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Znd. Eng. Chem. Res. 1 9 9 5 , 3 4 , 1132-1139
Transfer-LineHeat Exchanger Fouling during Pyrolysis of Hydrocarbons. 1. Deposits from Dry Cracked Gases Grete Bach and Gerhard Zimmermann* Department of High Temperature Reactions at the Institute of Chemical Technology, Leipzig University, Permoserstrasse 15, 0-04303 Leipzig, Germany
Frank-Dieter Kopinke Section of Remediation Research, Centre for Environmental Research LeipzigHalle Ltd., Permoserstrasse 15, 0-04303 Leipzig, Germany
Simon Barendregt, Paul van den Oosterkamp, and Harry Woerde KTI International B. V., P.O. Box 86, 2700 AB Zoetermeer, The Netherlands Isobutane of technical quality was pyrolyzed in nitrogen as a diluent under standardized conditions (820 "C, 0.4 s, Ndisobutane = 0.4 g/g) in a specially developed, vertically positioned tubular flow reactor coupled with a micro electrobalance. After cooling the dry cracked gases to temperatures thought to be representative for the innertube surface temperatures of a TLE (transfer-line exchanger), the growth of carbon-rich deposits (known as TLE fouling) was measured continuously at the surfaces of material coupons in dependence on (i) the prehistory and (ii) the sort of materials as well as (iii) the temperatures. The results show clearly that on low alloyed steels catalytic reactions can play an important role in TLE fouling if dry cracked gases are used. The thermal and thermocatalytic conversion of organic material at high temperatures is usually accompanied by the formation of highly undesirable carbon-rich, solid products deposited on the inner surfaces of outside heated coils (Froment, 1990). Since the products manufactured this way are unstable at reaction temperatures, they have to be cooled rapidly to 350 or even 300 "C in order to stop subsequent reactions (Albright et al., 1983). Normally this takes place in a transfer-line heat exchanger (TLE) which is positioned immediately after the reactor coils. In the TLE tubes carbon-rich deposits are formed as well (Huntrods et al., 1989). Though the properties of the latter deposits differ enormously from those formed at higher temperatures in the reactor coils (Veleny et al., 19911, they often are the starting point of a shutdown of the corresponding cracking furnace to remove the deposits in question. Because every shutdown of plants worsens the process economy (Lobo, 19881, a lot of research and development was done in the past to overcome these shortcomings (see, for example, Zander (1989), Huntrods et al. (1989), and Ranzi et al. (1985)). Most of the published work dedicated to the formation of carbon-rich deposits in steam cracking refers to the cracking coils and to events which occur at temperatures between 750 and 1000 "C (see, for example, Albright (1976),Albright et al. (1983), Jackson et al. (19861, and Melo (1988)). They result in carbon-rich products which are commonly called coke-like products by reason of their morphology (Albright, 1988; Albright and Marek, 1988). In contrast to that, only little is known about the details of the formation of those carbon-rich products which are deposited in the TLE tubes a t temperatures between about 600 and 350 "C. They also consist mainly of coke-like products, but the hydrogen content is much higher due to nearly intact highly condensed aromatic systems. This causes other properties of the deposits, and therefore, the formation of these coke-like products is generally described as 0888-5885/95/2634-1132$09.00/0
fouling or more particular as TLE fouling. The events which contribute t o TLE fouling are still under controversial debate. Events taken into account are a mechanical transfer of coke-like particles from the reactor coils into the TLE (Albright et al., 19831, a physical condensation of the highest boiling cracked products at the TLE walls if their dew points have fallen (Kopinke et al., 19931, a chemical condensation of unsaturated cracked products from the gas phase to droplets followed by their destructive solidification after impinging upon the TLE tubes at temperatures above 350 "C (Lahaye et al., 19771, a chemical interaction between the deposited products and reactive gas phase species (Kopinke et al., 19931, and even a metal catalyzed polymerization (Dente et al., 1983) or decomposition (Bujanov et al., 1977) of unsaturated hydrocarbons at the inner surface of the TLE tubes followed by the thermal dehydrogenation of the thus formed polymers. From the above-mentioned possibilities, the metal catalyzed formation of carbonaceous deposits at the TLE material is hitherto without any experimental proof. In this paper we report on latest results of TLE fouling during pyrolysis of isobutane. Although steam is used as a diluent in industrial plants to lower the partial pressure of the hydrocarbon species, nitrogen was used as a diluent in this study to avoid any suppression of the hitherto still questionable catalytic route to TLE fouling by steam (SiMos et al., 1986) and to guarantee a high accuracy of continuous weighing without any disturbance by water evaporation. The results of investigations on TLE fouling in the presence of the more common steam will be the content of a following paper.
