I. CHEM. E. SYMPOSIUM SERIES NO. 85

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1 THE DESIGN OF EMERGENCY FAIL-SAFE SYSTEMS FOR THE CONTINUOUS SULFONATION OF AN AROMATIC COMPOUND M. E. Quinn*, E. D. Weir*, T. F. Hoppe* For processing in a continuous sulfonation unit near the region of high decompositional reactivity, effective failsafe systems have been proposed. Quantification of thermal parameters under standard processing, process deviation and process interupt conditions was performed by integrating various thermal measurement techniques. Each processing component was analyzed independently, as well as with respect to its interaction with other components. Due to the violence of the decomposition involved, effective emergency venting was considered unlikely. Various quenching and dumping systems were recommended as alternatives. INTRODUCTION During the start-up phase of a continuous system for the sulfonation of an aromatic compound, a thermal explosion occurred in a pump and recirculation line. Although the incident damaged the plant and interrupted production, no injuries were incurred. ANALYSIS OF INCIDENT The reason for the explosion was clear immediately after the incident. While starting up recirculation of the reaction mass, pump and line pluggage occurred. This problem was corrected and line circulation was regained. Four hours later an explosion occurred resulting in the blow-out of the pump seal, immediately followed by rupture of the circulation line. An investigation of the incident revealed that during pipe cleanout some insulation was removed, leaving an exposed, untraced portion of the line. This condition apparently led to slow solidification of the reaction mass and a dead-headed pump. Calculations based on pump data indicated that a temperature 60 C above the processing temperature could be reached within 5 minutes after dead-heading. It had been previously determined that the rate of decomposition is considerable at this temperature and that the total decompositional potential of the reaction mass is large ( 500 kcal/kg). Because of the high decompositional potential and close proximity of the processing temperature to the region of high exothermic activity, a comprehensive Process Operations Review and Hazards and Operability Study were undertaken. The Hazards and Operability Study (1) was selected for this plant review because it is especially suited for continuous plants. The initial phase, which is the topic of this paper, involved development of the following data: -Kinetics and thermodynamics of decomposition of the sulfonation mass. -Kinetics, thermodynamics and crystallization characteristics of the reaction of the sulfonation mass with cooling medium (water). After evaluation of the data, recommendations were made for normal operation (processing temperature, heating and cooling parameters, pumping) and emergency operation. *Ciba-Geigy Corporation, Toms River, New Jersey 31

2 SULFONATION PLANT A simplified schematic of a portion of the sulfonation plant is shown in Figure 1. The general reaction scheme is also given. A cascade of three 100 gallon reaction vessels is followed by a 2000 gallon surge vessel. The fast sulfonation reaction is nearly completed in reactor Rl yielding 89% wt. ArSO 3H. The product is subsequently crystallized by the simultaneous addition of the reaction mass and water into vessel Cl. In each vessel and in the transfer lines the potential dangers are: -Overheating (too high wall temperature) -Underheating (too low wall temperature, crystallization, loss of heat transfer) -Heat accumulation (insufficient heat transfer) -Contamination (water leakage) THERMAL STABILITY STUDY The objectives of the thermal stability study were: 1. To develop a data base for the thermal stability of standard and off-standard sulfonation masses. 2. To translate thermal data into heat flow data for scale-up of fail-safe operations. In order to properly evaluate thermal data and remove instrument related factors, it is necessary to integrate various thermal analytical techniques (2). The following methods were used in this study: Temperature Programmed Differential Thermal Analysis (Dynamic DTA) was used in the dynamic mode (5 C/min heating ramp) to quantitatively determine heats of decomposition. This technique ensures that the material's full potential is observed by exposing the sample to all possible temperature regimes. Because thermal effects are strongly dependent upon experimental conditions (thermal inertia factor, heating rate, unstirred sample), the development of relative kinetics and onset points must be viewed with caution, especially with autocatalytic mechanisms. Isothermal Differential Thermal Analysis was used to investigate induction times and the effects of thermal aging. Constant Temperature Stability (GTS) is an isoperibolic method in which the surrounding temperature of a 2 gram sample and reference is held constant for a selected time period (3). A positive deviation in the temperature of the sample over the reference is noted as exothermic activity. This technique provides an inexpensive and simple determination of induction time or time dependency of an onset point. With Accelerating Rate Calorimetry (ARC) the kinetics of an exothermic decomposition may be measured under near adiabatic conditions (4). With this data, predictions of heat generation and response time under loss of cooling or loss of agitation in the plant are made. Considerable caution must be exercised with utilization of these data because of the possible distortion of kinetic data due to the thermal inertia factor. Heat Accumulation Testing involves placing material in an insulated Dewar Flask into an oven at constant temperature. Owing to the insulating factor small quantities of heat can accumulate in the flask. This factor significantly increases the sensitivity of this method over the CTS and DTA methods. Information relative to induction times is obtained. Thirty samples were evaluated in an investigation of the decompositional kinetics. These included incident material, standard sulfonation mass, thermally aged sulfonation mass, reduced and excess SO 3 material, and reduced and excess H 2SO 4 material. Some process deviations did alter the decompositional kinetics at temperatures well above the processing temperature. But, in most cases, these did not significantly increase the danger of processing at the proposed temperature, Tp. The process deviations which affected the kinetics 32

