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Inherent safety index module (ISIM) to assess inherent safety level during preliminary design stage
process safety and environmental protection 86 (2008) 113–119
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/psep
Inherent safety index module (ISIM) to assess inherent safety level during preliminary design stage Chan T. Leong, Azmi Mohd Shariff * Process Safety Research Group, Department of Chemical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia
article info
abstract
Article history:
One of the acceptable methods to quantify the level of inherent safety is based on the
Received 9 January 2007
inherent safety index. This paper reviews presently available techniques for quantification
Accepted 9 July 2007
of inherent safety level in a design and addresses the shortcoming of current techniques by proposing direct integration of process design simulator with inherent safety index. This integrated index is known as inherent safety index module (ISIM) and it is one of the
Keywords:
modules developed in a newly proposed framework to determine inherent safety level in the
Inherent safety
preliminary design stage. This framework is an enhancement of the framework developed
Risk
earlier by Mohd Shariff et al. [Mohd Shariff, A., Rusli, R., Chan, T.L., Radhakrishnan, V.R. and
Consequences
Buang, A., 2006, Inherent safety tool for explosion consequences study, J Loss Prev Process
Safety index
Ind, 19: 409–418]. This new framework allows process information from process design
Preliminary design
simulator to be extracted and analyzed for the determination of inherent safety level (ISL), consequences and probability of unwanted incidences. The availability of such information at earlier stage of design will help process designers to obtain ISL that will assist them in producing safer designs by the application of inherent safety principles in a more efficient and cost effective manner. This paper also discusses the overall concept of the proposed framework to produce an inherent safety tool. A case study is provided to illustrate the benefit of having inherent safety index known to process designers during preliminary design stage. With the right information, modification to process conditions can be carried out and this is likely to produce a safer process plant. # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Safety should be considered and addressed in the whole life cycle of a process system or a facility (Greenberg and Cramer, 1991). There are many established methodologies to identify, analyze, prioritize and manage risks arising from different stages of a plant as illustrated by Taylor (1994). The conventional safety methodologies as shown in Fig. 1 are often carried out in parallel with design process, after much of the process simulation has been completed. Zwetsloot and Askounes Ashford (1999) noted that conventional safety approaches alone are unable to avoid or reduce the risk of serious chemical accidents. They also noted that any re-design done as a result of unfavorable safety performance, after the detailed design stage of the process life cycle would be very expensive compared to alteration in
the early stage, i.e. during conceptual design stage. Also, modification could be carried out relatively easier during preliminary design stage. Khan and Amyotte (2002) reflected similar finding in their work, which concluded that an inherently safer approach is a cost-optimal option considering the lifetime costs of a process and its operation. Their subsequent research showed that inherent safety can be incorporated at any stage of design and operations; however, its application at the earliest possible stages of process design (such as process selection and conceptual design) yields the best results. The principles defining inherent safety as shown in Table 1 were formalized by Kletz (1991). These principles aim to reduce or eliminate hazards by modifying the design (using different chemicals, hardware, controls, and operating conditions) of the plant itself. The conventional safety approaches
* Corresponding author. Tel.: +60 5 3687570; fax: +60 5 3656176. E-mail address: [email protected] (A.M. Shariff). 0957-5820/$ – see front matter # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2007.10.016
114
process safety and environmental protection 86 (2008) 113–119
Table 1 – General principles of inherent safety Principles Intensification Substitution Attenuation
Limitation of effects
Simplification
Error tolerance
Definition Reduction of the inventories of hazardous materials Change of hazardous chemicals substances by less hazardous chemicals Reduction of the volumes of hazardous materials required in the process. Reduction of operation hazards by changing the processing conditions to lower temperatures, pressures or flows The facilities must be designed in order to minimize effects of hazardous chemicals or energies releases Avoidance of complexities such as multi-product or multi-unit operations, or congested pipe or unit settings Making equipment robust, processes that can bear upsets, reactors able to withstand unwanted reactions, etc.
shown in Fig. 1, on the other hand aim to reduce risk of a process by adding protective barriers to mitigate impact. Despite the attractiveness of being able to proactively identify and reduce risk, the principles of inherent safety have not been widely adopted in the industries. The lack of experience and knowledge (field and ‘‘real world plant’’) of the designers who are applying these principles and the lack of recognized methodology to review the agreement of different process alternatives according to the inherent safety principles are among the crucial obstacles to the implementation of this safety philosophy (Moore, 1999). A study by Mansfield et al. (1996), which assessed the familiarity and application of inherent safety among designers and companies, concluded that although many designers know of the basic principles of inherent safety, they are not always clear about how to apply them. There is also a general lack of familiarity with the specific advantages of adopting an inherently safer approach to process design. Rushton et al. (1994) emphasized the need for a computer aid that will perform comprehensive inherent safety analysis at each key decision point in the process life. The key benefits of automation are substantial reduction in time and effort,
Fig. 2 – Problems of implementing inherent safety (Kletz, 1991).
enhanced decision-making, improved documentation, and better understanding of the process. The other reasons for lack of implementation of inherent safety in actual designs are summarized by Kletz (1991) and shown in Fig. 2.
