Ind. Eng. Chem. Res. 1994,33, 185-190
185
Ethylene from Methane by a One-Stage Process: Product Distribution along a Tubular Reactor Marian Taniewski,' Alfred Lachowicz, Rita Lachowicz, Dymitr Czechowicz, and Krzysztof Skutil Silesian Technical University, 44-101 Gliwice, Poland
-
A one-stage process involving CH4 catalytic oxidative coupling over Li/MgO at -730 "C t o C2 hydrocarbons and thermal decomposition of the formed C2H6 to C2H4 at 850 "C in the postcatalytic zone (PCZ) was studied in a multisectional stainless steel tubular reactor of a scaled-up laboratory unit. Alternative forms of the process with admission of C2H6 or C3Hs t o PCZ were also examined. Product distribution along the catalyst bed and along the PCZ in the large steel tubular reactor was determined and, on this ground, a verification of a one-stage concept provided. A one-stage process was shown to be generally acceptable. It was confirmed, however, that severe operating conditions applied in PCZ for securing effective C2H6 dehydrogenation increased C2H4 decomposition to H2 and a coke deposit, COZ reduction to CO (by reactions with H2, a coke, and hydrocarbons), and some other surface-sensitive transformations. Under applied operating conditions, the C2H4/C2H6ratio was increased from 1.52 (standard coupling) to 4.89 (one-stage process), whereas the total C2 yield decreased from 15.4% t o 13.596, respectively. Introduction Conversion of CHI to C2 hydrocarbons by catalytic oxidative coupling (COC) has attracted wide attention as a potential route to C2H4 or fuels, especially after the appearance of the first promising reports in the mid-80's by Keller and Bhasin (1982),Hinsen et al. (19841, Ito and Lunsford (1985), and Otsuka et al. (1985). Since then, many hundreds of research papers, mostly concerned with catalysts and the mechanism of this complex heterogeneous-homogeneous process, have been published and already reviewed by Sinev et al. (19891, Anderson (19891, Hutchingset al. (1989),Amenomiyaet al. (1989),Lunsford (1990), and some others. However, the development of the industrial process still remains in its very early stage with the main efforts focused on the search for a sufficiently selective, active, and stable catalyst. Only modest results have been achieved in this field (CZyield usually about 20%, sometimes up to 30%,CH4 conversion 20-30% and C2 selectivity 70-80%). There is a controversy as to the chances to improve these results, which is based on opposing views about the appearance or absence of the selectivity and yield limitations. It is likely that this problem will be soon clarified. Some research groups, including ours, already undertook preliminary studies on certain technological engineering aspects of the future process. One of them concerns the method of the transformation of C2H6, coproduced with C2H4 by COC, to CzH4. The prospects for such a COC process which would lead exclusively to CzH4, without C2H6, are not very bright, in spite of some progress achieved with chlorinated catalysts or additives, a high O2/CH4 ratio, and high temperatures, etc., as reported by Otsuka (1987), Burch et al. (19881, Lachowicz and Taniewski (1988a, 1988b), Ahmed and Moffat (1989 and other papers), Taniewski et al. (1990a), Lachowicz et al. (1990), and others. A typical process would be therefore, multistage, involving CH4 COC, C2H6 isolation, and C2H6 steam cracking (or oxidative dehydrogenation) carried out in separate installations. A concept of a one-stage process, with both reaction steps, Le., CHI COC to CzH4-C& mixtures and thermal
* To whom correspondence should be addressed.
