Regeneration of plasma initiator

Abstract: Plasma initiators that have become deactivated due to the accumulation of carbonaceous deposits, sulfur deposits, or both thereon can be regenerated by contact with an oxygen-containing gas in the presence of microwave radiation.

Description: This invention relates to a method for regenerating a plasma initiator using an oxygen-containing gas. Microwave energy has been used to convert methane to other hydrocarbons. For example, U.S. Patent 4,574,038 discloses that methane can be converted to ethylene and hydrogen in a batch process at pressures of from 0.3 to 1 atmosphere by subjecting the methane to microwave radiation in the presence of a metal powder catalyst. Another example of methane conversion using microwave energy is U.S. Patent 3,663,394. Microwave energy has also been used to treat liquid hydrocarbons. For example, U.S. Patent 3,616,375 discloses a method of decreasing the sulfur content of crude oil and other petroleum products using microwave energy. As another example, U.S. Patent 4,234,402 discloses that a variety of petroleum products can be hydrogenated and desulfurized by contact with hydrogen and microwave energy. As yet another example, U.S. Patent 4,279,722 discloses that a number of petroleum refinery operations can be improved by subjecting the hydrocarbon reactants and catalysts to microwave energy. In addition, German Patent 2,535,119 discloses a method for conducting chemical reactions by subjecting a catalyst particle in a fluid medium containing the chemical reagents to microwave energy. According to the present invention a method for regenerating a plasma initiator used to initiate an electric discharge in a process for the conversion of hydrocarbons using microwave radiation, which plasma initiator has become at least partially deactivated due to the accumulation of carbonaceous deposits, sulfur deposits, or both thereon, which method comprises contacting the initiator with an oxygen-containing gas in the presence of microwave radiation having a frequency of at least 0.3 GHz at ambient conditions of temperature and pressure for a period of time sufficient to remove at least a portion of the deposits therefrom. Our copending European patent application EP-A-0 435 591 (filed on the same date herewith and claiming priority from U.S. Serial Nos. 457,426 and 457,428) discloses the conversion of methane using microwave energy in the presence of at least one plasma initiator. However, during conversion, the initiator becomes at least partially deactivated due to the formation of carbonaceous deposits thereon, such that methane conversion will decrease with time. Similarly, deposits of sulfur could accumulate on the initiator during reaction of sulfur-containing non-hydrocarbon compounds ( e . g . the dissociation of H₂S, dimethyl sulfide, or other sulfur containing materials). We have found that the activity of a plasma initiator to methane conversion can be at least partially restored ( i . e . regenerated) by contacting the initiator with an oxygen-containing gas for a period of time sufficient to remove ( i . e . burn) at least a portion (preferably all) of the carbonaceous deposits, sulfur deposits, or both from the initiator. The precise contact time required to regenerate the initiator will vary broadly depending on the amount of carbon or sulfur present, the oxygen concentration of the gas, and the like. For example, the contact time can vary from a few minutes or less to 12 hours or more. Typically, however, the contact time will vary from about 15 minutes to about 6 hours, preferably from about 30 minutes to about 2 hours. Similarly, the oxygen concentration in the oxygen-containing gas can vary broadly. For example, the oxygen concentration can vary from 0.5 mole% or less to 30 mole% or more. Typically, however, the oxygen concentration will range from about 2 to about 20 mole%. The oxygen-containing gas used in this invention can be essentially any gas containing oxygen that can remove carbonaceous deposits, sulfur deposits, or both from a plasma initiator. Suitable oxygen-containing gases include air, CO₂, ozone, O₂, H₂O, or mixtures therefore. A diluent such as nitrogen or helium may be present in the oxygen-containing gas as well. The plasma initiator regenerated according to this invention is essentially any material capable of accumulating an electric charge when placed in an electromagnetic field and then dissipating the charge (or initiating an electric discharge), for example, by ionizing a gas environment. This includes