The process is particularly advantageous for the removal of trace quantities of hydrocarbons from the gas.

According to one embodiment of this invention there is provided a process of converting hydrocarbon material in a gas to carbon dioxide and water, said process comprising: exposing the gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon material to carbon dioxide and water.

According to another embodiment of this invention there is provided a process of selectively converting hydrocarbon material in a gas to carbon dioxide and water, said process comprising: exposing the gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively the selected hydrocarbons to carbon dioxide and water.

The latter embodiment can be especially advantageous when the gas is carbon dioxide and the at least one compound is water.

According to a further embodiment of this invention there is provided a process of purifying a gas containing hydrocarbon impurities said process comprising: exposing the gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon impurities in the gas to carbon dioxide and water; and removing at least one compound selected from the group consisting of the carbon dioxide and water from the gas.

According to another further embodiment of this invention there is provided a process of selectively purifying a gas containing hydrocarbon impurities said process comprising: exposing the gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively the selected hydrocarbon impurities to carbon dioxide and water; and removing at least one compound selected from the group consisting of the carbon dioxide and water from the gas.

Advantageously the gas further comprises an oxidant in an amount effective to prevent reduction of the metal oxide to metal. The oxidant is typically selected from the group consisting of oxygen and air, in an amount effective to prevent reduction of the metal oxide to metal. In the processes of the invention the gas further comprises an oxidant and the process further comprises monitoring the oxidant content in the gas after exposing the gas to the metal oxide and adjusting the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.

Typically the oxidant is oxygen including pure oxygen or an oxygen source such as air or oxygen enriched air.

The processes of the invention are particularly useful in converting trace amounts of hydrocarbons in a gas to carbon dioxide and water. According to a further embodiment of this invention there is provided a system for convening hydrocarbon material in a gas to carbon dioxide and water, said system comprising: a reactor having an inlet and outlet and a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water therein, for exposing the gas to the metal oxide at an elevated temperature and for a time sufficient to convert the hydrocarbon material to carbon dioxide and water; means for feeding the gas into the inlet operatively associated with the inlet; means for cofeeding an oxidant into the inlet operatively associated with the inlet; means for controlling and adjusting the amount of oxidant cofed into the inlet operatively associated with the means for cofeeding; means for monitoring the oxidant content in the gas from the outlet operatively associated with the outlet and the means for controlling and adjusting to enable adjustment of the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal. According to yet a further embodiment of this invention there is provided a system for converting selectively selected hydrocarbons of a hydrocarbon material in a gas to carbon dioxide and water, said system comprising: a reactor having an inlet and outlet and a metal oxide capable of converting selectively selected hydrocarbons of a hydrocarbon material in the gas to carbon dioxide and water therein, for exposing the gas to the metal oxide at an elevated temperature and for a time sufficient to convert selectively selected hydrocarbons of the hydrocarbon material to carbon dioxide and water; means for feeding the gas into the inlet operatively associated with the inlet; means for cofeeding an oxidant into the inlet operatively associated with the inlet; means for controlling and adjusting the amount of oxidant cofed into the inlet operatively associated with the means for cofeeding; means for monitoring the oxidant content in the gas from the outlet operatively associated with the outlet and the means for controlling and adjusting to enable adjustment of the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.

Generally in the systems of the invention the oxidant is oxygen. Generally the above systems include means for preheating the gas and means for heating the reactor. The means for heating the reactor are generally operatively associated with the reactor.

According to a particular embodiment of the invention there is provided a process for purifying carbon dioxide gas containing hydrocarbon impurities said process comprising: exposing the carbon dioxide gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon impurities in the carbon dioxide gas to carbon dioxide and water.

The latter embodiment may include the additional step of: removing the water from the carbon dioxide gas.

According to yet a further particular embodiment of the invention there is provided a process for selectively purifying carbon dioxide gas containing hydrocarbon impurities said process comprising: exposing the carbon dioxide gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively a selected group of the hydrocarbon impurities in the carbon dioxide gas to carbon dioxide and water. The latter embodiment may include the additional step of: removing the water from "the carbon dioxide gas. The carbon dioxide and/or water can be removed by techniques well known in the art.

In the above embodiments a mixture of the gas to be purified or from which hydrocarbons are to be removed and an oxidant such as oxygen or air may be used. The amount of oxidant used is generally slightly above stoichiometric for oxidation of the hydrocarbons desired to be oxidised in the gas, so that the metal oxide catalyst does not eventually reduce to the base metal.

To achieve the oxidation of the hydrocarbons the gas is passed over a metal oxide at an elevated temperature. The actual temperature that it is necessary to obtain for the process to work is dependent upon the type of gas to be purified, the impurities to be removed and the metal oxide used. For example, the gas may be passed over a metal oxide catalyst at a temperature greater than 150°C and generally within the range of 180 - 600°C. Advantageously, the gas to be purified is passed over the metal oxide at a temperature of between 200 - 400°C, more typically 210 - 375°C, even more typically between 225 - 335° C, yet even more typically between 225 - 325°C or 225 - 300°C, at pressures of up to 300 bar, advantageously 1 - 250 bar, typically 10 - 40 bar, and more typically 15 - 35 bar and hourly space velocities (GHSV at STP) typically between 3 and 36000, more typically 3 and 24000 litre feed gas per kg of catalyst per hour (1/kg/hr), typically 120 to 18000 1/kg/hr. Typically the contact time of the gas with the metal oxide catalyst is between 1 sec and 5 minutes, more typically 0.15 seconds and 20 seconds and even more typically between 0.25 and 5 seconds. Generally the gas to be purified is preheated prior to contact with the metal oxide catalyst.