1.1.
Indices for quantification of inherent safety level (ISL)
One of the challenging aspects on the implementation of the inherent safety principles would be to convince the stakeholders and/or process owners about the benefits of investing in inherent safety features. Process designers are often faced with the question, ‘‘How can the benefits of implementing the inherent safety features are quantified?’’ Quantification is a challenging aspect unless a definitive comparison can be made such as two plants with identical requirements were built, with one using inherent safety features and the other one without. However, this would have been a very cost ineffective manner to be carried out. One of the potential solutions to the above queries is using inherent safety indices. The pioneering index was proposed by Edwards and Lawrence (1993). Heikkila (1999) improved the method by including an additional aspect to the index system.
Fig. 1 – Safety analysis program (Taylor, 1994).
process safety and environmental protection 86 (2008) 113–119
Table 2 – Inherent safety index parameters Edwards and Lawrence (1993) Inventory Temperature Pressure Heat of main reaction Heat of side reaction Flammability Explosiveness Corrosiveness Toxicity Chemical interaction Type of equipment Safety of process structure
X X X X – X X – X – – –
Heikkila (1999) X X X X X X X X X X X X
115
All the indices described above rely on manual extraction of design parameters such as from process design simulator for the calculation of inherent safety index. The study can only be done reaction by reaction within the process. It is difficult to analyze the inherent safety level for all process streams in a plant due to the complexity of the processes involved as process conditions change. Manual extraction of process design data limits the effective utilization of the indices above especially for repetitive quantification of inherent safety level as process conditions or chemicals change in the design stages. This paper proposes a framework to address the shortcoming for meaningful use of inherent safety indices during preliminary design stage.
1.2. Integrated risk estimation tool (iRET) for inherent safety application Table 2 compares the parameters used in the two-index system proposed by Lawrence and Heikkila. The parameters were then adopted by Palaniappan et al. (2002) to develop an expert system for the application of inherent safety in chemical process design. Apart from their attempt to develop computer software tool, they also proposed three additional supplementary indices—worst chemical index (WCI), worst reaction index (WRI), and total chemical index (TCI) to overcome shortcoming in earlier indices. The WCI is the summation of the maximum values of the flammability, toxicity, reactivity, and explosiveness indices of all the materials involved in a reaction step. Similarly, the WRI is the sum of the maximum values of the individual indices for temperature, pressure, yield, and heat of reaction of all the reactions involved in the process. The TCI is a measure of the number of hazardous chemicals involved in the route. Gupta and Edwards (2003) developed a graphical method to apply inherent safety index in evaluating six potential routes to produce methyl methacrylate (MMA) in an attempt to graphically show the comparisons. Khan and Amyotte (2004) proposed a new indexing technique which is intended to be applicable throughout the lifecycle of process design. The new index technique is known as integrated inherent safety index (I2SI) and has three indices, i.e. hazard index (HI), inherent safety potential index (ISPI) and inherent safety cost index (ISCI). The HI is intended to be a measure of the damage potential of the process after taking into account the process and hazard control measures. The ISPI, on the other hand, accounts for the applicability of the inherent safety principles (or guidewords) to the process. The HI is calculated for the base process (any one process option or process setting will be considered as the base operation or setting) and remains the same for all other possible options. The HI and ISPI for each option are combined to yield a value of the integrated index as shown in the following equation: I2SI ¼
Mohd Shariff et al. (2006) introduced a feasible framework for inherent safety application in preliminary design stage by integrating process design simulator with consequences of an unwanted event such as explosion. A demonstrative tool was developed by integrating process design simulation software, HYSYS with an explosion model developed in MS Excel spreadsheet. The demonstration tool named as an integrated risk estimation tool (iRET) used a TNT equivalent explosion model. The results from the case studies demonstrated that it is possible to determine consequences during preliminary design stage hence providing crucial information that can be used to improve plant safety. The algorithm involved for iRET is shown in Fig. 3. Apart from the benefits of having early indication of explosion consequences, the tool also eliminates the need to manually transfer information from process design simulator into consequence analysis software. This saves time and reduces chances of data entry error. The concept used in iRET proved very practical for the implementation of inherent safety principles in the preliminary design stage. However, there was no evidence to show how the tool can be utilized to
ISPI HI
Both the ISPI and the HI range from 1 to 200; the range has been fixed considering the maximum and minimum likely values of the impacting parameters. This range gives enough flexibility to quantify the index. For example, an I2SI value greater than unity denotes a positive response of the inherent safety guideword application (inherently safer option). The higher the value of the I2SI, the more pronounced the inherent safety impact.
Fig. 3 – Integrated risk estimation tool, iRET (Mohd Shariff et al., 2006).
