The integration of Dow's fire and explosion index (F&EI) into process design and optimization to achieve inherently safer design

For the processing industries, it is critically to have an economically optimum and inherently safer design and operation. The basic concept is to achieve the best design based on technical and business performance criteria while performing within acceptable safety levels. Commonly, safety is examined and incorporated typically as an after-thought to design. Therefore, systematic and structured procedure for integrating safety into process design and optimization that is compatible with currently available optimization and safety analysis methodology must be available.The objective of this paper is to develop a systematic procedure for the incorporation of safety into the conceptual design and optimization stage. We propose the inclusion of the Dow fire and explosion index (F&EI) as the safety metric in the design and optimization framework by incorporating F&EI within the design and optimization framework. We first develop the F&EI computer program to calculate the F&EI value and to generate the mathematical expression of F&EI as a function of material inventory and operating pressure. The proposed procedure is applied to a case study involving reaction and separation. Then, the design and optimization of the system are compared for the cases with and without safety as the optimization constraint. The final result is the optimum economic and inherently safer design for the reactor and distillation column system.     

Simple graphical method for inherent occupational health assessment

The concept of inherently safer design was introduced to design a fundamentally safer process so that hazards can be avoided or minimized rather than controlled or managed. The ideology has later been extended to the environmental, but not health criteria due to its complicated underlying principles. Even though health risk methods are already established, majority are for existing plants assessment. Early consideration of health aspect starting from process design stage however, has received much less attention. This paper introduces a simple graphical method to evaluate the inherent occupational health hazards of chemical processes during the R&D stage. A survey was conducted to identify the important health parameters for the graphical method development, involving nine world inherent safety and health experts. Based on their input, process mode, material volatility, operating pressure and chemical health hazard (toxicity and adverse effect) are the significant factors affecting inherent health hazards of chemical processes. The choice of parameters was bounded by the information availability at this stage. The method was applied on six routes to methyl methacrylate and ten routes to acetic acid. The parameters were plotted for each subprocess of the alternative routes. The ‘healthiest’ route was selected based on thorough hazards assessment across all the subprocesses. The first case study reveals the tertiary butyl alcohol as the ‘healthiest’ one as it poses relatively lower, or at least comparable hazards to the other routes due to exposure and health impacts. Meanwhile the acetic acid case study indicates ethanol oxide and ethyl oxide based routes as the inherently healthier as they operate at lower operating pressure besides posing comparable hazards level for the other three parameters, compared to the other routes. The case studies show that the inherent occupational health of a chemical process can already be evaluated easily in the R&D stage with the simple graphical method proposed.     

Study of high-tech process furnace using inherently safer design strategies (II). Deposited film thickness model

In the low-pressure chemical vapor deposition (LPCVD) process, there are many factors that could affect the thin film thickness and homogeneity in the thin film deposition process. Besides temperature, the volume flow rate of the reactant gas, the Mole fraction, and pressure are all affecting factors. On the basis of the forecast mode result of the thermal model of wafer temperature distribution in the previously published article, this study used the optimal method to calculate the conversion rate of the surface of each wafer under simulated conditions of various furnace temperatures and Mole fractions, as well as silane flow rates and pressures, and finally calculated the thin film growth rate and homogeneity of the surface of each wafer.This study validated four types of process conditions, and found that the results were very close to the experimental values in the previous studies, when conditions such as temperature, Mole fraction of silane, flow rate and pressure in the process were changed. This study thus concluded that the proposed process film thickness mode could meet the requirement of the process capacity. This study found that adjusting the heating zone temperature to regulate the film thickness growth rate was able to control uniformity of the film thickness. These feasible adjustment measures could be very effective on the industry commercial processes.Furthermore, the film thickness mode established in this study was used to discuss the application level of the inherently safer design strategies. This study mode could be used to adjust the temperature, mole fraction (mf), flow rate (F) and pressure (P) in the process for a single machine. Under the condition of meeting the film thickness process recipes required by the process, the silane flow rate was lowered through the adjustment of the control system. The study took the silane supply system as the basis of the case study. Based on a daily required silane consumption of 64.4 L, the daily required amount was 1.5 cylinders in the case of using 40 L high-pressure cylinders. The total daily required amount would be three 40 L high-pressure cylinders, including one more reserve amount on top of the daily required amount, which could match the inherently safer design philosophy and the principles of intensification, attenuation and limitation of effects.In terms of the factory evaluation, among all of the 12-inch furnace machines in Taiwanese wafer factories, nine out of 47 sets use silane, and the total daily consumption reaches 579.6 L. The total required amount, including the reserve, is 27 cylinders for 40 L high-pressure cylinders. As for the setting of the gas cabinet, the required amount could be reduced, since the gas supply pipeline could be designed in a multiple circuit. The minimum reactant gas input amount could be obtained in combination with the proposed film thickness mode, and the safety could be improved greatly, in accordance with the inherently safer design philosophy and the principles of intensification, attenuation and limitation of effects. In addition to the process capacity, this study also took the elevation of the inherently safer level into consideration, which could be provided to the industry for more comprehensive design consideration.     

