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Reactor runaway in pyrolysis gasoline hydrogenation
® 1997 Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G.F, Froment, B. Delmon and P. Grange, editors
245
Reactor Runaway in Pyrolysis Gasoline Hydrogenation E. Goossens^ R. Donker^ and F. van den Brink^ * DSM Research, Industrial Catalysis Section, P.O. Box 18,6160 MD Geleen, The Netherlands ^ DSM Hydrocarbons, P.O. Box 606, 6160 AP Geleen, The Netherlands
December 23, 1994 the wall of the first stage pyrolysis gasoline hydrogenation reactor in DSM's NAK3 steam cracker ruptured, requiring an emergency shutdown of the plant. A mixture of gasoline, hydrogen and nickel catalyst escaped through the crack and immediately caught fire. There were no personal injuries, but property damage was substantial. There had been no domino effects, nor had there been any danger to the surrounding area. One week after this severe fire NAK3 was put back into operation except for the first stage pygas hydrogenation. Thermodynamic calculations, analysis of the process data and of the catalyst and reactor wall indicated that the bulge and subsequent rupture of the reactor wall were caused by a brief local temperature excursion to 700-750 "C at an operating pressure of 30 bar. This temperature excursion can be explained by maldistribution of the liquid and thus poor heat transfer caused by local excessive carbon formation in the reactor in the period preceding the incident. Disruption of the heat removal can cause a chain of exothermic reactions, viz. hydrogenation of olefins, aromatics, as well as hydrocracking. Results of the investigation as well as the improvements made in the operation and procedures are presented in this paper.
1. INTRODUCTION 1.1 Process description and reactor design The C5+ fraction from a steam cracker contains alkadienes and alkenyl aromatics that will easily form gums, which are detrimental for end uses like automotive gasoline and aromatics production. These compounds need to be removed in order to stabilise the C5+. In the first stage pyrolysis gasoline (pygas) hydrogenation they are selectively hydrogenated over Ni/Al203 catalyst. Mono-olefins and aromatics are allowed to pass the reactor. The reactor is followed by a gas/liquid separator and a distillation unit (figure 1).
246 hydrogen
• fresh hydrogen
{XI—.j quench fresh Cc
Um ^uenchf
,R601 A,
I*
Hi V601
-M-
start up diluent
—
I
JdistillationJ
I
rQ
MPV6101 to storage
Figure 1.
Schematic representation of pygas hydrogenation unit
The hydrogenation reactor is a trickle-phase unit with two catalyst beds. The system comprises of three phases: hydrogen, liquid C5+ and catalyst. A mixture of hydrogen, C5+ and, optionally, hydrogenated product is fed to the reactor and passed over distributors to the first catalyst bed (3 m). Prior to entering the second catalyst bed (10 m), the feed is mixed with recycled hydrogenated product (quench) and redistributed. Quench is used to control the temperatures in the catalyst beds. The inlet temperature varies from approximately 40 °C (Start Of Run) to 95 °C (End Of Run). The temperature gradient is approximately 60 ° per bed. The temperatures in the catalyst bed are monitored by 18 thermocouples equally divided over 3 thermowells (figure 2). plan view
"TZI" Figure 2.
Arrangement of thermowells and thermocouples in first stage pygas hydrogenation reactor
From the gas/liquid separator, after the reactor, hydrogen is recycled to the inlet of the reactor and made up with fresh hydrogen. The liquid, i.e. hydrogenated product, is partly used
247 for quench, the excess amount being pumped to the distillation section. The distillation section is not in operation during startup of the reactor. In the first phase of start-up, hydrogenated product is recirculated through the reactor without charging raw, i.e. non-hydrogenated, C5+. In this phase, an exothermic effect occurs due to the heat of adsorption of hydrocarbons on the catalyst surface. As the start-up progresses an increasing amount of raw C5+ is charged while a balancing amount of hydrogenated product is sent to storage via MPV 6101. 1.2. The incident December 23, 1994, a few minutes before 18.00 h, the wall of the first-stage pygas hydrogenation reactor ruptured during operation. The incident took place during the start-up of the reactor. Due to the operating pressure of 30 bar a mixture of gasoline, hydrogen and nickel catalyst was blown out of the reactor. The mixture immediately caught fire, resulting in a jet of flames of 40 m. The plant was shut down. There were no personal injuries, no danger to the surroundings, no domino effects but the hydrogenation unit sustained substantial damage. One week after the incident NAK3 was on stream again except for the first-stage pygas hydrogenation unit.
