Znd. Eng. Chem. Res. 1991, 30,1081-1086
1081
The Uno-Quattro Coil: High Severities for Increased Ethylene Selectivity Patrick M. Plehierst and Gilbert F. Froment* Laboratorium voor Petrochemische Techniek, Rijksuniuersiteit Gent, Krijgslaan 281, B9000 Gent, Belgium
The Uno-Quattro (UQ) reactor, a new cracking coil for olefin production, combines short residence time with an appropriate process gas temperature profile, so as to achieve a favorable product distribution and a high ethylene selectivity in particular. The present paper discusses the results of a pilot test program. Aspects of coke formation are addressed as well. Introduction The advantages of cracking at short residence time and relatively high temperatures are well known. Moreover, Van Damme et al. (1984) and Plehiers and Froment (1987a),among others, have shown that a considerable gain in selectivity can be achieved by the proper combination of the process gas temperature profile and residence time in the cracking coil. A linear process gas temperature profile, with which temperature rises linearly with the reactor coordinate, leads to higher selectivities than the common, convex temperature profile, which achieves the major part of the temperature rise in the first section of the reactor. Obviously, combining a short residence time with a linear process gas temperature profile would be very advantageous. In a pilot-plant reactor, like the one described by Van Damme and Froment (19821, linear process gas temperature profiles can easily be achieved by modifying the heat input in the individually fired cells of the furnace. In commercial units, however, the coils are suspended in a furnace that has little or no flexibility in the firing pattern. Also, firebox simulations (Plehiers and Froment, 1989) indicate that a substantial modification of the process gas temperature profile inside the coil by changing the firing pattern is very difficult. Moreover, applying increased heat flux densities in the last part of the reactor leads to unfavorable operation with respect to tube metal temperatures and coking. As these simulations show, the process gas temperature profile is mainly determined by the geometry of the reactor. For achieving a linear temperature profie, a design must be found that allows for a high heat input in the part of the coil, without using high heat fluxes. The new UQ coil (from Uno-Quattro) achieves this by making use of the reversed split coil concept, proposed by Plehiers and Froment (1987a). It is a two-pass coil that combines the advantages of a short residence time and an almost linear temperature profile. A pilot model of the UQ coil has been designed, constructed, and tested in the Laboratorium voor Petrochemische Techniek (LPT). It is described in the next section. The main features of the laboratory's pilot unit are described by Van Damme and Froment (1982),while a detailed description of the analysis train is given by Dierickx et al. (1986). In the test program, the flow and heat-flux distributions in the reactor have been investigated. Product distributions for naphtha cracking have been obtained and compared with an extensive data base available in the LPT. Aspects of coke formation are discussed as well. Description of the UQ Pilot Reactor The LPT pilot furnace consists of seven cells that can be fired independently (Figure 1). In cell 1,the incoming 'P.M.P. is a Senior Research Aasiitant of the Belgian Nationaal Fonds voor Wetenschappelijk Onderzoek.