Inherent safety concept based proactive risk reduction strategies: A review

The growing scale and complexity of process industries have brought safety, health, and environmental issues to the forefront. As a result, proactive risk reduction strategies (RRSs) are commonly employed to address these issues by reducing the frequency or mitigating the consequences of potential incidents. Among these strategies, inherent safety, which is a proactive measure of loss prevention and risk management, is considered to be the most effective method. This review aims to provide a comprehensive analysis of RRSs for achieving inherency, as well as techniques for evaluating the performance of inherent safety, health, and environmental aspects. Background information is presented, including the development and implementation of the inherently safer process design, as well as the approaches for achieving inherently healthier and environmentally friendlier processes. Subsequently, the execution approaches and practical applications of other RRSs are discussed to highlight the distinctiveness and benefits of inherent safety. Next, this study examined the characteristics of inherency assessment tools (IATs) based on available information at different process stages. Furthermore, the evaluation methods and historical development of IATs are investigated from the perspectives of safety, occupational health, and environmental considerations, followed by a statistical analysis of IATs. It is concluded that the no-chemical hazards-based IATs have not been extensively studied yet, which may improve the safety level of process plants from the perspective of comprehensive inherency risk reduction. As a way forward, future research opportunities are proposed to promote the implementation of greater optimized risk management.     

化工过程开发中本质安全化设计策略

本质安全化设计是预防人为失误及设备失效、降低化工过程风险应优先采用的技术。在比较传统设计方法与本质安全化设计方法的基础上,讨论了化工过程开发各阶段实现本质安全的机会,认为在开发初期,实施本质安全化的成本低,难度小;通过分析可行性研究、工艺研究、概念设计、基础设计、工程设计等阶段本质安全化设计的影响因素、设计目标和设计方法,探索化工过程开发中本质安全化设计策略,提出了化工过程本质安全化设计流程。通过工艺过程本质安全设计、工艺流程的简化和优化、不同设计方案的本质安全度评估等措施,可提高化工过程本质安全水平。     

Advanced process control system with MPC as a new approach for layer of protection analysis

Layer of protection analysis (LOPA) is a widely used method to support process safety in the chemical industries. In the LOPA, the process is classified into many layers, one of such layers considers the basic process control system (BPCS) which commonly uses PID controllers. This kind of controllers cannot deal with constraints. For this reason, the main purpose of this work is to provide a framework to enhance the control layer in the LOPA, which consists of a model predictive control (MPC) with safety features. These features include: sublayers in the controller system (such as real time optimization, target calculation, and MPC), safety constraints, and guarantee of stability by adopting an Infinite Horizon MPC (IHMPC). Here, we propose an approach for control-inspired view to process safety, replacing the BPCS by an Advanced Process Control System (APCS). Moving forward with these concepts, first, a literature review emphasizes the content, showing two perspectives for the APCS. The APCS is designed for two varieties of controllers, a basic IHMPC and IHMPC with zone control to compare the performance. In this framework, the first sublayer consists of a real time optimization (RTO) structure, that calculates the optimal operating condition for the process controller, which computes the control action. Besides, RTO has an additional constraint called the safety index, based on the protection of process operational. RTO and basic IHMPC communicate directly, while for IHMPC with zone control there is an inner sublayer called Target Calculation, it computes a feasible target to the controller, working as another safety strategy in APCS. After that, we demonstrate both structures applied to a CSTR reactor. From the case study, we compared both controllers, and evaluated the effect that the safety index constraint causes in the setpoints, outputs, and control actions. The use of safety constraint in RTO proved to be a safe strategy for the control layer, as well as IHMPC with zone control presented a safer profile than basic IHMPC. Furthermore, the results show that safety constraint affect the economic goal, decreasing its value.     