2. RESULTS AND DISCUSSION The investigation was led by a multidisciplinary team of experts from DSM. The regulatory authorities were regularly informed of the progress of the investigation. In the phase of gathering information, production staff were interviewed and various companies operating similar processes were visited. All companies were quite open in discussing their experiences. The problems described here did not seem to be unique. The investigation focused on reconstruction of process data, catalyst characterization, material characterization and thermodynamic calculations in order to establish the cause of the crack and more importantly, to avoid similar problems in the future. 2.1 Process data Reactor R601A had been recharged with fresh catalyst one month prior to the incident. Catalyst was activated and the reactor started up in accordance with the operating instructions. Fully hydrogenated Cg-fraction, free of olefins and aromatics, was used for the startup. The catalyst used had not been presulphided for historical reasons and fear of odour nuisance. Although fresh, unsulphided, nickel catalysts are known for their poor initial selectivity, the process was very stable at the time of the rupture (figure 3). As the plot clearly indicates, temperatures are in the normal operating window and gradually increase from the top to the bottom of the reactor. By the end of the startup, the feed had almost completely been replaced by raw C5+, when the reactor wall ruptured for no apparent reason.
248
^%^ TS6022 ...w.^. TS6019 ys^
TS6016
^ ^ . TS6013 .V
TS6010
time (Dec. 23, 94)
Figure 3.
Process conditions on the day of the incident
From analysis of the process data it became clear that a runaway must have occurred a week before the rupture. This runaway was initiated by a sudden increase in the amount of hydrogenated product being sent to storage via MPV 6101, thereby simultaneously increasing the amount of raw C5+ being charged to the reactor (figure 4).
Figure 4.
Process conditions one week before the incident (hourly averages)
Despite the fact that production staff acted according to operating procedures, temperatures in the reactor rose to 320 °C in 1.5 hours. After another 3.5 hours, temperatures had returned to their normal levels and the hydrogen compressor was restarted. As temperatures immediately started to rise it was decided to shut down, depressurize and purge the reactor with nitrogen. All thermocouples except two indicated a decrease in temperature to normal level: thermocouples TS 6019 and TS 6022 remained at a high level for two days (figure 5).
249
o
Dec. 16, 94
date/time
Figure 5.
Process conditions one week before the incident (hourly averages)
Two days after the initial startup, the reactor was quenched with fully hydrogenated C5, fraction which resulted in a normal temperature level for all thermocouples. After assessment of the situation and the sequence of events, it was concluded that the reactor had not been outside its design specifications (T, P). The restart made on December 21 went very smooth up until to the moment of the rupture. 2.2 Catalyst characterization Characterization by means of TPR, elemental analysis, XRD and SEM proved that the fresh catalyst used was identical to the catalyst previously used. The relative difference in dispersion was 12%. Because of the low absolute metal surface area of approximately 4 m^.g"^ and a high metal loading of 10%, this difference in dispersion is too small to account for any difference in activity. Samples were taken from the catalyst left in and that blown out of the reactor. Excessive carbon formation was observed in many places in the reactor (figure 6): up to 50 wt% C was found in various samples.
1 crack second bed hard coal with pulverized catalyst catalyst with some coal
Figure 6.
Carbon formation throughout the reactor after the incident
250 In the first catalyst bed, carbon had been deposited along the reactor wall and in the centre of the bed on the support grid. Only little carbon had formed near the three thermowells. In what was left of the second bed, carbon deposits were observed along the wall. Carbon deposits were harder and thicker in the vicinity of the crack. Scanning and transmission electron microscopy revealed three types of carbon: amorphous, graphitic and whiskers (figures 7 and 8).