decomposition of C2H6 to C2H4 carried out in the same reactor was put forward by IFP under the name of "oxypyrolysis" (Mimoun et al., 1989, 1990), by CSIRO/ BHP-"OXCO" (Edwards et al. 1989,1990,19921,and by our group (Taniewski et al., 1990a, 1992,1993; Lachowicz et al., 1990, 1992). Such a process, as compared with a multistage technology, could be attractive for its lower capital and operating costs. One can expect, e.g., lower overall energy consumption due to higher efficiency of a heat utilization (direct utilization of a heat of exothermic CHI oxidative transformations by endothermic C2H6 dehydrogenation). A heat of COC can be also utilized for decomposition of the hydrocarbons injected from the outside to the postcatalytic zone (PCZ), e.g., of recycled CzH6, of LPG, etc. Possible forms of such a process may differ in reactor design (tubular and fluid bed, etc.), temperature level and profile (e.g., 1850 "C in the whole reactor or only in the COC area and adiabatic in the PCZ and 1800 "C in the COC area and nearly isothermal 1850 "C in the additionally heated PCZ, etc.), and in other aspects. The investigations by CSIRO/BHP were concerned mostly with a one-stage fluid-bed process, whereas by IFP and by ourselves, with a one-stage process carried out in a tubular reactor. Our previous studies aimed at giving more insight into the COC mechanism and into the nature and the significance of various contributing reactions in PCZ under the severe operating regime necessary to secure a high C2H6 to C2H4 conversion (Taniewski et al., 1990b). It was obvious, that the real situation in PCZ would be different from that in the conventional C2H6 steam cracking. Instead of large amounts of steam water, with its peculiar, beneficial role, the effluent gases from the COC area would contain, as diluents of C2H6, mainly hydrocarbons (CHI, CzH4,and C3+),Hz, CO,, and onlyminor amounts of formed H2O. In the series of our papers (Taniewski et al., 1992, 1993) were reported the results of studies on the main contributing reactions in PCZ, such as transformation of C2H6 to C2H4 and Ha, decomposition of hydrocarbons to H2 and a coke deposit, conversion of hydrocarbons by HzO and COz,reduction of C02 by Hz and by coke deposit, and the water-gas shift reaction, etc. Particular attention has been paid to the reactions not studied before, at least in this context, such as decomposition of C2H4 and other
0888-~885/94/2633-0~85~04.50/00 1994 American Chemical Society
186 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
- -
=tl OTHERS
Figure 1. Experimental apparatus (description in text).
hydrocarbons to the elements, affected by various investigated surfaces and reactions of gasification of a coke deposit, etc. Extending our initial findings (Taniewski et al., 1992, 19931, the present study was undertaken. For a final verification of a one-stage tubular concept, it was found necessary to examine it on a somewhat larger scale and to study the product-distribution changes occurring in the course of the process, i.e., along the COC area and particularly along the PCZ. The results of such observations carried out in a specially designed scaled-up laboratory demonstration unit are the subject of a present study. A stainless steel reactor was applied as a model of the future industrial reactor. No attempts were undertaken at this stage to maximize the absolute values of the yields, as could be possible by a choice of the catalyst, optimization of the parameters, or application of the most inactive construction material. It was hoped that observed regularities and general conclusions drawn from this study would be valid also for other catalysts and for other regimes of COC than those applied in this work.
Experimental Section Apparatus. A scaled-up laboratory unit was composed of the feed-gas premixers, a reactor with necessary controlling devices, and facilities for feeding and sampling (Figure 1). Premixing of the feed components operation was carried out in three carbon steel pressure vessels (PV) at 30 dm3 each, equipped with a special system of valves. Mixtures of strictly defined compositions were prepared here under pressure up to 2 MPa. A block of premixers was connected with a stainless steel reactor (R) via a reducing valve (RV), a feeding control valve (FCV), and an electrical heater (PEH). The feeding system was equipped with a flow meter (FM), a gas meter (GM), a water U-tubemanometer (M),andawatersafetyseal(WS). A heater (PEH), used for preliminary heating of the feed, was a tube inserted into a 2-kW resistance furnace equipped with temperature controlling (TC) and automatic regulating (ARS) devices. The reactor was a 38.3-mm 0.d. (32.3-mm i.d.),650-mm long stainless steel tube (1H18N9T) with specially constructed heads on both ends and with inserted concentrically a 6-mm 0.d. thermowell containing NiCr-Ni thermocouple, movable along all the reactor length. In the various points of the reactor (Figure 21, there were welded into the wall five stub pipes (6-mm o.d., 3-mm i.d., 120-mmlong), reaching the reactor axis. By this way, the samples for analysis were taken out of the reactor center
Td A
Temperature, ' C 790 800
900
p" I
_ _ _ _ _ 56 _ _ _ _ 600
700 800 900 Temperature, "C
Figure 2. Construction of the reactor and applied temperature profiles along its length (see text).
