GB1565059A - Oxidation of hydrocarbon components in lean natural gas - Google Patents

Description

(54) OXIDATION OF HYDROCARBON COMPONENTS IN LEAN NATURAL GAS (71) We, ENERGIAGAZDALKODASI INTÉZET of 33-34 Bem rampart Budapest II, Hungary, a body corporate organized under the laws of Hungary, do hereby declare the invention for which we pray that a patent may be granted to us. and the methods by which it is to be performed, to be particularly described in and by the following statements: The invention concerns a process for the treatment of natural gas and in particular relates to the oxidation of natural gas with a high content of inert components with the aid of a plasma beam itself produced from the gas.

According to the invention there is provided a process of treating lean natural gases to oxidise the hydrocarbon components thereto which comprises separating the lean natural gas from water carried by the gas. electrically transforming a smaller portion of the natural gas into a plasma beam as a working gas. and transforming a greater portion of the natural gas, by addition oxygen or air admixture. into a technological gas. expanding the smaller portion of the gas. prior to its transformation into a plasma beam. and utilizing it as a cooling medium for the electrodes of a plasma generator in a recuperatory preheater svstem to enhance the formation of the plasma beam. warming up the technological gas by passing the gas in heat exchange relationship with the plasma beam in a recuperatory preheater system, to preheat the technological gas and blowing the warmed up technological gas through the plasma beam.

The invention has two aspects or applications. In the first application insufficient oxygen or air is mixed with the natural gas for complete combustion of the hydrocarbons and the product gas is substantially water and carbon monoxide. said carbon monoxide being formed in part from the hydrocarbons and in part from the carbon dioxide in the natural gas.

In the second application sufficient oxygen or air is mixed with the natural gas for complete combustion of the hydrocarbons and the product gas is substantially water and carbon dioxide. said carbon dioxide being in part derived from the hydrocarbons and in part present initiallv in the natural gas.

Considering the first aspect. the exploitability of natural gases with a high content of inert components, principally carbon dioxide. herein lean natural gases. for combustion technology and the chemical industry is considerablv restricted bv the fact that currently there is no high output oxidation apparatus available that is of simple construction. reliable operation and relativelv low energy requirements, capable of satisfying the conditions for partial oxidation of hydrocarbons and reduction of the carbon dioxide. for such types of natural gas.

The known types of oxidation equipment. constructed for high quality natural gases, whether they operate continuously or cvclically. consume significant amounts of diverse catalyst materials which are often difficult to obtain. and thus they cannot be employed for lean natural gases because the basic conditions of economic operation are not met, from the point of view of investment and running costs. Thev require a great deal of space for installation. Their constructional form is complicated. Their instrumentation and automation of operation. and the provision of the necessarv catalysts. are expensive and difficult and may require importation of materials.

The essential of the invention in the first aspect is to eliminate or reduce these difficulties by a process wherein the oxidation of lean natural gases is achieved without the use of catalysts and with the practical exploitation of a plasma beam or jet produced from the gas itself, ensuring not only the conditions of incomplete oxidation of hydrocarbons but also of reduction of carbon dioxide.

This aspect of the invention is based on the discovery made by experimentation that the oxidation of natural gases which are difficult to employ for combustion and which contain principally carbon dioxide and some nitrogen, as well as a well-controlled partial oxidation of hydrocarbon components and reduction of carbon dioxide to carbon monoxide, can all be achieved without complicated equipment and without the use of catalysts by making use of a plasma beam or jet produced from the gas itself, in a closed space having cooled walls and a mixture of natural gas and oxygen or air is blown through the plasma, which mixture contains relative to the proportion of the combustible components of the gas to be oxidised, insufficient amounts of oxygen or air.

We now consider more specifically the second aspect of the invention.

In the composition of natural gases containing carbon dioxide and classified as natural gases of high inert component content, there is in addition to the carbon dioxide 0.5 to 2.5 volume % of nitrogen as well as up to 10% by volume of hydrocarbon components, mainly methane and others which are heavier than methane, of unpleasant odour, oily and partially emulsified (aerosol-like). The removal of these latter combustible components from, and thus the enrichment with carbon dioxide of, the basic gas is not an easy task but is important since the undesirable attendant gases and vapours considerably limit the possibilities of direct industrial use of natural carbon dioxide gas in spite of the fact that such natural gas is available in abundant quantities. The present process is however, practicable both from the point of view of quality and of quantity.By removal of the combustible components the natural carbon dioxide is also freed from the unpleasantly smelling oily contaminations and can also be used in the pharmaceutical and food industries.

Several processes have been developed for purifying natural gases rich in inert components and containing combustible components in addition to the carbon dioxide, amongst which is a distillation process combined with expansion and deep cooling. using large towers.