Figure 7.
SEM photograph of whisker-type carbon formed during pygas hydrogenation
1^
Figure 8.
SEM photograph of graphite carbon formed during pygas hydrogenation
251 It is well known that the type of carbon formed strongly depends on the reaction conditions. Atomic carbon and precursors formed by dissociative adsorption of hydrocarbons are transformed to polymeric coke, filamentous coke and graphitic carbon at progressively higher temperatures [1-4]. Polymeric hydrocarbons and graphitic films encapsulate the metal surface and deactivate the catalyst. Filamentous carbon (whiskers) generally do not deactivate the metal surface because the metal particle is lifted from the surface. When present in large amounts, however, the catalyst bed will plug and pellets will break up. The relation between the structure of the precursor and the kinetics and morphology of the coke formed is not well understood. An important parameter in the formation and structure of coke deposited is the presence of hydrogen. In the presence of hydrogen, carbon precursors are gasified which keeps the catalytic surface clean and thus active. Moreover, in a hydrogen-lean atmosphere, the catalyst will take its hydrogen from any hydrocarbons present, thereby enhancing the deposition of coke. In the sequence of events the hydrogen flow was stopped according to operating procedures prior to the incident (i.e. stop the feed of one of the reactants). This slows down the hydrogenation rate but the dehydrogenation and thus the rate of coke formation, will strongly increase. Near the crack, large amounts of whisker type carbon were characterized. It is well known that filamentary carbon has a low tensile strength but a high compressive strength [5]. Locally, these carbon deposits had led to pulverization of the catalyst. Lab tests confirmed that the catalyst tends to grow whisker in a CH4/H2 atmosphere. Initially, the investigation committee was put on the wrong track by the potential compressive forces exerted by filamentary carbon. Later on it became clear from the material characterization (section 2.3) that this compressive force on the wall was not necessary for the crack to occur. No a-Al203 was detected in any of the samples taken inside of the reactor, which would have indicated temperatures higher than 1000 °C. Some samples from outside of the reactor were found to contain a-Al203. Whether this phase was formed prior to the rupture or during the fire that followed caimot be decided. It was clear from elemental analysis that there had been a sulphur gradient present across the reactor: the concentration decreased concurrently in axial direction. This indicates that the feed contained little sulphur. Sulphur is known to enhance catalyst selectivity by selectively poisoning the hydrogenation sites that are most active. 2.3 Material characterization The reactor wall is made of 13CrMo44 steel. The crack, which was approximately 30 cm long and 2 cm wide, was in the centre of a large bulge that had a height of 10 cm. For further characterization a section of 1.5*1.5 m was cut out of the reactor wall, half of which was sent to the regulatory authorities. It was manifest that the wall within a radius of 0.3 m around the crack had briefly been at temperatures of 700-750 "C. At the operating pressure of 30 bar, a wall made of this material will fail at this temperature range. The geometry of the bulge and crack found on the reactor wall are as expected under these conditions. Other potential causes like corrosion, hydrogen embrittlement (low T), Nelson hydrogen attack (high T), material defect at the time of fabrication, creep or low cycle fatigue could be ruled out after an extensive study.
252 2.4 Thermodynamics In order to evaluate the conditions during various stages of operation, thermodynamic calculations were performed. Complete hydrogenation of fresh C5+ results in an adiabatic temperature rise of approximately 600 °C. In the presence of quench, i.e. hydrogenated product containing only olefins and aromatics, this temperature will be approximately 520 "C. If, however, hydrocracking occurs the adiabatic temperature can rise to about 1000 °C. In the case of maldistribution and thus bad heat transfer, temperatures higher than 700 "C can easily be reached. The fact that such temperatures were not detected by the thermocouples indicates that there had been a hot spot near the reactor wall.
