Ethylene from Methane and Naphtha by an Integrated Process

Oct 1, 1997 - Institute of Organic Chemistry and Technology, Silesian Technical ... Both air and oxygen can be used as oxidants in the coupling step...
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Ind. Eng. Chem. Res. 1997, 36, 4193-4197

4193

Ethylene from Methane and Naphtha by an Integrated Process Marian Taniewski* and Dymitr Czechowicz Institute of Organic Chemistry and Technology, Silesian Technical University, 44-101 Gliwice, Poland

An integrated one-stage process of ethylene production from methane and naphtha, consisting of exothermic oxidative coupling of CH4 over the catalyst fixed bed and endothermic pyrolysis of naphtha and of the formed C2H6 in the postcatalytic zone of the same reactor, was studied under varied operating parameters including CH4/naphtha ratio, temperatures of the catalytic and postcatalytic zones, and flow rates of the reagents. The rationale of an integrated onestage concept has been confirmed. High yields of C2H4 and coproducts were obtained. An additivity of the yields of oxidative coupling and pyrolysis was observed under experimental conditions. The increase in C2H4 yield, as compared with pyrolysis of naphtha, was proportional to the contribution of the coupling component. The C2H4/C2H6 mole ratio was found to be as high as 4-5 for oxidative coupling with partial pyrolysis of C2H6 and about 10 for the integrated process. Both air and oxygen can be used as oxidants in the coupling step. Depending on relative contributions of the coupling and pyrolysis components to the integrated process, an overall balance of CH4 could be negative (consumption) or positive (production). In the latter case, the integrated process could be based only on a liquid feedstock, with a recycle of CH4. Introduction The catalytic oxidative coupling (COC) of methane could be applied in the future either as an independent process for the production of ethylene (or liquid fuels) from natural gas or as a part of a complex technological scheme of ethylene production, consisting of COC of CH4 and steam cracking of hydrocarbon fractions. The economics of COC of CH4 as an add-on unit for naphtha cracking was discussed recently by Hoebink et al. (1995). The manufacture of ethylene by a combination of the two above-mentioned processes in one plant does not necessarily mean operation in two separate units. A method which would integrate both processes in one reactor, and, additionally, which would include a secondary transformation of a part of the formed C2H6 to C2H4 in the same reactor, can also be considered. Such a process, called below “an integrated process”, is the subject of the present study. The concept of direct utilization of the heat of exothermic catalytic oxidative coupling (COC) of CH4 to C2H4 and C2H6 by carrying out a second step of endothermic thermal decomposition of C2H6 to C2H4 in the same reactor, instead of in a separate unit, after isolation of C2H6, was put forward by several research groups (Mimoun et al., 1989, 1990; Edwards et al., 1990, 1992; Taniewski et al., 1990a, 1992, 1993, 1994; Lachowicz et al., 1990, 1992). Our own investigations concerning such a one-stage process of CH4-to-C2H4 transformation were conducted both in the small-scale and in the scaled-up laboratory demonstration units, with flow tubular reactors, where COC of CH4 was carried out over a Li/MgO fixed bed. The thermal decomposition of C2H6, contained in the effluent gases from the COC area, occurred in the hot postcatalytic zone (PCZ) (Taniewski et al., 1994). The nature and significance of various contributing reactions in PCZ were studied in detail, and the possibility of obtaining high C2H4 yields and high C2H4/C2H6 ratios was demonstrated. Some modifications of the one-stage process were also examined. The basic form of this process, * Author to whom correspondence should be addressed. Telephone: 48-32-372014. Fax: 48-32-372094. E-mail: [email protected]. S0888-5885(96)00711-7 CCC: $14.00