Any suitable metal oxide or mixture thereof capable of converting hydrocarbon material in the gas to carbon dioxide and water, or in the case of selective conversion, one which is capable of convening selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, may be used in the process of the invention. The preferred choice of metal oxide will depend on the impurity to be removed from the gas. Examples of suitable metal oxides for the removal of trace amounts of hydrocarbons from a carbon dioxide feed gas include, but are not limited to: copper oxide including copper (II) oxide, manganese oxide including manganese dioxide, silver oxide including silver (II) oxide, iron oxide including iron (III) oxide, ruthenium oxide including ruthenium dioxide, rhenium oxide including rhenium (VI) oxide, cobalt oxide including cobalt (II) oxide, osmium oxide including osmium dioxide, tungsten oxide including tungsten (VI) oxide, molybdenum oxide including molybdenum dioxide and molybdenum trioxide, rhodium oxide including rhodium dioxide, technetium oxide including technetium (VI) oxide, chromium oxide including chromium (III) oxide, cadmium oxide including cadmium (II) oxide, iridium oxide including iridium dioxide, tin oxide including tin dioxide, antimony oxide including antimony (III) oxide, bismuth oxide including bismuth (III) oxide, zinc oxide including zinc (II) oxide, and nickel oxide including nickel (II) oxide and mixtures of any of the foregoing (e.g. manganese dioxide/iron (III) oxide). Any valence forms of the above oxides or mixtures thereof may be used as metal oxide catalysts in the processes of the invention. Advantageously, when conversion of selected hydrocarbons is not required, the metal oxide M_wO_z is one in which the metal moiety of the metal oxide can change its valence state and thus can undergo the following reactions: aM_w ⁿ⁺O_z + b(hydrocarbons) + gas - cM_k ⁽ⁿ-^χ)+O_p + dCO2 + eH2θ + gas (I) fM_k ⁽ⁿ-^χ)+O_p + gθ2 → hM_w ⁿ⁺O_z (II) where M is the metal ion, O is the oxygen radical, w, n, z, k, p, x and a to h are parameters that are needed to balance both sides of the equations and are dependent on the nature of the particular metal oxide used and the degree of reduction as well as the reaction conditions. For example, if manganese dioxide were used as the metal oxide then depending on the reaction conditions, equations (I) and (II) may be rewritten as (IA) and (IIA): a'Mn(IV)O2 + b' (hydrocarbons) + gas-»c'Mn₂(III)O3 + d'CO2 + e'^O + gas (IA)

2Mn₂(III)θ3 + O2 - 4Mn(IV)O2 (IIA) where a' to e' are parameters that are needed to balance both sides of the equations and are dependent on the nature of the particular metal oxide used and the degree of reduction as well as the reaction conditions.

Advantageously, when conversion of selected hydrocarbons is required, the metal oxide M_wO_z is one in which the metal moiety of the metal oxide can change its valence state and thus can undergo the following reactions: a"M_w ^π+O_z + b Xunselected hydrocarbons) + b "(selected hydrocarbons) + gas — _c ^» _Mk ⁽n- ⁾⁺θp + + bi"(unselected hydrocarbons) + d"CO2 + e"H2θ + gas (IB) fM_k ⁽ⁿ-^χ)+O_p + gO → hM_w ^{n +}O_z (IIB) where M is the metal ion, O is the oxygen radical, w, n, z, k, p, x and a" to e", f and h are parameters that are needed to balance both sides of the equations and are dependent on the nature of the particular metal oxide used and the concentration and nature of impurities as well as the reaction conditions.

It is worth noting that in equation (I) there is no formation of CO which is to be contrasted with the situation when a noble metal catalyst is used in place of a metal oxide in equation (I) as shown below in equation (IC): a^{,, ,}(Noble metal)+b^,"(hydrocarbons)+gas- a"^,(Noble etal)+c"^,CO+d^,"Cθ2+e"Η2θ

+ f" 'H2+gas (IC) where a" ' to f " ' are parameters that are needed to balance both sides of the equations and are dependent on the nature of the particular noble metal used and the reaction conditions. Since CO is toxic then for many applications it is necessary to remove it from the gas and this necessitates an additional production step, such as distillation. The latter step is either not required or is facilitated (eg when the metal oxide catalyst selectively converts C2 + hydrocarbons to carbon dioxide and water) when a metal oxide of the invention is used in place of a noble metal (unless other impurities such as inert gases such as nitrogen or argon are present and it is desired to remove them). The gas containing hydrocarbons may be carbon dioxide, nitrogen, or any inert gas such as argon, neon or helium or air, for example.

An alternative to flushing the reaction vessel with oxygen or air includes the co-feeding of pure oxygen with the gas to be purified. This alternative allows for the substantially simultaneous oxidation of the hydrocarbons on the metal oxide and the reoxidation of the partly reduced metal oxide by the oxygen and thus a continued processing of the feed gas can be achieved. Further, complete oxygen consumption in the catalytic reactor can be achieved by monitoring oxygen concentration in the gas exiting the reactor with an oxygen sensor and linking the sensor with a feedback mechanism that appropriately controls the amount of input oxygen into the reactor whereby the oxygen concentration in the gas exiting the reactor approaches zero or very low concentrations.

When a metal oxide is used to remove hydrocarbons from a feed gas of carbon dioxide, the metal oxide may undergo reduction all the way to the base metal. However, the metal oxide can be quickly and cheaply reformed by exposing the spent or used metal to a source of oxygen. This may be achieved by periodically flushing the reaction vessel with a source of oxygen. For example, the reaction vessel may be periodically flushed with air or oxygen. In this case it may be advantageous to have two reactors whereby one reactor is used until the metal oxide is substantially used up and substantially ineffective or only partially effective and the other reactor is flushed with oxygen or air to regenerate the metal oxide before it is switched in place of the first-mentioned reactor and so on. In the embodiment of the invention wherein oxygen is co-fed with a gas to be purified, the amount of oxygen co-fed into the reactor is generally such so as to afford complete oxidation of all hydrocarbon impurities or selected hydrocarbon impurities. The level of oxygen that may be co-fed with the gas to be purified will substantially depend on the particular gas to be purified and the concentration and nature of the impurities in the gas. Where, for example, carbon dioxide gas has C1-C7 hydrocarbons it is desirable to remove C2-C7 hydrocarbons (including non aromatics such as linear and cyclic hydrocarbons (including ethylene) and aromatics) since some of them form azeotropic mixtures with carbon dioxide and cannot be removed easily by distillation, then the amount of oxygen added and the temperature at which the gas is exposed to the metal oxide is such that the C2-C7 hydrocarbons are oxidised to carbon dioxide and water, without CH4 being substantially simultaneously oxidised, and there is a slight excess of oxygen in the exit gas to prevent the metal oxide being reduced to the metal. Using this approach the usage of pure oxygen is kept at low levels.