3. CONCLUSIONS 3.1 Investigation On the basis of the results discussed here it can be concluded that the rupture of the reactor on December 23, 1994 was caused by a brief local temperature excursion to 700750 °C at an operating pressure of 30 bar. The geometry of the bulge and crack are consistent with this scenario. The hot spot can be explained in terms of liquid maldistribution. Maldistribution hampers heat transfer, leading to unwanted side reactions like hydrogenation of mono-olefins, aromatics and even hydrocracking of hydrocarbons. These reactions are highly exothermic and thus amplify the hot spot, i.e. a high temperature enhances vaporization of the feed, which in its turn results in a decrease of heat transfer due to a lower heat capacity of the vapour (positive feedback, figure 9).
Figure 9.
Formation of hot spot during pygas hydrogenation
The run-up to this situation started with the runaway that took place one week before the rupture. The strong increase in the concentration of fresh C5+ during startup, in combination with a decrease in hydrogen flow, caused the temperature excursion that was
253 observed. This in turn caused excessive carbon formation throughout the reactor, upsetting liquid distribution in the next startup. This eventually resulted in the, undetected, temperature excursion to 700-750 "C on December 23. Catalyst characterization confirmed that large amounts of carbon deposits, having various morphologies, were formed. 3.2 Lessons learned and actions taken The incident demonstrates that a runaway reaction can occur in a trickle-phase reactor loaded with a supported nickel catalyst. Presulphiding the catalyst prior to start up will minimize the possibility of a nmaway and improves the intrinsic safety of the process. Presulphiding can be performed in situ as well as ex situ. In situ sulphiding, by spiking the feed with a sulphur-containing compound like dimethyl sulphide, will result in a sulphur front moving through the reactor bed. Ex situ sulphiding is to be preferred in that it gives a uniform concentration of sulphur throughout the bed right from the start. A critical stage in the operation of a pygas hydrogenation reactor is the startup. The pygas hydrogenation reactor is started up using a specific Cs/Cg stream with a predetermined bromine number, diene number and aromatics content. The new operating procedures ensure that the quench, i.e. hydrogenated product, and hydrogen recirculation used are maximized when wall temperatures exceed 200 °C. They also ensure that a minimum linear velocity of the liquid and a minimum hydrogen:liquid ratio are maintained during operation. These improvements will minimize the chances of excessive carbon formation. As an extra source of information 28 thermocouples have been installed on the outside of the reactor wall. The alarm threshold for these thermocouples has been set at 200 °C. Furthermore, a research program has been set up to investigate the kinetics of runaway reactions during operation of the pygas hydrogenation. Experimental data will be gathered by using model compounds under various conditions (P, T, contact time) and studying their relation with the morphology of the carbon deposits. This incident has once again shown the importance of adequate response to near misses, in this case the runaway which was observed one week before the rupture. A thorough check of the catalyst bed after any runaway is called for in order to prevent future problems. In many cases this will imply that the reactor needs to be opened and the catalyst discharged. The sequence of events that eventually led to the incident had not been identified in the HAZard and OPerability study (HAZOP), although the individual events like runaway and carbon formation were known [6,7]. A thorough Process Safety Analysis (PSA) in an early stage of the process design could have revealed the possibility of this sequence of events. The PSA should include an extensive study of incidents in similar units. This illustrates the importance of a PSA prior to the HAZOP study. Finally, the incident and subsequent analysis illustrate the importance of an open discussion within the industry and academia in topics concerning safety, health and environment.
254 4. ACKNOWLEDGEMENTS Parts of this paper have been reproduced with permission of the American Institute of Chemical Engineers. Copyright © 1996 AIChE. All rights reserved [8]. The authors are indebted to prof. J. Geus and Dr. M. Hoogenraad of Utrecht University for performing the TEM analysis and the catalytic test in the formation of carbon whiskers.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
J. Rostrup-Nielsen and D.L. Trimm, J. Catal., 1977, 48, 155-165. C.H. Bartholomew, Catal. Rev. - Sci. Eng., 1982, 24(1), 67-112. R.T.K. Baker, M.A. Barbier, P.S. Harris, F.S. Feates and R.J. Waite, J. Catal, 1972, 26, 51-62. J. Rostrup-Nielsen, Catalysis - Science and Technology (Eds. J.R. Anderson and M. Boudart), Springer-Verlag, New York, 1-117. M. Hoogenraad, Ph.D. Thesis, Utrecht University, 1996. J.L. Figueiredo and J.J.M. Orfao, Sprechsaal, 1986,112(12), 1139-1142. D.L. Trimm, Progress in Catalyst Deactivation - Proc. NATO Adv. Study Inst. Catal. Deact. (Ed. J.L. Figueiredo), Algarve Portugal, May 18-29, 1981, 31-43. R.A. Donker, Proceedings of the 8th Ethylene Producers' Conference & 5th World Congress of Chemical Engineering, 1996, 5, in press.