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process safety and environmental protection 86 (2008) 113–119
estimate risk without the inclusion of probability estimation in the framework. The present research work expands the concept shown in Fig. 3 to include ISL quantification feature as a part of effort to address suggestions by the industries that ISL quantification methodologies be simplified and made adoptable during early plant design stages. This suggestion was the conclusion from a survey by Gupta and Edwards (2002) which received 63 responses including 36 from industries and consultants, 24 from academic and R&D organizations and 3 from regulatory bodies from 11 countries. The quantification of ISL is determined using an integrated system of inherent safety index, consequences estimation and probability estimation with process design simulator.
2. Inherent safety level (ISL) quantification using inherent safety index module (ISIM) The approach for the quantification of ISL follows the same concept as proposed by Mohd Shariff et al. (2006). A new framework which is proposed to allow process information from process design simulator, in this case HYSYS, be extracted and analyzed for the determination of inherent safety level, consequences and probability of unwanted incidences, as shown in Fig. 4. The availability of such information at earlier stage of design will help process designers to obtain ISL. The process designers can use the ISL as a tool to revise and produce safer designs by the application of inherent safety concept in a more efficient and cost effective manner. This paper discusses only the inherent safety index module (ISIM) as given in Fig. 4 to demonstrate its feasibility to measure inherent safety level during preliminary process design stage. Research is currently on-going to develop the entire framework and results will be published separately. The data such as pressure, temperature, flow rate and composition are extracted from process design simulator (HYSYS) using macros in Microsoft Excel to calculate inherent
safety indices. Heikkila’s (1999) classification approach is adopted as this method is very basic, easy to be implemented and does not require extensive use of proprietary monographs. Pressure, temperature and flow rate can be directly evaluated for their respective inherent safety indices, ranging from 0 to 5 (the worst). The composition of each stream is used to calculate the flammability limits of the mixture and translated to explosiveness. Based on the obtained indices, streams with unfavorable ISL are identified and improvements can be carried out by process designers by applying one or more of the inherent safety principles as outlined in Table 1. The consequences and probability of unwanted incidents can also be assessed using the complete tool which is being developed currently. With the integration with process simulator, it is expected the impact of changes in process conditions to inherent safety level and consequences can be effectively determined during preliminary process design stage.
3.
Case study
An acrylic acid production process is used as a case study to demonstrate the capability of ISIM for inherent safety application in the preliminary process design stage. The simulation model of an acrylic acid plant which is used in this case study was originally produced for academic purposes by Soo (2004) using HYSYS process design simulator. In this case study, ISIM is integrated with HYSYS to demonstrate its capability to determine stream that is more inherently unsafe. Design changes are then made to the identified stream following the principle of inherent safety for the improvement of safety level. For instance, a stream scoring a level 4 index is deemed to be more inherently unsafe compared to a stream scoring level 2. Since safety does not have an absolute level, the application of the as low as reasonable practicable (ALARP) principle is necessary to reduce the index in conjunction with process requirements. For instance, if a process can only work at a particular temperature or pressure, which may score high
Fig. 4 – Framework to determine ISL at preliminary design stage.
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process safety and environmental protection 86 (2008) 113–119
Fig. 7 – Inherent safety index for stream temperature in original design.
Fig. 5 – Parallel reactors in original design of acrylic acid production plant (Soo, 2004). Fig. 8 – Inherent safety index for stream explosiveness in original design. index, other means of increasing safety level, should be explored rather than directly reducing pressure and/or temperature. This means not all scoring high index streams can be reduced to a lower index. Modification of the process conditions has reached the limit once the desired process design objectives such as product quality have been compromised. The simulation process of acrylic acid production plant has a total of 33 streams all together. The main feature of this acrylic acid production process is the twin parallel reactors (R101A and R101B) as shown in Fig. 5 above that are capable to convert propene and air mixture into acrolein. Acrolein is further oxidized in another reactor to produce acrylic acid and other by-products. The mixture is further refined in distillation towers to produce 99.99% pure acrylic acid. For this case study, only the indices for temperature, pressure and explosiveness are considered in order to show the application of ISIM. Other parameters such as toxicity, corrosiveness, etc. are also important but not considered in this case study. The process information data for all 33 streams in the acrylic acid plant are easily transferred to ISIM due to the advantage of having integrated system with HYSIS. The results of the indices are shown in Figs. 6–8 in order to illustrate the concept proposed in the framework. From these figures, we observed that pressure parameters are all scoring
index of 0, which means the system is operating at low pressure regime and pressure is not an inherent safety concern for this process. Figs. 7 and 8 show that there are streams classified as levels 3 and 4. At this point, there is no definitive manner to determine which level is safe or otherwise. It can only be concluded that a higher index is more inherently unsafe compared to a lower one. For the purpose of illustration, this case study will demonstrate how these streams can be modified to achieve a lower index. The indices of related process streams are tabulated in Table 3. Pressure index is no longer being considered in the analysis as indicated earlier. This table clearly shows that integration of an inherent safety index module (ISIM) with process design simulator will allow potentially dangerous streams to be clearly identified up front during initial design. The process design engineers will have the opportunity to modify the process to produce an inherently safer option using the inherent safety principles as described in Table 1. In this particular case study, the streams scoring high indices (levels 3 and 4) are traced to streams originating from the parallel reactors (R101A and R101B) system as shown in Fig. 5. The principle of simplification from the inherent safety concept as given in Table 1 is adopted in order to show the
Table 3 – ISL for parallel reactors streams in original design Stream
Fig. 6 – Inherent safety index for stream pressure in original design.
