FR2717403A1 - Elimination of light hydrocarbon(s) from gaseous mixts. - Google Patents

Abstract

A process for the elimination of light hydrocarbons in gaseous mixts. is carried out by decomposition using an electrical discharge over a temp. range of 300-700 K.

Description

 The present invention relates to a method for eliminating light hydrocarbons contained in poor gas mixtures, in particular in mine gases or industrial gases, this by their electro-chemical oxidation and their transformation into CO2 and H2O using electric discharges. high voltage.

 Environmental protection increasingly requires taking into account increasing quantities of residual gases containing hydrocarbons produced by industries. The toxicity of these gases or vapors and their stability in the atmosphere vary from one gas to another; all, however, have mutagenic and sometimes carcinogenic properties. They also contribute much more to the greenhouse effect than carbon dioxide. Efficient and energy-efficient methods for the annihilation of hydrocarbon compounds are therefore topical.

The literature tells us about methods of neutralizing hydrocarbons by incineration, for which their recovery is of no economic interest. For low concentrations, catalytic combustion is used combined with absorption processes [see
T. Banaszak, M. Miller, W. Rybak, A. Zieba, Thermal-Catalytic Incineration of
West Gases, First Int. Conf. on Combustion Technologies for a Clean
Environment, Vilamoura, Portugal, 3-6 September 1991, vol. 2, p. 13-19].

For concentrations of the order of 100 to 500 ppm, specific absorption or adsorption methods are used, for example with benzene or styrene. These gases are often absorbed in basic or carbonated solutions or on activated carbon.

Catalytic incinerators also have an important role in eliminating the hydrocarbons contained in gas mixtures [see
A. Musialik-Piotrowska, K. Syczewska, B. Mendyka, Dobor katalizatora do spalania specyficznych zanieczyszczen organicznych powietrza, I Symp.

Ograniczania Zanieczyszczen do Atmosfery, Pol-Emis'92, Wyd. Inst. Ochrony
Srodowiska Politechniki Wroclawskiej, p. 71-80]. These incinerators contain a catalytic bed (for example that of platinum-palladium on t-Al203 or vanadium oxide V203 on a Cu-Cr-Mn support) which can only be active in the temperature range between 473 and 723 K. This is why a preheating of the gas to be treated is essential like that based on the combustion of an additional energy fuel such as natural gas [see JE Kuznetscov, KJ Smat, SJ Kuznetsov, Oborudovanye dla Sanitarnoi
Ochistki Gazov, Tekhnika, Kiev, 1989].

 Good results for removing hydrocarbons from gases can be obtained by chemisorption in Krumeier towers or in scrubbers sprayed with sodium hypochlorite. The two approaches have the disadvantage of rejecting chlorine into the atmosphere and generating residual products [see H. Pfeiffer, VDI-Ber., 561 (1985), 53].

 The complex composition of the gases to be purified is often a source of difficulty as regards the choice of the purification method. The gases often contain compounds with different chemical properties which makes it difficult to apply a single sorbent or chemistry-sorbent. Excess moisture, the presence of a mist, dust or droplets (eg polymers) can prevent the use of adsorption. Catalytic or thermal combustion may be impossible if the temperature of the gas to be purified is too low or if the calorific effect of combustion is insufficient due to too low a concentration of the combustible molecule in the gas. Hydrogen sulfide is often present in the gas to be purified, which limits or completely prevents the use of a catalyst.

 An original method of air purification using biofilters has also been proposed [see M. Szklarczyk, J.D. Rutkowski, Polish patent application No. 153092 (1991)]. The functioning of biofilters is based on two parallel processes: a sorption of the impurities contained in a gas on the support of a filtering material and a biological decomposition of these impurities. The impurities adsorbed on the support, mainly organic molecules, are broken down by living organisms that populate this support. As a result of these transformations, carbon dioxide and water are obtained. Inorganic molecules, such as hydrogen sulfide or ammonia, are also adsorbed and transformed, but in this case transformation products such as sulfuric or nitric acids cause deactivation of the biofilter. Another disadvantage of the method is the very low ratio required between the flow rate of the gas to be purified and the surface of the biofilter, 2 which is between 20 and 500 m3 / h per m of cross section of the support (not to mention the need for '' have filters with very large specific surfaces).

 It has unexpectedly appeared that it is possible to eliminate hydrocarbons originating from mine gases or industrial gaseous effluents without the need to preheat these gases and to use catalytic beds, or without having recourse to large bio-filtering surfaces. and very long contact times between them and the gases to be treated. According to the present invention, such elimination of light hydrocarbons (in other words: volatile, having a concentration exceeding fifty ppm by volume at room temperature or higher) contained in the gas mixtures is possible using sliding electrical discharges.

The method according to the invention electrochemically transforms the hydrocarbons contained in a gas to non-toxic compounds, such as
CO2 and H20. The treated gases contain little or no hydrocarbons, which eliminates or severely limits the emission of hydrocarbons into the atmosphere.

 The process according to the invention is carried out in a reactor fitted with a generator of sliding electrical discharges in which an electro-chemical oxidation of the CnHm hydrocarbons takes place.

 The method according to the invention is particularly effective in the temperature range from 300 to 700 K, under atmospheric pressure and for gas flow rates between 30 and 120 Nl / min (Nl here means the liters measured under normal conditions); the flow rates indicated are adapted to the small reactor described below.