In a further embodiment of the invention there is provided a two step process for the purification of a gas wherein the process comprises feeding the gas over a metal oxide and subsequently distilling the gas. This embodiment of the invention is of particular advantage to maximise the removal of different impurities.

The selection of a one step or two step process according to the invention will depend on the impurities in the gas to be purified, the level of purification required and a range of other time and cost factors. In the example of purifying carbon dioxide containing methane and higher hydrocarbons, a two step process is advantageous as the first step may be most efficiently used to remove trace amounts of higher hydrocarbons whilst the second step may be used to most efficiently remove trace amounts of nitrogen and methane. The efficiency of oxidation of the hydrocarbon is dependent upon a number of factors. One of the important factors is the residency of the gas to be purified in relation to the metal oxide. Ideally, the residency time is sufficiently long so that substantial conversion of all the required hydrocarbons to be converted (the required hydrocarbons may correspond to all the hydrocarbons in the gas or a selected portion of the total hydrocarbons in the gas) occurs so that they are substantially converted to carbon dioxide and water. However, if a two step process is being employed the ideal residency time is sufficient to cause substantial conversion of the higher hydrocarbons to carbon dioxide and water, as the lower hydrocarbons such as methane will be removed by the second step. The efficiency of oxidation of the hydrocarbon is also dependent on the nature of the hydrocarbons. Thus typically the reactivities of hydrocarbons are such that Cη > C(_ > C5 > C4 > C3 > C2 > C 1. The optimum residency time of the gas is dependent on a number of factors including the type of gas to be purified, the metal oxide selected, the temperature selected and the impurities to be removed.

To optimise the residency time a number of features may be altered. These include the path length of the gas over the metal oxide, pressure, temperature of the reactor, the flow rate of the gas and the volume of the reactor.

The metal oxide may be in the form of powder, granules, discs, pellets, monoliths or other suitable form. The metal oxide may be in the form of pure metal oxide or alternatively it may be held together with a binder and/or may be coated or deposited on a support or carrier by techniques well known in the art (e.g. by vacuum deposition, impregnation, electrodeposition or by the coating techniques described in AU76028/87, the contents of which are incorporated herein by cross reference). Suitable binders or support materials include but are not limited to alumina including α-alumina, mullite, cordierite, mullite aluminium titanate, magnesia, zirconia, zirconia spinels, titania, silica-alumina including amorphous silica-alumina, and clays and mixtures thereof. Small amounts of other materials such as zirconia, titania, magnesia and/or silica may be present. The amount of binder may be 3 - 50wt% of the catalyst, more typically 5 to 30wt% base on the total weight of the catalyst. Typically, the metal oxide catalyst has a surface area to volume ratio of at least 0.5π gm, more typically between 25 and 500π g, and even more typically between 5 and 250m²/g.

The reactor may be a single pass reactor packed with the metal oxide catalyst, such as fo example a paniculate metal oxide catalyst disposed in a fixed bed within the reactor or metal oxide catalyst deposited on or impregnated in a ceramic foam carrier (e.g. cerami foams made from the aforementioned refractory oxides particularly alumina and α-alumina disposed within the reactor, or a multiple pass reactor packed with the metal oxide catalyst The catalyst may be arcanged fixedly within the reactor so as to provide a high tortuosity fo the feed gas (typically between 1.0 and 10.0, more typically 1.3 to 4.0; "tortuosity" wit reference to a fixed catalyst bed is the ratio of the pathlength of gas flowing through the be to the length of the shortest straight line through the bed). The reactor may be operated s that the feed gas contacts the metal oxide catalyst under isothermal conditions or adiabati conditions ("adiabatic" referring to reaction conditions wherein substantially all heat loss an radiation from the catalyst bed is prevented except for the heat leaving in the exit gas fro the reactor).

Advantages of the metal oxide catalysts used in the process of the invention are as follows: (i) Metal oxide catalysts used in the process of the invention have been found to operat at low temperatures relative to noble metal catalysts. The advantages of this are as follows: Firstly, the reactor feed gas only has to be preheated to lower temperature which translate into smaller size of heat exchangers which are capital intensive. Secondly, higher levels o hydrocarbon impurities can be removed as greater adiabatic temperature rise can b accommodated for the identical exit stream temperature.

(ii) Metal oxide catalysts used in the process of the invention have been found to b more selective than noble metal catalysts. Currently used noble metal catalysts combust al hydrocarbons indiscriminately and tend to produce carbon monoxide and hydrogen. Tw kinds of selectivity are desirable and at least one type of the following selectivities has bee found to be a feature of the metal oxide catalysts used in the process of the invention: First, selective removal of different hydrocarbons. The metal oxide catalysts used in the process o the invention have been found to be capable of oxidising higher hydrocarbons (which ar difficult to separate by distillation) without substantially oxidising methane. This limits adiabatic temperature rise and saves oxygen. Methane can then be removed by distillation and possibly used as a process heat source. Second, metal oxides used in the process have been found capable of converting hydrocarbons to CO₂ and H₂O only, with no CO or H₂ detected in the exit gas. (iii) Metal oxide catalysts used in the process of the invention have been found to be capable of working under conditions of total oxygen consumption. This results in oxygen saving and improvement in product quality, (iv) Metal oxide catalysts used in the process of the invention can operate in redox mode. Thus the metal oxide catalysts can be used for hydrocarbon removal without oxygen addition to the feed. This is important when the use of oxygen is not economic. An alternative is to cofeed air, but it is detrimental as an excessive amount of nitrogen in exit gas may adversely affect the efficiency of the distillation stage. The purification system of the invention ideally includes two reactors containing the metal oxide catalyst which can exist in oxidised and reduced state. The reactor with oxidised catalyst is used for oxidation of hydrocarbons, while the one containing oxygen depleted catalyst is reoxidised with air. The feed to both reactors could be switched at regular intervals.