R100 R101A R101A R101A R102A R102A R102A 5
Feed Top Bot Feed Top Bot
Temperature index
Pressure index
3 3 4 4 3 4 4 4
0 0 0 0 0 0 0 0
Explosiveness index 2 2 2 0 2 2 0 0
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process safety and environmental protection 86 (2008) 113–119
Fig. 9 – A modified processes with a single reactor.
Fig. 10 – Inherent safety index for stream temperature in original and modified design.
application of ISIM in the preliminary design stage. The objective is to reduce the complexity of the process that will reduce the number of streams having high ISL value for temperature and explosiveness. The improvement is made to the parallel reactors, which resulted in a single reactor as shown in Fig. 9. This modification resulted in the elimination of one reactor and its inlet and outlet stream. After the modification, ISIM is again used to analyze the ISL of the modified process. The inherent safety indices for the original and modified cases are compared and the results are shown in Figs. 10 and 11. It can be concluded that the modification had resulted in reduction in number of streams temperature having ISL 3 and 4. This has indirectly improved the overall safety of the plant. The explosiveness index has also reduced. The modification to the process took very little time and with little cost since the design is still at the simulation stage. This would have been expensive if the concern is only detected further into the design stage, for instance, when the complete process and instrumentation diagram had been prepared and equipment sized.
This case study demonstrates that quantification of inherent safety level can be carried out in the simulation stage of process design and with such information, an inherently safer design option can be proposed. In this particular case, the process has been simplified by combining both reactors as one. From a mechanical and instrumentation stand point, lesser main equipment and lesser auxiliary equipment may improve overall reliability of the plant. It is important to stress that the final selection decision has to be made after considering other factors such as economic of larger reactor and operational flexibility of having only one reactor. The demerit of this design option is obviously the larger inventory to be handled by the single reactor. It is very important to note that the application of different inherent safety principles to the same problem may derive different solutions to improve the safety level. It is better to consider all the relevant inherent safety principles according to the hierarchy proposed by Kletz (1991) and careful consideration should be given to select the best solution in improving the safety level. As an example for the case of the reactor above, it is not always the case that combining two reactors together will provide the best solution to improve the safety level. In many cases, multiple units rather than one big reactor will provide better solution. Therefore, careful consideration and attention must be given in the application of inherent safety principles to ensure the best solution that gives the safest condition. Experience and knowledge on the design aspects are very important for effective application of the principles of inherent safety especially to review the agreement of different alternatives as highlighted by Moore (1999). A more detailed study is currently being carried out to address the issues raised in the above paragraph. An enhanced safety index determination technique is also being carried out which will be more suitable to be used with the proposed framework. A study using a heuristic approach is also in progress to provide easy application of inherent safety principles in process design.
4.
Conclusion
This paper has conclusively shown that an inherently safer process can be designed since the inherent safety level (ISL) of the original process can be determined at the preliminary design stage using inherent safety index module (ISIM) as proposed in the framework given in Fig. 4. A more rigorous result could be determined once the entire system described in the framework given in Fig. 4 is completed.
Acknowledgement This paper is written in memories of our mentor, the late Professor Dr. V.R. Radhakhrishnan.
references
Fig. 11 – Inherent safety index for stream explosiveness in original and modified design.
Edwards, D.W. and Lawrence, D., 1993, Assessing the inherent safety of chemical process routes: is there a relationship between plant costs and inherent safety? Trans IChemE B: Process Safety Environ Prot, 71B: 252–258.
process safety and environmental protection 86 (2008) 113–119
Greenberg, H.H. and Cramer, J.J., 1991, Risk Assessment and Risk Management for the Chemical Process Industry. (Van Nostrand Reinhold, New York). Gupta, J.P. and Edwards, D.W., 2002, Inherently safer design– present and future, Trans IChemE, Process Safety Environ Prot B, 80: 115. Gupta, J. P, and Edwards, D.W., 2003, A simple graphical method for measuring inherent safety, J Hazard Mater, 104: 15–30. Heikkila, A.M., 1999, Inherent safety in process plant design, an index-based approach, PhD Thesis, Helsinki University of Technology, Finland. Khan, F.I. and Amyotte, P.R., 2002, Inherent safety in offshore oil and gas activities: a review of the present status and future directions, J Loss Prev Process Ind, 15: 279–289. Khan, F.I. and Amyotte, P.R., 2004, Integrated inherent safety index (I2SI): a tool for inherent safety evaluation, Process Safety Prog, 23(2): 136–148. Kletz, T.A. 1991, Plant Design for Safety—A User Friendly Approach (Second ed.), ). Mansfield, D.P., Kletz, T.A. and Al-Hassan, T., 1996, Optimizing safety by inherent offshore platform design, In Proceedings of the 1st International Conference on Health, Safety and Environment. The Netherlands: The Hague.
