Article pubs.acs.org/IECR
Novel Methodology for Inherent Safety Assessment in the Process Design Stage Preeti Gangadharan,† Ravinder Singh,† Fangqin Cheng,‡ and Helen H. Lou*,† †
Dan F. Smith Department of Chemical Engineering, Lamar University, P.O. Box 10053, Beaumont, Texas 77710, United States Institute of Resources and Environment Engineering, Shanxi University, Wucheng Road 92, Taiyuan City, 030006 Shanxi, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Hazards are intrinsic to a material or its conditions of storage or use [Hendershot, D. C. Inherently safer chemical process design. J. Loss Prev. Process Ind. 1997, 10 (3), 151−157]. Inherently safer designs aim to avoid hazards by design, rather than by add-on measures. The importance of inherent safety has been increasingly stressed in chemical process industries in recent years. It is the most suitable safety approach, particularly in the process design stage. This paper describes a new comprehensive inherent safety index (CISI) for use in the early process design stage. The CISI assigns equipment safety scores to individual units in the process based on chemical, process, and connectivity scores. The chemical score considers the weighted severity score of each chemical in the unit as well as the reactivity score. The reactivity score is calculated separately for the mixture of chemicals in each unit. Since hazards can be compounded by the existence of highly interconnected units, the concept of the connectivity score is introduced. Case studies involving biodiesel and methyl methacrylate processes are used to demonstrate the new safety assessment methodology. The results of the assessment are used to compare the processes based on inherent safety, and they can potentially serve as a valuable aid to clearly identify key areas for improvement in a root-cause analysis.
1. INTRODUCTION Inherently safer design, a philosophy introduced by Kletz,2−4 focuses on the elimination of hazards or reduction of the magnitude of hazards rather than the control of hazards.5 Since “safety” is a fuzzy concept, it is never possible for a process to be 100% safe. Rather, inherent safety is a relative characteristic,6,7 and it is most appropriate to describe one process as inherently safer than another. There are a number of process hazards associated with a chemical process. Some examples are high temperature, high pressure, toxicity, reactivity, and explosiveness of chemicals. All these contribute to the relative safety of the process. In order to choose from a number of alternatives, it is essential that the inherent safety be quantified. The Dow Fire & Explosion Hazard Index8 and the Mond Index9 are two widely used methods in process industries. These indices are mainly related to the fire and explosion rating of a plant. Another popular method for safety analysis is HAZOP (hazard and operability analysis).10 HAZOP studies are normally conducted using P&ID (piping and instrumentation diagrams) to find out possible process disturbances and their consequences. However, such details are not available early in the design stage. Over the past few decades, there have been several attempts to quantify the inherent safety of processes in the design stage.11−17 Some of these make use of fuzzy logic,13 an expert system called i-Safe,14 a graphical method,15 SREST-Layer Assessment,18 etc. The challenge is to develop a methodology that is not exceedingly complex or timeconsuming while at the same time it includes enough detail and depth to provide a realistic idea of the inherent safety. The inherent safety method introduced by Edwards and Lawrence19 had seven parameters: temperature, pressure, yield, inventory, explosiveness, toxicity, and flammability. These © 2013 American Chemical Society
parameters are grouped into two subcategories: process and chemical safety. Scores for each category were given in the range of 1−10. Heikkilä12 improved upon this index by adding certain new parameters (type of equipment, safety of process structure, chemical interaction, equipment layout) and altered the scoring table to a 0−4 scale. The structure of Heikkilä’s index is shown in Table 1. In this method, the calculations of the inherent safety Table 1. Structure of the Inherent Safety Index12 chemical inherent safety index, ICI heat of main reaction heat of side reaction, max chemical interaction flammability explosiveness toxic exposure corrosiveness process inherent safety index, IPI inventory process temperature process pressure equipment safety inside battery limits outside battery limits safe process structure
Received: Revised: Accepted: Published: 5921
symbol
score
IRM IRS IINT IFL IEX ITOX ICOR
0−4 0−4 0−4 0−4 0−4 0−4 0−2
II IT IP IEQ ISBL OSBL IST
0−5 0−4 0−4 0−4 0−3 0−5
November 17, 2012 February 5, 2013 April 4, 2013 April 4, 2013 dx.doi.org/10.1021/ie303163y | Ind. Eng. Chem. Res. 2013, 52, 5921−5933
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hazard compared to a similar process with a more simplified design. Also, the connection between relatively unsafe units will have a higher hazard. The new methodology that takes these factors into consideration is described in a sequential manner in section 5. The inclusion of reactivity score in the calculation of equipment safety index is also an important development. Heikkilä’s index12 considers chemical interaction but only takes into account the maximum value. The authors believe that chemical reactivity should be considered for the mixture of chemicals present in each unit. This would ensure that the most hazardous interaction leads to a higher score, while also accounting for the incremental contributions of every possible hazardous reaction. The reactivity for any group of chemicals present together can be easily predicted using NOAA’s CAMEO software21 or the Chemical Reactivity Worksheet (CRW).22 The equipment inherent safety index for each unit is calculated as the sum of the chemical safety index and the process safety index.