modified by injection of C2H6 or C3H8 from the outside to PCZ, was studied to simulate processes with recycled C2H6 or with the admission of LPG, respectively (in the latter case, the integrated process would utilize all components of natural gas). To extend our previous investigations on various modifications of a one-stage process, the present study was undertaken on the possible integration of COC of CH4 with pyrolysis of naphtha or of any other liquid hydrocarbon fraction in the same reactor. This idea was never studied before experimentally. Contrary to COC of CH4 alone, or to the above-mentioned previously studied modifications of a one-stage process, the modification now considered would not be based exclusively on natural gas but at least in part also be based on oil hydrocarbon fractions. In this respect, a one-stage process would not differ from a combination of independent units of COC of CH4 and of steam cracking of oil fractions, producing ethylene in the same factory. Since CH4 is consumed in COC and produced in pyrolysis, it can be anticipated that a combination of both processes in one reactor or in two reactors may lead, depending on their relative contributions, to the production of C2H4 either based on both raw materials (natural gas and oil fractions) or exclusively on oil fractions, with a recycle to the COC area of CH4 produced in excess. The integrated process could be treated, depending on the circumstances, either as COC of CH4 with limited contribution of pyrolysis, mainly for utilization of heat, or as pyrolysis of liquid hydrocarbons with some participation of COC of CH4, as a route leading to the increase in C2H4 production without necessarily increasing the consumption of the liquid feed. The attractiveness of the integrated process, based on transformations of CH4, naphtha, and the C2H6 in one reactor, may combine the benefits common to all one-stage modifications, such as an efficient heat utilization, high C2H4/C2H6 ratios obtained in one step, lower capital and operating costs, etc., with those deriving from the partial involvement of CH4 (from natural gas or as a recycle from gas separation units) in C2H4 production, based at present on transformation (steam cracking) of C2+ hydrocarbons. © 1997 American Chemical Society

4194 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

During the oxidative coupling alone, a gaseous feed was introduced to the upper part of the reactor, whereas the side pipe was closed. When pyrolysis alone was carried out, a naphtha was injected into the reactor through a side pipe and N2 was introduced into the catalyst zone. In the course of the integrated process, a feed for COC was passed through a catalyst bed and naphtha was introduced from a side pipe. Analysis. Gaseous products were analyzed with the use of a Hewlett-Packard 5890 Series II gas chromatograph equipped with TCD, FID, computer, and HP ChemStation G 1201A. Details concerning the columns and conditions have been described earlier (Taniewski et al., 1994). Results and Discussion Figure 1. Simplified scheme of the apparatus (description in the text).

Experimental Section Apparatus. All experiments were carried out in a small laboratory unit, with a reactor equipped with all necessary controlling devices and facilities for feeding and sampling. A simplified scheme of this unit is shown in Figure 1. The reactor was a 226-mm-long, 13.2-mm-i.d. quartz tube with a side pipe, situated 145 mm from the top of the reactor. The upper part of the reactor, above the side pipe, was filled with the catalyst bed supported on quartz wool (catalyst zone). The lower part (pyrolysis zone) was empty. A feed for COC was introduced to the reactor through a flow controller (FC) and a flowmeter (FM). Naphtha for pyrolysis was injected via a side pipe to the pyrolysis zone. The effluent gases from the reactor were sent to the liquid-phase separator and, after passing through a cross valve (CV), were directed to a FM or to the sample collector (SC). The reactor was inserted in a furnace with two heating zones, each equipped with a separate system of automatic temperature regulators and controls, which enabled independent heating of both parts of the reactor. A movable NiCr-Ni thermocouple, positioned in the center of the reactor, enabled the determination of a temperature gradient along the reactor length. Catalyst. A catalyst, 1% Li/MgO, was prepared by the impregnation method, as described previously (Taniewski et al., 1990b). A 7.2-cm-long catalyst bed contained 7.0 cm3 of a catalyst using 0.3-0.6 mm pellets. Feed. Two types of mixtures were used for COC of CH4: (1) CH4-air, containing approximately (vol %) 30 CH4, 15 O2, and 55 N2; (2) CH4-O2, containing approximately (vol %) 66 CH4, 33 O2, and 1 N2. Gaseous mixtures were prepared from CH4 (97.4 vol %) and air or O2 (technical grade). Nitrogen (technical grade) was employed as the inert gas in pyrolysis. Naphtha for pyrolysis was supplied by the Czechowice-Dziedzice Refinery. Its boiling range was 50-175 °C; the calculated average molecular weight was 146.3 g/mol, and the density at 20 °C was 0.735 g/cm3. The content of C and H (mass %) was 84.75 and 14.70, respectively. Procedure. Three types of processes were examined: COC of methane alone, pyrolysis of naphtha alone, and the integrated process composed of COC and pyrolysis, both carried out in the same reactor.

