A TORCH DEVICE FOR CHEMICAL PROCESSES

Abstract: In a plasma torch (1) which consists of two or more tubular electrodes (2, 3) which are located coaxially inside one another and are designed for chemical treatment of the reactant, a lead-in tube (4) for supply of reactant is located coaxially in the internal electrode (3) and consists of a liquid-cooled tube provided with a heat-insulating layer (10, 11) on the outer surface. Furthermore the lead-in tube (4) can be moved in the axial direction for positioning of the nozzle in relation to the plasma flame. The lower part of the lead-in tube (4) is exposed to high temperatures which can lead to erosion and lacerations, and this part is therefore provided so as to be replaceable. Moreover the tube can be provided with a narrowing in the outlet opening in the form of a venturi nozzle, in order to increase the exit velocity of the reactant. Between the lead-in tube and the internal electrode an annular passage is formed into which plasma-forming gas (13) can be introduced. The plasma-forming gas (13) which flows along the lead-in tube will simultaneously cool it.

Description: A torch device for chemical processes The present invention concerns a lead-in tube for the supply of a reactant to a plasma torch. The plasma torch is used for the chemical treatment of a reactant, and it can be supplied with both plasma-forming gas and reactant. From Norwegian patent no. 164 846 there is known an electrically insulated supply tube for admixtures, which is provided centrally in an internal electrode in a plasma torch designed for submersion in a metallurgical smelt. In US 4 122 293 there is described an external liquid-cooled supply tube for the supply of gas, admixture and electric current to a hollow electrode which is used in an electric arc smelting furnace. Furthermore, EP 0 178 288 describes a nozzle for a plasma torch specially designed for heating a metallurgical melting pot. The nozzle has an electrode tip attached to a liquid-cooled electrode holder which simultaneously acts as a supply tube for plasma-forming gas and electric current. The electrode tip has a central boring for the plasma-forming gas and the outlet of the boring is designed first as a Laval nozzle and thereafter as a diffuser to permit the gas to be sprayed when it leaves the electrode. During chemical treatment of a reactant, for example during pyrolysis, it is essential that the gas has the correct temperature when it reaches the plasma flame. If the temperature of the gas exceeds a certain value it will react too early. This is undesirable as decomposition products can be formed before the gas reaches the plasma flame, and this can lead to precipitation of such products in the lead-in device and on the electrodes. It has been found that the known designs of supply devices for gas produce unsatisfactory results when used in a plasma torch which is utilized for chemical treatment of reactant. Thus it is an object of the present invention to provide a lead-in device wherein the required temperature and correct rate of reactant supplied to such a plasma torch are achieved. This object is achieved by a lead-in tube which is characterized by the features in the claims presented. The plasma torch is composed of tubular electrodes located coaxially inside one another. In its simplest form the torch consists of two electrodes, an external electrode and an internal electrode. The plasma torch can also be provided with more electrodes. The electrodes can be hollow, provided with cooling channels for the transport of a coolant. All types of solid materials with good thermal and electrical conductivity can be used for liquid-cooled electrodes. It is preferable to use solid electrodes. Solid electrodes are usually constructed of a material with a high melting point and with good conductivity, such as graphite. The reactant is fed in through a separate lead-in tube located coaxially in the internal electrode. The term reactant refers to pure gas or gas mixed with liquid particles or solid particles with which chemical reactions will take place in the plasma .flame. When the lead-in tube is heated in the plasma zone, it is necessary to cool it. It is therefore provided with channels for transport of a coolant. The cooling channels can for example be formed by providing the tube with an internal dividing plate which ends some distance above the bottom of the lead-in tube. The direction of flow of the coolant is provided in such a way that the lowest temperature is obtained in the inner part of the lead-in tube. It is important for the reactant to have the correct temperature when it is fed into the plasma zone. The desired temperature for methane for example can be in the range of 650 to 700 degrees C. By measuring the temperature at the outlet nozzle of the lead-in tube, for example by means of thermocouples located in the tube, the temperature of the coolant can be adjusted so that the reactant reaches the desired temperature when it leaves the outlet nozzle. The outer surface of the lead-in tube and especially the lower surface which faces the plasma flame is supplied with a heat- insulating coating. The lead-in tube with insulating coating has a smaller diameter than the internal diameter of the inner electrode. In the annular passage which is formed between the lead-in tube and the inner electrode, plasma-forming gas or reactant can be supplied. The plasma-forming gas or reactant is at a low temperature when it is supplied and will therefore further contribute to the cooling of the lead-in tube. The plasma-forming gas may for example be an inert gas such as nitrogen or argon, which normally will not participate in or affect the chemical reaction occurring in the plasma flame. The reactant can also be used as a plasma-forming gas. The lead-in tube can be moved in the axial direction to enable the nozzle to be adjusted in order to achieve a favourable position in relation to the plasma flame. Advantageous temperature conditions are thereby obtained in the reactant when it reaches the plasma zone and optimal efficiency is achieved in the chemical process. In the plasma torch consumable electrodes can be used which will have some degree of melting loss, thus altering the length of the electrode. For this reason it is also advantageous if the lead-in tube can be moved so that it can be readjusted and follow the wear on the electrode. The nozzle or the lower part of the lead-in tube which faces the plasma flame are provided so as to be replaceable. This part of the lead-in tube is exposed to high temperatures so that erosion and lacerations can occur on the tube. It is therefore advantageous for the nozzle to be capable of replacement at set intervals. The nozzle of the lead-in tube can be provided with a conical narrowing, a venturi or Laval nozzle. The reactant will thereby achieve a higher flow rate, thus feeding it more rapidly towards the plasma flame. The gas rate of flow is a parameter for achieving the best possible operating conditions in a plasma torch designed for chemical processes. Since the venturi is replaceable, a nozzle can be chosen which offers optimal gas flow rate for the reactant in use. With a lead-in tube according to the invention the object is achieved of being able to supply the reactant at the desired temperature and at the correct rate of flow and with the outlet nozzle in the right position in relation to the plasma flame, thereby preventing the reactant from reacting before it reaches the reaction area. This also prevents precipitation of reaction or decomposition products in the nozzle of the lead-in tube and on the electrodes. Within the scope of the invention the lead-in tube can be used for many different types of plasma torch, such as a plasma torch described in the applicant's Norwegian application no. 91 4907. The lead-in tube for a plasma torch according to the present invention will be described in more detail with reference to drawings which schematically illustrate a preferred embodiment. Figure 1 is a vertical section through a plasma torch with lead-in tube according to the present invention. Figure 2 is a vertical section of a second design of the lead- in tube for a plasma torch according to the present invention. In figure 1 the plasma torch is indicated by 1. Here it is provided with two electrodes, an external electrode 2 and an internal electrode 3. The electrodes 2 and 3 are preferably circular and tubular and are located concentrically inside each other. They can be solid or hollow provided with cooling channels for the transport of a coolant. Solid electrodes are preferably constructed of a material with a high melting point and with good electrical conductivity such as graphite or silicon carbide. All types of solid materials with good electrical and thermal conductivity, e.g. copper, can be used for liquid-rooled electrodes. The plasma torch is provided with a lead-in pipe 5 for reactant. The lead-in pipe 5 is preferably composed of a material with good thermal conductivity, such as copper. The tube has an interior wall 6 and an exterior wall 7 and is equipped with an internal dividing plate 8 which ends some distance above the bottom of the tube, thereby forming a channel for coolant. ■ αe supply of coolant is provided in such a way that the )olant flows into the channel along the inner surface of the tube 6 and flows out of the channel along the outer surface 7. This is indicated by arrows. With the indicated direction of flow the object is achieve" that the lowest t -mperature is ob¬ tained in the inner surface of the lead-in tute. The outer surface 7 and especially the lower surface 9 of the tube are provided with a heat-insulating coating 10 and 11. The reactant is fed to the plasma flame through the lead-in tube 5. This is illustrated by the arrow marked 12. The term reactant refers here to pure gas or gas mixed with fluid particles or with solid particles with which chemical reactions will take place in the plasma flame. Between the lead-in tube and the internal electrode and between the internal and the external electrodes annular passages are formed. Through these passages plasma-forming gas can be supplied. This is illustrated by arrows 13 and 14. The plasma- forming gas may for example be an inert gas such as nitrogen or argon, which normally will not participate in or affect the chemical reaction occurring in the plasma flame. The plasma-forming gas which is fed in through the annular passage between the lead-in tube and the internal electrode is indicated by arrows 13. This gas can be precooled and will further contribute to the cooling of the lead-in tube. The lead-in tube 5 for the reaction gas can be moved in the axial direction. The equipment for moving the tube is not illustrated in the drawing. The object of moving the lead-in tube is to enable the nozzle to be adjusted so that it attains the correct position in relation to the plasma flame. The nozzle or the lower part of the lead-in tube is replaceable. The interior and exterior walls of the tube are preferably equipped with a threaded section to enable the nozzle to be screwed off and replaced if this part of the tube should be damaged. The threaded section is indicated by the reference number 16 for the interior tube wall and 17 for the exterior tube wall. In figure 2 there is illustrated a lead-in tube 5 wherein the nozzle or the lower part of the lead-in tube which faces the plasma flame is designed in a conical form, thus producing a narrowing towards the outlet of the pipe in the form of a venturi nozzle 15. The design of the lead-in tube and the reference numbers are outwith this alteration in accordance with figure 1. When the reactant is forced through the nozzle 15 it will achieve a higher rate of flow and it will be fed more rapidly towards the plasma flame. The nozzle or the lower part of the lead-in tube is replaceable. The interior and exterior walls of the tube are preferably provided with a threaded section to enable the nozzle to be screwed off and replaced in the case of wear and tear. The threaded section is indicated by the reference number 16 for the interior tube wall and 17 for the exterior tube wall.