hydrocarbons and the dilution water are evaporated and superheated. They are further preheated and niixed in cell 2. In cell 6, the reaction mixture is heated to the coil inlet temperature, between 400 and 500 OC. The UQ reactor proper is located in cell 7. A more detailed representation of the UQ reactor is given in Figure 2. The first part of the reactor consists of a single tube with an internal diameter of 10 mm. It is 1853 mm long. Since the length of the tube is fixed by the furnace geometry, the only way to limit the heat transfer to this section was to surround the tube with a layer of refractory brick of about 20-mm thickness. The thermal conductivity of the brick is 0.5 W/(mK). In the second half of the reactor, the process gas flow is distributed over four parallel tubes. The internal diameter of these tubes is 10.75 mm (1/2 in. Sch 80). The total length of the reactor is 3890 mm. The process gas temperature is measured by means of 15 thermocouples. The temperature is measured at the coil inlet, T,, at the end of the low-temperature tube, T2, at the coil outlet, T15,and in three locations in each of the four high-temperature tubes, T3to TI&The temperature at the end of the low-temperature tube, T2, is kept below 680 "C, so that fouling in the restrictors (see below) is minimal. This is done by adapting the heat supply to cell 6, The pressure is measured at the coil inlet and outlet. Maldistribution of flow and heat in the four high-temperature tubes would lead to differences in temperature, conversion, and rate of coke formation, which would be harmful to a smooth operation. Extreme care was taken to achieve identical flow distributions and heat-flux profiles for each of the four tubes. Flow restrictors are inserted at the inlet of the hightemperature tubes to distribute ,the flow evenly over the tubes. They are 50 mm long and have an internal diameter of 5 mm. The restrictors are designed to create a pressure drop of 8-10 kPa in normal operation. The four hightemperature tubes have an estimated pressure drop of 3 kPa. Restrictors were preferred rather than critical flow venturis for two reasons. In the first place, the nozzle diameter of the latter would be only 1.5-2.0 mm, making them very sensitive to fouling. Also, the inlet pressure of the critical flow venturis, i.e., the pressure in the lowtemperature tube, would be too high. This would lead to an unnecessarily high residence time in this section of the reactor. The flow restricton were protected from excessive heating by heat shields. To obtain an identical heat flux in the four high-temperature tubes, care was taken to place the tubes as symmetrically as possible with respect to the burners that are mounted in the side walls of the furnace. Experimental Program Flow Distribution Tests. The flow distribution was tested with air at ambient temperature. The outlet
0888-5885/91/2630-1081$02.50/00 1991 American Chemical Society
1082 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 Table I. Flow Rates in the Four High-Temwrature T u b (L/h Air at 101 kPa and 18 O C ) flow rate in tube test 1 2 3 4 av flow std dev 1 487.8 480.0 500.0 495.9 491.2 9.2 2 731.7 731.7 736.2 750.0 737.4 8.7 3 937.5 975.6 980.4 952.4 961.5 20.1 4 1403.6 1379.3 1363.6 1392.1 17.2 1384.6 1739.1 1739.1 5 1751.8 1777.8 1752.0 18.2
I
%
std dev
spread f10 115 f30 f30 k50
1.87 1.18 2.09 1.24 1.04
T (‘ti
I
3
1 low-T tube
4 high-T tubes
_Is
900
700
0 thermocouple
Figure 1. The UQ pilot reactor. T15 858
500
I
/I\
I in
0
2
4
Figure 3. Axial process gas temperature profile in the UQ pilot (naphtha 5.5 kg/h, b = 0.6 kg/kg, COP = 200 kPa). Table 11. Characterization of the Naphtha average molecular weight 85.67 H/C molar ratio 2.264 specific gravity 0.691 g/cmg sulfur content 400-450 ppm by weight nc 3 4 5 6 7 8
9 10 11
total
639
Figure 2. The UQ pilot reactor. Detail of the reaction section and process gas temperature distribution (2’ in O C , naphtha 5.5 kg/h, b = 0.6 kg/kg, COP = 200 kPa).