index (ISI), computed as the sum of the chemical and process safety subindices, are made on the basis of the worst-case scenario. For example, the chemical safety index considers the greatest sum of flammability, explosiveness, and toxic exposure subindices. In the case of inventory, process temperature, and process pressure, the maximum expected values are used. However, comparing two processes only on a worst-case basis may give a totally different result from comparing them using all available data. It is possible that the worst-case scenario of one process is less safe than another process; however, when scores for the whole process (and not just the worst case) are considered, it might actually be safer. Also, ISI only considers the maximum value and not the quantity of the materials or chemicals used. It also does not consider the number of equipment in the plant and the complexity of the process. To overcome some of these limitations, the enhanced inherent safety index (EISI)20 was proposed. In the EISI, the severity (i.e., flammability, toxicity, etc.) score is multiplied by the flow rate with the basis taken as 1000 kg/h flow. The scores for individual chemicals are added together to obtain the total chemical inherent safety index. In the process inherent safety index, scores are given for individual equipment and multiplied by the number of equipment. The scores of all the equipment are added together to get the total process safety index. Although the EISI made it possible to compare similar processes which would normally obtain the same score in the earlier index, there is still scope for improvement, as will be demonstrated in section 2 and the case study (section 3).
IEISI = ICSI + IPSI
where IEISI is the equipment inherent safety index, ICSI is the chemical safety index, and IPSI is the process safety index. The total inherent safety index (ITISI) is given by k
ITISI =
∑ (IEISI)i i=1
where k is the number of units in the process.
2. COMPREHENSIVE INHERENT SAFETY INDEX This paper introduces the comprehensive inherent safety index (CISI), which adopts an object-oriented approach for inherent safety analysis. Just like the object-oriented approach in software programming, a major advantage of the CISI is that it provides a clear modular structure, where each equipment forms a separate entity with its own safety score. This enables the consideration of two major factors that was not possible in earlier indices: (i) the severity of reactions of the combination of chemicals present in each unit (ii) connection scores between two units based on their equipment safety scores In the CISI, the chemical and process safety subindices are calculated for each individual unit. Therefore, each individual unit has its equipment safety index that takes into account the following: (i) the flammability, explosiveness, toxicity, and corrosiveness of each chemical in the unit (severity multiplied by flow rate) (ii) the reactivity hazard due to the particular mixture of chemicals in that unit (iii) hazards due to process conditions in that particular unit (pressure, temperature, etc.) This approach allows process safety to be visualized in a network-type framework with the individual units as nodes, thus providing a more intuitive way to assess the total hazard. The results can be represented on a block diagram, table, or Pareto chart to immediately give a clear idea of the individual units and connections that contribute most significantly to the safety score. The connection scores account for the number and severity of connections among units. As the complexity of the design increases, so does the hazard. A process that has a larger number of loops or highly interconnected units will pose a greater safety
The chemical safety index considers the severity of each chemical in the unit multiplied by their flow rate through the unit, and the severity of the reactions of these chemicals when present together multiplied by the total flow rate. The process safety index considers the hazard due to process conditions such as temperature, pressure, and inventory. The calculation will be illustrated in detail in a case study on biodiesel production20 in section 3. Some Other Factors for Consideration. In addition to the factors mentioned above, there are a few other criteria that are important in the safety assessment of processes. Health-Related Factors. Health differs from safety in terms of the time for the effect to appear. Safety deals with acute (serious) short-term events. Health is a chronic matter because it takes some time before the effects on people’s health can be identified and the impact might persist over a long time. This time factor compounds the task of assessing the occupational healthiness of chemical plants.23 OSHA (Occupational Safety and Health Administration) has a comprehensive health hazard classification system24 as part of their Regulations (Standards29 CFR), based on acute toxicity and target organ toxicity. There are some very good papers18,23,25−28 that deal with the assessment of Scheme 1. Transesterfication Reaction