245
Reactor Runaway in Pyrolysis Gasoline Hydrogenation E. Goossens^ R. Donker^ and F. van den Brink^ * DSM Research, Industrial Catalysis Section, P.O. Box 18,6160 MD Geleen, The Netherlands ^ DSM Hydrocarbons, P.O. Box 606, 6160 AP Geleen, The Netherlands
December 23, 1994 the wall of the first stage pyrolysis gasoline hydrogenation reactor in DSM's NAK3 steam cracker ruptured, requiring an emergency shutdown of the plant. A mixture of gasoline, hydrogen and nickel catalyst escaped through the crack and immediately caught fire. There were no personal injuries, but property damage was substantial. There had been no domino effects, nor had there been any danger to the surrounding area. One week after this severe fire NAK3 was put back into operation except for the first stage pygas hydrogenation. Thermodynamic calculations, analysis of the process data and of the catalyst and reactor wall indicated that the bulge and subsequent rupture of the reactor wall were caused by a brief local temperature excursion to 700-750 "C at an operating pressure of 30 bar. This temperature excursion can be explained by maldistribution of the liquid and thus poor heat transfer caused by local excessive carbon formation in the reactor in the period preceding the incident. Disruption of the heat removal can cause a chain of exothermic reactions, viz. hydrogenation of olefins, aromatics, as well as hydrocracking. Results of the investigation as well as the improvements made in the operation and procedures are presented in this paper.
1. INTRODUCTION 1.1 Process description and reactor design The C5+ fraction from a steam cracker contains alkadienes and alkenyl aromatics that will easily form gums, which are detrimental for end uses like automotive gasoline and aromatics production. These compounds need to be removed in order to stabilise the C5+. In the first stage pyrolysis gasoline (pygas) hydrogenation they are selectively hydrogenated over Ni/Al203 catalyst. Mono-olefins and aromatics are allowed to pass the reactor. The reactor is followed by a gas/liquid separator and a distillation unit (figure 1).
246 hydrogen
• fresh hydrogen
{XI—.j quench fresh Cc
Um ^uenchf
,R601 A,
I*
Hi V601
-M-
start up diluent
—
I
JdistillationJ
I
rQ
MPV6101 to storage
Figure 1.
Schematic representation of pygas hydrogenation unit
The hydrogenation reactor is a trickle-phase unit with two catalyst beds. The system comprises of three phases: hydrogen, liquid C5+ and catalyst. A mixture of hydrogen, C5+ and, optionally, hydrogenated product is fed to the reactor and passed over distributors to the first catalyst bed (3 m). Prior to entering the second catalyst bed (10 m), the feed is mixed with recycled hydrogenated product (quench) and redistributed. Quench is used to control the temperatures in the catalyst beds. The inlet temperature varies from approximately 40 °C (Start Of Run) to 95 °C (End Of Run). The temperature gradient is approximately 60 ° per bed. The temperatures in the catalyst bed are monitored by 18 thermocouples equally divided over 3 thermowells (figure 2). plan view
"TZI" Figure 2.