(v) Noble metal catalysts are expensive (approx. $150 per kg) as compared to the metal oxide catalysts used in the process of the invention.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the invention are described below with reference to the following drawings in which:

Fig. 1 depicts schematically a system for purifying carbon dioxide; Fig. 2 illustrates the percentage conversion of the methane as a function of the temperatures trialed for a feed gas comprising 1.5% of methane in carbon dioxide fed over copper oxide supported on alumina at a flow rate of 100 cc/min at various temperatures;

Fig. 3 depicts schematically a reactor system used in various experiments; Fig. 4 depicts schematically a reactor system used in various experiments; Fig. 5 illustrates the dependence of the ethane concentration in the reactor effluent on the inlet gas flow at a reactor temperature of 225 °C;

Fig. 6 illustrates the dependence of the ethane concentration in the reactor effluent on the inlet gas flow at a reactor temperature of 250°C;

Fig. 7 illustrates the dependence of the ethane concentration in the reactor effluent on the inlet gas flow at a reactor temperature of 275 °C;

Fig. 8 illustrates the dependence of the ethane concentration in the reactor effluent on the inlet gas flow at a reactor temperature of 300°C;

Fig. 9 illustrates the changes in methane and ethane concentration in the reactor effluent at different oxygen concentrations in the feed at a reactor temperature of 225 °C, inlet gas flow 4.5L/min;

Fig. 10 depicts schematically a flowsheet a process for the removal of C2+ hydrocarbons from carbon dioxide;

Fig. 11 illustrates the maximum gas flow rates allowable for the removal of 100% of C2 + hydrocarbons as a function of reactor temperature; Fig. 12 illustrates the dependence of catalyst weight and preheater surface area on the process temperature; BEST MODE AND OTHER MODES OF CARRYING OUT THE INVENTION

Figure 1 depicts a system 100 for selectively converting hydrocarbon material in carbon dioxide gas to carbon dioxide and water. Carbon dioxide (typically 15-35bar, ambient temperature, containing C1 + hydrocarbons and above) from natural carbon dioxide gas well 101 is passed via line 201 through desulphuriser 102 where it undergoes desulphurisation (a representative impurity composition after vaporiser and desulphurisation of natural carbon dioxide is: CH4 0.9 to 1.3 mole% , C2H6 < 0.1 %, C3Hg < 200ppm V/V, -C4 & i-C4 < lOOppm, n-C5's < 50ppm, Cg's < lOppm, as well as ppb levels of aromatics and approximately 0.2 to 0.4 ppm to total sulphur compounds (H2S, COS)) and then passes via line 202 to optional drier 103 where it is dried before passing via line 203 into mixer 104 where it is mixed with pure oxygen which also passes into mixer 104 from pure oxygen source 105 via line 204 in an amount slightly above stoichiometric (1 - 600ppm or more, typically 10 - 150ppm, more typically 30 - 60ppm, and even more typically about 50ppm) for C2+ hydrocarbons (i.e. hydrocarbons containing two or more carbon atoms) in the carbon dioxide gas. The carbon dioxide/oxygen mixture from mixer 104 then passes via line 205 through first heat exchanger 106 where it is heated so that the temperature of the exit heated carbon dioxide/oxygen mixture is typically in the range 205-280°C. The heated carbon dioxide/oxygen mixture from first heat exchanger 106 then passes via line 206 through second heat exchanger 107 where it is heated so that the temperature of the exit heated carbon dioxide/oxygen mixture therefrom is typically in the range 225-300°C. The heated carbon dioxide/oxygen mixture is then passed through catalytic reactor 108 via line 207 under adiabatic conditions (reactor 108 containing Nissan-Girdler N-150 catalyst in pellet form: 1- 6mm diameter, typically 3mm diameter, available from Nissan-Girdler Catalyst Co., Ltd, Shinjo Bldg. 9-2, Kandatacho 2-chome, Chiyoda-ku, Tokyo, Japan, resident time typically 0.15 sees - 20 sees). In reactor 108 the C2+ hydrocarbons in the carbon dioxide /oxygen mixture are converted selectively to carbon dioxide and water. The temperature of the gas exiting reactor 108 which is typically in the range 260-335°C. The gas exiting reactor 108 (carbon dioxide + water -I- methane) is then passed through the appropriate part of first heat exchanger 106 via line 208 to heat the incoming feed carbon dioxide/oxygen mixture into first heat exchanger 106 before exiting first heat exchanger 106, typically at a temperature of 70-80°C, and passing via line 209 through drier 109 which removes water from the carbon dioxide/water/methane mixture. The oxygen concentration of the exit gas from reactor 108 is monitored by oxygen sensor 110 via line 210. The amount of oxygen from pure oxygen source 105 into mixer 104, and thus indirectly in the exit gas from reactor 108, can be readily and automatically adjusted to the desired level by feeding back an output signal related to the oxygen concentration in the exit gas from reactor 108 from oxygen sensor 110 via line 211 to a gas flow controller 111 operatively associated with pure oxygen source 105 via line 212 which controller 111 is adapted to automatically adjust the amount of oxygen from pure oxygen source 105 flowing into mixer 104. Thus, in operation oxygen concentration of the exit gas from reactor 108 is typically indirectly adjusted via oxygen sensor 110 and gas flow controller 111 so that it is in the order of 5ppm to 150ppm, more typically 50ppm and in this way the metal oxide catalyst in reactor 108 is prevented from eventually reducing the base metal. Oxygen sensor 110 could alternatively be set up to monitor oxygen concentration of gas exiting drier 109. The carbon dioxide/methane mixture from drier 109 passes via line 213 into cooler 112 where it is cooled to -15 - -25°C, typically -20°C, before passing into distillation column 113. The carbon dioxide/ methane is then distilled in column 113 whereby condenser 115 which is typically operated at -40°C, minimises the loss of carbon dioxide from gas that flows into condenser 115 via line 216 by liquefying carbon dioxide therein whereby it flows back into column 113 via line 217. Gas such as methane which is being distilled from the carbon dioxide is vented via line 218 and vent 116. Purified carbon dioxide in liquid form passes to carbon dioxide storage tank 114 via line 215. One of the advantages of distilling a carbon dioxide/methane mixture as opposed to a carbon dioxide/hydrocarbon mixture which includes hydrocarbons equal to and above C2+ is that the loss of carbon dioxide during the former distillation may be of the order of 1 % whereas during the latter distillation the loss of carbon dioxide may be of the order of 10% or more. The process of the invention is now described with reference to the following examples: EXAMPLE 1

A feed gas comprising 1.5% of methane in carbon dioxide was fed over copper oxide catalyst BASF R3-11 at a flow rate of 100 cc/min at various temperatures. Fig. 2 illustrates the percentage conversion of the methane as a function of the temperatures trialed.

As shown in Fig. 2, 100% conversion of the methane occurred at the elevated temperature of 550°C.

