S. T. Tiew and D. C. FOO, Centre for Green Technologies/Department of Chemical and Environmental Engineering, University of Nottingham Malaysia, Selangor, Malaysia; and R. M. RAZALLI, IGL Services Sdn Bhd., Kuala Lumpur
In the chemicals, petroleum refining and natural gas industries, gas flaring systems are commonly installed for safe operation to burn associated and non-associated gases. Gas flaring is a common process in hydrocarbon processing plants during maintenance or abnormal situations.1 In addition to providing safe operations, a gas flaring system also helps to reduce air pollution from hydrocarbons. As most components in hydrocarbons are greenhouse gases (GHGs), they show greater greenhouse effect than carbon dioxide (CO2) with higher carbon equivalent value [e.g., methane (CH4) with 30 CO2-e–36 kg CO2-e].2,3 It is therefore necessary to design a flaring system to provide fast conversion to reduce the GHG emissions into the atmosphere.
Safety is a critical element for every operating plant. An appropriate flare system should always be available and capable of performing its intended purpose through all emergency plant conditions. According to American Petroleum Institute (API) Standard 537,4 flare-related components should be designed to operate and properly perform under specified service conditions for a minimum of 5 yr without the need of an outage of the operating facility. Therefore, the design of the flaring system is important to satisfy the minimum requirement to ensure the safety of an operating plant.
Process simulation may be used to optimize the gas flaring system, as it provides comprehensive visualization and reveals debottlenecking potential for a given system. In the work by Davoudi, et al.,5 a proprietary simulation toola was used to assess two flaring networks in the South Pars gas processing plant in the Middle East. The results showed that the backpressure of flaring networks was overestimated, so the designed gas sweeping flowrate can be reduced by one-third.5
As gas flaring systems are considered unprofitable, it is important to reduce their capital cost. A cost-reduction exercise can be conducted in several ways (e.g., equipment and pipe sizing reduction). Equipment reduction can be performed through integrating flaring networks. Such work was reported by Pemii, et al.,6 who integrated flare networks that are in close proximity, resulting in an optimized flare system of lower cost and environmental impact. Pipe size reduction can be carried out through flaring system debottlenecking studies, in both steady-state and dynamic models. Dynamic models require the calculation for time-dependent pressure safety valve (PSV) opening rate and time-dependent heat transfer rate. This is generally more time-consuming and requires computation resources. Therefore, the optimization of the flaring system may be attempted through a steady-state model. Assumptions made include maximum heat transfer rate and maximum allowable relieving rate. This eliminates the computation for time-dependent variables.
This article details a gas flaring system that was designed by considering different emergency scenarios. Cost reduction was conducted by performing pipe analysis for each scenario. The proposed framework made use of the simulation modela for the design of a gas flaring system. Further evaluation of the gas flaring systems was performed by installing additional equipment. Pipe analysis was conducted for the optimization of the gas flaring system.
Methodology. FIG. 1 shows the flow chart for designing the gas flaring system proposed in this work. First, the simulation model of the process was first generated using a commercial simulation software.a Physical properties of molecules (e.g., mole fraction, heat capacity, latent heat) and equipment operating conditions (e.g., temperature, pressure) were exported to Microsoft Excel. Other equipment data—such as volume of equipment, maximum allowable working pressure (MAWP), maximum allowable accumulated pressure (MAAP) and liquid inventory—were also extracted to Microsoft Excel. API Standard 521 was used as the major design code in this report.7 The emergency scenarios considered were based on API Standard 521.
Based on API Standard 521,7 the gas flaring system design was constrained by allowable backpressure (the maximum backpressure that can be withstood by a PRV in the gas flaring system) and Mach number The allowable backpressure of PSVs varies with different manufacturers—however, the allowable backpressure cannot exceed 21% of the set pressure of the PSVs (according to API Standard 520).8
Conversely, the Mach number is defined as the actual fluid velocity over sonic velocity through the fluid at the associated temperature. The Mach number limit is to prevent the flow from entering transonic flow, which may create shock waves and dramatically increase drag. According to API Standard 521,7 the Mach number of a flare header is allowed up to 0.5 for short-term, peak, infrequent flow that is mainly seen during emergency situations. The Mach number of tailpipes can be as high as 0.7. The Mach number of 0.2 for continuous flaring is not considered in this work, as routing flaring is excluded during the design.7 TABLE 1 shows the maximum allowable Mach number in different situations.
The maximum relieving rate of each PSV in each scenario was calculated based on API Standard 521.7 Next, the gas flaring system was designed using the simulation toola and considering all scenarios. To determine possible improvements to the design, a pipe analysis was performed for all flare headers to reduce their pipe size.
To prevent equipment overpressure, backpressure was used as a limiting factor during the pipe analysis (i.e., the pipe size was reduced to the maximum allowable backpressure). Backpressure tolerance is defined as the difference between the allowable backpressure and exerted backpressure. Due to safety considerations, backpressure tolerance is prohibited to be a negative value. When the pipe size was reduced, the reduction of backpressure tolerance of the PSVs remains identical. Therefore, the PSV with the lowest backpressure tolerance was selected to perform the pipe analysis.