Extensive preliminary studies were undertaken to examine the degree of accuracy and reproducibility of our experiments. The results (not included in the tables) indicated the following: (1) the reproducibility of all types of runs was satisfactory, with C2H4 yield varying by (1.2%; (2) the material balance of COC was always good (C balance equal to 96-99%, H and O balances showing a deficit, evidently caused by H2O formation, as seen from a H/O atomic ratio equal to 2 ( 0.1:1); (3) the applied catalyst samples were stable during at least 33 h (O2 conversion exceeded 99%), and afterwards, they underwent a slow deactivation, with a gradual loss of selectivity. All observations and conclusions were next fully confirmed during the main series of experiments. The main experiments were carried out in series, each composed of three runs carried out under identical basic operating conditions: COC of CH4, pyrolysis of naphtha (Pyr), and the integrated process (COC + Pyr). Such procedures enabled the examination of the course of individual and integrated processes, the role of operating conditions, and the degree of additivity of the results, without a danger of uncontrolled changes in the catalyst activity and selectivity. To make corresponding runs comparable inside the series, the flow rate of nitrogen introduced in Pyr was kept equal to the flow rate of gases exiting the catalyst bed area in COC of CH4. The CH4-air mixture was used in series I-VII and in some runs of series VIII-X. The CH4-O2 mixture was used in some runs of series VIII-X. The results of the three series of experiments carried out to examine the effect of naphtha flow rates on the yields of products in the integrated process (also expressed in the form of flow rates) are given in Table 1. All runs were conducted at the temperatures 720 (COC) and 800 °C (Pyr) with the same flow rate (3.1 mmol min-1) and composition (CH4:O2 ) 2.0) of CH4air mixture, so the results in Table 1 illustrate also the influence of the changes in contribution of both component processes to the combined integrated process. Apart from the experimental results obtained in the integrated process (COC + Pyr), there are also given some calculated values, the sums of the results being obtained in COC and Pyr (∑calc). A comparison of experimentally obtained and calculated values clearly indicates that under experimental conditions, a good additivity of the yields of C2H4 and of all other products is observed. This fact may indicate the insignificance of interactions between products and negligible losses of the products formed by COC in the PCZ area. On this basis, however, one should not reject the possibility

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4195 Table 1. Catalytic Oxidative Coupling (COC) of CH4, Pyrolysis of Naphtha (Pyr), and Integrated Process (COC + Pyr) series I type of run feed flow rate, mg min-1 CH4 naphtha products flow rate, mg min-1 CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C4H6 C5-C6 C6H6 H2 CO CO2