Without basic experimental data the economy of the proposal for a purification process based on combustive oxidation is difficult to discuss because one cannot make reliable deductions about the geometry and construction of the combustion space for its optimum size or the operational parameters for ensuring complete combustion. Moreover, the indicated requirement for a catalyst is likely to have an unfavourable influence on running costs The essential of the present invention in its second aspect is to obviate or reduce the above-mentioned disadvantages and to reduce gas purification costs by removing the combustible components of the basic gas with the aid of a plasma beam. produced from the gas to be purified. by means of an oxidative process by feeding in an amount of oxygen adjusted to the volume percentages of the hydrocarbon components and odorous contaminants.The further purification of the final product. namely the removal of water vapour and nitrogen and possible excess oxygen remaining in the gas is effected by a low temperature distillation process.

The invention in this second aspect is based on the discovery obtained experimentally that in the purification of natural gases having a high content of inert components. mainly carbon dioxide, after removal of water from the gases preferably by an expansion method they can be used as a working gas without further treatment or preparation apart from expansion to form a plasma beam and for this the electrical power requirement is surprisingly low. Obviously the thus formed plasma, having a temperature of several thousands of "K is capable of ensuring the perfect oxidation of combustible components and other oily contaminants in the gas to be purified and blown therethrough after a previous step of mixing with the appropriate proportion of oxygen.

Where natural gas at high pressure is available. the gas to be purified is expanded to cool it and is used not only as a working gas but also as a coolant for the intensive cooling of the electrodes of the plasmatron employed. The process also makes it possible effectively to remove by distillation any water vapour. excess oxygen and nitrogen from the final product, by using a complex energy utilization principle. A further advantage to be expected of the process is that it makes it possible to conduct continuous operation without introducing complicated equipment.

The accompanying diagrammatic drawing illustrates an example of apparatus for use in either aspect of the invention which has been verified by laboratory experiments.

Lean natural gas. freed from its water content but otherwise unpurified. arrives through a sliding lock 1 at a gas distributor 2 from which a small quantity of gas, to be used as working gas, passes via valve 3 and cooling circuit 6 to the working space of a plasmatron 4. The plasmatron 4 has a ring electrode and a consumable advancing rod electrode. The electrodes are connected to the conductors of a DC current supply 5. The electrodes are cooled as shown by using expanded working gas which is thus preheated. whereby to assist in promoting the conditions for the ionization. For the additional continuous cooling of the mutually insulated electrodes cold water may be circulated in the cooling circuit 6.The plasma beam or jet is formed by instantaneously short-circuiting the electrodes or by arc Ignition, after adjustment or regulation of the electric supply and the throughput of working gas.

The larger quantity of gas passes via a valve 7 to a gas mixer 8 where it is mixed with oxygen or air introduced via a valve 9. The mixture thus obtained is passed to the inlet pipe 11 of a cooling jacket surrounding a reactor 10. The cooling jacket functions as a countercurrent recuperator. The thus pre-heated mixture of gas and oxygen or air leaves through pipe 12 and flows through an inlet pipe 13 to gas distributor manifold 14 disposed at the bottom of the apparatus. The manifold 14 has angled and appropriately dimensioned bores arranged in a circle for blowing the gas mixture through the plasma beam. The reaction products are discharged from the combustion space of the apparatus via a pipe 15 at the top.

The amount of direct current required for producing the plasma is small for these gas types and the electric power source 5 required can be provided even on site at a gas field by using a DC generator driven by a gas turbine operated by the gas to be dissociated.

A further advantage of energy economy is represented by the fact that in addition to the gas wells producing natural carbon dioxide on the same gas fields there occur greater gas sources containing combustible components. The gases of these latter wells can be directly utilized as the working fluid of gas turbine-driven generators and thus the electrical energy required for the plasma for the types of gas to be purified can be ensured at the gas fields themselves.

A process according to the first aspect of the invention using the above described apparatus was developed on a laboratorv scale bv means of a plasmatron having internal arcs and watercooled electrodes made of tungsten and molybdenum. using a current of strength not exceeding 30 A and a voltage between 200 and 400 V. using a natural gas of high inert component content. e.g. containing carbon dioxide of more than 606it by volume and by using a gas sampler of double-walled construction the internal wall of which was of a quartz glass while the outer wall was a Pyrex (Registered Trade Mark) glass tube. The maximum starting load for forming a stable plasma was 5.8 kilowatts.The temperature of the plasma 5 mm above the outflow opening of the plasmatron was between 8500 and 10,0e)0 K, as monitored by an emission colour analvser. The basic or starting gas. serving also as the working gas, and the cold gas mixture comprising the working gas, oxygen and air ingested due to imperfect sealing as well as the composition of the gas formed. had the composition given by the Table below. as established by gas chromatography examination: Components In the starting In the mixture of gas + O2/air gas: in the cold: after oxidation C1 29.756it b.v 29.19 0.283'it b.v.