Arrangement of thermowells and thermocouples in first stage pygas hydrogenation reactor
From the gas/liquid separator, after the reactor, hydrogen is recycled to the inlet of the reactor and made up with fresh hydrogen. The liquid, i.e. hydrogenated product, is partly used
247 for quench, the excess amount being pumped to the distillation section. The distillation section is not in operation during startup of the reactor. In the first phase of start-up, hydrogenated product is recirculated through the reactor without charging raw, i.e. non-hydrogenated, C5+. In this phase, an exothermic effect occurs due to the heat of adsorption of hydrocarbons on the catalyst surface. As the start-up progresses an increasing amount of raw C5+ is charged while a balancing amount of hydrogenated product is sent to storage via MPV 6101. 1.2. The incident December 23, 1994, a few minutes before 18.00 h, the wall of the first-stage pygas hydrogenation reactor ruptured during operation. The incident took place during the start-up of the reactor. Due to the operating pressure of 30 bar a mixture of gasoline, hydrogen and nickel catalyst was blown out of the reactor. The mixture immediately caught fire, resulting in a jet of flames of 40 m. The plant was shut down. There were no personal injuries, no danger to the surroundings, no domino effects but the hydrogenation unit sustained substantial damage. One week after the incident NAK3 was on stream again except for the first-stage pygas hydrogenation unit.
2. RESULTS AND DISCUSSION The investigation was led by a multidisciplinary team of experts from DSM. The regulatory authorities were regularly informed of the progress of the investigation. In the phase of gathering information, production staff were interviewed and various companies operating similar processes were visited. All companies were quite open in discussing their experiences. The problems described here did not seem to be unique. The investigation focused on reconstruction of process data, catalyst characterization, material characterization and thermodynamic calculations in order to establish the cause of the crack and more importantly, to avoid similar problems in the future. 2.1 Process data Reactor R601A had been recharged with fresh catalyst one month prior to the incident. Catalyst was activated and the reactor started up in accordance with the operating instructions. Fully hydrogenated Cg-fraction, free of olefins and aromatics, was used for the startup. The catalyst used had not been presulphided for historical reasons and fear of odour nuisance. Although fresh, unsulphided, nickel catalysts are known for their poor initial selectivity, the process was very stable at the time of the rupture (figure 3). As the plot clearly indicates, temperatures are in the normal operating window and gradually increase from the top to the bottom of the reactor. By the end of the startup, the feed had almost completely been replaced by raw C5+, when the reactor wall ruptured for no apparent reason.
248
^%^ TS6022 ...w.^. TS6019 ys^
TS6016
^ ^ . TS6013 .V
TS6010
time (Dec. 23, 94)
Figure 3.
Process conditions on the day of the incident
From analysis of the process data it became clear that a runaway must have occurred a week before the rupture. This runaway was initiated by a sudden increase in the amount of hydrogenated product being sent to storage via MPV 6101, thereby simultaneously increasing the amount of raw C5+ being charged to the reactor (figure 4).
Figure 4.
Process conditions one week before the incident (hourly averages)
Despite the fact that production staff acted according to operating procedures, temperatures in the reactor rose to 320 °C in 1.5 hours. After another 3.5 hours, temperatures had returned to their normal levels and the hydrogen compressor was restarted. As temperatures immediately started to rise it was decided to shut down, depressurize and purge the reactor with nitrogen. All thermocouples except two indicated a decrease in temperature to normal level: thermocouples TS 6019 and TS 6022 remained at a high level for two days (figure 5).
249
o
Dec. 16, 94
date/time
Figure 5.
Process conditions one week before the incident (hourly averages)
Two days after the initial startup, the reactor was quenched with fully hydrogenated C5, fraction which resulted in a normal temperature level for all thermocouples. After assessment of the situation and the sequence of events, it was concluded that the reactor had not been outside its design specifications (T, P). The restart made on December 21 went very smooth up until to the moment of the rupture. 2.2 Catalyst characterization Characterization by means of TPR, elemental analysis, XRD and SEM proved that the fresh catalyst used was identical to the catalyst previously used. The relative difference in dispersion was 12%. Because of the low absolute metal surface area of approximately 4 m^.g"^ and a high metal loading of 10%, this difference in dispersion is too small to account for any difference in activity. Samples were taken from the catalyst left in and that blown out of the reactor. Excessive carbon formation was observed in many places in the reactor (figure 6): up to 50 wt% C was found in various samples.