COC

Pyr

∑calc

14.21 23.04

-5.761 0.347 1.785 -0.083 0.143 0.001 0.019 0.084 0.055 0.197 8.056

3.508 0.500 6.713 0.066 1.959 0.029 0.560 1.058 0.631 4.549 0.207

-2.253 0.847 8.498 -0.017 2.102 0.030 0.579 1.142 0.631 4.549 0.262 0.197 8.056

series II COC + Pyr COC

Pyr

14.20 23.16

14.38 35.10

-2.521 0.754 8.741 -0.017 2.377 0.038 0.584 1.286 0.847 4.596 0.279 0.261 8.208

-5.669 0.350 1.860 -0.085 0.143 0.001 0.013 0.063 0.051 0.128 8.730

∑calc

5.563 0.894 10.298 0.115 3.269 0.052 0.860 1.710 1.274 6.534 0.032

-0.106 1.224 12.158 0.030 3.412 0.053 0.873 1.773 1.274 6.534 0.083 0.128 8.730

series III COC + Pyr COC

Pyr

14.32 35.45

14.27 60.59

-0.710 1.177 11.723 0.038 3.750 0.075 1.164 1.874 1.761 6.766 0.369 0.243 8.717

-5.458 0.280 1.521 0.026 0.134 0.046 0.124 0.080 0.390 8.685

8.922 1.882 15.765 0.260 6.252 0.144 1.968 2.760 3.804 4.471 0.489

∑calc

COC + Pyr 14.30 60.59

3.464 2.162 17.286 0.286 6.386 0.144 2.102 2.884 3.804 4.471 0.569 0.390 8.685

2.895 2.078 17.061 0.277 6.634 0.151 2.500 3.112 4.984 10.366 0.578 0.523 8.547

Table 2. Comparison of Naphtha Pyrolysis and Integrated Process V0* = 23 mg min-1 type of run yield per pass, mg/100 mg of naphtha C2H6 C2H4 C3H6 yield of C2H4 from COC/total yield of C2H4, % yield of C2H4 from Pyr/total yield of C2H4, %

V0* = 35 mg min-1

V0* = 61 mg min-1

Pyr

COC + Pyr

∆abs

∆rel

Pyr

COC + Pyr

∆abs

∆rel

Pyr

COC + Pyr

∆abs

∆rel

2.17 29.14 8.50

3.25 37.74 10.26 22.8

1.09 8.61 1.76

50.09 29.55 20.72

2.55 29.34 9.31

3.32 33.07 10.58 11.2

0.77 3.73 1.26

30.33 12.70 13.57

3.11 26.02 10.32

3.46 27.93 10.98 6.8

0.36 1.91 0.66

11.47 7.36 6.35

77.2

that for the larger scale and steel reactors, and particularly in the case of a higher contribution of COC to the integrated process, some losses of C2H4 could arise, as a result of transformation (decomposition) of C2H4, especially of the part which, being formed in the COC zone, might stay too long in PCZ (cf. Taniewski et al., 1994). As can be seen from the results in Table 1, with increasing naphtha feed rate, i.e., with a rising contribution of naphtha pyrolysis in the integrated process (series I-III), an obvious increase in the yields of C2H4 and its main coproducts was observed. In all cases, the inclusion of COC into the integrated process increased the yield of C2H4. In all series, high C2H4/C2H6 ratios were obtained, as a result of relatively severe conditions in PCZ. This was typical for a one-stage process. This ratio was found to be 4-5:1 for COC and about 10:1 for the integrated process. In accordance with expectations, the balance of the yield of CH4 underwent significant variation from the net consumption in series I (high consumption) and II (low consumption) to the net production in series III. This observation confirms the initial suggestion that the integrated process could be based on either of two feedstocks, i.e., CH4 and liquid hydrocarbons, or be self-sufficient with respect to CH4, only on the liquid feed with a recycle of excess CH4. One of the reasons for possible industrial interest in commercialization of the integrated process could be the willingness to intensify C2H4 production in the pyrolysis of naphtha without additional naphtha consumption. It seems reasonable to analyze again some results from Table 1, presented now in the form of yields per pass of naphtha, as a function of a naphtha flow rate (V0*). Such a presentation is given in Table 2 and contains the yields of main products obtained in pyrolysis and in the