C2 2.73 " 1.86 0.014 C3 0.76 " 0.52 0.005 C4 0.42 " 0.27 0.031 C5 0.26 " 0.16 0.028 C6 0.17 " 0.14 0.049 C7 0.13 " O.Oh 0.052 Cx 0.13 " 0.04 0.047 Hydrocarbons 34.35 " 32.24 0.509 in toto CO2 62.30 " 58.87 6.070 2 0.32 " 6.22 0.240 N2 3.030 " 2.67 2.260 CO 0.00 " 0.00 57.510 H2O 0.00 " 0.00 33.411 Total: 100.00 'ol. r/(. 100.00 100.000 viol.% The data checked from the Table unambiguously verify the possibility of the practical realization, on an industrial scale, of the process according to the invention both for the partial oxidation of hydrocarbons and for the reduction of the carbon dioxide component to carbon monoxide.The removal of water generated and of possible excess oxygen and nitrogen can be achieved by known processes and thus no further problems arise.

The unpurified state of the lean natural gas used as a working gas does not hinder the establishment of a stable plasma and by emission colour analysis it has also been extablished that with this type of gas no hydrogen cyanide is formed.

By comparison with currently used processes the process according to the first aspect of the invention can be realized more economically not only from the point of view of investment but also of running costs, for the following reasons: Without complicated construction and use of catalyst materials it can ensure the successful use of lean natural gases and provide the necessary conditions for the partial oxidation of hydrocarbons and the reduction of the carbon dioxide component to carbon monoxide; The space required for installing the apparatus and the amount of energy required for the process is small for known types of plasma; The electrical power supply can be ensured even at the gas field, remote from the national grid, e.g. with the aid of a DC generator driven by a gas turbine power plant operated with the gas to be oxidised.

Apart from the removal of water the process does not require further purification. nor pretreatment or preparation of the gas; and The provision of the constructional materials, machines, fittings and instruments required to realize the process is unlikely to be difficult or to involve importation.

As with the first aspect of the invention a control examination of the feasibility of the process according to the second aspect of the invention was conducted in laboratory conditions. The basic gas from which water has been removed which otherwise was unpurified was bottled directly at the gas well and contained, in addition to the carbon dioxide. 4.58 volume % of hydrocarbon contamination, n. 16 volume % of oxygen and 1.94 volume% of nitrogen. In the formation of the plasma beam from the basic gas the maximum starting load of the DC motor was below 6 KW which reduced to around 2 KW once the plasma became self-maintaining. The basic gas diluted with oxygen, as well as with air due to imperfections in sealing had a carbon dioxide content in the cold of 49.9 volume %, after blowing through the plasma.The final product obtained was checked with the aid of control data and with a Drager-type (Registered Trade Mark) apparatus. and a significant amount of water, a small amount of nitrous gas and a little oxygen were found, while a great decrease in the hydrocarbon components and enrichment of the carbon dioxide component were shown.

Comparing the process according to the second aspect of the invention with the hitherto known and used solutions the advantages set out below can be enumerated, as follows: a) The removal of the combustible components of natural gases containing mainly carbon dioxide and rich in inert components by blowing through a plasma jet and adding appropriate amounts of oxygen can be achieved at the gas field itself, without much space requirement or constructional intricacies; b) In the formation of plasma the gas to be purified is also used as working gas and is expanded during the technological process so as to serve as a coolant also;; c) The electric power requirement of the process is low and can be ensured even at a location removed from the national grid. at the gas field itself, next to the wells providing the basic gas, with the aid of electric motors driven by gas turbines operated with natural gases rich in inert components and having a calorific value of at least 3000 kcal/Nm3.

d) Both from the viewpoint of investment and economy the small space requirement, the simplification of the technological processes and the complex utilization of the gas energy represent advantages.

WHAT WE CLAIM IS: 1. A process of treating lean natural gases to oxidise the hydrocarbon compounds thereon which comprises separating the lean natural gas from water carried by the gas; electrically transforming a smaller portion of the natural gas into a plasma beam as a working gas. and transforming a greater portion of the natural gas, by addition of oxygen or air, into a technological gas; expanding the smaller portion of the gas. prior to its transformation into a plasma beam. and utilizing it as a cooling medium for the electrodes of a plasma generator in a recuperator preheater system to enhance the formation of the plasma beam: warming up the technological gas by passing the gas in heat exchange relationship with the plasma beam in a recuperatory preheater system to preheat the technological gas: and blowing the warmed up technological gas through the plasma beam.