After the pipe size reduction, the exerted backpressure of all PSVs was examined to ensure that it fell below the allowable backpressure. A rating calculation in the simulation toola was used to ensure the feasibility of the system in terms of backpressure and Mach number. If there is no further reduction in pipe sizing, the optimized gas flaring systems will undergo cost analysis. Cost correlations from Stone, et al.,9 were used to calculate the cost of pipes, given as Eqs. 1 and 2:
CP = [127LD1.21/ 100] (where 1 in. < D < 24 in.) (1)
CP = [139LD1.07/ 100] (where 30 in. < D < 60 in.) (2)
where D is the diameter of the pipe [in inches (in.)] and L is the length of the piping [in feet (ft)].
After selecting the gas flaring system, the addition of equipment (e.g., reactor, distillation) was considered before any decisions were made. If there is additional equipment, a re-evaluation is required for all flaring systems. In the absence of additional equipment, a detailed piping drawing of the chosen flare system was drawn in the simulation toola.
Case study. A case study on an ethanol production plant was evaluated for its flare system design; the process flow diagram is shown in FIG. 2. The process flow of the ethanol production plant begins with the fermentation of a glucose mixture, followed by the purification of ethanol. The glucose mixture is sent to two glucose fermenters (GF-001, GF-002) that are operated at 35°C and 2 bar. Dilute ethanol is produced in the fermenters and sent to the ethanol distillation column (DC-001) for purification; this column (DC-001) is operated at 120°C and 2 bar. Ethanol leaving the top stream of DC-001 has a purity of 60% and is sent to the azeotrope distillation column (DC-002) for further purification. Ethylene glycol is added to DC-002 as solvent to ease the separation of the ethanol-water mixture; DC-002 is operated at 135°C and 1.8 bar. Ethanol is purified to 90% before it leaves DC-002 and is sent to a storage tank. As ethylene glycol is an expensive solvent, a recovery column (DC-003) was installed to purify ethylene glycol for its recovery to DC-002. Note that DC-003 is operated at 150°C and 1.6 bar.
As ethanol is produced and purified in the plant, a gas flaring system is required to prevent accumulation of ethanol in the process during emergencies. Ethylene glycol is also a hazardous chemical, and due to these materials, fermentation and distillation are two hazardous sections that were considered in the gas flaring design. The fermentation section is the main reaction section in the plant where a large amount of ethanol is produced, while the distillation and accumulator sections involve ethanol purification where high-purity ethanol and large amounts of ethylene glycol exists.
To design the gas flaring system, various emergency scenarios were screened based on the major potential circumstances. During the process operation, emergency scenarios may occur due to closed outlets, utility failure, external fire and chemical reaction. Among these, the external fire scenario is the most hazardous as high temperatures and pressure may build within the equipment, causing overpressure and rupture of equipment.
According to API Standard 521,7 there are three external fire scenarios: open pool fire, confined pool fire and jet fire. Among these scenarios, a confined pool fire is not considered as the plant is in an open area where the air-to-fuel ratio is sufficient for continuous combustion. As a jet fire will only occur when the reservoir pressure is > 3 bar,1 jet fire is excluded from the scenarios as the highest operating pressure in the plant was 2 bar, which is lower than the required pressure. Therefore, only open pool fire is considered in this case, where constant heat radiation is emitted from the fire due to the stabilized combustion of open pool fire.
A closed outlet is the most frequent scenario and can be caused by stream blockage, controller failure and reverse flow. The inadvertent closure of a valve at the outlet of the equipment during operation can expose the equipment to a pressure that exceeds its MAWP. All closed outlet scenarios are independent scenarios, as the outlet blockage of a stream does not cause the blockage of other streams in the process. To study the design of a gas flaring system, both the most hazardous scenario and the most frequent scenario were considered to achieve a better vision of the gas flaring system design. In the process, a PSV is installed for each piece of equipment to relieve its accumulated gases, avoiding any equipment overpressure. A summary of the sources of the gas flaring design is shown in TABLE 2.
Design approach. The base case of the gas flaring system of the ethanol production plant was designed using the simulation toola, with Peng-Robinson as its thermodynamic model. The worst-case scenario, shown in TABLE 2, was considered for the design of the gas flaring system. To reduce the capital cost, the size of the piping in the gas flaring system is to be reduced by decreasing the governing flowrate, which is the highest relieving rate within the selected scenario.
FIG. 3 shows four different configurations of the considered gas flaring system. In Alternative 1 (top left), the sub-headers of the distillation section, fermenter section and accumulator section were integrated, leading to a reduced number of sub-headers that are connected to the flare header. The relieving rate from the PSVs in each section were connected to a sub-header, where a total of three sub-headers were used. The governing flowrate in Alternative 1 was determined as 39,269 kg/hr, which has the total relieving rate of PSV-004 and PSV-005.
