Journal of Applied and Industrial Sciences, 2014, 2 (4): , ISSN: (PRINT), ISSN: (ONLINE)

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1 Research Article 181 Research Article Estimation of Blast over Pressures of Possible Explosion in a Furnace in Khartoum Refinery by Using MATLAB Software Mohamed Almustafa *1, M. H. M. Abuuznien 2 and Ibrahim A. Ibrahim 3* (1) Faculty of Graduate Studies and Scientific Research, University of Karary, Khartoum- Sudan *Corresponding Author mmustafagader@gmail.com (2) Department of Chemical Engineering, Faculty of Engineering, University of Karary, Khartoum- Sudan abuuznien77@yahoo.com Telephone: (3) Department of Chemical Engineering, Faculty of Engineering, University of Karary, Khartoum- Sudan *Corresponding author E mail: iahmed153@yahoo.com Telephone: (Received: June 02, 2014; Accepted: August 22, 2014) ABSTRACT Due to the notable industrial development, safety side start to be taken more seriously. Fires and explosions in the process industries, although rare, do occur, and can cause loss of life and damage to the environment. In Khartoum refinery, in the Crude Catalytic reforming unit during naphtha hydro-treating process a vapor cloud explosion or a physical explosion can occur this may happen by a releasing of a compressed fuel gas inside the furnace during its shutdown. This case may occur because of improper maintenance or any failure in the process. The aim of this work is to analyze the effects of possible explosions by using several methods of analysis for safety purposes. Data collection from the field includes the furnace and process parameters which required for the methods of analysis. These data were analyzed through MATLAB software. The analysis showed that the explosion can produce an overpressure that will cause a significant damage to structures, equipment and loss of life. It is shown that applying these methods will upgrading the performance of prevention and mitigation of explosions. I. INTRUDUCION Naphtha is hydro-treated before sent to the reforming section. It is heated, blended with hydrogen and sent to the hydrotreating reactor. With the action of the catalyst; sulfur, nitrogen and unsaturated olefins in the naphtha will react with hydrogen to give rise to hydrogen sulfide, ammonia, and saturated hydrocarbons, while the metal impurities will be adsorbed onto the catalyst. Hydrogen Produced during the reactions is recycled and reused. The hydro-treated naphtha is then sent to a stripper to get rid of the associated lighter hydrocarbons and impurities. The bottom product of the stripper is heavy naphtha which is sent to the reforming section to produce high octane number gasoline [1]. A reboiler (furnace) will provide heat into the stripping tower using fuel gas mostly composed from methane & ethane. Fuel gas from the refinery network is fed to the furnace at the bottom where the burners are located. The combustion flue gases travel upwards providing heat by radiation to the radiant section tubes. Before leaving the furnace via the fuel gas stack, they come in contact with the convection section tubes to exploit the remaining heat. Fuel gas flow rate is controlled with a valve located before the burners [2]. During the start-up of the unit, this valve must be checked for any malfunctioning, a passing valve will continue delivering fuel gas which will fill the internal chamber of the furnace resulting in a fuel gas cloud [3]. This may lead to one of two fatal cases, physical explosion and/or vapor cloud explosion. Physical explosion occurs due to the sudden release of energy, by releasing a compressed fuel gas that formed inside the furnace. A second case can happen if the released fuel gas is mixed with air and forms a vapor cloud inside the furnace. Starting-up the process again or existing of any source of ignition in the presence of this cloud, a vapor cloud explosion will occur [4]. II. MATERIALS AND METHODS First Case: Physical Explosion There are four methods to estimate the energy of physical explosion for a pressurized gas. These methods are represented by Brode s equation, isentropic expansion, isothermal expansion and thermodynamic availability [5]. i. Brode s equation E = Energy of explosion (J). P 1 = Ambient pressure (Pa). P 2 = Burst pressure of the vessel (Pa). V = Volume of the expanding gas in the vessel (m 3 ) ϒ = Heat capacity ratio for the gas (unit less) ii. Isentropic expansion equation iii. Isothermal expansion equation R g is the ideal gas constant and T 1 is the absolute ambient Temperature (K) iv. Thermodynamic availability equation [6] 4