88.8

93.2

integrated process, as well as the values of the absolute and the relative yield increase (∆abs and ∆rel), expressed in mg/100 mg of naphtha and in % of the yield in pyrolysis, respectively. The data in Table 2 demonstrate the potential importance of the increased yields of C2H4 and its coproducts, as related to a naphtha feed, if a conventional pyrolysis (steam cracking) were replaced by the integrated process. It is obvious from the results that the increase in C2H4 yield is proportional to the contribution of COC to the integrated process. The additivity of the results of both individual processes (COC and Pyr) and the integrated process (COC + Pyr) observed in this work, under applied experimental conditions, may suggest that the influence of changes in operating conditions on the overall yields in the integrated process could also be predicted on the basis of our knowledge of corresponding effects exerted in individual processes. However, such an expectation should be verified, at least in the case of those parameters which could be essential from the point of view of technological and engineering nature of the future process. To this important group of parameters belong the temperature of the catalytic bed, the temperature of PCZ, and the type of oxidant used for COC. The knowledge of the influence of temperature changes could be decisive for choosing the type of an integrated process, either with different or with uniform temperatures in both zones of the reactor. The replacement of air by oxygen would certainly lead to the desirable increase in concentrations of hydrocarbons in effluent gases. However, at the same time, it may affect the selectivity of COC by inducing undesirable oxidation reactions and influence the yields of pyrolysis products by changing the dilution in PCZ. Experimental studies

4196 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 3. Effect of Temperature on Naphtha Pyrolysis and on Integrated Process (Flow Rates: CH4 + Air = 3 mmol min-1, Naphtha = 60 mg min-1) series IV type of run temp, °C yield per pass, mg/100 mg of naphtha CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C4H6 C5-C6 C6H6 H2

series V

series VI

series VII

Pyr 720

COC + Pyr 720, 720

Pyr 760

COC + Pyr 720, 760

Pyr 800

COC + Pyr 720, 800

Pyr 800

COC + Pyr 800, 800

6.68 2.11 13.52 0.43 9.97 1.03 8.93 4.28 23.78 7.84 0.42

-0.94 2.87 16.52 0.31 10.67 0.87 11.76 5.34 22.81 6.94 0.58

10.88 2.79 21.07 0.50 12.67 0.74 7.73 5.53 13.54 10.91 0.64

2.84 3.30 23.39 0.34 12.63 0.56 7.26 5.48 12.59 11.33 0.79

14.28 2.82 26.05 0.43 11.42 0.39 4.45 5.42 5.76 16.15 0.80

5.88 3.35 28.63 0.27 11.57 0.28 4.43 5.31 6.60 14.85 0.97

14.71 2.66 26.39 0.38 10.12 0.28 3.59 4.86 7.30 12.05 0.83

3.15 2.78 27.90 0.17 9.77 0.11 3.73 4.32 5.12 10.13 0.93

Table 4. Effect of Oxidant in Catalytic Oxidative Coupling of CH4 on Integrated Processa series VIII type of oxidant flow rate of naphtha, mg min-1 yield per pass, mg/100 mg of naphtha CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C4H6 C5-C6 C6H6 H2 a

series IX

air 23.16

O2 19.82

-10.89 3.26 37.75 -0.07 10.26 0.17 2.52 5.55 3.66 19.85 1.21

-26.11 4.12 38.30 -0.02 8.46 1.70 5.02 4.99 27.76 1.55

series X

air 35.45

O2 41.89

air 60.59

O2 58.92

-2.00 3.32 33.07 0.11 10.58 0.21 3.28 5.29 4.97 19.09 1.04

1.86 4.15 32.93 0.36 10.04 0.27 2.88 4.81 5.87 25.12 1.20

4.78 3.43 28.16 0.46 10.95 0.25 4.12 5.14 8.23 17.11 0.96

1.53 3.82 28.83 0.29 9.89 0.21 3.05 4.39 3.92 18.24 1.01

Temperature: COC, 720 °C; Pyr, 800 °C. CH4:O2 ) 2:1. Feed rates: CH4 + air, 3 mmol min-1; CH4 + O2, 2 mmol min-1.