pressure of the coil was atmospheric. The flow rates of air in each of the tubes were measured with a wet gas meter. The results are given in Table I in L/h. Within the experimental uncertainty the total flow is uniformly distributed over the four tubes, even at flow rates far below the design value of 8-10 Nm3/h (2-2.5 Nms/h per tube). Temperature Distribution Tests. Prior to performing the actual cracking experiments, a number of test runs were carried out to check the uniformity of the tempera-
PIANO Analysis per Carbon Number in w t I nP iP A N 0 unknown 0.2188 1.7334 0.3192 18.4550 11.0888 1.3916 0.5426 12.6444 14.4577 0.9038 4.2487 0.1136 0.0300 4.4737 5.6588 0.9946 4.1934 2.6569 3.4915 1.7734 2.4741 0.6600 1.9226 2.5774 0.2067 0.7770 0.3431 0.3440 0.1032 0.8590 0.3415 0.690 43.307 38.279 3.879 13.189 0.656
ture distribution. The tests were carried out with naphtha under normal operating conditions. Figure 2 gives a typical result. The largest temperature difference between two tubes is about 8 OC. Figure 3 shows a typical axial process gas temperature profile in the reactor. The temperature varies almost linearly with the axial reactor coordinate. Naphtha Cracking Runs. All cracking runs were performed with one and the same light naphtha. A characterization of the naphtha is given in Table 11. With this naphtha, Herrebout and Froment (unpublished data) recently carried out a large number of cracking runs in the classical single-tube pilot reactor that has been described previously by Van Damme and Froment (1982). The experimental conditions cover the entire range of industrial operating conditions, with the exception of very low res-
Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 1083 Table 111. Values of the Most Important Process Variables for the Naphtha Cracking Experiments LPT data base set UQ1 set UQ2 naphtha flow rate, kg/h 2.0-5.0 5.5 6.7 steam dilution, kg/kg 0.24.4-0.8 0.6 0.6 coil outlet press., kPa 200 200 200 700-930 830-886 845-905 coil outlet temp, O C temp profile linear-convex UQ UQ total residence timea 0.2-0.44.8 0.25 0.19 residence time in high-temp 0.15 0.11 tubes, s O 8
H2 yield (ut%)
I
t1
l5
COP = 200 kPa
. 0
uo uo
1
low e , high 6 ,iineor
2
d
10
= (1/FdSoVbt/Rr) dV.
idence time. The values of the most important process conditions are listed in Table 111. Combinations of residence time (0.2, 0.4,and 0.8 s), steam dilution (0.2, 0.4, and 0.8 kg/kg)and types of temperature profile (linear and convex) were investigated. Two sets of experiments were performed with the same naphtha in the UQ pilot reactor. For the set UQ1, the residence time in each of the four high-temperature tubes, where most of the reaction occurs, OQ,is about 0.15 s. In the set UQ2, this residence time is decreased to 0.11 s. The process conditions for the UQ experiments are also given in Table 111. The experimental results are presented in Figures 4-16 and are discussed in the next section. The zones in the figures indicate the minimum and maximum values of the various yields, extracted from the LPT data base. The naphtha conversion is used as a measure for the cracking severity. It is a weighted and normalized sum of the conversions of 12 key components in the naphtha (Van Damme et al., 1981):
m
high
05
'
.
0,
low 6 .convex
-
XN 1961
ion
95
90
85
Figure 4. Hydrogen yield as a function of naphtha conversion. CHq yield l w t % l
1.
COP = 200 kPa
20
15
0
uo 1 uo 2
database
-
XN 1%)
10
-
85
90
1
100
95
Figure 5. Methane yield as a function of naphtha conversion.
The key components are n-pentane, n-hexane, n-heptane, 2-methylbutane, 2-methylpentane, 2-methylhexane, 3-methylpentane, 3-methylhexane, 2,3-dimethylbutane, cyclopentane, methylcyclopentane, and methylcyclohexane. They represent 75 mol % of the naphtha characterized in Table 11. Only experiments with material balances closing to within 2 w t % are included in the discussion. Error analysis of the pilot plant shows errors in the order of magnitude 1.5-2.0% relative to the yields of the components considered.
Results and Discussion Because of the short residence time and the almost linear process gas temperature profile, the UQ reactor has tc operate a high-temperature level in the exit zone. To achieve the same naphtha conversion, the coil outlet temperature in the set of experiments UQ1 is a few degrees higher than that of the experiments at 8 = 0.2 s of the LPT data base. Because of the very short residence time in the UQ2 set of data, the coil outlet temperature is between 5 and 10 OC higher. It is well-known that higher temperatures, combined with an appropriate short residence time, favor the olefins selectivity. Figure 4 illustrates that the UQ reactor leads to a high hydrogen yield. This is explained below. Figure 5 indicates that, for a given naphtha conversion, the methane yield is only slightly affected by the process conditions. This means that when the same ethylene yield is aimed at, the UQ reactor shows a substantial decrease in the
CzH2 yield lwt%l
1.0
00
'
05
. . .