metal initiators, non-metal initiators (including semiconductors), and composites of metal and non-metal initiators. As used herein, "composite" is meant to include mixtures (or combinations) of metals and non-metals. Examples of suitable metal initiators are tungsten, iron, nickel, copper, their alloys, or mixtures thereof. Preferred metal initiators are tungsten, iron, or mixtures thereof. Examples of suitable non-metal initiators include carbon, alumina, manganese dioxide, magnetite, nickel oxide ( e . g . NiO), iron oxide ( e . g . Fe₃O₄), calcium aluminate, cobalt oxide, chromium nitride, iron sulfide ( e . g . FeS₂, Fe 1-x S) copper sulfide ( e . g . Cu₂S) or mixtures thereof. Carbon, iron oxide, and calcium aluminate are preferred, with carbon being particularly preferred. Silica is not a suitable non-metal initiator. However, silica composited with a metal initiator or another non-metal initiator would be a suitable plasma initiator. Each plasma initiator is preferably elongated. The term 'elongated' includes both inherently elongated forms, such as wire, and also powder or other discrete particles arranged in elongated form. Preferably when the initiator is elongate it is in an inherently elongated form. The elongated initiator may be formed, combined, or bent in any convenient shape ( e . g . straight, helix, spiral, and the like). Preferably, each initiator should be formed such that there are points or sharp edges at the ends or on the surface of each initiator. The regeneration process of this invention occurs in the presence of microwave energy. The frequency of the microwave source can vary broadly, but will have a frequency of at least 0.3 GHz; with frequencies centered around 0.915, 2.45, 5.80, or 22.0 GHz being presently preferred in North America; particularly frequencies centered around 0.915, 2.45, or 5.80 GHz; especially frequencies centered around 0.915 or 2.45 GHz. The microwave energy may be continuous or pulsed. If pulsed, the duration of on-time pulses can vary broadly, but typically will range from about 1 nanosecond to about 20 seconds, preferably from about 1 millisecond to about 10 seconds, and most preferably from about 0.01 to about 0.2 seconds. The duration of off-time rests can vary broadly as well, but typically will range from about 1 nanosecond to about 100 seconds, preferably from about 0.003 to about 60 seconds, and most preferably from about 0.3 to about 5 seconds. Although this invention has been described above with respect to the formation of carbonaceous deposits from converting methane, these deposits (or for that matter the sulfur deposits) could have originated from any reactions that would form such deposits. The regeneration process of this invention is practiced at ambient temperature and pressure. By comparison, if microwave radiation is not used, the regeneration would have to be conducted at a temperature of at least 350°C, more typically at least 400°C, to remove the deposits. This invention will be further understood by reference to the following Examples which are not intended to restrict the scope of the appended claims. The Examples include references to the accompanying drawings in which:- Figure 1 is a graph of methane conversion versus time which shows the regeneration of an iron wire plasma initiator using an air/helium mixture; and Figure 2 is a graph of methane conversion versus time which shows the regeneration of a tungsten wire plasma initiator using air. Example 1 - Regeneration of Iron Wire Using an Air/Helium Mixture A methane/hydrogen mixture (1:4 mole ratio) flowing at 25 ml/min (milliliters/min) at atmospheric pressure was contacted with 2.9 gm of iron wire (about 0.03 inches - 0.76 mm - in diameter and cut into 45 mm lengths) in a reactor fabricated from WR430 waveguide bounded by quartz plate glass windows and positioned approximately one-quarter waveguide wavelength from a short circuit plate. The reactor was irradiated with microwave radiation centered at a 2.45 GHz frequency and pulsed in an on/off cycle (0.14 seconds on in a total cycle of 3.5 seconds) with an average power ranging from 7.5 to 10 watts. The methane/hydrogen mole ratio was then decreased from 1:4 to 1:1, with the methane conversion being calculated according to the following equation: The results (as illustrated in Figure 1) show that a reduction of the methane/hydrogen mole ratio from 1:4 to 1:2 had little effect on methane conversion. However, a further reduction to 1:1 resulted in a significant decrease in methane conversion (to less than 5%). In addition, carbonaceous