2. A process as claimed in claim 1 wherein the amount of oxygen being admixed to said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   The data checked from the Table unambiguously verify the possibility of the practical realization, on an industrial scale, of the process according to the invention both for the partial oxidation of hydrocarbons and for the reduction of the carbon dioxide component to carbon monoxide. The removal of water generated and of possible excess oxygen and nitrogen can be achieved by known processes and thus no further problems arise.

   The unpurified state of the lean natural gas used as a working gas does not hinder the establishment of a stable plasma and by emission colour analysis it has also been extablished that with this type of gas no hydrogen cyanide is formed.

   By comparison with currently used processes the process according to the first aspect of the invention can be realized more economically not only from the point of view of investment but also of running costs, for the following reasons: Without complicated construction and use of catalyst materials it can ensure the successful use of lean natural gases and provide the necessary conditions for the partial oxidation of hydrocarbons and the reduction of the carbon dioxide component to carbon monoxide; The space required for installing the apparatus and the amount of energy required for the process is small for known types of plasma; The electrical power supply can be ensured even at the gas field, remote from the national grid, e.g. with the aid of a DC generator driven by a gas turbine power plant operated with the gas to be oxidised.

   Apart from the removal of water the process does not require further purification. nor pretreatment or preparation of the gas; and The provision of the constructional materials, machines, fittings and instruments required to realize the process is unlikely to be difficult or to involve importation.

   As with the first aspect of the invention a control examination of the feasibility of the process according to the second aspect of the invention was conducted in laboratory conditions. The basic gas from which water has been removed which otherwise was unpurified was bottled directly at the gas well and contained, in addition to the carbon dioxide. 4.58 volume % of hydrocarbon contamination, n. 16 volume % of oxygen and 1.94 volume% of nitrogen. In the formation of the plasma beam from the basic gas the maximum starting load of the DC motor was below 6 KW which reduced to around 2 KW once the plasma became self-maintaining. The basic gas diluted with oxygen, as well as with air due to imperfections in sealing had a carbon dioxide content in the cold of 49.9 volume %, after blowing through the plasma.The final product obtained was checked with the aid of control data and with a Drager-type (Registered Trade Mark) apparatus. and a significant amount of water, a small amount of nitrous gas and a little oxygen were found, while a great decrease in the hydrocarbon components and enrichment of the carbon dioxide component were shown.

   Comparing the process according to the second aspect of the invention with the hitherto known and used solutions the advantages set out below can be enumerated, as follows: a) The removal of the combustible components of natural gases containing mainly carbon dioxide and rich in inert components by blowing through a plasma jet and adding appropriate amounts of oxygen can be achieved at the gas field itself, without much space requirement or constructional intricacies; b) In the formation of plasma the gas to be purified is also used as working gas and is expanded during the technological process so as to serve as a coolant also;;

   c) The electric power requirement of the process is low and can be ensured even at a location removed from the national grid. at the gas field itself, next to the wells providing the basic gas, with the aid of electric motors driven by gas turbines operated with natural gases rich in inert components and having a calorific value of at least 3000 kcal/Nm3.

   d) Both from the viewpoint of investment and economy the small space requirement, the simplification of the technological processes and the complex utilization of the gas energy represent advantages.

   WHAT WE CLAIM IS: 1. A process of treating lean natural gases to oxidise the hydrocarbon compounds thereon which comprises separating the lean natural gas from water carried by the gas; electrically transforming a smaller portion of the natural gas into a plasma beam as a working gas. and transforming a greater portion of the natural gas, by addition of oxygen or air, into a technological gas; expanding the smaller portion of the gas. prior to its transformation into a plasma beam. and utilizing it as a cooling medium for the electrodes of a plasma generator in a recuperator preheater system to enhance the formation of the plasma beam: warming up the technological gas by passing the gas in heat exchange relationship with the plasma beam in a recuperatory preheater system to preheat the technological gas: and blowing the warmed up technological gas through the plasma beam.
2. 2. A process as claimed in claim 1 wherein the amount of oxygen being admixed to said

   greater portion of the lean natural gas is sufficient for complete combustion of its combustible components, contaminants of the gas being removed by oxidization from said technological gas while traversing the plasma beam so as to render it substantially pure.
3. 3. A process as claimed in claim 1 wherein the amount of oxygen being admixed to said greater portion of the lean natural gas is less than that needed for complete combustion of its combustible components, the hydrocarbons forming carbon monoxide and water.
4. 4. A process according to claim 1 substantially as herein described with reference to, and as shown, in the accompanying drawing.