COP = 200 kFu
. 0
UP1
uo 2
low @,high6 , linear
ci4Uu.e high e.Iow 6 ,convex
XN
90
95
1W
-
l%i
Figure 6. Acetylene yield as a function of naphtha conversion.
methane output. This is typical of high-severity operation in general. The acetylene yield is given versus the naphtha conversion in Figure 6. At relatively low severities, the acetylene yield obtained by cracking in the UQ reactor slightly exceeds that of cracking at low residence time and high dilution. More important differences are obtained as the conversion is increased. Figure 7 shows that, for a given conversion, the UQ ethylene yield exceeds that of experimentv performed at the shortest residence time (8 = 0.2 s) and the highest dilution (6 = 0.8 kg/kg) with the classical pilot reactor and its more c ~ n v e xtemperature
1084 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 C3H& yield Iwt%l
35
1 I
0
iiP 1
8
UP2
t
wA
COP = 200 k h
l5
low e , high d , linear
high B , low
6
t
6 , linear
high e , low
6
,convex
XN i%l
85
90
95
100
Figure 7. Ethylene yield as a function of naphtha conversion.
95
90
85 L-L__.-A-I
15
low e , hqh
,convex
05
I
t
100
Figure 9. Sum of propadiene and methylacetylene yields as a function of naphtha conversion. C3Hg yield i w t % l
C2Hg yield i w t % l
1 COP = 200 kPn
-
0
uo
8
UQ2
v
1
low B , highe. linear
I
XNI%1
2
85
90
95
100
5' a5
2
Figure 8. Ethane yield as a function of naphtha conversion.
profile. A net gain in ethylene yield of about 0.5-1.0 wt % can be achieved. With the high temperatures in the UQ coil, the ethane yield is low, as is clear from Figure 8. Ethyl radicals can either abstract hydrogen from a feed molecule to form ethane or undergo decomposition to produce ethylene and hydrogen. At high temperature, the decomposition reaction, which has a higher activation energy, is favored. This also explains the high hydrogen yield. From Figure 10, it follows that the propylene yield of the UQ sets of data is lower than that of the shortest residence time in the data base. The difference becomes smaller as the naphtha conversion is increased. This lower propylene yield finds its origin in the primary distribution of the naphtha cracking. In the naphtha studied here, propylene is mainly formed through hydrogen abstraction on secondary carbon atoms. These reactions have a lower activation energy than abstractions on primary carbon atoms, which favor the formation of ethylene. Because of the high temperature in the exit section of the UQ reactor, the propylene formation is slightly lowered. A similar effect is observed for the isobutene yield. A fraction of the loss in propylene is compensated for by an important increase in the propadiene and methylacetylene yields, as can be seen from Figure 9. These producta result from the decomposition of allyl types of radicals, which is favored by high temperatures.
I
90
95
high e dotabase
IOU e
x,i%l 1 100
Figure 10. Propylene yield as a function of naphtha conversion. 1.3-CqHg yield Iwt%)
COP 0
I
uo
200 kPo 1
up2
3 L
I w 95 1W Figure 11. 1,3-Butadieneyield 88 a function of naphtha conversion.
m
The UQ reactor leads to a high butadiene yield, which is typical of short residence time cracking. In Figure 11, the only points at which the butadiene yield exceeds that of the UQ coil were obtained at a higher dilution of 0.8 kg/kg. The yield of 1- and 2-butene are represented in Figures 12 and 13. They are little influenced by the process conditions. For isobutene, however, there is a net decrease with respect to the experimental results of the data base (Figure 14), at least at lower conversions. As was already the case with propylene, the differences become
Ind. Eng. Chem. Res., Vol. 30,No. 6, 1991 1085
"1
COP = 200 k h
high 6,Iow
b,
convex
low e , hlgh
6
,linear
COP = 200 kPa 0
database
UP 1
m UQ 2
01
90
85
1
100
95
Figure 12. 1-Butene yield as a function of naphtha conversion.