or some additives were introduced to the reactor. The sampling point locations enabled the observations of the composition changes along the COC zone (S 1-4) and the PCZ (S 5 and S 6). The reactor was heated by five independent heating coils of 1000-1300W each (EH 1-5), with separate automatic regulation and controlling systems. Additional heating of the reactor heads was applied to compensate heat losses. Due to a flexible heating system, desirable temperature profiles (see below) were obtainable. The effluent gas was cooled in an air cooler (AC)and a water cooler (WC) and then sent to a gas meter and collector (SC).
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 187 Catalyst. A 1% Li/MgO catalyst prepared as described previously (Taniewski et al., 1990b) was applied in the series of runs with a catalyst bed. A 32.5-cm long catalyst bed contained 274 cm3 (360 g) of a catalyst (1.0-1.5 mm pellets). It was supported on a steel net made of the same stainless steel as the reactor and on a quartz wool. A small precatalytic zone was packed with quartzite chips. PCZ was left unpacked. Feed. Mixtures were prepared from the following components: CHI, containing ( ~ 0 1 % ) 97.42 CH4,1.29 N2, 0.90 C2H6,0.191 C3Ha,0.076 C4H10,0.025 C5H12, and 0.04 c6+; C2H6, pure grade, >99%, supplied by Aldrich Chemical Co.; C2H4,polymer grade, >99.9%, supplied by MZRiP Plock; C3H8, pure grade, >99%, supplied by Fluka AG; and air, N2, H2, and C02, technical grade. Procedure. Experiments were carried out in the reactor used either empty or packed with a catalyst in its upper part (Figure 2). Air was used as an oxidant (02/CH4molar ratio about 0.5, i.e., outside the explosive limits). The feed flow rate was approximately 2.74 dm3 (STP) min-l (GHSV about 600 h-l, a residence time in PCZ about 1.3 SI.
Three different temperature profiles were applied (Figure 2) as follows: profile 1-nearly isothermal (-730 "C) in an upper zone of the reactor and nearly isothermal (-850 "C) in its lower part (a profile for the comparative noncatalytic runs carried out in an empty reactor), profile 2-as profile 1but with the hot spot in the upper part (a profile of the basic runs carried out in the reactor with a catalyst bed and a "hot" PCZ), and profile 3-as profile 2 in the upper part of the reactor but with nonisothermal, not heated PCZ (a profile for the comparative runs in the reactor with a catalyst bed and a "cold" PCZ). Analysis. Gas-tight syringes (20-50 cm3) with 130170-mmlong needles were applied for sampling to enable axial gas concentration determinations. Gas pipettes were used for analyses of the feed or the collected products. A gas analysis was carried out with the use of a HewlettPackard 5890 Series I1 gas chromatograph equipped with TCD, FID, computer, and HP ChemStation G 1201A. An activated carbon column (3-mm i.d., 1.8 m) was used for determination of H2, air, CO, and C02. A column (5-mm i.d., 2.2 m) packed with A1203-KzC03 was applied for determination of CH4, CZH6, C2H4, C3H8, C3H6, C4H10, C4Ha, and C4H6. 0 2 and N2 were determined in a column with molecular sieves 5A (4-mm i.d., 3.5 m). No water measurements were conducted.