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Mohd Shariff, A., Rusli, R., Chan, T.L., Radhakrishnan, V.R. and Buang, A., 2006, Inherent safety tool for explosion consequences study, J Loss Prev Process Ind, 19: 409–418. Moore, A.D., 1999, http://process-safety.tamu.edu/Research/ Inherent%20Safety/Isappr.html (12 February 2006). Palaniappan, C., Srinivasan, R. and Tan, R., 2002, Expert system for the design of inherently safer processes. 2. Flowsheet development stage, Ind Eng Chem Res, 41: 6711–6722. Rushton, A.G., Edwards, D.W. and Lawrence, D., 1994, Inherent safety and computer aided process design, Trans IChemE B, 72: 83–87. Soo, E.W. 2004, Acrylic Acid Production Plant, B. Eng. Final Year Plant Design Project. (Universiti Teknologi PETRONAS, Malaysia). Taylor, J.R. 1994, Risk Analysis for Process Plant, Pipelines and Transport. (E&FN Spon, UK). Zwetsloot, G.I.J.M. and Askounes Ashford, N., 1999, Encouraging inherently safer production in European firms: a report from the field, J Hazard Mater, in Special Issue on Risk Assessment and Environmental Decision Making, Amendola, A. and Wilkinson, D. (eds), pp. 123–144.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/psep
Inherent safety index module (ISIM) to assess inherent safety level during preliminary design stage Chan T. Leong, Azmi Mohd Shariff * Process Safety Research Group, Department of Chemical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia
article info
abstract
Article history:
One of the acceptable methods to quantify the level of inherent safety is based on the
Received 9 January 2007
inherent safety index. This paper reviews presently available techniques for quantification
Accepted 9 July 2007
of inherent safety level in a design and addresses the shortcoming of current techniques by proposing direct integration of process design simulator with inherent safety index. This integrated index is known as inherent safety index module (ISIM) and it is one of the
Keywords:
modules developed in a newly proposed framework to determine inherent safety level in the
Inherent safety
preliminary design stage. This framework is an enhancement of the framework developed
Risk
earlier by Mohd Shariff et al. [Mohd Shariff, A., Rusli, R., Chan, T.L., Radhakrishnan, V.R. and
Consequences
Buang, A., 2006, Inherent safety tool for explosion consequences study, J Loss Prev Process
Safety index
Ind, 19: 409–418]. This new framework allows process information from process design
Preliminary design
simulator to be extracted and analyzed for the determination of inherent safety level (ISL), consequences and probability of unwanted incidences. The availability of such information at earlier stage of design will help process designers to obtain ISL that will assist them in producing safer designs by the application of inherent safety principles in a more efficient and cost effective manner. This paper also discusses the overall concept of the proposed framework to produce an inherent safety tool. A case study is provided to illustrate the benefit of having inherent safety index known to process designers during preliminary design stage. With the right information, modification to process conditions can be carried out and this is likely to produce a safer process plant. # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Safety should be considered and addressed in the whole life cycle of a process system or a facility (Greenberg and Cramer, 1991). There are many established methodologies to identify, analyze, prioritize and manage risks arising from different stages of a plant as illustrated by Taylor (1994). The conventional safety methodologies as shown in Fig. 1 are often carried out in parallel with design process, after much of the process simulation has been completed. Zwetsloot and Askounes Ashford (1999) noted that conventional safety approaches alone are unable to avoid or reduce the risk of serious chemical accidents. They also noted that any re-design done as a result of unfavorable safety performance, after the detailed design stage of the process life cycle would be very expensive compared to alteration in
the early stage, i.e. during conceptual design stage. Also, modification could be carried out relatively easier during preliminary design stage. Khan and Amyotte (2002) reflected similar finding in their work, which concluded that an inherently safer approach is a cost-optimal option considering the lifetime costs of a process and its operation. Their subsequent research showed that inherent safety can be incorporated at any stage of design and operations; however, its application at the earliest possible stages of process design (such as process selection and conceptual design) yields the best results. The principles defining inherent safety as shown in Table 1 were formalized by Kletz (1991). These principles aim to reduce or eliminate hazards by modifying the design (using different chemicals, hardware, controls, and operating conditions) of the plant itself. The conventional safety approaches
* Corresponding author. Tel.: +60 5 3687570; fax: +60 5 3656176. E-mail address: [email protected] (A.M. Shariff). 0957-5820/$ – see front matter # 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2007.10.016
114
process safety and environmental protection 86 (2008) 113–119
Table 1 – General principles of inherent safety Principles Intensification Substitution Attenuation
Limitation of effects
Simplification
Error tolerance
Definition Reduction of the inventories of hazardous materials Change of hazardous chemicals substances by less hazardous chemicals Reduction of the volumes of hazardous materials required in the process. Reduction of operation hazards by changing the processing conditions to lower temperatures, pressures or flows The facilities must be designed in order to minimize effects of hazardous chemicals or energies releases Avoidance of complexities such as multi-product or multi-unit operations, or congested pipe or unit settings Making equipment robust, processes that can bear upsets, reactors able to withstand unwanted reactions, etc.