EXAMPLE 2

The primary objective of this example is to remove ethane and higher hydrocarbons (i.e. C₂+ hydrocarbons) from carbon dioxide using oxide catalysts. Particular objectives were to meet the following criteria: operate at pressure (approx. 25 bar) operate at lowest possible temperature remove all C2 + hydrocarbons selectively without removing methane no carbon monoxide is formed in the reaction SUMMARY OF RESULTS Four metal oxide catalysts were evaluated: BASF R 3-11 (supported copper oxide), CSIRO (30% MnO₂ γ-Al₂θ₃), Nissan-Girdler N-150 (mixture of manganese and iron oxides) and BASF 04-110 (supported V₂O₅). All catalysts were used in oxidised state. Evaluation was carried out at 25 bar pressure in two modes: batch mode (without added oxygen) and cofeed mode (with 1 % oxygen added)

Two catalysts containing manganese oxide (Nissan-Girdler N-150 and CSIRO) were identified as best performers. They removed C₂+ hydrocarbons completely while leaving > 90% of methane unreacted at 225-250°C in cofeed mode (Tables IB and 2B) and at 250-300° C in batch mode (Tables IA and 2A).

Copper based catalyst (BASF R 3-11 ) required temperatures about 350°C to achieve C₂+ removal in cofeed mode (Table 3B). It was much less active and selective in batch mode (Table 3 A) and removed 80% of methane while leaving traces of ethane at 500°C. Carbon monoxide has not been detected (detection limit 10 ppm) under any experimental conditions with Mn or Cu containing catalysts.

Vanadium containing catalyst (BASF 04-110, Tables 4A and 4B) did not show expected activity under any conditions. Furthermore, it produced between 10 and 100 ppm of carbon monoxide in most experiments. In all experiments in cofeed mode oxygen conversion did not exceed 70% . Two catalysts (Nissan-Girdler N-150 and BASF R 3-11) were run for several hours in cofeed mode at temperatures 250°C and 350°C respectively. REACTOR SYSTEM

The reactor system used in all experiments is depicted in Figure 3. The flows of carbon dioxide, nitrogen and oxygen are controlled with mass flow controllers (1-3). The four way switching valve allows either nitrogen or carbon dioxide (or carbon dioxide/oxygen mixture in cofeed mode) to be passed through the reactor vessel (5) containing the catalyst. The needle valve (6) allows the diversion of a small fraction of the exit gas into gas chromatographs (7,8). The reactor pressure is monitored with electronic pressure transducer and can be varied by adjusting the backpressure regulator (10) . Both gas streams (i.e. through the reactor and through the bypass line) are merged before the backpressure regulator to avoid flow surges during the changeover.

ANALYTICAL PROCEDURE

Two gas chromatographs are used for analysis of the reactor effluent. Methane, ethane, oxygen, nitrogen and carbon monoxide concentrations are monitored using gas chromatograph equipped with packed column (Carbosphere 80/100 from Alltech) and TCD detector. Second gas chromatograph with capillary column (BP 1) and FID detector is used for analysis of all hydrocarbons. Methane concentration as measured by both instruments is used as a crosscheck of both analyses. Both gas chromatographs are calibrated using commercially available standard mixtures containing known concentrations of all components. The detection limits are approximately 1 ppm for hydrocarbons and 10 ppm for the remaining components. EXPERIMENTAL PROCEDURE

A sample of catalyst (4.9g, + 125 -500 μm) was placed in the reactor. Flows of carbon dioxide and nitrogen were adjusted to 100 mL/min and oxygen (in cofeed experiments only) to 1 mL/min. The system was pressurised to 25 bar at ambient temperature. The reactor effluent was analysed to provide reference concentrations of all components prior to the reaction.

Batch mode operation. The reactor temperature was raised to the required level under nitrogen flow to avoid the depletion of oxygen from the metal oxide. When the reactor temperature stabilised, carbon dioxide was passed through the catalyst bed for approximately

20 minutes. At the end of this period the analyses of the exit gas were done. The flow of nitrogen was then switched back, the reactor temperature increased to the next level and the whole procedure repeated.

Cofeed mode operation. The procedure was identical to that used in batch mode except that the mixture of carbon dioxide and oxygen is flowing through the catalyst bed at all times.

ANALYSIS OF GAS "AS RECEIVED"

The contents of hydrocarbons in carbon dioxide is presented below.

Variations in hydrocarbon concentrations in Tables 1 to 4 are caused by two factors. Firstly, prior to each experiment, the gas was analysed at ambient temperature (25°C) after passing through the catalyst bed. This can distort the result through adsorption. Secondly, we noticed variations in analyses with changes of the temperature of the cylinder bank which is located outside the building. This can be caused again by adsorption/desorption from the cylinder walls and/or evaporation of liquids.

TABLE IA. CATALYST NISSAN-GIRDLER N-150, BATCH MODE

^a) Not Detectable

TABLE IB. CATALYST NISSAN-GIRDLER N-150, COFEED MODE

EXAMPLE 3

The primary objective of this example was to obtain the data necessary for designing a process system which removes C₂+ hydrocarbons from carbon dioxide using a Nissan - Girdler N-150 catalyst and which meets the following criteria:

Plant capacity 100 tpd. Operating pressure 25 bar. Operating temperature between 225°C and 300°C. Minimum oxygen consumption.

In addition to the above criteria the process should achieve the highest possible selectivity, i.e. total removal of C₂+ hydrocarbons without substantial oxidation of methane.

SUMMARY OF RESULTS.