were therefore needed. The results of some experiments on the effects of the factors considered here are presented in Tables 3 and 4. The results in Table 3 show the effect of pyrolysis temperature on the product yields in the integrated process, both in the mild regime of pyrolysis (series IV and V) and in the severe “ethylene regime” (series VI and VII). In all cases, the integrated process gave significantly higher C2H4 and total C2 yields, as compared with pyrolysis alone. The yields of higher olefins, formed only from naphtha, were similar in pyrolysis and in the integrated process. The temperature dependencies were shown to be typical, resembling those observed for steam cracking itself. As seen from series IV and VII, a uniform temperature regime in both zones of the reactor could also be applied. The C2H4/C2H6 mole ratio was varied from about 5:1 at 720 °C in both zones to about 10:1 at 800 °C in both zones. The final choice of temperature profiles in both zones could be based on local conditions, desirable products, and other circumstances. The results in Table 4 confirm our previous findings concerning oxidants in COC of CH4 (Lachowicz et al., 1990). They show that the air could be replaced by O2, with practically no negative consequences on the yields. It was also found that in the integrated process composed of COC of CH4 and of naphtha pyrolysis, the air could be replaced by O2, giving, under identical conditions, practically the same yields of C2H4 and other products. Such results may indicate that the choice of oxidant could be based only on economic factors (the cost of air separation, the cost of product separation, etc.).

As shown before (Taniewski et al., 1994), the basic form of a one-stage process with its severe operating conditions in PCZ, as required by pyrolysis of C2H6 formed in COC, corresponded to an ethylene regime of C3+ steam cracking. This maximizes the yield of C2H4. The modification of this process with injection of C2H6 to PCZ had a similar character. The other modification, with injection of LPG to PCZ, provided the chance to achieve a limited product flexibility of the C2H4/C3H6 ratio. This modification of a one-stage process, with injection of liquid hydrocarbons to PCZ, would present, among the modifications considered until now, the highest flexibility of the C2H4/higher olefins ratio. This is especially true if carrying out the process in the unit which allows changes of the injection point location and, therefore, of the residence time. As pointed out previously, one of the characteristic features of the integrated process is that methane could be either consumed or produced. In the latter case, it would be possible to make the process independent of the outside sources of methane. The net yield of CH4 in the integrated process depends, obviously, on its consumption in the COC step, being proportional to the total flow rate of the CH4 and on its formation in pyrolysis, which is proportional to the flow rate of naphtha (Table 1). Under the experimental conditions shown in Table 1, the consumption of CH4 prevails over its production below the naphtha/CH4 feed flow rate ratio of about 3 mg/mg, whereas the production of CH4 prevails over consumption above this value. The changes in absolute values of the flow rates would obviously alter the course of the process (conversion, yields). The

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4197

changes in conversion of methane in COC, defined as the mass of reacted CH4/mass of introduced CH4 ratio, or the changes in the yield of CH4 in pyrolysis, expressed as the mass of formed CH4/mass of introduced naphtha ratio, would influence the net production of methane in the integrated process. Therefore, the value of 3 mg/ mg should be treated only as an approximate order of magnitude estimate of the border ratio value. One of the obvious advantages of the integrated process would be the possibility to utilize the heat of exothermic reactions in COC for endothermic pyrolysis. Calculations based on typical results of COC of CH4 (Table 1, series II) gave ∆H993 ) -12.2 kJ/g of introduced CH4 (-30.9 kJ/g of reacted CH4). Calculations of the heat demand for pyrolysis, assuming n-decane to be the applied feed, show that pyrolysis requires about 0.66 kJ/g of the feed. Thus, one can conclude that the amount of naphtha could be about 18 times greater than the amount of methane. However, it must be noted that this value indicates only an order of magnitude, since a heat demand for heating and evaporation was not taken into account in the calculations for the sake of simplicity. For such a high naphtha/methane ratio, the COC process would serve only as a source of a heat, with only negligible influence on the total ethylene yield. Much more beneficial is the type of integrated process with moderate values of naphtha/methane ratio, which may give all possible positive effects, such as an effective heat utilization, an increase in C2H4 yield, a balance between CH4 formation and consumption, etc. Conclusions This study examined the rationale of an integrated one-stage process of ethylene production from methane and naphtha, consisting of exothermic oxidative coupling of CH4 over the catalyst fixed bed and endothermic pyrolysis of naphtha and of the product C2H6 in the postcatalytic zone of the same reactor. Within a range of operating parameters (CH4/naphtha ratio; temperature of the catalytic and postcatalytic zones; flow rates of reagents), it was demonstrated that the concept is realistic. Additivity of the yields of oxidative coupling and pyrolysis was observed, showing that under experimental conditions the losses in PCZ of the products formed in COC are very low and pyrolysis in PCZ is not affected by the effluent gases from COC. High yields obtained in naphtha pyrolysis indicated that applied dilution with nitrogen or with effluent gases from COC corresponded to the conditions of conventional steam cracking. Desirably high C2H4/C2H6 ratios were obtained in the integrated process. The increase in C2H4 yields in the integrated process above the yields in pyrolysis of naphtha was proportional to the contribution of a coupling component. Both air and oxygen can be used as oxidants in the coupling step. Depending