shown in Fig. 1, on the other hand aim to reduce risk of a process by adding protective barriers to mitigate impact. Despite the attractiveness of being able to proactively identify and reduce risk, the principles of inherent safety have not been widely adopted in the industries. The lack of experience and knowledge (field and ‘‘real world plant’’) of the designers who are applying these principles and the lack of recognized methodology to review the agreement of different process alternatives according to the inherent safety principles are among the crucial obstacles to the implementation of this safety philosophy (Moore, 1999). A study by Mansfield et al. (1996), which assessed the familiarity and application of inherent safety among designers and companies, concluded that although many designers know of the basic principles of inherent safety, they are not always clear about how to apply them. There is also a general lack of familiarity with the specific advantages of adopting an inherently safer approach to process design. Rushton et al. (1994) emphasized the need for a computer aid that will perform comprehensive inherent safety analysis at each key decision point in the process life. The key benefits of automation are substantial reduction in time and effort,
Fig. 2 – Problems of implementing inherent safety (Kletz, 1991).
enhanced decision-making, improved documentation, and better understanding of the process. The other reasons for lack of implementation of inherent safety in actual designs are summarized by Kletz (1991) and shown in Fig. 2.
1.1.
Indices for quantification of inherent safety level (ISL)
One of the challenging aspects on the implementation of the inherent safety principles would be to convince the stakeholders and/or process owners about the benefits of investing in inherent safety features. Process designers are often faced with the question, ‘‘How can the benefits of implementing the inherent safety features are quantified?’’ Quantification is a challenging aspect unless a definitive comparison can be made such as two plants with identical requirements were built, with one using inherent safety features and the other one without. However, this would have been a very cost ineffective manner to be carried out. One of the potential solutions to the above queries is using inherent safety indices. The pioneering index was proposed by Edwards and Lawrence (1993). Heikkila (1999) improved the method by including an additional aspect to the index system.
Fig. 1 – Safety analysis program (Taylor, 1994).
process safety and environmental protection 86 (2008) 113–119
Table 2 – Inherent safety index parameters Edwards and Lawrence (1993) Inventory Temperature Pressure Heat of main reaction Heat of side reaction Flammability Explosiveness Corrosiveness Toxicity Chemical interaction Type of equipment Safety of process structure
X X X X – X X – X – – –
Heikkila (1999) X X X X X X X X X X X X
115
All the indices described above rely on manual extraction of design parameters such as from process design simulator for the calculation of inherent safety index. The study can only be done reaction by reaction within the process. It is difficult to analyze the inherent safety level for all process streams in a plant due to the complexity of the processes involved as process conditions change. Manual extraction of process design data limits the effective utilization of the indices above especially for repetitive quantification of inherent safety level as process conditions or chemicals change in the design stages. This paper proposes a framework to address the shortcoming for meaningful use of inherent safety indices during preliminary design stage.
1.2. Integrated risk estimation tool (iRET) for inherent safety application Table 2 compares the parameters used in the two-index system proposed by Lawrence and Heikkila. The parameters were then adopted by Palaniappan et al. (2002) to develop an expert system for the application of inherent safety in chemical process design. Apart from their attempt to develop computer software tool, they also proposed three additional supplementary indices—worst chemical index (WCI), worst reaction index (WRI), and total chemical index (TCI) to overcome shortcoming in earlier indices. The WCI is the summation of the maximum values of the flammability, toxicity, reactivity, and explosiveness indices of all the materials involved in a reaction step. Similarly, the WRI is the sum of the maximum values of the individual indices for temperature, pressure, yield, and heat of reaction of all the reactions involved in the process. The TCI is a measure of the number of hazardous chemicals involved in the route. Gupta and Edwards (2003) developed a graphical method to apply inherent safety index in evaluating six potential routes to produce methyl methacrylate (MMA) in an attempt to graphically show the comparisons. Khan and Amyotte (2004) proposed a new indexing technique which is intended to be applicable throughout the lifecycle of process design. The new index technique is known as integrated inherent safety index (I2SI) and has three indices, i.e. hazard index (HI), inherent safety potential index (ISPI) and inherent safety cost index (ISCI). The HI is intended to be a measure of the damage potential of the process after taking into account the process and hazard control measures. The ISPI, on the other hand, accounts for the applicability of the inherent safety principles (or guidewords) to the process. The HI is calculated for the base process (any one process option or process setting will be considered as the base operation or setting) and remains the same for all other possible options. The HI and ISPI for each option are combined to yield a value of the integrated index as shown in the following equation: I2SI ¼
Mohd Shariff et al. (2006) introduced a feasible framework for inherent safety application in preliminary design stage by integrating process design simulator with consequences of an unwanted event such as explosion. A demonstrative tool was developed by integrating process design simulation software, HYSYS with an explosion model developed in MS Excel spreadsheet. The demonstration tool named as an integrated risk estimation tool (iRET) used a TNT equivalent explosion model. The results from the case studies demonstrated that it is possible to determine consequences during preliminary design stage hence providing crucial information that can be used to improve plant safety. The algorithm involved for iRET is shown in Fig. 3. Apart from the benefits of having early indication of explosion consequences, the tool also eliminates the need to manually transfer information from process design simulator into consequence analysis software. This saves time and reduces chances of data entry error. The concept used in iRET proved very practical for the implementation of inherent safety principles in the preliminary design stage. However, there was no evidence to show how the tool can be utilized to
ISPI HI
Both the ISPI and the HI range from 1 to 200; the range has been fixed considering the maximum and minimum likely values of the impacting parameters. This range gives enough flexibility to quantify the index. For example, an I2SI value greater than unity denotes a positive response of the inherent safety guideword application (inherently safer option). The higher the value of the I2SI, the more pronounced the inherent safety impact.