o
XN
85
i
1%) c
90
95
100
Figure 15. Benzene yield as a function of naphtha conversion. rC&
aromatics yield I w t % l
I COP = 200 kPa
: ::;
hgh e,low
6
,convex
database
COP = 200 kPa 0
high e
low e. high s.linear
UP1 u s g
UQ 2
0
85
-
90
I
I
95
100
Figure 13. Sum of %-butenesyields as a function of naphtha conversion.
XN 1%)
0
85
90
95
100
-
Figure 16. Sum of Cs to CB aromatics yields as a function of naphtha conversion.
i-CqH8yleld I w t % l
Figure 14. Isobutene yield as a function of naphtha conversion.
portant role in the UQ reactor, converting alkyl aromatics into benzene. These reactions are also favored by the high hydrogen yield (cf. Figure 4). Also, cyclization of large radicals can occur in the inlet tube, where the temperature is low, leading to aromatics formation by dehydrogenation of the intermediates, in the four high-temperature tubes. The moderate slopes of the yield a t high conversions for the UQ data in Figures 16 and 16 indicate that the aromatics production from secondary reactions is low. This , yields: the olefin yields remain is also obvious from the C high at high conversion. With the UQ reactor, a benzene-rich gasoline cut is produced. In all cases, the fuel oil yield obtained with the UQ reactor is low (2.0-2.5 w t %) as compared to the one obtained in the database experiments (3.0-5.0 wt %).
smaller and eventually disappear as conversion increases. High temperatures do not favor addition reactions, which have a low activation energy. These reactions are mainly responsible for the disappearance of olefins. Figure 15 shows that the benzene yield obtained with the UQ reactor is rather high. Total aromatics yield (benzene + toluene + xylenes + styrene + ethylbenzene), on the contrary, is low, even at high conversion (Figure 16). This indicates that dealkylation reactions play an im-
Coke Formation in the UQ Reactor At one time, the UQ reactor was operated continuously for 13 h. The pressure drop was 19 kPa for the UQ1 set and 24 kPa for the UQ2 set. It increased only slightly during this time: the additional pressure drop caused by coking was less than 1.5kPa. This is a first indication that coke formation in the UQ reactor is not excessive. The coke was then burned off with a controlled flow of air. At regular time intervals, the carbon oxides were
1
0
1
85
COP
200 kPo
.
, 90
, 95
,ow
@'d
XNI%1
I 100
Ind. Eng. Chem. Res. 1991,30, 1086-1092
1086
measured by gas chromatography. The total coke yield was 0.71 g of carbon formed per kilogram of hydrocarbon fed, or 0.071 w t 9%. The average rate of coke formation (averaged over the total duration of the experiment and over the total wall surface in the reaction section proper) amounted to 13.5 g/(m2.h). As mentioned in Table 111, the coil outlet temperature ranged from 830 to 905 "C. Coking experiments performed with a full-range naphtha, with a naphtha-ethane mixture (Plehiers and Froment, 1987b),and with light gases at outlet temperatures ranging from 820 to 850 "C and at comparable hydrocarbon partial pressures led to coke yields of 0.064.27 wt % and average rates of coke formation ranging from 3.1 to 13.9 g/(m2.h). Clearly, no excessive coke formation occurs in the UQ reactor. Also, there are no indications of special fouling problems in the restrictors.
Nomenclature COP = coil outlet pressure, kPa COT = coil outlet temperature, "C Fh = logarithmic average total molar flow rate, mol/s p t = total pressure, kPa R = gas constant = 8.3143 J/(mol.K) T = temperature, "C V = volume, m3 XN = naphtha conversion x i , N = conversion of key component i in naphtha
Conclusions Flow restrictors suffice to achieve a homogeneous flow distribution in the reactor. In addition, careful positioning of the tubes with respect to the burners leads to practically identical temperature profiles in the four high-kmperature tubes. These are two important points that ensure a good operability of the reactor. The product distribution is favorable. The total olefin selectivity is high. The UQ reactor features particularly high ethylene yields and a low fuel oil production. Although the process gas temperature reaches a high value close to the exit, the coking rates remain within acceptable limits.
