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Consequence Modeling for Accidental Events of SO2 Release in a Detergent Manufacturing Company

Article type: Original article Article history: Received: 14 August 2019 Revised: 7 October 2019 Accepted: 3 November 2019 DOI: 10.29252/jhehp.5.4.5

evaluate industrial facilities based on various scenarios. Consequence analysis is a tool used by process engineers to assess industrial activities to determine the extent of the risks and possible casualties caused by accidents [1]. In the present study, the consequences of possible process events from the furnaces and transmission pipelines of sulfur dioxide (SO2) gas have been investigated using the PHAST7.11 software.
SO2 is a colorless gas, which reacts on the surfaces of many solids and airborne particles. It is non-explosive and has a suffocating odor, weighing almost twice as air. SO2 is soluble in water and raindrops and could convert into sulfurous acid. Furthermore, sulfur trioxide (SO3) is combined with water vapor and forms sulfuric acid. It is estimated that SO2 remains in the air for 2-4 days on average [3,4]. Sulfur oxides are among the most important atmospheric pollutants, which are produced through the combustion of combustible materials (especially in fossil fuel power plants), causing damages to humans, plants, animals, objects, and equipment [5]. Some of these injuries in humans include allergic rhinitis, lung inflammation, lung cancer, dyspnea, coughing, sneezing, and decreased lung capacity [6].
Considering the severe toxicity of SO2 gas, the present study aimed to evaluate the consequences of SO2 discharges from pipelines during the processing and transfer of SO2 gas.

Study Design
The methodology of the present study was based on the guidelines of the American Society of Chemical Engineers (AICHE) to assess the consequences of the release of chemicals. Various methods that are used to evaluate these consequences often have a relatively similar framework, with some differences in the details and stages. In the current research, in addition to visiting the industrial site and studying the production process, the knowledge and experiences of the employees and records of the previous accidents in the industrial unit were exploited to select the proper scenario. After scenario selection, the hazard and operability study (HAZOP) technique was used to analyze the conditions. After screening the results, the worst and most likely scenarios were selected. Finally, the modeling and evaluation of the consequences of the selected scenario of SO2 emissions from the production transmission pipelines were carried out using the PHAST software.

Study Area
A large-scale detergent production company was selected as the location of the study. In this industrial complex, the sulfur was required to be solidly integrated into the refinery, which would enter the furnace in the liquid form after melting. Combustion would occur in the furnace (sulfur, air, and flame), resulting in the production of the SO2 gas, so that the produced SO2 would be transferred by pipes for the next stage of the process. To model the SO2 emissions using the software, basic data were required, which are described in detail in the following sections.

Technical and Process Data for the Studied Equipment
The required technical information of the equipment used to produce and transfer SO2 for modeling included the pipe length, internal diameter of the pipe, number of the flanges connected to the pipe, number of the welding points in the pipe path, and reservoir height. In addition, the important process data for modeling were the internal pressure of the pipe and internal temperature of the pipe ( Table 1).

Climate Data
A digital anemometer (model: MASTECH 61-650 CE) was used to collect the data on the wind speed, humidity, and temperature in each month of 2017. Moreover, the data on the wind rose were collected from the Meteorological Organization, and the atmospheric stability conditions were also evaluated, including six classes (A-F). Consequently, the F stability was considered as the worst condition ( Table  2).

Consequences of the Toxicity of the Released Materials in the Environment
The threshold limit value-short-term exposure limit (TLV-STEL) criteria were used to evaluate the toxicity of the SO2 emissions. The SO2 amount of the TLV-STEL within the occupational exposure limits is 0.25 ppm [7]. Many of the methods that are used for consequence assessment often have a relatively similar framework, with the differences mainly in the details and stages [8].

First Step: Scenario Selection
Four scenarios were applied for the two-ton transmission pipeline to evaluate the consequences of SO2 release. As is shown in Table 3, the criteria for scenario selection were based on the previous studies conducted by DNV specialists [9]. The selected scenarios for the two-ton transmission pipeline were as follows : -First scenario: Continuous release from a rupture with the diameter of five millimeters ; -Second scenario: Continuous release from a rupture with the diameter of 25 millimeters; -Third scenario: Continuous release from a rupture with the diameter of 100 millimeters; -Fourth scenario: complete pipe rupture For the scenarios with the diameter of five, 25, and 100 millimeters, six different directions were considered for gas release (Table 4), including horizontal, horizontal impingement, horizontal angle, horizontal impingement angle, down-impinging on the ground, and vertical.