The appropriate data required for the design of the process for the removal of hydrocarbons from carbon dioxide were obtained. The process is based on the Nissan - Girdler N-150 catalyst containing manganese and iron oxides. The experimental data fully confirmed the feasibility of selective removal of hydrocarbons in the ethane - hexane range under conditions of total oxygen consumption. No oxidation of methane or formation of carbon monoxide was detected under a wide range of- experimental conditions and the process can be operated at temperatures below 300°C. These results are of fundamental importance for the process economics. The experimental results were used for the design of the operating system. The minimum reactor and preheater sizes required at different process temperatures were calculated and an appropriate process flowsheet was developed. All objectives have been met. THE REACTOR SYSTEM. The reactor system was modified to accommodate higher gas flow rates and a larger amount of catalyst (up to 100 g). The configuration of the reactor as used in all experiments is presented in Figure 4. The flow of gas from the bank of cylinders (1) was controlled by a Brooks mass flow controller (3) up to a flowrate of 14 L/min. When higher flowrates were required, the mass flow controller (3) was replaced with a needle valve. The flow of high purity oxygen (2) was controlled with a Brooks mass flow controller (4) in all experiments. Both streams were mixed and passed through a preheater (5). The preheater consisted of a stainless steel vessel (1 L capacity) filled with low surface area zirconia spheres. The preheated gas mixture was then passed through a stainless steel reactor vessel (120 mL capacity) filled with the Nissan-Girdler N-150 catalyst. The coaxial furnace consisting of a tubular aluminium block heated by six cartridge heaters ensured constant temperature along the reactor length.

The temperature of the catalyst bed was monitored with a thermocouple placed in a well extending along the reactor axis. This allowed the measurements to be taken at all levels of the catalyst bed.

The reactor pressure was controlled by a Tescom backpressure regulator (7) and the exit gas flow rate was measured by a Toyo ML 2500 gas flow meter.

A small fraction of the reactor effluent was periodically directed into gas chromatographs for analysis. Two gas chromatographs were used: a Hewlett - Packard 5890 equipped with a capillary column (BP1 from SGE) and a FID detector for the analysis of methane and C₃ + hydrocarbons and a Shimadzu GC-8A equipped with a packed column (Carbosphere 80 from Alltech) and a TCD detector for the analysis of methane, ethane, oxygen and carbon monoxide. OPTIMISATION OF SPACE VELOCITY AT DIFFERENT TEMPERATURES.

The experimental procedure used in all experiments was as follows. The reactor was packed with 100 g of catalyst and the pressure was maintained at 25 bar in all experiments. The flowrate of the gas - oxygen mixture (measured at ambient temperature and pressure) was adjusted to the desired level. The temperatures of the preheater and the reactor were adjusted so that the temperature differences at the top and at the bottom of the catalyst bed were within 5°C from the required level. The system was then left for approximately 30 minutes to reach a steady state. After this period the reactor effluent was analysed and the above procedure was repeated for all temperatures and gas flowrates studied. For each temperature the gas flowrates were varied within such a range that ethane conversion varied from 95% to 5%.

In every experiment carried out all hydrocarbons higher than ethane (i.e. C₃ - C₆) were oxidised before the conversion of ethane reached 100% . The maximum space velocities at which total removal of C₂+ hydrocarbons was achieved were therefore conveniently determined by extrapolating the concentrations of ethane found in the reactor effluent to zero.

The results of experiments carried out at temperatures 225°C, 250°C, 275°C and 300°C are presented in Figures 5 - 8. No traces of carbon monoxide were found in the product stream (detection limit for CO was 10 ppm) and all methane remained unconverted under all conditions used. This result fully confirmed an excellent suitability of a metal oxide catalyst system for the selective removal of higher hydrocarbons from carbon dioxide.

In all cases linear dependence between gas flowrates and ethane concentrations in the exit gas was observed. The gas flow rates at the intersections of the linear regression lines drawn through ethane concentrations with the X - axis were taken as the maximum allowable gas flowrates for the complete removal of all C₂+ hydrocarbons. They were subsequently used for the calculation of the minimum reactor size in the process design.

Oxygen concentration in the feed gas was 0.3% (v/v) in all experiments. This means that at ethane conversions above 90% total oxygen consumption occurred. Any higher conversion would occur at the expense of the depletion of oxygen from the catalyst lattice. The maximum allowable flow rates for 100% removal of C₂+ hydrocarbons determined under conditions of total oxygen consumption for each temperature represent therefore the "worst case" scenario. In this context it is important to find out if further reduction of the reactor size could be achieved if higher oxygen concentrations in the feed are used. To determine the sensitivity of the reaction to the variation in oxygen concentration a further experiment was carried out at 225°C and with a constant inlet gas flow (4.5 L/min). The results of this experiment are presented in Figure 9. They indicate that a substantial increase in the amount of oxygen added results in a relatively small decrease in ethane concentration. Furthermore, oxidation of methane begins to occur in the presence of excess oxygen.

It appears that addition of oxygen above the amount required by the reaction stoichiometry does not substantially improve the performance of the catalyst. PROCESS DESIGN. The process flowsheet used for all calculations referred to in this report is presented in Figure 10. The feed gas (RAWGAS stream) is mixed with oxygen (OXYGEN stream) and passed through the indirect heat exchanger (GASHTR1) where it is preheated by cooling the hot reactor effluent stream (GASOUT).

Since the concentration of C2+ hydrocarbons in the raw feedgas is low, the combustion of these species in the presence of Nissan - Girdler N-150 catalyst produces a relatively small temperature rise across the combustor (REACTOR) and hence the thermal energy carried with the reactor effluent stream GASOUT is insufficient to raise the temperature of the HOTGAS stream to the required reactor inlet temperature.

A booster heater (GASHTR2) is therefore necessary which will, in actual practice, supply the balance of thermal energy. The heat duty associated with the booster heater and the heat loss from the combustor are shown as QFEEDHT and QHTLOSS streams respectively.

The reactor effluent GASOUT, after cooling in the preheater (GASHTR1) leaves the system as the PRODUCT stream which will be further refined in a distillation unit to obtain a food and beverage grade carbon dioxide product. Since the present scope of the project has been rather limited, any issues related to the process engineering of the distillation unit have not been defined here.

The assumptions used in the process design of the catalytic combustion system are presented in Table 1. The material and energy balance calculations around each unit of the process system were carried out using the HYSIM process simulation package. For the sake of simplicity, the catalytic combustor was modelled in HYSIM as an adiabatic stoichiometric reactor, though in actual practice heat losses through the wall insulation could cause the reactor exit temperature to be slightly less than that predicted according to the simulation.

This possibility has been duly acknowledged while sizing the preheater, GASHTR1, by assuming that the inlet temperature for the stream GASOUT into the preheater will be 3 to 4 degrees less than the value predicted, i.e. roughly 10 to 12 % heat loss through the reactor.