on the relative contributions of the component coupling and pyrolysis processes to the integrated process, the overall balance of CH4 could be negative (consumption) or positive (production). In the latter case, the integrated process could be based only on the liquid feedstock, with a recycle of the excess CH4. Acknowledgment Financial support from the Polish Scientific Research Committee (Research Project 3 P405 004 07) is gratefully acknowledged. Literature Cited Edwards, J. H.; Do, K. T.; Tyler, R. J. Reaction Engineering Studies of Methane Coupling in Fluidised-bed Reactors. Catal. Today 1990, 6, 435. Edwards, J. H.; Do, K. T.; Tyler, R. J. The OXCO Process. A New Concept for the Production of Olefins from Natural Gas. Fuel 1992, 71, 325. Hoebink, J. H. B. J.; Venderbosch, H. M.; van Geem, P. C.; van den Oosterkamp, P. F.; Marin, G. B. Economics of the Oxidative Coupling of Methane as an Add-on Unit for Naphtha Cracking. Chem. Eng. Technol. 1995, 18, 12. Lachowicz, R.; Skutil, K.; Taniewski, M. Oxidative Condensation of Methane to Ethylene on Lithium-Magnesium Catalysts. Neftekhim. 1990, 30, 656. Lachowicz, R.; Skutil, K.; Taniewski, M. Technological Aspects of Methane to Ethylene Transformation. Khim. Tekhnol. (Kiev) 1992, 1, 3. Mimoun, H.; Robine, A.; Bonnaudet, S.; Cameron, C. J. Oxidative Coupling of Methane Followed by Ethane Pyrolysis. Chem. Lett. 1989, 2185. Mimoun, H.; Robine, A.; Bonnaudet, S.; Cameron, C. J. Oxypyrolysis of Natural Gas. Appl. Catal. 1990, 58, 269. Taniewski, M.; Lachowicz, R.; Skutil, K. Studies on Oxidative Coupling of Methane to Ethylene over Lithium-Magnesium Catalysts. Przem. Chem. 1990a, 69, 541. Taniewski, M.; Skutil, K.; Lachowicz, R. Verification of the Reaction Scheme for the Oxidative Coupling of Methane over a Li/MgO Catalyst. Chem. Stosowana 1990b, 34, 215. Taniewski, M.; Skutil, K.; Lachowicz, R.; Lachowicz, A.; Dudek, B.; Czechowicz, D. Transformations of the Products of Methane Oxidative Coupling in the Post-Catalytic Zone of the Convertor. Catal. Today 1992, 13, 529. Taniewski, M.; Skutil, K.; Czechowicz, D. Possible Contribution of Oxidative Transformation and Decomposition of Hydrocarbons in the Zone of Pyrolysis of Methane Oxidative Coupling Reactor. Pol. J. Appl. Chem. 1993, 37, 109. Taniewski, M.; Lachowicz, A.; Lachowicz, R.; Czechowicz, D.; Skutil, K. Ethylene from Methane by a One-Stage Process: Product Distribution along a Tubular Reactor. Ind. Eng. Chem. Res. 1994, 33, 185.

Received for review November 8, 1996 Revised manuscript received June 16, 1997 Accepted June 23, 1997X IE960711Z

X Abstract published in Advance ACS Abstracts, August 15, 1997.