1 crack second bed hard coal with pulverized catalyst catalyst with some coal
Figure 6.
Carbon formation throughout the reactor after the incident
250 In the first catalyst bed, carbon had been deposited along the reactor wall and in the centre of the bed on the support grid. Only little carbon had formed near the three thermowells. In what was left of the second bed, carbon deposits were observed along the wall. Carbon deposits were harder and thicker in the vicinity of the crack. Scanning and transmission electron microscopy revealed three types of carbon: amorphous, graphitic and whiskers (figures 7 and 8).
Figure 7.
SEM photograph of whisker-type carbon formed during pygas hydrogenation
1^
Figure 8.
SEM photograph of graphite carbon formed during pygas hydrogenation
251 It is well known that the type of carbon formed strongly depends on the reaction conditions. Atomic carbon and precursors formed by dissociative adsorption of hydrocarbons are transformed to polymeric coke, filamentous coke and graphitic carbon at progressively higher temperatures [1-4]. Polymeric hydrocarbons and graphitic films encapsulate the metal surface and deactivate the catalyst. Filamentous carbon (whiskers) generally do not deactivate the metal surface because the metal particle is lifted from the surface. When present in large amounts, however, the catalyst bed will plug and pellets will break up. The relation between the structure of the precursor and the kinetics and morphology of the coke formed is not well understood. An important parameter in the formation and structure of coke deposited is the presence of hydrogen. In the presence of hydrogen, carbon precursors are gasified which keeps the catalytic surface clean and thus active. Moreover, in a hydrogen-lean atmosphere, the catalyst will take its hydrogen from any hydrocarbons present, thereby enhancing the deposition of coke. In the sequence of events the hydrogen flow was stopped according to operating procedures prior to the incident (i.e. stop the feed of one of the reactants). This slows down the hydrogenation rate but the dehydrogenation and thus the rate of coke formation, will strongly increase. Near the crack, large amounts of whisker type carbon were characterized. It is well known that filamentary carbon has a low tensile strength but a high compressive strength [5]. Locally, these carbon deposits had led to pulverization of the catalyst. Lab tests confirmed that the catalyst tends to grow whisker in a CH4/H2 atmosphere. Initially, the investigation committee was put on the wrong track by the potential compressive forces exerted by filamentary carbon. Later on it became clear from the material characterization (section 2.3) that this compressive force on the wall was not necessary for the crack to occur. No a-Al203 was detected in any of the samples taken inside of the reactor, which would have indicated temperatures higher than 1000 °C. Some samples from outside of the reactor were found to contain a-Al203. Whether this phase was formed prior to the rupture or during the fire that followed caimot be decided. It was clear from elemental analysis that there had been a sulphur gradient present across the reactor: the concentration decreased concurrently in axial direction. This indicates that the feed contained little sulphur. Sulphur is known to enhance catalyst selectivity by selectively poisoning the hydrogenation sites that are most active. 2.3 Material characterization The reactor wall is made of 13CrMo44 steel. The crack, which was approximately 30 cm long and 2 cm wide, was in the centre of a large bulge that had a height of 10 cm. For further characterization a section of 1.5*1.5 m was cut out of the reactor wall, half of which was sent to the regulatory authorities. It was manifest that the wall within a radius of 0.3 m around the crack had briefly been at temperatures of 700-750 "C. At the operating pressure of 30 bar, a wall made of this material will fail at this temperature range. The geometry of the bulge and crack found on the reactor wall are as expected under these conditions. Other potential causes like corrosion, hydrogen embrittlement (low T), Nelson hydrogen attack (high T), material defect at the time of fabrication, creep or low cycle fatigue could be ruled out after an extensive study.
252 2.4 Thermodynamics In order to evaluate the conditions during various stages of operation, thermodynamic calculations were performed. Complete hydrogenation of fresh C5+ results in an adiabatic temperature rise of approximately 600 °C. In the presence of quench, i.e. hydrogenated product containing only olefins and aromatics, this temperature will be approximately 520 "C. If, however, hydrocracking occurs the adiabatic temperature can rise to about 1000 °C. In the case of maldistribution and thus bad heat transfer, temperatures higher than 700 "C can easily be reached. The fact that such temperatures were not detected by the thermocouples indicates that there had been a hot spot near the reactor wall.