The composition of the raw feed gas used in the reactor calculations is described in Table 2. It represents an average composition after several analyses of the raw gas "as received". The maximum space velocities required for total removal of C2 + hydrocarbons at different temperatures were calculated as described in the previous section and are presented in Figure 11. Accordingly, the minimum required amounts of catalyst, reactor sizes and the adiabatic temperature rise for different operating conditions have been calculated and the results presented in Table 3. While setting the ratio of reactor diameter to packed length, constancy of residence time for reactants between the laboratory reactor and its scaled up version was maintained.

The pressure drop across the catalyst bed has been estimated using the Ergun equation for packed beds. Since the catalyst pellets are cylindrical, the mean voidage for the packed bed was corrected for pressure loss calculations using the Foumeny and Roshani correlation (Chemical Engineering Science, Volume 46, No. 9, pp. 2363, 1991). The calculated pressure drop across the combustor and the overall system pressure loss are presented in Table 4 together with the estimated surface area for the preheater (GASHTR1) and the heat duty for the booster heater (GASHTR2). The overall heat transfer co-efficient for the preheater was conservatively estimated as approximately 175 W/m2.°C assuming the shell and tube type configuration. However, if any other configuration is chosen for the purpose of compactness of the equipment and economics of cost, this value may need to be reassessed for the new configuration in order to correctly calculate the surface area of the preheater. The above results can be summarised as follows. The process temperature has a substantial effect on the amount of catalyst (and the reactor size) as well as on the surface area of the preheater (GASHTR1) as shown in Figure 12. Since these items are the major components of the overall cost of the process, the optimum operating temperature for the combustor can be determined after choosing and costing the relevant equipment. The overall system pressure loss is less than 500 kPa under all conditions and may not significantly be affected by the choice of the configuration for the heat exchanger, GASHTR1. The heat duty QFEEDHT associated with the booster heater, GASHTR2 does not vary substantially with the combustor inlet temperature (Table 4), i.e. the temperature of the stream GASIN. However, during any indirect mode of the heat transfer, this heat duty may pose a problem depending upon the source of thermal energy available on site and will have an effect on the surface area of GASHTR2. By an approximate rule, any hot stream offering thermal energy to the stream HOTGAS in the counter current mode will require its own temperature to be at least 50 °C higher than the desired temperature for the stream GASIN. The amounts of catalyst required have been calculated on the basis of the laboratory results obtained with a 1/8 inch pellet size. If bigger pellets are to be used, the adjustment to the amounts of catalyst required will have to be made in order to achieve the same levels of global reaction rates. The appropriate conversion factors for this task should be available as a ". ? function of different temperatures for the different pellet sizes from the catalyst manufacturer.

During actual operation, the concentration of hydrocarbons in the GASIN stream may vary with time, and hence it is desirable to have a continuous control over the oxygen concentration in the lower region of the catalyst bed. The use of, for example, a ceramic oxygen sensor with a feedback to the oxygen supply controller would serve a dual purpose. It will, first, allow to minimise the oxygen consumption and, second, will also prevent depletion of oxygen from the metal oxide in cases when higher hydrocarbon concentrations in the feedstock are encountered.

The final system configuration for the catalytic combustion process described here and the operating parameters will have to be chosen in the context of the existing purification plant into which it will be finally incorporated.

TABLE 1. Assumptions used in process design.

TABLE 2. Inlet gas composition

TABLE 3. The reactor parameters.

TABLE 5. The process parameters for GASIN stream temperature 225°C.

TABLE 6. The process parameters for GASIN stream temperature 250°C. Stream RAWGAS OXYGEN FEEDGAS HOTGAS

Description

Vapour frac. 1.0000 1.0000 1.0000 1.0000

The following examples illustrate the application of metal oxide catalysts to CO₂ purification. All examples here refer to the purification of carbon dioxide However, the same method can be used to oxidise hydrocarbons in any other gas which does not react with oxygen, e.g. helium, neon, nitrogen, argon etc.

EXAMPLE 4

This demonstrates the removal of methane from carbon dioxide using different metal oxides. Methane is the least reactive hydrocarbon - any longer chain alkane and alkene reacts faster (see following examples on selective removal).

Experimental Conditions: Catalyst weight 5 g, inlet gas flow 100 mL/min carbon dioxide containing 2% methane and 5 % oxygen, ambient pressure.

EXAMPLE 5

This demonstrates selective removal of ethylene using commercially available metal oxide catalyst without oxidising methane.

Experimental conditions: Catalyst weight 50 g. inlet gas flow 965 mL/min of carbon dioxide, 15 mL/min methane, 5mL/min ethylene and 15 mL/min oxygen, pressure 21 bar. Catalyst Nissan - Girdler N-150 (mixture of Iron and Manganese oxide)

CO was never detected in these experiments. • Catalysts which save oxygen.

Same as above. Quote examples where oxygen consumed 100% .

DISCUSSION The examples demonstrate the ability of metal oxides of the invention to remove and in particular to remove selectively various hydrocarbons from the gas. To some degree this selectively is determined by the reactivity of the hydrocarbon impurities. However, the removal of selected hydrocarbons or the degree of removal of selected hydrocarbons can be also controlled by the choice of catalyst and process conditions (especially temperature). The reaction selectivity at different temperatures and with different catalysts is illustrated in Example 2. The differentiation of hydrocarbons reactivity over different metal oxides is best illustrated by comparing Tables 2B and 3B. Manganese oxide catalyst is capable of selectively oxidising C2+ hydrocarbons without affecting methane (Table 2B). By contrast, copper based catalyst is less active and selective and under conditions required for removal of all C2+ hydrocarbons approximately 21 % of methane is also oxidised (Table 3B). It is important to stress that all results present in Example 2 were obtained under conditions of incomplete oxygen conversions ( <70%). Yet despite the presence of excess oxidant, the selectivity of metal oxide catalysts is clearly visible. This remains in contract with noble metal catalysts, which are not selective and once the reaction is initiated the oxygen consumption reaches 100% and hydrocarbons are combusted indiscriminately. The effect of contact time on the selective removal of hydrocarbons is shown Example 3. In this case the oxygen concentration in the feed was adjusted to the level required for the removal of C2+ hydrocarbons only. Figures 5-8 illustrate that the removal of the desired amounts of ethane and higher hydrocarbons at constant temperatures can be achieved by variation of the contact time, which in turn can be adjusted by variation of either gas flow or the pressure.