3 were strongly considered in evaluating the emergency systems. For example, residual SO 3 significantly altered the time train of the runaway resulting from a water leak, due to the additional heat generated. For the sake of brevity only the experimental results for the incident material will be presented. That is the material taken from the recirculation line between vessels R3 and S1. The heat of decomposition as measured by Dynamic DTA was -440 kcal/kg with an onset point of Tp +100 C (Figure 2). When the sample was thermally aged at Tp +40 C for 7 hours and run under Dynamic DTA, substantial degradation of the material was observed (Table 1). Evaluation of the material by Constant Temperature Stability, indicated no reaction during 8 hours at Tp +20 C. Induction periods of 1 to 3 hours were observed from Tp +50 C to Tp +60 C, suggesting an autocatalytic mechanism No heat accumulation was observed over 30 days at Tp +10 C in a sealed Dewar Flask. The onset point as measured by ARC was approximately 100 C less than the DTA onset (Figure 3). This difference cannot be attributed to differing sensitivity levels alone. If this were the case, estimations predict that the difference would be on the order of 50 C, rather than 100 C. However, the accumulated experimental time at onset was 11 hours by the ARC method versus 20 minutes by the Dynamic DTA test. So again, the time/temperature dependence or autocatalytic nature of the decompositional kinetics has been verified. Heat generation rates were determined from ARC (Table 2). The following conditions inherent to the test method were used in scale-up: 1. Raw data were developed under unstirred conditions. 2. Mathematic compensation was used to remove thermal inertia of sample container. 3. Zero order kinetics were assumed. The time to maximum rate (TMR) was defined as the time to reach 200 C. At this temperature violent decomposition and off-gassing occur under adiabatic conditions. Under loss of plant cooling, this may represent the response time before a catastrophe occurs. HEATS OF DILUTION In the evaluation of a process the compatibility of the reaction mass and heat transfer medium must be considered. Because of the possibility of triggering a thermal runaway by a water/steam leak in this case, a complete knowledge of the consequences was considered essential. The amount of heat liberated by the water addition to the sulfonation mass was measured with a Bench Scale Calorimeter (5). The basic design of the apparatus is a stirred tank reactor with fast circulation of oil through the jacket. The reactor unit contains a programmable cascaded controller which adjusts the temperature of the circulation loop. Heat of reaction, kinetics, rate of heat evolution and heat capacity are all readily attainable. Water added to the sulfonation mass will react with residual SO 3, hydrate ArSO 3H molecules and dilute the acid solution. The parameter selected to relate heats of dilution was percent ArSO 3H by weight. The results are graphically presented in Figure 4 in terms of cumulative dilution heat to 30% ArSO 3H. Heat generation from water leakage into the reactor represents a considerable hazard. An adiabatic temperature increase of 71 C is possible, which could trigger a thermal runaway of very high potential. Based on the measured heats of dilution, adiabatic temperature profiles for various water leaks (1, 5, 10 gpm) were developed. The results are shown for vessels Rl and S1 in Figures 5 and 6. A temperature rise in Rl of 60 C is expected within 3 minutes of start of a 10 gpm water leak. These data help to specify emergency system response times. Thermal stability of the diluted sulfonation mass was investigated by ARC (Table 3). The results provide information for processing and pumping the diluted sulfonation mass during normal operation, and describe the relative 33