Claims: PATENT CLAIMS 1. A lead-in tube for the supply of reactant to a plasma torch designed for chemical treatment such as pyrolysis or cracking of the reactant, wherein the plasma torch comprises two or more tubular or consumable electrodes located coaxially inside each other, and wherein the outer surface and the lower surface of the fluid-cooled lead-in tube are provided with a thermally insulating coating and is located coaxially in the internal electrode, characterized in that the lead-in tube can be moved in the axial direction in order to adjust the nozzle in relation to the plasma flame, and that the lower part is provided with a conical narrowing in the form of a venturi nozzle which is replaceable, thus enabling a nozzle to be chosen which provides optimum gas velocity for the reactant which is used. 2. A lead-in tube for the supply of reactant according to claim 1, characterized in that elements for measuring temperature are located at the outlet nozzle for adjustment of the coolant in order to obtain the correct temperature in the reactant which is used.

Inventors: HAUGSTEN KJELL, HOX KETIL, HUGDAHL JAN, LYNUM STEINAR, MYKLEBUST NILS, HAUGSTEN, KJELL, HOX, KETIL, HUGDAHL, JAN, LYNUM, STEINAR, MYKLEBUST, NILS
Applicants: KVAERNER ENG, KVAERNER ENGINEERING A.S
Application year: 1992
Main classification: H05H1
IPC codes: H05B7, H05H1