Results and Discussion Tables 1-6 summarize the main results of this study. Being an extract from the full set of obtained experimental results, they contain only some selected information concerning the dominant products. All data are presented in the form of the yields per pass expressed in mol (of the formed products but also of the unconverted feed component)/100 mol of the basic component of the feed introduced to the reactor. They were calculated from the equation: yield of i per pass Yi= where VOis the feed flow rate, dm3 (STP)min-'; Vxis the reaction mixture flow rate at the sampling point, dm3(STP)min-1; ni is the volume fraction of i in the reaction mixture at the sampling point; and noy is the volume fraction of the basic component y in the feed. The values
Table 1. CHI Oxidation in Empty Reactor run number 1 temperature profile, number feed composition, vol %
1
30.68 68.90 2.190 2.020
CH4 air feed flow rate, dm3 min-l effluent gas flow rate, dm3 min-1 yield per pass, mo1/100 mol of CH4 inlet (feed) sampling point 1
2 3 4 5 outlet (collected)
CHI
coz
co
te
tr tr
100.00 99.07 100.80 96.67 95.13 82.43 69.96
tr tr
1.63 3.43 7.17
2.60 5.10 11.90 22.37
t r = traces.
nw and ni were-determined from the analyses, VOwas measured, and Vxwas calculated from the Nz balance. Concentrations of the components, the yield obtained from converted feed, and the selectivities,etc., can be easily calculated from presented data. For example, CH4 conversion = 100 - YcH,(% ) C2selectivity =
IC, 100 (%I 100 - YCH,
C2yield = 2 Yc,( % ) In such cases when feed conversion was negligibly low and error in calculated yield would be very high, a mark, nc (not calculated), was put into the yield column of the table. The yields in the tables are related, in each case, to the indicated sampling points and sometimes also the indicated time of experiment. At the end of each table are given the overall yields calculated from the collected effluent gas. Preliminary blank runs confirmed high stability of CH4 and CO2 in an empty reactor under applied conditions (temperature profile 1)as well as CH4 stability under the same conditions in an empty reactor precovered internally with a coke deposit of C2H4 origin. It has to be noted, however, that in some cases, decomposition of CH4 may occur in this temperature region as a result of a specific wall effect (Taniewski et al., 1992, 1993). In the presence of applied amounts of air (run 1, Table 11, in an empty reactor, CH4 was still practically stable in a low-temperature zone simulating the COC regime but was markedly converted to CO, C02, H2, and H2O at higher temperatures corresponding to the PCZ conditions. Such results confirm the following conclusions: (i) PCZ ought to be oxygen-free (for detailed quantitative examination of the effect of 0 2 on CH4, C2H6, and C2H4, see Taniewski et al., 1993), (ii) CH4 activation in the COC process is predominantly catalytic rather than homogeneous, and (iii) CO dominates over COz among CO, formed in a gas phase under applied conditions (cf.Lachowicz at al., 1990). Run 2 (Table 2) shows that under experimental conditions, in an empty reactor, C02 reduction by H2 is slow in a low-temperature area and much faster in a hightemperature zone. As expected, ACO2:AHz:ACO = l:l:l, and the volume of noncondensable gases decreases in the course of the reaction. Run 3 (Table 2) clearly demonstrated that in a high-temperature zone of an empty
188 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 2. COz Reduction by H2 and Coke Deposit in Empty Reactor run number 2 temperature profile, number feed composition, vol %
1
con
1
10.42 8.58 81.00 2.186
Hz Nz
feed flow rate, dm3 min-l effluent gas flow rate, dm3 min-l yield per pass, mo1/100 mol of COz inlet (feed) sampling point 1 2 3 4 5 6 outlet (collected) a
run number 3
2.111
coz
19.47
co
coz
nc nc 74.12 69.57 65.62
1.62 2.83 4.15 10.31 13.65
55.31
23.96
nc nc nc 87.39 35.26 4.37167.03" 31.77
Hz
100.00
82.31
nc nc 95.96 93.93 85.04 70.88
100.00
80.53 2.108 2.299
co nc 0.76 2.89 16.59 133.24 189.63162.61" 142.25
After 30 min.