The effect of temperature on the degree of removal of ethylene at constant time and pressure is shown in Example 5. In this case the amount of oxygen in the feed was adjusted to the level required for the oxidation of ethylene only. The results again confirm that excellent selectivities can be achieved using certain metal oxide catalysts. Further, various degrees of selectivity can be achieved by kinetic control of the oxidation such as by varying the temperature at which the gas is exposed to the metal oxide. Thus the lower the temperature the more selectively oxidised the hydrocarbon with the higher the number of carbon atoms therein (i.e. at low temperatures i.e. say, for example, about less than about 335°C and above about 150°C and depending on the particular temperature and the particular metal oxide, C7 hydrocarbons oxidise before C6 hydrocarbons which in turn oxidise before C5 hydrocarbons etc. The conditions required for the particular selectivity can be determined by routine experiment). The Table in Example 5 illustrates selective oxidation of ethylene without affecting methane.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is therefore to be understood that the invention includes all such variations and modification which fall within the spirit and scope of what has been described.

Claims

1. A process of converting hydrocarbon material in a gas to carbon dioxide and water, said process comprising: exposing the gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon material to carbon dioxide and water.

2. A process of selectively converting hydrocarbon material in a gas to carbon dioxide and water, said process comprising: exposing the gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively the selected hydrocarbons to carbon dioxide and water.

3. A process of purifying a gas containing hydrocarbon impurities said process comprising: exposing the gas to a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert the hydrocarbon impurities in the gas to carbon dioxide and water; and removing at least one compound selected from the group consisting of the carbon dioxide and water from the gas.

4. A process of selectively purifying a gas containing hydrocarbon impurities said process comprising: exposing the gas to a metal oxide capable of converting selectively selected hydrocarbons of the hydrocarbon material in the gas to carbon dioxide and water, at an elevated temperature and for a time sufficient to convert selectively the selected hydrocarbon impurities to carbon dioxide and water; and removing at least one compound selected from the group consisting of the carbon dioxide and water from the gas.

5. The process of claim 1 wherein the gas is carbon dioxide.

6. The process of claim 2 wherein the gas is carbon dioxide.

7. The process of claim 3 wherein the gas is carbon dioxide and the at least one compound is water.

8. The process of claim 4 wherein the gas is carbon dioxide and the at least one compound is water.

9. The process of any one of claims 1 to 8 wherein the gas further comprises an oxidant in an amount effective to prevent reduction of the metal oxide to metal.

10. The process of any one of claims 1 to 8 wherein the gas further comprises an oxidant which is selected from the group consisting of oxygen and air, in an amount effective to prevent reduction of the metal oxide to metal.

11. The process of any one of claims 1 to 8 wherein the gas further comprises oxygen in an amount effective to prevent reduction of the metal oxide to metal.

12. The process of any one of claims 1 to 8 wherein the elevated temperature is in the range from 150°C - 600°C.

13. The process of any one of claims 1 to 8 wherein the elevated temperature is in the range from 200°C - 335°C.

14. The process of any one of claims 1 to 8 wherein the metal oxide is selected from the group consisting of copper oxide including copper (II) oxide, manganese oxide including manganese dioxide, silver oxide including silver (II) oxide, iron oxide including iron (III) oxide, ruthenium oxide including ruthenium dioxide, rhenium oxide including rhenium (VI) oxide, cobalt oxide including cobalt (II) oxide, osmium oxide including osmium dioxide, tungsten oxide including tungsten (VI) oxide, molybdenum oxide including molybdenum dioxide and molybdenum trioxide, rhodium oxide including rhodium dioxide, technetium oxide including technetium (VI) oxide, chromium oxide including chromium (III) oxide, cadmium oxide including cadmium (II) oxide, iridium oxide including iridium dioxide, tin oxide including tin dioxide, antimony oxide including antimony (III) oxide, bismuth oxide including bismuth (III) oxide, zinc oxide including zinc (II) oxide, and nickel oxide including nickel (II) oxide and mixtures thereof.

15. The process of any one of claims 2, 4, 6 or 8 wherein the selected hydrocarbons are hydrocarbons containing two or more carbon atoms.

16. The process of any one of claims 1 to 8 wherein the gas further comprises oxidant and the process further comprises monitoring the oxidant content in the gas after exposing the gas to the metal oxide and adjusting the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.

17. The process of any one of claims 1 to 8 wherein the gas further comprises oxygen and the process further comprises monitoring the oxygen content in the gas after exposing the gas to the metal oxide and adjusting the amount of oxygen in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxygen in the gas after exposing the gas to the metal oxide such that the amount of oxygen in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.

18. A system for converting hydrocarbon material in a gas to carbon dioxide and water, said system comprising: a reactor having an inlet and outlet and a metal oxide capable of converting hydrocarbon material in the gas to carbon dioxide and water therein, for exposing the gas to the metal oxide at an elevated temperature and for a time sufficient to convert the hydrocarbon material to carbon dioxide and water; means for feeding the gas into the inlet operatively associated with the inlet; means for cofeeding an oxidant into the inlet operatively associated with the inlet; means for controlling and adjusting the amount of oxidant cofed into the inlet operatively associated with the means for cofeeding; means for monitoring the oxidant content in the gas from the outlet operatively associated with the outlet and the means for controlling and adjusting to enable adjustment of the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.

19. A system for converting selectively selected hydrocarbons of a hydrocarbon material in a gas to carbon dioxide and water, said system comprising: a reactor having an inlet and outlet and a metal oxide capable of converting selectively selected hydrocarbons of a hydrocarbon material in the gas to carbon dioxide and water therein, for exposing the gas to the metal oxide at an elevated temperature and for a time sufficient to convert selectively selected hydrocarbons of the hydrocarbon material to carbon dioxide and water; means for feeding the gas into the inlet operatively associated with the inlet; means for cofeeding an oxidant into the inlet operatively associated with the inlet; means for controlling and adjusting the amount of oxidant cofed into the inlet operatively associated with the means for cofeeding; means for monitoring the oxidant content in the gas from the outlet operatively associated with the outlet and the means for controlling and adjusting to enable adjustment of the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide whereby there is a detectable amount of oxidant in the gas after exposing the gas to the metal oxide such that the amount of oxidant in the gas prior to the exposing of the gas to the metal oxide is an amount effective to prevent reduction of the metal oxide to metal.