Alternative 2 (top right) was designed to reduce the size of the flare header by having two separate flare headers (i.e., Flare Headers 1 and 2). Each fermenter is connected to a flare header, segregating the governing flowrate. The governing flowrate of the first header is the total flowrate from PSV-001, PSV-002 and PSV-003 (i.e., 35,154 kg/hr), while that of the second header originates from PSV-004 (i.e., 19,634 kg/hr).
In Alternative 3 (bottom left), the governing flowrate is further segregated. The highest flowrate of PSV in Flare Header 1 was connected to Flare Header 2, which significantly reduced the governing flowrate in Flare Header 1. Overall, the governing flowrate of Flare Header 1 originates from PSV-002 and PSV-003 (i.e., 22,068 kg/hr), while the governing flowrate of Flare Header 2 header originates from PSV-004 (i.e., 19,634 kg/hr).
In Alternative 4 (bottom right), the pipe size of the sub-header based on Alternative 1 is reduced. This decreases the governing flowrate of Sub-header 1 from 39,268 kg/hr to 19,634 kg/hr, while remaining the governing flowrate of Flare Header 1. After the simulation of the four alternative designs, the optimization of piping is then carried out for each alternative. The pipe size was reduced until the simulated backpressure reached the allowable backpressure. For all alternatives, the smallest available size (2 in.) is sufficient for the relieving rate of the closed outlet scenario. Therefore, the optimization of pipes in the Accumulator Section (PSV-006 to PSV-008) is infeasible. The summaries of optimization are shown in TABLE 3.
As shown in FIG. 4, there is high backpressure tolerance of PSV-004 and PSV-005 in Alternative 1. However, further optimization is infeasible, as PSV-002 limits the reduction of the flare header. Additionally, the size reduction of the sub-header is also infeasible, as the exerted backpressure will exceed the allowable backpressure when decreasing one pipe size. Alternatives 2 and 3 give similar backpressure of PSVs due to the similar design of the two alternatives. The segregation of PSV-004 and PSV-005 allowed the size reduction of pipes in the fermenter section. Further pipe size reduction is infeasible as the exerted backpressure exceeded the allowable backpressure. In Alternative 4, the backpressure profile shows a similar trend with Alternative 1—the improvement in Alternative 4 was performed based on Alternative 1. All alternatives are feasible as the exerted backpressure for each PSV is below the allowable backpressure. Therefore, the selection of alternatives falls on the comparison of cost.
The PSVs in each optimized gas flaring system were examined to ensure that the exerted backpressure is below the allowable backpressure (FIG. 4). As the sizes of the flare stack and horizontal knockout drum are identical among the alternatives, the estimated costs for both equipment are excluded. Therefore, the comparison was achieved by calculating the cost of pipes in each alternative using Eqs. 1 and 2. Based on TABLE 4, Alternative 1 was selected as the optimum design due to its lowest cost among the design approaches: it indicated a cost reduction of $20,152 from the base case.
Takeaway. Because a gas flaring system is unprofitable, it is beneficial to reduce its capital cost—this can be achieved through pipe size reduction. To design a gas flaring system, various emergency scenarios were screened based on significant circumstances that often occur. In this work, a gas flaring system was designed for an ethanol production plant where a fire scenario was chosen as the worst-case scenario. Various configurations of the gas flaring system were simulated using a proprietary simulation toola to explore alternatives with lower capital cost. The optimization of each alternative was conducted through the analysis of backpressure tolerances. Compared to the base case, results showed that the piping cost of the flaring system design could be reduced by 39% after optimization. HP
NOTES
a Aspen HYSYS® with Aspen Flare System Analyzer™ (AFSA)
LITERATURE CITED
SHIE TECK TIEW is a chemical engineering graduate from the University of Nottingham, Malaysia. His research interests are in the areas of process safety, process design and optimization, and chemical product design. His final year research project included work on the design of fragrance molecule using machine-learning and computer-aided molecular design. Tiew also has experience in sustainability during his summer internship at Kuala Lumpur Kepong Berhad. The author can be reached at jackwintiew@hotmail.com.
DOMINIC C. Y. FOO is a Professor of Process Design and Integration at the University of Nottingham, Malaysia. He was the winner of the Innovator of the Year Award 2009 of the Institution of Chemical Engineers (IChemE), and the Outstanding Asian Researcher and Engineer and Top Research Scientist Malaysia 2016, among other honors. He is Editor-in-Chief for Process Integration and Optimization for Sustainability, Subject Editor for IChemE Transaction Process Safety and Environmental Protection, and an editorial board member for several journals. Foo has published and edited 10 technical books, and is certified as a PEng (Board of Engineer Malaysia), a Chartered Engineer (Engineering Council UK), and an ASEAN Chartered Professional Engineer. The author can be reached at dominic.foo@nottingham.edu.my.
RAZMAHWATA BIN MOHAMAD RAZALLI is a Lead Engineer for IGL Services Sdn Bhd, and earned both BA and MEng degrees. With more than 25 yr of experience in the oil and gas industry in both design and operations, he has upstream and downstream experience in process safety, measurement and allocation, process engineering and offshore technical support, and is also a training provider. The author can be reached at razmahwata.razalli@igl.com.my.