Table 3. Cd€r Decomposition in Empty Reactor run number 4 temperature profile, number feed composition, vol ?& CZH4
1
7.22
Nz
92.78 1.250 feed flow rate, dm3 min-1 1.250 effluent gas flow rate, dm3 min-1 yield per pass, moll100 mol of CzH, CzHd inlet (feed) 100.00 sampling point 88,83192.08" 3 74.94182.320 4 53.10174.93" 5 23.33/69.39 6 61.04b/69.13c outlet (collected)
Hz 9.43/5.01° 32.26/17.15O 66.50/23.48a 82.38135.36" 52.61bI32.72'
*
0 After 60 min. Collected between 0 and 90 min. Collected between 90 and 120 min.
precoked reactor (by prolonged C2H4 treatment), C02 intensively reacted with a coke deposit (cf. Taniewski et al., 1992, 1993). The rate of this reaction declined with time, apparently with the consumption of a coke, or at least, of its more active forms. An expected increase of gas volume was observed in this case. The found ratio ACOZ/ACO N 0.5 corresponded to a simple stoichiometry, COZ + C = 2C0, which indicated by the way, that in this particular case a coke was rather poor in hydrogen. The results in Table 2 confirmed that C02 formed in the COC zone by oxidation of the radicals or of CO on the surface of the catalyst having oxidative properties (cf. Taniewski et al., 1990b) or by catalytic conversion of CO with formed HzO (Lachowicz et al., 1992) would be subsequently partly reduced in PCZ to CO by action of H2 and a coke deposit (and possibly hydrocarbons as well). The results (not included in the tables) of a run conducted with a C02-Hz-N2 mixture passed through a precoked reactor fully confirmed the view about C02 parallel reduction pathways. Run 4 (Table 3) carried out with C2H4 in a fresh empty reactor illustrated a course of precoking operation and a behavior of C2H4 at experimental conditions. A strong dependence of the rate of C2H4 disappearance on the temperature and on the character of the surface was observed. C2H4 was decomposing less intensively in a lowtemperature zone and faster in the zone simulating PCZ (HZbeing a major gaseous product in both cases). C2H4conversion degree and parallel H2-formation yield grew along the reactor length and declined with time (Table 3), apparently as a result of a coke deposition on the active metal surface. C/H balance indicated that in this run a
deposit (CH,) still contained a certain amount of hydrogen. The results of run 4 demonstrated a possible level of C2H4 losses in PCZ and the significance of the catalytic coking effect exerted by some surfaces, at least until a coke layer was deposited. Some losses of C2H4 in the one-stage process, occurring at elevated temperatures in PCZ, are probably connected, among other reasons, with a long residence time in PCZ of this part of C2H4 which was formed in the COC area (residence time of C2H4 gradually formed in PCZ from C2H6 being, naturally, shorter). A special run (not included) with an injection of the same amount of CZH4 as in run 4 to the Nz stream in point 5, thus simulating the behavior of C2H4 forming in PCZ (points 4-6), demonstrated a much lower level of the losses (-6%). The next series of experiments were conducted in the reactor packed, in its upper part, with a catalyst bed (Figure 2). Three forms of the one-stage CH4 transformation process were examined, a basic one (Table 4, run 5), one modified by injection of CzH6 to PCZ (Table 5, run 7), and one modified by injection of CBHS(representing also LPG) to PCZ (Table 6, run 10). In all runs, carried out in the presence of air, practically full 0 2 consumption in the COC area was observed. In Table 4, the one-stage process (run 5, profile 2) was compared with standard COC (run 6, profile 3). As shown, in run 6 (not heated PCZ), no changes in concentration of C2H6, C2H4, H2, C02, and CO were observed along PCZ, thus indicating a negligible contribution of C2H6 dehydrogenation and hydrocarbon decomposition, as well as of C02 reduction. In run 5, desirable C2H6 to CzH4 conversion