Toward an inherently safer design and operation of batch and semi-batch processes: The N-oxidation of alkylpyridines

This work shows an application of inherent safety principles to a reaction widely used in the pharmaceutical industry. More specifically, it incorporates the teachings of Trevor Kletz into the design of an inherently safer process for the N-oxidation of alkylpyridines. This reaction is of interest because of the hazards resulting from the undesired, gas-generating decomposition of hydrogen peroxide, the oxidizing agent. The generation of oxygen, combined with the flammability of the alkylpyridines, represents a serious fire and explosion hazard for this process. The purpose of this paper is to demonstrate how an inherently safer process can be potentially achieved by designing improved reactors and by assessing conditions that reduce or eliminate the hazards. Furthermore, it is shown that such improvement in safety increases the efficiency of the process and results in a cost reduction.     

Systematic inherent safety and its implementation in chlorine liquefaction process

As a proactive safeguard, inherent safety has been regarded as the top hierarchy for loss prevention and risk management due to its salient features in eliminating or significantly reducing risks at source rather than mitigating them by add-on protections. Simultaneously, various assessment tools have been developed for ranking and selecting inherently safer designs or modifications. However, there still lacks a metric that can systematically incorporate various hazardous factors, which may hinder most industries from utilizing it to a full extent. To address this limitation, this work developed a Systematic Inherent Safety Metric (SISM) for measuring the inherently safer modifications. Firstly, the conceptual framework of SIS was proposed based on 5M1E (man, machine, material, method, measurement, and environment). Subsequently, analytic hierarchy process and fuzzy comprehensive evaluation were adapted to conduct risk identification and assessment. Finally, taking chlorine liquefaction process as a case study, the applicability and efficacy of SIS were validated based on PDCA (plan-do-check-action) cycle. The results show that the SISM value has improved from the relatively dangerous (RD) to the relatively safe (RS) after implementing SIS, thus demonstrating that the revised design is inherently safer than the base design.     

Inherent risk assessment methodology in preliminary design stage: A case study for toxic release

At preliminary design stage, process designers normally lack of information on the risk level from process plant. An inherently safer process plant could be designed if the information of risk levels could be known earlier at the preliminary design stage. If the risk level could be determined, there is a possibility to eliminate or reduce the risk by applying the well-known concept: inherent safety principle. This paper presents a technique to determine the risk levels at preliminary process design stage using a 2-region risk matrix concept. A model to calculate the severity and likelihood of a toxic release accident was developed in Microsoft Excel spreadsheet. This model is integrated with process design simulator, iCON to allow for data transfer during preliminary design stage. 2-region risk matrix is proposed and used to evaluate the acceptability of the inherent risk based on the severity and likelihood rating. If the inherent risk level is unacceptable, modification for improvement can be done using the inherent safety principles. A case study has been carried out to illustrate the benefit of applying this newly developed technique. It was successfully shown that an inherently safer plant could easily be designed by applying this technique.     

Process safety aspects in water-gas-shift (WGS) membrane reactors used for pure hydrogen production

The syngas produced by coal gasification processes can be utilized in Pd-based water-gas-shift membrane reactors for the production of pure H₂. Pd/alloy composite membrane reactors exhibit comparative advantages over traditional packed bed reactors such as simultaneous reaction/separation in one compact unit and increased reaction yields. Furthermore, the development of comprehensive process intensification strategies could further enhance membrane reactor performance resulting in a substantially smaller and functional, inherently safer, environmentally friendlier and more energy efficient process.A systematic non-isothermal modeling framework under both steady state and dynamic/transient conditions for a catalytic high temperature water-gas shift reaction in a Pd-based membrane reactor has been developed to characterize the dynamic behavior of the process system at various operating conditions from a process safety standpoint. In particular, various reaction conditions as well as key process variables such as feed temperature and flow rate, catalyst loading, driving force for H₂ permeation are considered as they are critically related to various safety aspects in the operation of a Pd-based membrane reactor. Within the proposed framework, process parameters and operating conditions which may induce hazards and compromise process safety are identified, analyzed and characterized. Finally, the proposed approach is evaluated through detailed simulation studies in an illustrative case study involving a real Pd-based membrane reactor used for pure hydrogen production and separation that exhibits complex behavior over a wide operating regime.     