4 reactivity of a sulfonation mass involved in a water leak. Similar reactivity was observed at Tp +30 C to Tp +70 C for samples with 89 to 60% ArSO 3H. With 30% ArSO 3H, the reactivity decreased substantially in this region. At temperatures greater than 200 C, however, a large exothermic decomposition would still ensue. Crystallization points for the diluted samples were determined with Dynamic DTA (Figure 7). These values were needed to evaluate fail-safe options with respect to reduced heat transfer from crusting and ineffective discharge due to line pluggage. Of course, these data are also necessary for specifying pumping and heating/cooling parameters during normal operation. It is interesting to note that the standard reaction mass (89% ArSO 3H) crystallizes at Tp -20 C, whereas the 80% ArSO 3H sample crystallizes at Tp +20 C. FAIL-SAFE OPERATIONS Because the off-gas potential and decompositional energy of the sulfonation mass is extremely high, effective emergency venting under runaway conditions was considered unlikely. As an alternative, an effective fail-safe system, independent of utilities, which prevents an unsafe temperature elevation was required. In addition to the fail-safe system, a temperature alarm system was considered necessary. The alarm concept allows corrective action to be taken as a function of the circumstances or cause. For example, for a temperature rise due to heat contribution from decomposition alone, calculations show that plant personnel would have approximately one hour to regain cooling, restart agitation, etc. before activation of the fail-safe system. The following fail-safe options were considered for each vessel: 1. Rapid discharge - The vessel is emptied through a bottom outlet by gravity or by pressurizing the vessel. 2. Dilution with inert material - Inert material in an overhead tank is flooded by gravity into the vessel. 3. High pressure water deluge - Water is sprayed into the vessel at 225 gpm with vessel overflow into a catch basin. Four of the design models are presented. Rapid Discharge of Vessel S1 This model assumes a 10 gpm water leak into 700 gallons of sulfonation mass at Tp. The alarm is initiated at Tp +20 C, and vessel feed and cooling water are curtailed. Discharge is initiated at Tp +30 C with a discharge rate of 100 gpm. To develop a temperature vs. time profile within the reactor, the following heat generation and absorption contributions were considered: Heat Generation = Heat of Dilution Heat Absorption = Sensible Heat of Reaction Mass and Water The profile was developed for small time intervals taking volume reduction into account (Figure 6). No problems with solidification are predicted because Tp +30 C is reached at 88% RSO 3H, only one minute after start of water leakage. Crystallization will not occur above this temperature. The rapid temperature acceleration near the end of the discharge is due to the reduced heat sink in terms of mass and the increased heat of dilution per reaction mass. For the short response times and temperatures involved, heat of decomposition and jacket cooling have nearly negligible effects. Dilution with Inert Material in Vessel S1 Rapid cooling may be achieved by flooding the reaction mass with an inert material. This case considers dumping 100% H 2SO 4 into 1000 gallons of sulfonation mass at Tp +30 C. A 1000 gallon tank would be suspended 3m above vessel S1 for this purpose. To develop a temperature vs. time profile within the reactor, the following contributions were considered: Heat Generation = Heat of Decomposition Heat Absorption = Sensible Heat of Inert Material and Reaction Mass 34