took place, which led to a significant increase in the C ~ H ~ / C ~ ratio. H G A total C2 hydrocarbons yield in run 5 was, however, somewhat lower (at about 15%)than in run 6, evidently mainly as a result of partial CZH4 decomposition in PCZ (as indicated also by excess of Hz). A weak COZreduction to CO was observed in this case. Table 5 presents the results of the one-stage process with addition of C2H6 to point 4 of PCZ (run 8)as compared with the simultaneously studied basic mode of the process (run 7, analogous to run 5) and with the blank run 9 added in point 4 of PCZ to the N2 stream passed through the COC zone). As demonstrated, (i) the CH4 ultimate yield in run 8 was somewhat higher (conversionwas lower) than in run 7, apparently due to some CHI formation in the initiation step of CzHs-chain decomposition; (ii) the C2H6 yield was very low and CzH4/C& ratios were very high in all three runs, thus indicating that sufficiently severe conditions were applied in PCZ; (iii) the C2H4 yield
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 189 Table 4. Comparison of Catalytic Oxidative Coupling of CHI with or without Transformations in Postcatalytic Zone
temperature profiie, number feed composition, vol % CH4 air feed flow rate, d m 3 min-1 effluent gas flow rate, d m 3 min-1 yield per pass, moVl00 mol of CH4 inlet (feed) sampling point 1 2 3 4 5 6 outlet (collected) fiial Cz yield per pass, mo1/100 mol of C& fiial CzHJC2& ratio fiial CZyield, %
c&
c2&
65.34 66.14 65.70 66.16 66.47 65.62 65.93
3.21 3.31 3.21 3.03 1.80 1.10 1.15
run number 5
run number 6
2
3
30.32 69.27 2.88 2.49
30.32 69.27 2.83 2.33
CZH4
H2
C02 CO CH4 C2& 100.00 0.99
CzH4
Hz
COz CO
4.54 4.52 5.00 4.82 5.75 5.41 5.62
2.82 1.33 0.97 2.10 5.16 12.16 11.44
65.74 20.0 0.8 66.14 19.9 0.4 65.09 1.0 64.02 19.4 1.5 64.13 19.5 4.2 64.28 63.52
4.39 4.44 4.62 4.57 4.47 4.62 4.65
3.29 0.99 0.84 1.46 0.97 0.97 0.97
20.4 20.2 19.4 20.3 20.1
100.00 1.00
3.11 3.24 3.11 3.00 3.03 3.03 3.05
6.77 4.89 13.5
7.70 1.52 15.4
Table 5. One-Step Process with Injection of CsHt to Postcatalytic Zone run number 7 run number 8 temperature profiie, number feed composition, vol % CH4 air
2
2
32.51 67.04
32.51 67.49
run number 9
2
100.00 14.07
N2
Cz& added to sample point 4, moVl00 mol of C& yield per pass, mo1/100 mol of C& inlet (feed) sampling point 1 2 3 4 5 6 outlet (collected)
14.07 CHI C2& 100.00 0.99
C2H4 HZ
64.47 65.01 63.57 64.71 63.33 64.02 64.54
5.81 5.38 5.26 5.48 6.23 6.08 6.05
3.95 3.57 3.25 3.20 1.71 1.04 1.02
1.54 0.97 0.82 1.74 4.96 11.36 11.84
C& C2H6 100.00 0.98
C2&
64.92 3.34 inj 64.84 5.20 66.61 1.77 67.47 1.91
5.40 0.86
H2
CHI CzHs
inj 16.27 20.68 0.80 2.57 14.87 26.59 1.32 0.65 14.86 26.59 1.40 0.65
Table 6. One-Step Process with Injection of CsHe to Postcatalytic Zone run number 10 temperature profiie, number feed composition, vol % CH4 air Nz CsHe added to sample point 4,moUl00 mol of CH4 yield per pas, moVl00 mol of C& inlet (feed) sampling point 3 4 5 6 outlet (collected)
0.7 0.4 0.6 0.5 0.4
C2H4 HZ
10.29 13.44 9.82 16.09 9.92 16.64
run number 11
2
2
32.30 67.26 100.00
C& CsHe 100.00 0.25 0.97 65.91 71.95 76.11 74.54
0.17 inj 0.55 0.10 0.10
in run 8 was markedly increased due to C2H6 injection, its value being close to the s u m of the yields obtained in runs 7 and 9;and (iv)the estimated C2H4 selectivityfrom added C2H6 in runs 8 and 9 was equal to approximately 70%. Table 6 contains the results of an analogous series of runs with C3H8 admitted to the product stream in PCZ (run 10) and with C3He injected into the N2 stream (a blank run, 11). For the sake of space, there were not included in Table 6 the results of the simultaneously conducted run examining the basic form of the one-stage process (the results being very close to those of run 7). As expected, C3H8almost fully decomposesin PCZ, the major products being C2H4 and CHd (- 1:l). The very low C3H6 yield, declining with the reactor length (residence time), suggests that, under applied severe operating conditions,