Second Step: Analysis of the Conditions
For the accurate analysis of the cause of incidents, the HAZOP method was used to assess the risk of the SO2 production and transfer processing operations. In addition, the consequences and causes of SO2 emissions were investigated based on the selected scenarios ( Table 5). The HAZOP technique is a specific structure for hazard identification and assessment in process units by a team consisting of various engineering expertise. The method has been acknowledged as the basis for the identification of the process risks in the design and operation of industrial units [10]. After the process evaluation, an experienced team in HAZOP was formed in the presence of several experts, including technicians, electrician and instrumentation administrators, production administrators, chemical engineering personnel, and health and safety executives. Regular meetings were organized with the team, and the HAZOP method was used to investigate and identify the hazards. In this evaluation, the P&ID map was applied to identify the parameters involved in the process, as well as the causes of the deviation based on these parameters. Finally, the consequences of the deviations were identified.

Third Step: Modeling of the Incident
The simulation of the incidents was carried out after considering all the influential factors in the incidents based on the HAZOP method and described details in the previous sections. The modeling of the release of toxic SO2 was performed based on various parameters, such as the technical features of the furnace and pipes, process features of the furnace and pipes, and atmospheric conditions. Moreover, the PHAST 7.11 software was used to evaluate the toxic effects of SO2 emissions and their modeling [9].
After the modeling, the effects of the incidents were evaluated. To this end, the amount of the casualties and losses resulting from the outcomes of the scenario was calculated. The purpose of the evaluation was to determine the mortality rate in the study. The mortality rate of the individuals affected by the gas poisoning scenario was obtained by calculating the Y-factor variable using Equation 1.    The inappropriateness of the size or weight of the furnace stones makes it possible to reject and add more air inside the furnace, which increases the furnace temperature.
Perforation of boiling points -Gas leakage

Fourth Step: Evaluation of Damage
At this stage, the outputs of the software simulation were interpreted and matched the findings based on the map of the geographical locations of the study area and combining other data, such as the number of the workers at the site, distance from the point of emission (Table 6), and amount of the damage caused by leakage (e.g., possibility of death/injury).

Result and Discussion
According to the data on the climatic conditions and technical specifications of the equipment in the four scenarios modeled in the present study, per 15 employees who were working at approximately 10 meters of length and height of 1-5 meters, the gas leak scenario with the diameter of 100 millimeters and gas emission with the horizontal impingement angle mode was associated with the worst conditions and most significant consequences. Approximately 1.84 seconds after the gas leak and SO2 emission at the distance of 10 meters and height of 1.39 meters, the concentration was approximately 39,339 ppm in August, which represented the worst conditions compared to the other seasons of the year. In this scenario, the possibility of death was estimated at 49%, which equaled almost seven individuals ( Figure 1).
Air temperature, wind speed, and atmospheric stability are among the most important influential factors in the dispersion of released gas. Depending on weather conditions and atmospheric stability, warm seasons were observed to have the highest SO2 emission concentration in the present study, so that the highest concentration of released gas was observed in August.According to the current research, the main consequences of the emissions of toxic SO2 to the environment were toxicity to humans and damage to plants, animals, objects, and equipment [9]. The TLV-STEL criteria are fundamentally used for the evaluation of the consequences of gas emissions based on the guidelines of the American Conference of Governmental Industrial Hygienists (ACGIH).
These guidelines show the highest concentration of chemicals in the air, to which individuals could be exposed for 15 minutes with a 60-minute interval four times per day without causing eye/throat injuries or irreversible health effects. The standard TLV-STEL for SO2 is 0.5 ppm [7]. Some of the main signs of exposure to this gas include pulmonary reactions and stimulation of the lower respiratory tract. Based on the scenarios in the present study, the leakage of the gas from the transmission pipeline caused its release in various ways, which affected the individuals, plants, animals, and equipment that were in the distribution pathways, leading to consequences such as death, plant destruction, and damage to animals and equipment [11].