5 It was assumed that the temperature rose 30 C above Tp due to loss of cooling, and that no water leaks were experienced. For this case, cooling to Tp would be accomplished with 385 gallons of 100% H 2SO 4 with an efflux time of 3 minutes through a 2" line. Because H 2SO 4 is considerably more dense than the reaction mass, cooling could conceivably still be accomplished under loss of agitation if the dispersion were uniform. Note that for flooding with 96% H 2SO 4 a significantly larger volume, 1100 gallons, is required with an efflux time of 4 minutes through a 4" line. High Pressure Water Deluge in Vessel Rl High pressure fire water with an associated vessel overflow was considered for use in emergency cooling. This case assumes a 10 gpm water leak into vessel Rl with overflow at 75% capacity. When alarm is initiated at Tp +20 C, feeds and cooling water are terminated. Fire water is introduced at Tp +30 C at 225 gpm. To develop a theoretical temperature vs. time profile the following contributions were considered: Heat Generation = Heat of Dilution Heat Absorption = Sensible Heat of Water and Reaction Mass After 0.5 minutes of water leakage, Tp +30 C is reached. On an adiabatic basis, 0.5 minutes of fire water flow are required to cool to Tp. A lab simulation of water flooding in a 1 liter stirred reactor was carried out to verify the response time as well as to investigate line pluggage, pressure effects from steam generation and mixing efficiency. No significant steam pressures were developed. Overflow line pluggage was experienced at ambient conditions. Depending on the geometry, the plant overflow line may require tracing. In the event of agitation failure, water pressure is critical for adequate mixing, expecially with large volumes. A minimum pressure of 100 psig was recommended with a multiple nozzle unit to enhance mixing. The experimental temperature profile is shown for deluging a standard sulfonation mass (Figure 8). Cooling to Tp was accomplished in 0.7 minutes, with a maximum temperature of Tp +37 C. For deluging a 50% excess SO 3 batch with no agitation the maximum temperature was Tp +69 C, and cooling was successfully achieved. The feed lines, scrubber line and overflow to vessel R2 must be guaranteed closed before activation of water flooding. No trickling in of fire water prior to the 225 gpm deluge is permitted. Dilution with Inert Material in Vessel Rl This case assumes a 10 gpm water leak into Rl at Tp. An alarm is initiated at Tp +20 C and vessel feeds and cooling water are terminated. 100% H 2SO 4 is introduced at Tp +30 C at 100 gpm. The following contributions were considered: Heat Generation = Heat of Dilution Heat Abosrption = Sensible Heat of Inert Material and Reaction Mass On an adiabatic basis, cooling to Tp is achieved in 1.5 minutes (150 gallons). For the short response time involved, jacket cooling and heat of decomposition are negligible. For the smaller reactors in the cascade, Dilution with Inert Material is preferred over Rapid Discharge, because with a small reactor involved in a water leak the crystallization temperature may be reached early in the discharge process leading to plugging and solidification. The criticality of terminating feeds at Tp +20 C is to be stressed. Under loss of heat transfer, the adiabatic temperature rise from the sulfonation reaction is 270 C. A violent explosion would result. PUMPING RECOMMENDATIONS The following recommendations were made for pumping: Normal Operation - Undiluted Sulfonation Mass 1. Jacket transfer lines with circulating fluid at Tp +10 C. 35

6 2. Keep pump capacity and transfer lengths to a minimum. Emergency Operation 1. Install temperature, pressure and flow elements in pipe and pump with automatic shutdown capabilities. 2. Provide for easy discharge at Tp +30 C. CONCLUSION Because of the proximity of the processing temperature to the crystallization point on one side and the decompositional onset on the other side, technical safety measures are required. Based on the extremely high exothermic potential these measures must be fail-safe. A comprehensive thermal analysis permits quantitative design and verification of emergency options. Invaluable data relative to response time permit the implementation of non-hazardous and cost effective measures. REFERENCES 1. Hazard and Operability Studies, Chemical Industries Association Ltd., 93 Albert Embankment, London SE 1 7 TU. 2. T. Hoppe and E. D. Weir, "Integrated Testing for the Evaluation of Thermal Hazards", Inter. Conf. on Thermal Anal. 7th, 1982, p J. Lutolf, Staub-Reinhaltung der Luft, 31, 93 (1973). 4. D. I. Townsend, "Accelerating Rate Calorimetry", I. Chem. E. Symposium Series No. 68, 1981, p. 3/Q:l F. Brogli, G. Giger, H. Randegger and W. Regenass, "Assessment of Reaction Hazards by Means of a Bench Scale Heat Flow Calorimeter", I. Chem. E. Symposium Series No. 68, 1981, p. 3/M:l

7 Reaction Ar-H + SO 3 ArSO 3H Figure 1 Schematic of Sulfonation Plant and Reaction Figure 2 Dynamic DTA Recording of Sulfonation Mass (Incident Material) Figure 3 ARC Recording for Sulfonation Mass (Incident Material) 37

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