Experimental Section The pyrolysis apparatus includes the coupling of a tube reactor with a thermobalance. Its handling was described in more detail elsewhere (Kopinke et al., 1993). Therefore, only a brief description shall be given here. It is t o be restricted to the remarks which are essential for the matter of concern of this paper and the conclusions drawn up from the measured fouling rates.
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4,1995 1133 Table 1. Composition of Isobutane (Technical Quality) hydrocarbon isobutane n-butane propane propylene ethane/ethylene methane
content (wt %) 81.6 11.6 3.9 2.2 0.2 1.0
Isobutane of technical quality (Table 1)was cracked in a vertical arranged tubular quartz reactor (300 x 10 x 1.5 mm) under standardized conditions (T= 820 "C, z = 0.4 s, p = 0.1 MPa, C2HdC3Hc = 0.55 g/g in the cracked gas) and using nitrogen as a diluent (N2/ isobutane = 0.4 g/g). Under these conditions the conversion degree of the isobutane amounted to about 85%. The lower part of the reactor quartz tube is thought to embody the TLE (Figure 1). Inside this tube segment material coupons were positioned at the surface of which the fouling rates were measured continuously within a limit of accuracy of weighing of about f 5 pg per specimen under flow conditions. The specimen (dimension 20 x 7 x 1 mm) was positioned a t each desired place by a thin string of platinum connecting the specimen with the thermobalance directly through the reactor volume. The surface roughness is known to play an important rate in coking (Crynes and Crynes, 1987). Therefore, all the specimens used were polished by means of Sic paper resulting each in a roughness of 0.25 pm. To avoid any influence of surface dirtiness of the studied coupons on the coking rates, all of them were precleaned by an ultrasonic treatment in acetone for 20 min before the experiments. To make possible the measurement of the fouling rates at specimen surface temperatures which are comparable with those prevailing at the inner surface of operating TLEs, the hot cracked gas formed in the reactor had to be cooled to the desired experimental temperature by a well-tuned ratio of heating and heat emission between the reactor outlet and the tube segment simulating the TLE. The adjustment of TLE temperature was carried out with nitrogen only prior to each balance run. Compared with the amounts of carbon-rich products deposited a t coupon surfaces, those precipitated at the platinum string are usually negligible. Only in such cases where very small amounts of coke were formed the coke deposits on the string had to be measured separately. To be able to distinguish between the changes of the surface properties caused by burning off on the one hand and by mechanical removal of the carbon-rich deposits on the other hand, a two-step procedure was applied for mechanical surface cleaning. It consisted of a careful brushing away of the main parts of deposits followed by a cleaning in water in an ultrasonic bath (residence time 20 min). After this procedure, the blank coupons had a completely different appearance than those cleaned by burning off (darkly gray-colored against redcolored) and the surface oxide content was much lower.