3. CONCLUSIONS 3.1 Investigation On the basis of the results discussed here it can be concluded that the rupture of the reactor on December 23, 1994 was caused by a brief local temperature excursion to 700750 °C at an operating pressure of 30 bar. The geometry of the bulge and crack are consistent with this scenario. The hot spot can be explained in terms of liquid maldistribution. Maldistribution hampers heat transfer, leading to unwanted side reactions like hydrogenation of mono-olefins, aromatics and even hydrocracking of hydrocarbons. These reactions are highly exothermic and thus amplify the hot spot, i.e. a high temperature enhances vaporization of the feed, which in its turn results in a decrease of heat transfer due to a lower heat capacity of the vapour (positive feedback, figure 9).
Figure 9.
Formation of hot spot during pygas hydrogenation
The run-up to this situation started with the runaway that took place one week before the rupture. The strong increase in the concentration of fresh C5+ during startup, in combination with a decrease in hydrogen flow, caused the temperature excursion that was
253 observed. This in turn caused excessive carbon formation throughout the reactor, upsetting liquid distribution in the next startup. This eventually resulted in the, undetected, temperature excursion to 700-750 "C on December 23. Catalyst characterization confirmed that large amounts of carbon deposits, having various morphologies, were formed. 3.2 Lessons learned and actions taken The incident demonstrates that a runaway reaction can occur in a trickle-phase reactor loaded with a supported nickel catalyst. Presulphiding the catalyst prior to start up will minimize the possibility of a nmaway and improves the intrinsic safety of the process. Presulphiding can be performed in situ as well as ex situ. In situ sulphiding, by spiking the feed with a sulphur-containing compound like dimethyl sulphide, will result in a sulphur front moving through the reactor bed. Ex situ sulphiding is to be preferred in that it gives a uniform concentration of sulphur throughout the bed right from the start. A critical stage in the operation of a pygas hydrogenation reactor is the startup. The pygas hydrogenation reactor is started up using a specific Cs/Cg stream with a predetermined bromine number, diene number and aromatics content. The new operating procedures ensure that the quench, i.e. hydrogenated product, and hydrogen recirculation used are maximized when wall temperatures exceed 200 °C. They also ensure that a minimum linear velocity of the liquid and a minimum hydrogen:liquid ratio are maintained during operation. These improvements will minimize the chances of excessive carbon formation. As an extra source of information 28 thermocouples have been installed on the outside of the reactor wall. The alarm threshold for these thermocouples has been set at 200 °C. Furthermore, a research program has been set up to investigate the kinetics of runaway reactions during operation of the pygas hydrogenation. Experimental data will be gathered by using model compounds under various conditions (P, T, contact time) and studying their relation with the morphology of the carbon deposits. This incident has once again shown the importance of adequate response to near misses, in this case the runaway which was observed one week before the rupture. A thorough check of the catalyst bed after any runaway is called for in order to prevent future problems. In many cases this will imply that the reactor needs to be opened and the catalyst discharged. The sequence of events that eventually led to the incident had not been identified in the HAZard and OPerability study (HAZOP), although the individual events like runaway and carbon formation were known [6,7]. A thorough Process Safety Analysis (PSA) in an early stage of the process design could have revealed the possibility of this sequence of events. The PSA should include an extensive study of incidents in similar units. This illustrates the importance of a PSA prior to the HAZOP study. Finally, the incident and subsequent analysis illustrate the importance of an open discussion within the industry and academia in topics concerning safety, health and environment.
254 4. ACKNOWLEDGEMENTS Parts of this paper have been reproduced with permission of the American Institute of Chemical Engineers. Copyright © 1996 AIChE. All rights reserved [8]. The authors are indebted to prof. J. Geus and Dr. M. Hoogenraad of Utrecht University for performing the TEM analysis and the catalytic test in the formation of carbon whiskers.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
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