3.36
5.35
0.27
1.54 0.99 0.94
12.26 1.12 15.74 0.77 15.42 0.77
0.87 inj 15.81 12.59 1.48 25.21 11.79 0.15 24.37 9.17 0.09
13.77 11.19 8.55
2.02 0.59 0.39
9.02 10.90 8.67
the C3& formed rapidly decomposes (and perhaps in C3& decomposition dominates the route to C2H.4 over the parallel route to C3H6). The C2H4 yield in run‘l0 and its estimated selectivity from C3H8 were reasonably high. As expected, no similar decrease in C2H6 conversion as observed earlier at much lower temperatures with CrHlo admixtures (Taniewski et al., 1992) was noticed. Highly severe operating conditions which should be applied in PCZ for securing the extensive pyrolysis of C2& to C2H4 correspond, for possible C3+ hydrocarbons admixtures, to an “ethylene regime” of steam cracking, maximized for C2H4 production. Such conditions are created by high temperature, long residence time, low partial pressure of decomposed hydrocarbons, and perhaps also the accelerating effect of decomposing C2H6 on the
190 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
decomposition of higher alkanes. The selection of sufficiently low located injection points for admission of C3+ hydrocarbons to PCZ (shorter residence time) could be the simplest way to make operating conditions less severe and, as a consequence, to change the product distributions toward higher unsaturates.
Conclusions This study examined the changes in the reaction mixture composition along the flow tubular reactor in which a onestage process of CHI to CzH4 conversion, being composed of COC of CHI and of thermal dehydrogenation of CzH6, was carried out. In a scaled-up unit with a stainless steel reactor, a rationality of a one-stage concept has been verified. At sufficiently severe operating conditions in oxygen-free PCZ (-850 OC, residence time -1.3 s), a significant part of the CzH6 formed in the COC area (and, if wanted, also of recycled C ~ H or G other CZ+introduced to PCZ from the outside) could be effectively converted to CzH4. Whereas the main source of CzH4 losses in the COC area is its oxidation, some losses of CzH4 observed in PCZ are caused by its undesired thermal transformation, mainly decomposition to a coke deposit and Hz. These losses could be limited by optimization of the operating parameters and by a precoking operation, but some would remain (e.g., those from the contribution in the decomposition of this part of CzH4 which was formed in the COC zone and would have the longest actual residence time in PCZ). The one-stage process with its severe operating conditions in PCZ, as required by C2H6 pyrolysis, corresponds to a steam-cracking regime of high severity, maximizing the yield of CzH4 (from LPG or liquid hydrocarbons). In the case of a one-stage process with injection of C3+ hydrocarbons to PCZ, a limited product flexibility concerning the CzHdhigher olefins ratio could be achieved, e.g., by changing the location of the injection point.
Acknowledgment Participation of Mr. L. Orlidski in the experimental part is acknowledged with thanks. Financial support from the Scientific Researches Committee (Research Project 7 0214 91 01) is gratefully acknowledged. Literature Cited Ahmed, S.;Moffat, J. B. The Enhancement of the Oxidative Coupling of Methane on Oxide/SiOz Catalysts by Tetrachloromethane. Catal. Lett. 1989,2,309. Amenomiya, Y.;Birss, V. J.; Golendzinowski, M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal.Rev.Sci. Eng. 1990,32, 163. Anderson, J. R. Methane to Higher Hydrocarbons. Appl. Catal. 1989,47,177. Burch, R.; Squire, G. D.; Tsang, S. C. Comparative Study of Catalysts for the Oxidative Coupling of Methane. Appl. Catal. 1988,43, 105.
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Received f o r review October 1, 1993 Accepted October 31, 1993' e Abstract published in Advance A C S Abstracts, January 1, 1994.