Inherently safer design of solvent processes at the conceptual stage: Practical application for substitution

The implementation of inherently safer design concepts is considered beneficial to avoid hazards during early stages of design. The application of existing process design and modeling techniques that aid ‘substitution’, ‘intensification’ and ‘attenuation’ has been shown in this work. The techniques have been applied to solvent processes because of the inherent hazards associated with them, such as large inventories, and presence of highly toxic and flammable materials. For ‘substitution’, computer aided molecular design technique has been applied to select inherently safer solvents for a solvent operation. For ‘intensification’ and ‘attenuation’, consequence models and regulatory guidance from EPA RMP have been integrated into process simulation. Combining existing techniques provides a design team with a higher level of information to make decisions based on process safety. A case study has been shown for liquid extraction of acetic acid–water mixture. Suitable solvents were identified using ICAS 11.0-ProCAMD, and consequence models were integrated into Aspen plus simulator using a calculator sheet. Solvents such as 5-nonanone, 2-nonanone and 5-methyl-2-hexanone provide inherently safer options, but conventionally-used solvent, ethyl acetate, provides higher degree of separation capability. A conclusive decision regarding feasible solvents and operating conditions would depend on design requirements, regulatory guidance, and safety criteria specified for the process. Inherent safety has always been an important consideration to be implemented during early design steps, and this paper presents a methodology to incorporate the principles and to obtain inherently safer alternatives.     

Inherent risk assessment—A new concept to evaluate risk in preliminary design stage

Quantitative Risk Assessment (QRA) has been a very popular and useful methodology which is widely accepted by the industry over the past few decades. QRA is typically carried out at a stage where complete plant has been designed and sited. At that time, the opportunity to include inherent safety design features is limited and may incur higher cost. This paper proposes a new concept to evaluate risk inherent to a process owing to the chemical it uses and the process conditions. The risk assessment tool is integrated with process design simulator (HYSYS) to provide necessary process data as early as the initial design stages, where modifications based on inherent safety principles can still be incorporated to enhance the process safety of the plant. The risk assessment tool consists of two components which calculate the probability and the consequences relating to possible risk due to major accidents. A case study on the potential explosion due to the release of flammable material demonstrates that the tool is capable to identify potential high risk of process streams. Further improvement of the process design is possible by applying inherent safety principles to make the process under consideration inherently safer. Since this tool is fully integrated with HYSYS, re-evaluation of the inherent risk takes very little time and effort. The new tool addresses the lack of systematic methodology and technology, which is one of the barriers to designing inherently safer plants.     

Qualitative Assessment for Inherently Safer Design (QAISD) at preliminary design stage

Conventional hazard evaluation techniques such as what-if checklist and hazard and operability (HAZOP) studies are often used to recognise potential hazards and recommend possible solutions. They are used to reduce any potential incidents in the process plant to as low as reasonably practicable (ALARP) level. Nevertheless, the suggested risk reduction alternatives merely focus on added passive and active safety systems rather than preventing or minimising the inherent hazards at source through application of inherently safer design (ISD) concept. One of the attributed reasons could be the shortage of techniques or tools to support implementation of the concept. Thus, this paper proposes a qualitative methodology that integrates ISD concept with hazard review technique to identify inherent hazards and generate ISD options at early stage of design as proactive measures to produce inherently safer plant. A modified theory of inventive problem solving (TRIZ) hazard review method is used in this work to identify inherent hazards, whereby an extended inherent safety heuristics tool is developed based on established ISD principles to create potential ISD options. The developed method namely Qualitative Assessment for Inherently Safer Design (QAISD) could be applied during preliminary design stage and the information required to apply the method would be based on common process and safety database of the studied process. However, user experiences and understanding of inherent safety concept are crucial for effective utilisation of the QAISD. This qualitative methodology is applied to a typical batch reactor of toluene nitration as a case study. The results show several ISD strategies that could be considered at early stage of design in order to prevent and minimise the potential of thermal runaway in the nitration process.     