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Figure 1. Process flow diagram (PFD) of conventional biodiesel production process.20
Figure 2. Process flow diagram of heterogeneous biodiesel process.20
inherent health hazards in a process. In this paper, the health aspect is not dealt with as human health hazards are already covered in the environmental assessment section of the sustainability assessment methodology by the same authors.20,29 However, these factors are important if the inherent safety is being evaluated in a stand-alone assessment, i.e., when the safety assessment is not conducted as part of an overall sustainability assessment. Dust Explosion Hazard. Dust explosion poses one of the most serious and widespread explosion hazards in the process industry.30 Much research has been carried out to understand and control dust explosions. In particular, coal dust explosions, which can have severe consequences in coal mines and thermal power plants, have been studied in detail in numerous published works.31−33 The important factors that affect the fire/explosion probability of dusts include particle size, dust and oxidant concentrations, ignition temperature, maximum rate of pressure rise, and presence of flammable gases. In the context of preventing and mitigating dust explosions, inherently safe design is a more attractive alternative to adding preventive and mitigatory measures to an existing process.34 Amyotte and Khan35 proposed a framework for directing the concept of inherently safe process design specifically toward reducing the dust explosion hazard in industry. Dusts can be classified in terms of their ability to propagate a flame,36
Figure 3. Stepwise procedure for the calculation of CISI.
combustion class (CC)37 based on their behavior when subjected to a gas flame or hot platinum wire, or “Kst value”,38 5923
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Table 2. Flow Rate of Chemicals through Equipment in the Conventional Process chemical flow rate (kg/h) equipment name
MeOH
Pump 1 Reactor 1 Distillation Column 1 Pump 3 Heat Exchanger 1 Separator 1 Distillation Column 2 Pump 2 Heat Exchanger 2 Reactor 2 Separator 2 Distillation Column 3 Heat Exchanger 3
121 129 129 8 8 8
oil
FAME
glycerol
NaOH
water
55 55 55 55 55 55 1102 1102
1055 1055 1055 1055 1055 1055 3 3
109 109 109 109 109
40 10 10 10 10 10
10 42 42 40 40 89 4
8
109 109 109 52
H3PO4
123 123 123
Na3PO4
0.001
total 171 1400 1400 1277 1277 1326 1114 1105 1105 253.6 245.6 232 55
13.6 13.6
3
Table 3. Calculation of Chemical Severity (Conventional Process) severity/1000 kg flammability, IFL nonflammable combustible (flash point > 55 °C) flammable (flash point ≤ 55 °C) easily flammable (flash point < 21 °C) very flammable (flash point < 0 °C and boiling point ≤ 35 °C) explosiveness (UEL−LEL) (vol %), IEX nonexplosive 0−20 20−45 45−70 70−100 toxic limit (ppm), ITOX TLV > 10 000 TLV ≤ 10 000 TLV ≤ 1000 TLV ≤ 100 TLV ≤ 10 TLV ≤ 1 TLV ≤ 0.1 corrosiveness, ICOR carbon steel stainless steel better material needed chemical severity score (SC)
score
MeOH
oil
FAME
glycerol
NaOH
water
H3PO4
Na3PO4
0 1 2 3 4
3
1
1
1
0
0
0
0
0 1 2 3 4
2
0
0
0
0
0
0
0
2
5
5
4
4
0
5
5
0
0
0
0
1
0
1
0
7
6
6
5
5
0
6
5
0 1 2 3 4 5 6 0 1 2
which represents the maximum rate of pressure rise in a 1 m3 vessel when a dust is ignited. In industries, several tests can be conducted using the “Hartmann vertical tube”, “20l sphere”, etc., to determine the explosivity of dusts. In the early process design stage, these factors can only be judged in terms of minimum ignition energy (MIE), Kst values, etc., when data are available.
main component of which is triacylglycerol (TAG). When TAG reacts with alcohol, fatty acid methyl esters (FAME, or biodiesel) and glycerol will be generated. The catalyst can be either enzymes, acids, or bases.39 The transesterification reaction is illustrated in Scheme 1.40 In this study, two alternative biodiesel production processes are considered. The first case is the conventional homogeneous process, which produces biodiesel using alkali based catalyst (NaOH). The preferred methanol to oil molar ratio is 6:1. At 65 °C, a 93−98% conversion of TAG is achieved within 1 h. The transesterification reaction requires low water content (