Dierickx, J. L.; Plehiers, P. M.; Froment, G. F. On-line Analysis of Hydrocarbon Effluents. Calibration Factors and Their Correlation. J. Chromatogr. 1986,362,155-174. Plehiers, P. M.;Froment, G. F. Reversed Split Coil Improves Ethylene Yields. Oil Gas J. 19878,Aug 17, 41-49. Plehiers, P. M.;Froment, G. F. Cocracking and Separate Cracking of Ethane and Naphtha. Ind. Eng. Chem. Res. 1987b, 26, 2204-2211. Plehiers, P. M.;Froment, G. F. Firebox Simulation of Olefin Units. Chem. Eng. Commun. 1989,80,81-99. Van Damme, P. S.; Froment, G. F. Thermal Cracking Computer Control in Pilot Plants. Chem. Eng. Prog. 1982, 78 (9), 77-82. Van Damme, p. s.; Froment, G. Fa; Balthaar, w.B. Scaling UP of Naphtha Cracking Coils. Ind. Eng. Chem. Process Des. Dev. 1981, 20,366-376. Van Damme, P. S.; Willems, P. A.; Froment, G. F. Temperature, not Time, Controls Steam Cracking Yields. Oil Gas J. 1984,Sept. 3,
Acknowledgment We acknowledge the cooperation with and the support of the KTI group.
Greek Symbols 6 = steam dilution, kg/kg 8 = residence time, s 9 0 t J = molar fraction of key component i in the naphtha feed
Literature Cited
68-74.
Received for review August 27, 1990 Accepted January 28, 1991
High-pressure Hydrogenation of Nitrile Rubber: Thermodynamics and Kinetics Susmita Bhattacharjee,? Ani1 K.Bhowmick,*it and B. N.Avasthit Rubber Technology Centre and Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
Homogeneous catalytic hydrogenation of nitrile rubber (NBR) having various acrylonitrile contents
was carried out with tris(triphenylphosphine)chlororhodium(I) under high pressure. Thermodynamic parameters of hydrogenation reaction show that the reaction is thermodynamically feasible. Optimum reaction conditions were developed by varying the reaction parameters-temperature, time, pressure, catalyst concentration, and solvent. Reaction of NBR a t a catalyst concentration of 0.02 mmol, under 5.6-MPa hydrogen pressure a t 373 K in chlorobenzene for 11h, was found to be optimum for complete hydrogenation. The activation energy of the reaction was 22 kJ/mol. Carboxylated nitrile rubber and polybutadiene rubber were also used. The hydrogenated product was characterized by IR and NMR spectroscopies, iodine value, molecular weight, glass transition temperature (Tg), and stress-strain behavior. Introduction The hydrogenation of nitrile rubber has been used in order high heat and oxidative resistance, 88 well as good resistance to swelling, and improve the wear resistance of nitrile rubber (Hasimoto et al., 1988;Thormer
* Author to whom correspondence should be addressed. 'Rubber Technology Centre. 8 Department of Chemistry.
et al., 1984; Rawlinson and Djuricis, 1988). Bhattacharjee et al. (1991)have studied the degradation of nitrile rubber. Reports are available on the hydrogenation of nitrile rubber and polybutadiene rubber using homogeneous catalysts (Weinstein, 1984; Doi et al., 1986; Mohammadi and Rempel, 1987). However, in these reports optimized reaction parameters are not mentioned and, moreover, some of the conditions listed are contradictory. In India, the technology of hydrogenated nitrile rubber is being developed and a number of catalysts have been
0888-5885/91/2630-lO86$02.50/00 1991 American Chemical Society