Bayesian networks based laboratory retrofitting towards inherent safety: A risk-based implementation framework

Over the years, a number of high-profile laboratory accidents involving severe injuries, fatalities, and economic losses have been reported, prompting a significant increase in efforts towards laboratory safety. However, the dominant safety measures rely excessively on add-on safeguards such as sprinklers and respirators and pay little attention to reducing the hazardous factors at their sources. This study introduced the inherent safety concept to minimize laboratory hazards and developed a dedicated implementation tool called Generic Laboratory Safety Metric (GLSM). The Traditional Laboratory Safety Checklist (TLSC) was first used to represent the safety indicators, and then the Precedence Chart (PC) and Bayesian Networks (BN) methods were used to reconcile the safety indicators to develop the GLSM. The developed GLSM was subsequently demonstrated through a case study of a university laboratory. The results revealed that the safety level increased from 2.44 to 3.52 after the risk-based inherently safer retrofitting, thus creating laboratory conditions with a relatively satisfactory safety level. This work presented a set of generic solutions to laboratory retrofitting towards inherent safety with a novel GLSM as the implementation tool. The proposed GLSM would contribute to risk quantification and identification of key risk factors for assigning targeted and fundamental safety measures to achieve inherently safer laboratories.     

A hybrid machine learning model for predicting crater width formed by explosions of natural gas pipelines

Despite the existence of industry models for estimating the crater width formed by the explosion of natural gas pipelines, their applicability is still limited since the complex formation mechanisms. In this work, a novel hybrid model was developed to predict crater width formed by explosions of natural gas pipelines, using artificial neural networks (ANN) as the fundamental predictor. Based on the historical accident records, the proposed hybrid model was trained by the pipeline parameter, the operating condition, the installation parameter, and the crater width. A novel nature-inspired optimization algorithm, i.e., the Lévy-Weighted Quantum particle swarm optimization (LWQPSO) algorithm, was proposed to optimize the ANN model's parameters. Three machine learning models were developed for comparative reasons to predict the crater width. The use of precision and error analysis indicators assesses prediction performance. The results show that the proposed hybrid model (LWQPSO-ANN) has high prediction accuracy and stability, which outperforms QPSO-ANN-based benchmark hybrid models and the model without an optimizer (Support Vector Machine, SVM). The parameter sensitivities of the proposed algorithm, including the maximum number of iterations, population size and contraction-expansion coefficient, were determined. The proposed hybrid model is expected to support the quantitative risk assessment (QRA), Right-of-Way (ROW) definition and the inherently safer design of the underground parallel pipelines.     

Trevor Kletz's scholarly legacy: A co-citation analysis

Dr. Kletz is one the pioneers in the process safety area, known widely for his work on inherent safety design and loss prevention. He worked 38 years in Imperial Chemical Industries, and became a fulltime researcher only after his retirement. He published more than 200 papers and 15 books during his retirement. The intellectual basis analysis presented in this article shows that he frequently cited his books in his articles, indicating that his industrial experience was very influential to his scholarly contributions. Lawley, H.G. was one of the researchers whose work had most influence on Kletz's research. Among Dr. Kletz's publications, the article ‘What You Don't Have, Can't Leak’ has the highest impact, while his most influential book is ‘Process plants: A handbook for inherently safer design’. The references co-citation network is divided in two clearly connected components: his earlier work related to infra-red spectra, and his later work addressing process safety related topics, including inherent safety and hazard and accident analysis. Both his work and that of his followers is rooted in a similar intellectual basis within process safety research, in which particularly Dr. Kletz's earlier work forms an influential original body of knowledge rooted in his industrial experience. His career is a prime example of how process safety research has been strongly influenced by knowledge from industrial practice, illustrating that a continued strong connection between industry and academia can lead to very fruitful outcomes. It is hoped that the presented analysis can inspire especially young graduates with academic interests to first embark on an industrial career to gain industrial experience before aiming to contribute to academic process safety knowledge.     