Results and Discussion Kinetics of "LE Fouling during Isobutane Pyrolysis. A continuous measuring of TLE fouling rates over about 25 h a t quartz surfaces and a temperature of 500 "C in the presence of cracked gas from isobutane
Thermobalance Purge gas
n
I
Injection of gaseousand liquid feeds
I
+ I Cooler
Figure 1. Experimental setup of the reactor and simulated TLE.
pyrolysis reveals a slight increase of the measured fouling rates only (Figure 2). If, however, fouling rates are measured under the same conditions but a t coupons of 15Mo3 steel, the most used TLE material in the industrial practice, instead of quartz a completely different behavior was observed. After a steep increase during the first 10-15 min the fouling rate goes through a maximum and then it drops slowly. Finally, the weight increase reaches a quasi steady state value (compare, for example, Lobo (1988)). This behavior corresponds with the kinetics of catalytic reactions which run under catalyst deactivation (Backman and Froment, 1988). We consider that as a strong argument of the existence of a catalytic route of TLE fouling in the presence of dry cracked gases. Since iron is the only element which is available in concentrations above the trace level at 15Mo3 steel surfaces, we assign the observed catalytic activity to it. Obviously, the iron controlled surface properties are suppressed with the increase of carbonrich fouling products at the steel surface. We reached almost the same steady state fouling rate a t the surface of fresh (hitherto unused) 15Mo3 after a run time of about 30 h as that a t quartz coupons. This statement cannot be maintained for 15Mo3 coupons which were already treated by fouling experiments with
1134 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 fouling rate (ug/cm2 min) 400 I
5
0
15
10
20
25
run time (h) Figure 2. Dependence of fouling rate on run time at different materials and surface properties: 1, quartz; 2, fresh 15Mo3 steel; 3, 15Mo3 steel aRer five fouling/air cleaning procedures.
fouling rate (pg/cm2 min) 1 .ooo
800
600
400 Number of runs
'
13. 12 8.
200
6. 3 2 1
0
I
I
20
40
60
80
run time (min) Figure 3. Dependence of fouling rate on number of fouling/air cleaning procedures of a 15Mo3 steel coupon versus run time.
a following removal of the attached carbon-rich products by burning off with air. In Figure 2 such an example for the dependence of the fouling rate on the run time at a 15Mo3coupon, which was treated five times in that way, is additionally demonstrated. We failed t o reach definitely the steady state fouling rate even after some days. From calculations based on the kinetic equation for self-inhibiting reactions rf = dmddt = rAO)/(l r t (0)art)with rf = fouling rate bg/(cm"min)], rkO) = initial fouling rate (here maximum fouling rate) mf = mass of
+
Table 2. Average Roughness and BET Surface of 15Mo3 Steel Coupons Fresh and after FoulingMr Cleaning Procedures material av roughness b m ) BET surface (m2/g) 15Mo3 (fresh) 0.5 1.4 15Mo3 ( a h r 3 runs) 1.15 258
carbon-rich deposits @g/cm2),af= deactivation constant (cm2/pg),and t = duration of run (min), we found out that 525 h would be necessary to reach the experimen-
Ind. Eng. Chem.Res., Vol. 34, No.4,1995
be needed to suppress completely the catalytic influence of that TLE material. Finally, it is important to note that the curves given for the measured fouling rates at 15Mo3 represent mean values from several runs, which were carried out under strictly identical conditions but a t different coupons made from the same piece of material (fresh ones) and under standardized fouling and burning off conditions, respectively. Though the shapes of the curves in question correspond with each other, their levels can differ considerably (up to 50 or even 100 pgl(cm2min) during the first 10 min). We put that down to very small differences in the physical and/or chemical nature of the surface properties with a surprisingly high but hitherto not sufficiently interpretable effect on the fouling rate. Dependence of the TLE Fouling Rates on Material Aging. With reference to the 15Mo3 steel the results demonstrated in Figure 2 are basically in line with well-known observations from corresponding industrial plants (Froment, 1989): the higher the number of foulinglcleaning procedures the higher the observed extent of TLE fouling. Consequently, the run times between two necessary cleaning procedures are automatically shortened in TLEs with older tubes. To determine a relation between the number of cleaning procedures by burning off the deposits with air and the measured fouling rates, we treated a coupon of 13CrMo4.4 steel (another frequently used TLE material) with cracked gases from isobutane for 60 min. Then, the carbon-rich deposits were burned off and the procedure was repeated 13 times under comparable conditions each. The results are compared in Figure 3. The data show impressively that the fouling rates increase from run to run starting with a measured initial fouling rate of about 150 pgl(cm2min) in the first and ending with 2500 pgl(cmzmin) in the thirteenth run. The reason for the observed behavior seems to be a continuous increase in the roughness of the coupon surface
Figure 4. Appearance of oxide layer available on a 13CrMo4.4 steel coupon after 13foulinglair cleaning procedures (temperature 500
1136
"0.