2 182 Second Case: Vapor Cloud Explosion i. Tri-Nitro-toluene (TNT) Equivalency Three methods can be used to estimate the The TNT-equivalency method is originally used for overpressure as a function of distance from the explosion [5]. prediction of a far field effect of vapor cloud explosions. The principle is to convert the available combustion energy in a vapor cloud into an equivalent charge weight of TNT [5]. This method needs to evaluate firstly scaled distance 5 Z = Scaled distance (m/kg 1/3 ). R * = Distance from the explosion center (m). W = Mass of TNT (kg). From scaled distance and figure (1) scaled overpressure (P s ) and impulse (I s ) can be estimated The scaled overpressure is: 6 P s = Scaled overpressure (unit less). P o = Peak side-on overpressure (Pa). P a = Absolute ambient pressure (Pa). The term over -pressure always refers to a gauge pressure. Figure 1. Scaled overpressure and impulse curves for TNT explosion on a surface [5]. The equivalent mass of TNT is estimated using the following equation: 7 W = Equivalent mass of TNT (kg). ɳ = Empirical explosion efficiency (unit less) m = Mass of flammable gas (kg). E c = Heat of combustion of the gas (J/kg). E TNT = Energy of explosion of TNT (J /kg) A typical value for the energy of explosion of TNT which is equal to 4620 kj/kg. Notice that the scaled overpressure data in Figure (1) are for a TNT explosion on the ground at sea level. Overpressure correlations are frequently presented for free-air explosions, with a distant ground surface. To convert surface data to free air data, the mass of TNT is halved to account for the loss of energy reflected by the ground Surface [5]. ii. TNO Multi-Energy Method The TNO method identifies the confined volumes in a process, assigns a relative degree of confinement, and then determines the contribution to the overpressure from this confined volume. Semi-empirical curves are used to determine the overpressure [5]. 8

3 183 shape, is to assume maximum initial blast strength of 10 = Sachs-scaled distance from the charge (dimensionless). R = Distance from the charge (m). E = Charge combustion energy (J).Pa = ambient pressure (Pa). Equation above is represented by the following figure iii. Baker Strehlow Tang Method The Baker Stehlow Tang method is based on a flame speed as shown in table 1, which is selected based on three factors: the reactivity of the released material, the flame expansion characteristics of the process unit (which relates to confinement and spatial configuration) as shown in table 3, and the obstacle density within the process unit as shown in table 3 [7]. A set of semi-empirical curves is used to determine the overpressure, apply to free air blasts level, the energy of the cloud is doubled to account for the strong reflection of the blast wave [7]. 10 Figure 2. TNO multi energy model for (Vapor Cloud Explosion) VCE [5]. The blast peak side-on overpressure is calculated by the following equation: 9 The scale 1 to 10 is the initial blast strength. Figure 2 can be used for determine. If environmental and atmospheric conditions are such that vapor cloud dispersion can be expected to be very slow (during stable atmospheric conditions), the possibility of a vapor cloud detonation should also be considered if, in addition, a long ignition delay is likely. In that case, the full quantity of fuel mixed within detonable limits should be assumed for a fuel air charge whose initial strength is 10 [5]. The most conservative approach to this work, according to the confined furnace Figure 3. Baker-Stehlow Model for VCE [8]. This curve provides the scaled overpressure as a function of the Sachs scaled distance [8]. is classified as low, medium, and high according to the following recommendations of TNO. Methane and carbon monoxide are the only materials regarded as low reactivity, whereas only hydrogen, acetylene, ethylene, ethylene oxide. And propylene oxide was considered to be highly reactive. All other fuels are classified as medium reactivity. Fuel mixtures are classified according to the concentration of the most reactive component [9]. The Baker Strehlow pressure curves apply to free air blasts. Since the vapor cloud for this example is at ground level, the energy of the cloud is doubled to account for the strong reflection of the blast wave [11].

4 184 Table 1. Flame Speed in Mach Number for Soft Ignition Sources [10]. 1D Flame Expansion Case (not in use) (Unit less) High Medium Low D Flam Expansion Case (Unit less) High DDT DDT 0.59 Medium Low D Flam Expansion Case (Unit less) High DDT DDT 0.47 Medium Low D Flam Expansion Case High DDT DDT 0.36 Medium Low DDT = Deflagration to detonation transition

5 185 Table 2. Geometric Considerations for the Baker-Strehlow Vapor Cloud Explosion Model [10] Dimension Description Geometry 3D Unconfined volume almost completely free expansion 2.5D Compressor shelters with lightweight roofs; dense pipe racks Between 3-D and 2-D 2D Platforms carrying process equipment; space beneath cars; open-sided multistory buildings 1D Tunnels, corridors, or sewage systems Table 3. Confinement Considerations for the Baker-Strehlow Vapor Cloud Expansion Model [10] Type Obstacle Blockage Ratio per Plane Pitch for Obstacle Layers Geometry Low Less than 10% One or two layers of obstacles Medium Between 10% and 40% Two to three layers of obstacles High Greater than 40% Three or more fairly closely spaced obstacle layers In this work the reactivity of fuel gas is: medium, because ethane is more reactive than methane in fuel gas [12], flame expansion is: 2D according to the furnace shape and obstacle density is: high. The resulting flame speed from Table 3 is 1.6, and used figure 3 to get P S, then calculated P O from eq. 11: 11