Fig. 3 – Integrated risk estimation tool, iRET (Mohd Shariff et al., 2006).
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estimate risk without the inclusion of probability estimation in the framework. The present research work expands the concept shown in Fig. 3 to include ISL quantification feature as a part of effort to address suggestions by the industries that ISL quantification methodologies be simplified and made adoptable during early plant design stages. This suggestion was the conclusion from a survey by Gupta and Edwards (2002) which received 63 responses including 36 from industries and consultants, 24 from academic and R&D organizations and 3 from regulatory bodies from 11 countries. The quantification of ISL is determined using an integrated system of inherent safety index, consequences estimation and probability estimation with process design simulator.
2. Inherent safety level (ISL) quantification using inherent safety index module (ISIM) The approach for the quantification of ISL follows the same concept as proposed by Mohd Shariff et al. (2006). A new framework which is proposed to allow process information from process design simulator, in this case HYSYS, be extracted and analyzed for the determination of inherent safety level, consequences and probability of unwanted incidences, as shown in Fig. 4. The availability of such information at earlier stage of design will help process designers to obtain ISL. The process designers can use the ISL as a tool to revise and produce safer designs by the application of inherent safety concept in a more efficient and cost effective manner. This paper discusses only the inherent safety index module (ISIM) as given in Fig. 4 to demonstrate its feasibility to measure inherent safety level during preliminary process design stage. Research is currently on-going to develop the entire framework and results will be published separately. The data such as pressure, temperature, flow rate and composition are extracted from process design simulator (HYSYS) using macros in Microsoft Excel to calculate inherent
safety indices. Heikkila’s (1999) classification approach is adopted as this method is very basic, easy to be implemented and does not require extensive use of proprietary monographs. Pressure, temperature and flow rate can be directly evaluated for their respective inherent safety indices, ranging from 0 to 5 (the worst). The composition of each stream is used to calculate the flammability limits of the mixture and translated to explosiveness. Based on the obtained indices, streams with unfavorable ISL are identified and improvements can be carried out by process designers by applying one or more of the inherent safety principles as outlined in Table 1. The consequences and probability of unwanted incidents can also be assessed using the complete tool which is being developed currently. With the integration with process simulator, it is expected the impact of changes in process conditions to inherent safety level and consequences can be effectively determined during preliminary process design stage.
3.
Case study
An acrylic acid production process is used as a case study to demonstrate the capability of ISIM for inherent safety application in the preliminary process design stage. The simulation model of an acrylic acid plant which is used in this case study was originally produced for academic purposes by Soo (2004) using HYSYS process design simulator. In this case study, ISIM is integrated with HYSYS to demonstrate its capability to determine stream that is more inherently unsafe. Design changes are then made to the identified stream following the principle of inherent safety for the improvement of safety level. For instance, a stream scoring a level 4 index is deemed to be more inherently unsafe compared to a stream scoring level 2. Since safety does not have an absolute level, the application of the as low as reasonable practicable (ALARP) principle is necessary to reduce the index in conjunction with process requirements. For instance, if a process can only work at a particular temperature or pressure, which may score high
Fig. 4 – Framework to determine ISL at preliminary design stage.
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Fig. 7 – Inherent safety index for stream temperature in original design.