R 1
il
1I 4 d
Figure 5. ELM1 photo of the oxide layer shown in Figure 4 (enlargement 5000; reproduced a t 55% of the original).
tally determined fouling rate a t a quartz surface under the used conditions. According to that 22 days could
fouling rate (pg/cm2 min) 500
400
300
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
200
.A
. . . . . . . . . . . . . . . . .1 . . . . . . . .
100
0 0
20
40
60
80
100
120
run time (min) Figure 6. Comparison of fouling rates measured for 15Mo3 steel coupons with different surface properties: 1, fresh 15Mo3 steel; 2, 15Mo3 steel after five times buring off of carbon-rich deposits; 3, 15Mo3 steel after five times mechanicallultrasonic removal of carbonrich deposits.
1136 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 Table 3. Dependence of Fouling Rates on the Temperature Measured after a Run Time of 2 h for Different Materials material 15Mo3 13CrMo4.4 CrNi30.30 quartz
quasistationary fouling rate lug/(cm"min)l 350°C 400°C 450°C 500°C 14 40 1.8 6 10 32 1.2 4.5 0.6 0.6 0.7 0.6 0.6 0.7 0.4 0.5
caused by the formation of iron clusters thought to be the final product of carburization processes (compare, e.g., the phenomenon of carburization (Kinniard et al., 1986) and of metal dusting (high temperature hydrocarbon corrosion) of CrNi steels (Aarna et al., 1978; Albright, 1976))and ending in loosely adhered iron oxide clusters of hitherto unknown composition afier the combustion of the carbon-rich deposits. This assumption is supported by measurement of the surface roughness and by BET measunnents at a fresh
15Mo3 coupon and this one after three foulinglair cleaning procedures (Table 2). The roughness increased from 0.25 pm (for the fresh coupon) to 1.15 pm (aged coupon) and the BET surface from 1.4 to 258 m2g-l. An additional but indirect confirmation arises from the appearance (Figures 4 and 5) and the X-ray diffraction analysis of the iron oxide deposits. The surface of the coupon was covered with a voluminous red-colored layer identified by X-ray diffraction analysis as hematite (Fez031 with a particle size between 30 and 40 pm. Figure 4 gives an impression of its dispersity. The totality of the preceding described findings agrees with the observation that the increase in fouling rate with the number of foulinghir cleaning procedures (Figure 3) is caused by the highly dispersed metal and metal oxide clusters, respectively, which in turn are imagined to be the starting point for the catalytically supported formation of carbon-rich foulants. Additional studies are necessary, however, to
amount of coke (1 O3 pg/cm2 h)
X40CrNiSi32 25 X8CrNi18 10
lncoloy800
X2CrNiS114 15 X5CrNi20 80
Incone1600
materials .first
hour msecond hour
Figure 7. Fouling rates measured at different materials (500 "C). Table 4. Code Names and Compositions of the Tested Steels composition of tested steels (wt %) material 15Mo3 13CrMo4.4 X5CrAl23.5 XlOCrAl24 X8CrNiAl2 1.32 X8CrNi18.10 X50CrNi30.30 X5CrNi20.80 X2CrNiSi14.15 X4OCrNiSi35.25 Inconel 600 Incoloy 800
Fe 99 98 70-73 71-74 40-47 68-72 40 71 33-41 7.2 46
Ni
Cr
30-34 8-10 30 78-82 14 32-36 76.4 31
0.7-1.0 20-24 23-25 20-22 17-19 30 19-21 15 23-27 15.8 21
C 0.1 0.1 40.05