deposits were observed to have accumulated on the iron wires as the mole ratio decreased. At this point, the methane/hydrogen flow was stopped and the reactor purged with helium flowing at about 100 ml/min at atmospheric pressure. Air (10 ml/min) was then introduced into the reactor with the helium and the initiators irradiated with pulsed microwave radiation centered at a 2.45 GHz frequency in an on/off cycle (0.5 seconds on in a total cycle of 3.5 seconds) for 17 minutes. As shown in Figure 1, methane conversion using a methane/hydrogen mixture (1:4 mole ratio) flowing at 25 ml/min showed only marginal improvement in conversion (from less than 5% to about 8%). The methane/hydrogen flow was again replaced by helium, the reactor purged, and the above air/helium regeneration repeated with the exception that the time was extended to 60 minutes. Flow of the methane/hydrogen mixture (1:4 mole ratio at 25 ml/min) was reinitiated and the methane conversion calculated as above. Figure 1 shows that the methane conversion was almost the same as that before deactivation. Example 2 - Regeneration of Tungsten Wire Using Air Methane/hydrogen mixture (1:4 mole ratio) flowing at 25 ml/min at atmospheric pressure was contacted with 1.5 gm of tungsten wire (about 0.03 inches - 0.76 mm - in diameter and cut into 45 mm lengths) in the reactor of Example 1 and irradiated with microwave radiation centered at a 2.45 GHz frequency pulsed in an on/off cycle (0.14 seconds on in a total cycle of 3.5 seconds) with an average power of about 4.5 watts. As shown in Figure 2, this resulted in average steady state methane conversion of 39.7%. After running for about 19.5 hours at this level of methane conversion, hydrogen flow was stopped and the initiators allowed to deactivate (by formation of carbon deposits) for about 2.5 hours. An activity check using a methane/hydrogen mixture (1:4 mole ratio) flowing at 25 ml/min showed that methane conversion was essentially zero. Carbonaceous deposits were also observed to have accumulated on the tungsten wires. The flow of the methane/hydrogen mixture was then stopped and the reactor purged with air flowing at substantially greater than 100 ml/min. The initiators were then irradiated with microwaves in the manner described above for about 1.7 hours. Air was purged from the reactor using a methane/hydrogen mixture (1:4 mole ratio) and methane conversion calculated as above. Figure 2 shows that methane conversion was restored to about 28%. The data obtained in Examples 1 and 2 show that plasma initiators can be regenerated by contact with an oxygen-containing gas in the presence of microwave energy.

Claims: A method for regenerating a plasma initiator used to initiate an electric discharge in a process for the conversion of hydrocarbons using microwave radiation, which plasma initiator has become at least partially deactivated due to the accumulation of carbonaceous deposits, sulfur deposits, or both thereon, which method comprises contacting the initiator with an oxygen-containing gas in the presence of microwave radiation having a frequency of at least 0.3 GHz at ambient conditions of temperature and pressure for a period of time sufficient to remove at least a portion of the deposits therefrom. The method of claim 1 wherein the plasma initiator is a metal. The method of claim 2 wherein the metal is tungsten, iron, nickel, copper, their alloys, or mixtures thereof. The method of claim 3 wherein the metal is tungsten, iron, or mixtures thereof. The method of claim 1 wherein the plasma initiator is a non-metal other than silica. The method of claim 5 wherein the non-metal is carbon, alumina, or mixtures thereof. The method of claim 1 wherein the plasma initiator is a composite of a metal initiator and a non-metal initiator. The method of claim 1 wherein a plurality of plasma initiators are regenerated. A method of any preceding claim, wherein the plasma initiator is elongate. A method of any preceding claim, wherein the microwave energy is pulsed. A method of claim 10, in which the microwave energy is pulsed with the duration of on-time pulses ranging from 1 millisecond to 10 seconds and the duration of off-time pulses ranging from 0.003 to 60 seconds. A method of any preceding claim, wherein the oxygen concentration in the oxygen-containing gas is from 2 to 20 mole % and the contact time is from 15 minutes to 6 hours.

Inventors: MURPHY WILLIAM JOHN, MURPHY, WILLIAM JOHN
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1990
Main classification: B01J19
IPC codes: B01J19, H01J37, H05H1, C23F4