Fig. 5 – Parallel reactors in original design of acrylic acid production plant (Soo, 2004). Fig. 8 – Inherent safety index for stream explosiveness in original design. index, other means of increasing safety level, should be explored rather than directly reducing pressure and/or temperature. This means not all scoring high index streams can be reduced to a lower index. Modification of the process conditions has reached the limit once the desired process design objectives such as product quality have been compromised. The simulation process of acrylic acid production plant has a total of 33 streams all together. The main feature of this acrylic acid production process is the twin parallel reactors (R101A and R101B) as shown in Fig. 5 above that are capable to convert propene and air mixture into acrolein. Acrolein is further oxidized in another reactor to produce acrylic acid and other by-products. The mixture is further refined in distillation towers to produce 99.99% pure acrylic acid. For this case study, only the indices for temperature, pressure and explosiveness are considered in order to show the application of ISIM. Other parameters such as toxicity, corrosiveness, etc. are also important but not considered in this case study. The process information data for all 33 streams in the acrylic acid plant are easily transferred to ISIM due to the advantage of having integrated system with HYSIS. The results of the indices are shown in Figs. 6–8 in order to illustrate the concept proposed in the framework. From these figures, we observed that pressure parameters are all scoring
index of 0, which means the system is operating at low pressure regime and pressure is not an inherent safety concern for this process. Figs. 7 and 8 show that there are streams classified as levels 3 and 4. At this point, there is no definitive manner to determine which level is safe or otherwise. It can only be concluded that a higher index is more inherently unsafe compared to a lower one. For the purpose of illustration, this case study will demonstrate how these streams can be modified to achieve a lower index. The indices of related process streams are tabulated in Table 3. Pressure index is no longer being considered in the analysis as indicated earlier. This table clearly shows that integration of an inherent safety index module (ISIM) with process design simulator will allow potentially dangerous streams to be clearly identified up front during initial design. The process design engineers will have the opportunity to modify the process to produce an inherently safer option using the inherent safety principles as described in Table 1. In this particular case study, the streams scoring high indices (levels 3 and 4) are traced to streams originating from the parallel reactors (R101A and R101B) system as shown in Fig. 5. The principle of simplification from the inherent safety concept as given in Table 1 is adopted in order to show the
Table 3 – ISL for parallel reactors streams in original design Stream
Fig. 6 – Inherent safety index for stream pressure in original design.
R100 R101A R101A R101A R102A R102A R102A 5
Feed Top Bot Feed Top Bot
Temperature index
Pressure index
3 3 4 4 3 4 4 4
0 0 0 0 0 0 0 0
Explosiveness index 2 2 2 0 2 2 0 0
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Fig. 9 – A modified processes with a single reactor.
Fig. 10 – Inherent safety index for stream temperature in original and modified design.
application of ISIM in the preliminary design stage. The objective is to reduce the complexity of the process that will reduce the number of streams having high ISL value for temperature and explosiveness. The improvement is made to the parallel reactors, which resulted in a single reactor as shown in Fig. 9. This modification resulted in the elimination of one reactor and its inlet and outlet stream. After the modification, ISIM is again used to analyze the ISL of the modified process. The inherent safety indices for the original and modified cases are compared and the results are shown in Figs. 10 and 11. It can be concluded that the modification had resulted in reduction in number of streams temperature having ISL 3 and 4. This has indirectly improved the overall safety of the plant. The explosiveness index has also reduced. The modification to the process took very little time and with little cost since the design is still at the simulation stage. This would have been expensive if the concern is only detected further into the design stage, for instance, when the complete process and instrumentation diagram had been prepared and equipment sized.
This case study demonstrates that quantification of inherent safety level can be carried out in the simulation stage of process design and with such information, an inherently safer design option can be proposed. In this particular case, the process has been simplified by combining both reactors as one. From a mechanical and instrumentation stand point, lesser main equipment and lesser auxiliary equipment may improve overall reliability of the plant. It is important to stress that the final selection decision has to be made after considering other factors such as economic of larger reactor and operational flexibility of having only one reactor. The demerit of this design option is obviously the larger inventory to be handled by the single reactor. It is very important to note that the application of different inherent safety principles to the same problem may derive different solutions to improve the safety level. It is better to consider all the relevant inherent safety principles according to the hierarchy proposed by Kletz (1991) and careful consideration should be given to select the best solution in improving the safety level. As an example for the case of the reactor above, it is not always the case that combining two reactors together will provide the best solution to improve the safety level. In many cases, multiple units rather than one big reactor will provide better solution. Therefore, careful consideration and attention must be given in the application of inherent safety principles to ensure the best solution that gives the safest condition. Experience and knowledge on the design aspects are very important for effective application of the principles of inherent safety especially to review the agreement of different alternatives as highlighted by Moore (1999). A more detailed study is currently being carried out to address the issues raised in the above paragraph. An enhanced safety index determination technique is also being carried out which will be more suitable to be used with the proposed framework. A study using a heuristic approach is also in progress to provide easy application of inherent safety principles in process design.
4.
Conclusion
This paper has conclusively shown that an inherently safer process can be designed since the inherent safety level (ISL) of the original process can be determined at the preliminary design stage using inherent safety index module (ISIM) as proposed in the framework given in Fig. 4. A more rigorous result could be determined once the entire system described in the framework given in Fig. 4 is completed.
Acknowledgement This paper is written in memories of our mentor, the late Professor Dr. V.R. Radhakhrishnan.
references
Fig. 11 – Inherent safety index for stream explosiveness in original and modified design.
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