 The sliding electrical discharge (1) is installed between at least one pair of specially shaped electrodes (2a) and (2b) and placed along a gas flow (3) as shown in Figure 1. The discharge ignites at the place (4) where the distance between the electrodes is the smallest and it then propagates along the electrodes, pushed by the gas flow, to the place (5) where it disappears . The path traveled by the discharge from its birth to its disappearance depends on the geometry of the electrodes, the pressure of the gas, its nature, the linear speed of the gas, and the parameters of the electrical supply of the discharge. Under the influence of the discharge, several physical and electrical phenomena generate chemical transformations in the gas. The discharge is supplied by a direct or alternating voltage. The average discharge current is between 0.1 and 0.25 A (for a pair of electrodes) under voltage from 0.5 to 15 kV.

 Below we give examples of the implementation of the process, without this limiting its scope.

 The experiments were carried out in a cylindrical steel reactor with a volume of 2.7 1. The gas to be treated was preheated to temperatures of 300 to 700 K. In the ceramic cover of the reactor, three were housed pairs of electrodes connected to a special high voltage power supply. The reciprocal separation of the electrodes varied from 1 to 20 mm. A gaseous mixture of hydrocarbons and air (possibly with added steam), the flow rate of which was carefully measured using mass flowmeters, was introduced into the reactor through a nozzle with a diameter 2 mm. The pressure of the gas in the reactor was maintained at a constant level chosen between 0.1 and 0.5 MPa.

 The compositions of the incoming and outgoing gases were determined by gas chromatographic analysis with a flame ionization detector; the gas samples were transferred to analysis in rubber flasks. The amount of water vapor in the gases was measured by passing a certain volume of gas through anhydrous CaCl2.

According to the invention presented here, we observe an oxidation of hydrocarbons up to C02 and HzO. Among the products resulting from such a treatment we also observe minimal amounts of CO and C4 as well as
NOx whose content depends on the oxygen concentration of the incoming gas.

Example I.

We introduce into the reactor a mixture of majority air and methane (0.98, 1.98 or 2.98% vol.) At the temperature of 300, 380 or 450 K. The flow rate of the incoming gas has been stabilized at a value between 30 and 120 Nl / min. Under the effect of the electrical sliding discharge installed in the reactor we were able to observe the reaction
CH4 + 2 02 => C02 + 2 H20 (MI298 = - 803 kJ / mol) which could be confirmed by the presence of C02 in the outgoing gas as well as by the presence of H20 adsorbed on the mass of Caca2. We have also observed the following side reactions (albeit in minor proportions)
CH4 + 3/2 02 = CO + 2 H20 (H298 = - 520 kJ / mol),
C2H6 + 1/2 02 = C2H4 + H20 (H29s = - 105 kJ / mol),
C2H6 = C2H4 + H2 (H298 = + 137 kJ / mol), which gave, alongside the CO2 and H2O, small amounts of C2H4 and CO.

The results of Example I are presented in FIGS. 2 to 4 which show the conversion rate of methane in air as a function of the energy of the discharge reduced to one unit of volume of the treated gas (the specific energy ), the variable being the temperature T of the gas entering the reactor. The
Figures 2, 3 and 4 show the conversion rates for initial mixtures of 0.98, 1.98 and 2.98% CH4 in air, respectively. We see from Figure 2 that the methane conversion rate increases with increasing temperature of the incoming gas (which is preheated in a heat exchanger). An improvement in the conversion rate is noted when the specific energy injected into the gas is increased.

Example II.

 Ethane (C2H6) is also contained in natural gases and in mine gases. We therefore examined the behavior of this hydrocarbon in experiments of combustion assisted by sliding discharges of a methane + ethane mixture. The operating conditions were identical to those presented in Example I except that the gas entering the reactor was a mixture of air polluted with 1.98% CH4 and 0.16% C2H6. Figure 5 shows the results of the C2H6 conversion; there is a conversion rate close to 90%.

Example III.

 In this experiment we show the influence of water vapor on the electrically assisted decomposition of a mixture of methane in humid air. A dry air mixture containing 2.07% CH4 was introduced into the reactor at atmospheric pressure and at the initial temperature of 450 K.

In this mixture, water vapor was added at different controlled rates. As shown in Figure 6, the presence of water vapor causes a decrease in the methane conversion rate. The treatment time of the mixture in the reactor was 1.62, 2.29 or 4.26 s. We also note that the presence of H20 in the discharge contributes to the reduction of CO in the outgoing products (see Figure 7).

Example IV.

 In this experiment we show the influence of the gas pressure in the reactor on the reduction of hydrocarbons in the sliding discharge. Air containing 1.94 or 2.07% CH4 was introduced into the reactor as in Examples I to III. This mixture was treated in a sliding discharge which dissipated an energy of 0.17 or 0.29 kWh per 1 Nm of gas. The gas flow rate was set at 90 Nl / min. By acting on the valves of the gas circuit, we could maintain a constant pressure during the experiment between 0.1 and 0.5 MPa. The results show that an increase in pressure up to 0.35 MPa contributes to the increase in the methane conversion rate. An even higher pressure, however, causes this rate to decrease, as shown in Figure 8. The introduction of water vapor does not seem to influence the methane conversion rate.

 The experimental results presented in Examples I to IV confirm the efficiency of the process, object of the present invention, of conversion of light hydrocarbons in sliding discharges. The main products of the conversion are C02 and H20.

 It appeared that the elimination of ethane, despite its lower concentration, is more marked than the disappearance of methane, however more concentrated. As a general rule, the CH4 conversion rate is between 80 and 90%. For heavier hydrocarbons, the conversion rate will be even higher. The heavier the hydrocarbons, the more this rate will increase due to the growing thermodynamic instability of such molecules.

Claims (2)

 1. Method for removing light hydrocarbons present in gas mixtures, characterized by the decomposition of these hydrocarbons in an electrical discharge in a temperature range between 300 and 700 K.  2. Method according to claim 1 characterized in that this decomposition is carried out at atmospheric pressure or higher up to 0.5 MPa.