Risk assessment of chemical process considering dynamic probability of near misses based on Bayesian theory and event tree analysis

With the development of modern automatic control systems, chemical accidents are of low frequency in most chemical plants, but once an accident happens, it often causes serious consequences. Near-misses are the precursor of accidents. As the process progresses, near misses caused by abnormal fluctuation of process variables may eventually lead to accidents. However, variables that may lead to serious consequences in the production process cannot update the risk in the life cycle of the process by traditional risk assessment methods, which do not pay enough attention to the near misses. Therefore, this paper proposed a new method based on Bayesian theory to dynamically update the probability of key variables associated with process failure risk and obtain the risk change of the near-misses. This article outlines the proposed approach and uses a chemical process of styrene production to demonstrate the application. In this chemical process, the key variables include flow rate, liquid level, pressure and temperature. In order to study the dynamic risk of the chemical process with consideration of near misses, according to the accumulated data of process variables, firstly the abnormal probability of the variables and the failure rate of safety systems associated with the variables were updated with time based on Bayesian theory. On the basis of the dynamic probability of key process variables, an event tree of possible consequences caused by variable anomalies was established. From the logical relationship of the event tree, the probability of different consequences can be obtained. The results show that the proposed risk assessment method based on Bayesian theory can overcome the shortcomings of traditional analysis methods. It shows the dynamic characteristics of the probability of different near misses, and achieves the dynamic risk analysis of chemical process accidents.     

The feasibility of encouraging inherently safer production in industrial firms

Inherent Safety is generally recognised as an important concept in the design of chemical plants. It is, however, often regarded as the sole province of engineers. Inspired by the successful development in the last decade from cleaner technology towards Cleaner Production, we explored in this research the feasibility of what we call Inherently Safer Production. Four pilot cases were carried out, two in the Netherlands and two in Greece; three in existing plants, one at the design stage. A methodology to generate inherently safer technological options was developed and tested. In all four cases, a number of options for inherently safer production were identified, while more traditional safety options were also identified. The great majority of options was shown to have pay back times of less than 2 years. Overall, 25% of the options were implemented during the project.In existing plants, the option generation process can be organised as a collective learning and inspiring effort. This can be of considerable value on its own, apart from the implementation of the (technological) options identified. We conclude that Inherently Safer Production is a feasible concept, that it has great potential for simultaneous improvement of safety and economic performance, and that it deserves to be further developed.     

Safety is an inherently inconsistent concept

Some basic principles for philosophical definition work are introduced and then applied to safety and related concepts. Definitions are provided first for comparative safety concepts such as “safer than” and then for the monadic “safe”. It is shown that “safe” is an inherently inconsistent concept, i.e. it cannot be restored to consistency without giving up what we perceive as some of its central elements. The reason for this is that both absolute and relative conceptions of safety are entrenched in common usage of the term. In order to avoid the inconsistency a strategy of terminological ramification is proposed: We should distinguish between the two concepts “reasonably safe” and “absolutely safe”. Any usage of “safe” or “safety” simpliciter should be seen as an abbreviated reference to one of these two closely related, remarkably confusable, but still unmergable concepts.     

Optimizing the design of storage facilities through the application of ISD and QRA

Four strategies can be used to achieve safety in chemical processes: inherent, passive, active and procedural. However, the strategy that offers the best results is the inherent safety approach, especially if it is applied during the initial stages of a project. Inherently Safer Design (ISD) permanently eliminates or reduces hazards, and thus avoids or diminishes the consequences of incidents. ISD can be applied using four strategies: substitution, minimization, moderation and simplification. In this paper, we propose a methodology that combines ISD strategies with Quantitative Risk Assessment (QRA) to optimize the design of storage installations. As 17% of major accidents in the chemical industry occur during the storage process and cause significant losses, it is essential to improve safety in such installations. The proposed method applies QRA to estimate the risk associated with a specific design. The design can then be compared to others to determine which is inherently safer. The risk analysis may incorporate complex phenomena such as the domino effect and possible impacts on vulnerable material and human elements. The methodology was applied to the San Juanico tragedy that occurred in Mexico in 1984.