6 K Pa 186 III. RESULTS AND DISCUSSION Results for Physical Explosion Energy of explosion for a pressurized gas is estimated by four methods: Input Data are: P 1 = 1.01 bar, P 2 = 1.4 bar, V = m 3, ϒ = 1.26 Results of this case (physical explosion assumption) are shown in table (4) below: Results for Vapor Cloud Explosion Overpressure as a function of distance from VCE explosion estimated by three methods and shown in table (5) and figure (4): Table 4. Results for Physical Explosion Method Brode Isentropic expansion Isothermal expansion Thermodynamic availability Energy (bar m 3 ) Results Type Table 5. Results for Cloud Explosion at Different Distances Methods Unit TNT TNO Baker Strehlow ɳ % m Kg E C Kj/Kg E TNT Kj/Kg W Kg P R 3 m P R 50 m K Pa P R 100m Pa K Pa Po TNT Po TNO 100 Po Baker Strehlow Distance (m) 0.1 Figure 4. Cloud explosion at different distance Investigation the results of the analysis of the expected explosion, table 4 showed that the thermodynamic availability method produces the lowest value, while the Brode method produces the largest value. Table 5 showed the pressure resulting is very high compared to the tolerable pressure which could cause a real disaster by destroying of buildings, equipment and killing people on the surrounding [5]. The results showed a variation in the amount of pressure could occur depending on analysis method and distance from the center of the explosion, so in matter of safety the highest value must be chosen in terms of prevention and mitigations of risk, in this case that will be the results estimated by TNO Multi-Energy Method. TNT Equivalency and TNO Multi- Energy Method at 3 meters distances away from the VCE showed almost same results, but at 50 and 100 m it showed different results which correspond to the theory, the overpressure curve for TNT tends to over-predict the overpressure near the VCE, and under-predict at distances away from the VCE.

7 187 IV. CONCLUSION It is concluded that these methods can be used to give a reasonable results, in the area of safety, to predict the effects of explosions that may occur in industries. To get more accurate results, application of these methods needs precise assumptions, and taking into account the special conditions surrounding the subject of each individual case. The major concerns for anyone involved with risk assessment related to vapor cloud explosions is the overpressure as a function of distance from the explosion. Once these are known the damage effects can be estimated by using the standard tables of Damage Estimates Based on Overpressure for Process Equipment and Damage Estimates for Common Structures Based on Overpressure. ACKNOWLEDGEMENT This paper was produced from the Ph.D. Dissertation of the first author. The generous support and assistance provided by the graduate faculty of Karary University to complete this work is hereby duly acknowledged and appreciated. REFERENCES [1] Khartoum Refinery Co. Ltd 2006, Continues Catalytic Reforming Unit As Built Design Documents. [2] Khartoum Refinery Co. Ltd April 2009, Operation Manual for Continues Catalytic Reforming Unit Compiled by a group of KRC engineers. [3] Stephens, M. M. 1970, Minimizing Damage to Refineries, Washington, DC, U. S. Department of Interior, Office of Oil and Gas. [4] AICHE (1996a), Guidelines for Use of Vapor Cloud Dispersion Models, 2 nd, New York: American Institute of Chemical Engineers. [5] Crowl, D. A. 2003, UNDERSTANDING EXPLOSIONS, American Institute of Chemical Engineers, New York. [6] Crowl, D. A. 1992, Calculating the Energy of Explosion Using Thermodynamic Availability, J. Loss prev. Process Ind. [7] Baker, Q. A., C. M. Doolittle, et al. 1997, Recent Developments in the Baker-Strehlow, VCE Analysis Methodology, 31 st Loss Prevention Symposium, Houston, TX. New York: American Institute of Chemical Engineers. [8] M.J. Tang, Q.A. Baker 2000, Journal of Loss Prevention in the Process Industries, Comparison of blast curves from vapor cloud explosions. [9] Zeeuwen, J. P., and B. J. Wiekema 1978, the Measurement of Relative Reactivities of Combustible Gases, Conference on Mechanisms of Explosions in Dispersed Energetic Materials. [10] Baker, Q. A., M. J. Tang, et al. 1994, Vapor Cloud Explosion Analysis, 28 th Loss Prevention Symposium, Atlanta, GA. New York: American Institute of Chemical Engineers. [11] Tang, M. J. and Q. A. Baker 1999, A New Set of Blast Curves from Vapor Cloud Explosions Process Safety Progress. [12] Sami Matar, Ph.D., Lewis F. Hatch, Ph.D 1994, 2000 Chemistry of Petrochemical Processes, 2 nd edition, by Gulf Publishing Company, Houston, Texas. [13] Clancey, V. J. 1972, Diagnostic Features of Explosion Damage, 6 th International Meeting on Forensic Sciences, Edinburgh, Scotland.