Including Inherent Safety in the Design of Chemical Processes

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Including Inherent Safety in the Design of Chemical Processes Andrea Paulina Ortiz-Espinoza, Arturo Jimenez-Gutierrez, and Mahmoud M El-Halwagi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02164 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Industrial & Engineering Chemistry Research

Including Inherent Safety in the Design of Chemical Processes Andrea P. Ortiz-Espinozaa, Arturo Jiménez-Gutiérreza*, Mahmoud M. El-Halwagib a

Departamento de Ingeniería Química, Instituto Tecnológico de Celaya, Celaya, Gto. 38010, México b

Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA

ABSTRACT Past industrial accidents have raised major concern for the inclusion of safety as part of the design of a chemical process, an item typically carried out after the process design has been completed. In this work, a procedure for the inclusion of inherent safety that can be used to compare different technologies at the design stage is presented. The combination of two indices, one that rates the process routes and a second one that rates the process streams, provide the basis for a safety metric. Results from simulations of process flowsheets can be conveniently used to obtain the data needed for the application of the safety indices. The procedure was applied to two technologies for the production of ethylene and four alternatives for the production of methanol. In addition to the ranking of technologies from safety considerations, economic and environmental measures were also applied. The results provided the options with the best inherent safety properties, but conflicts with the other metrics were observed. It is shown how a given design can then be revised to improve one of the three properties under consideration. In particular, process modifications aiming to improve safety, guided from the values of the inherent safety indices, were explored. From the methodology, a selection of alternatives with suitable safety, economic and environmental indicators can be identified.

Keywords: inherent safety, environmental impact, natural gas, methanol, ethylene

*Corresponding author. E-mail: [email protected]

1. INTRODUCTION The typical approach for the comparison and selection of different alternatives to produce a given chemical is based on economic and technical aspects. Methodologies to include other factors, such as those related to sustainability issues and environmental considerations, have been recently proposed.1 One aspect of primary importance is generally left for analysis after the design has been completed, and that has to do with process safety. Incidents such as those observed in the plants of Flixborough, Seveso, Bhopal and Texas City have raised significant concern on safety considerations.2,3 If safety is to be included at the design stage of the process,

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an evaluation of the inherent safety properties of the process would provide a suitable basis for such purpose.4,5 The concept of inherent safety was first established by Kletz.6 The main purpose of inherent safety is the elimination, or minimization, of sources of hazards in a chemical plant, which is then translated into fewer control layers and protective add-ons. The main principles of inherent safety are elimination, minimization, substitution, moderation and simplification.6,7 Those principles are best applied at early stages of the design, since changes to the design are easily made, and the opportunities to apply the inherent safety principles reduce considerably as the process design moves forward.4,7,8 Also, the application of inherent safety principles at early stages of design can lead to a reduction in capital and operating costs.6.7 Despite these incentives, inherent safety principles have not found a common industrial application, mainly because of the lack of simple methodologies that can be applied by design engineers when the process is being developed.9 Some metrics have been proposed for the evaluation of inherent safety; such metrics take into account chemical and process parameters that are available at the early design stages.10 One approach consists in the application of a quantitative risk analysis (QRA), in which a frequency analysis is first carried out, followed by a consequence analysis to provide a measure of risk.11 Recent works have shown the application of QRA for the analysis of different distillation structures12 and for the selection of solvents in extractive distillation systems.13 While in those works the QRA approach provided a suitable tool for systems with a few units (i.e. one or two distillation columns), the procedure is rather complex to be applied when complete flowsheets are under consideration. Simpler metrics for the evaluation of inherent safety, therefore, need to be considered. A pioneering index to evaluate inherent safety was proposed by Edwards and Lawrence.14 Their Prototype Index for Inherent Safety (PIIS) is reaction oriented, and takes into consideration parameters that include inventory, flammability, explosiveness and toxicity, as well as operating conditions such as temperature, pressure and yield. To include other parts of the process, which may be also important sources of risk, the Inherent Safety Index (ISI) was proposed.15,16 ISI is divided into two indexes, and takes into account the same parameters as the PIIS, in addition to other factors related to equipment, process structure and corrosiveness properties. A third index, the i-Safe Index,17 takes sub-indexes values from PIIS and ISI, and adds information from reactivity ranking provided by the National Fire Protection Association. Palaniappan et al.17 presented the application of inherent safety principles for a case study in which ten process routes for the production of acetic acid were evaluated to identify the inherently safer alternatives. Although these indexes take into account several chemical properties for the species involved in the processes, they only consider individual components and overlook the properties of the stream mixtures. Also, the calculation of such indexes is sometimes inconvenient because of the amount of data that is subject to manual transferring procedures, which might increase the


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probability of human error.18 To overcome these limitations, other indices have been developed, among them the Process Route Index (PRI)18 and the Process Stream Index (PSI)19. PRI can be used to rank different processes according to inherent safety levels, while PSI serves to rank the streams within a process according to the individual risk they might pose. Both indexes are defined as a function of stream parameters such as flammability, temperature, pressure, and density. The PRI was benchmarked by Leong and Shariff 18 against the PIIS, the ISI, and the i-Safe indexes, as well as expert opinion, and resulted in high agreement with three of those sources (PIIS, ISI and expert opinion). In addition, PRI was able to distinguish between processes that were ranked at the same level of inherent safety by the ISI. PRI and PSI indexes have the limitation that they only rank processes or streams with respect to each other, and therefore not provide a quantitative measure of risk. In that aspect, QRA is a more complete technique for assessing risk. However, given the objective of this work to compare different flowsheets for complete processes, the use of QRA becomes a complex task, and for the purpose of comparing safety among process design alternatives and detecting sections with streams of higher risk, the combined use of PRI and PSI provides a useful tool for an initial comparative assessment of inherent safety. In addition to safety, economic factors, measured through the process return on investment, and environmental impact, measured through CO2 emissions, are evaluated, so that trade-offs among these criteria can be identified. We also show how the use of these metrics serves as a basis to consider process modifications aiming to improve a given metric, with an assessment of the effect on the others.

2. DESIGN APPROACH The conceptual methodology developed for the assessment of design alternatives considering economic, environmental and safety aspects is shown in Figure 1. A synthesis problem to convert a given set of feedstocks into products requires data on different routes of chemistry, properties for separation tasks and basic operating conditions to provide initial flowsheets for processes that meet a given demand for the chemical product. The flowsheet is then validated through rigorous process simulations to provide base-case flowsheets, that include details on mass and energy balances, equipment and operating conditions, and other relevant data for energy and environmental assessment. Such results, together with an economic scenario give an initial assessment in terms of the potential process profitability. In addition to these common steps, two other metrics are considered in this work, one related to the process environmental impact and the other one to the process inherent safety. Alternatives can then be ranked according to each of these items. If an alternative meets desired criteria on each metric, such a flowsheet can be selected as the final design; otherwise, process improvements, such as modifications on the flowsheet structure, operating conditions and/or the implementation of process integration strategies, can be explored aiming to improve one or more of such criteria.



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We take here process flowsheets already reported for the synthesis of methanol and ethylene production from shale gas as a basis for the application of the approach that includes economic, environmental and safety considerations. Simulations were carried out using the Aspen Plus process simulator, and data from the process streams, as well as results from energy and mass balances, were extracted for the evaluation of each metric. The levels of inherent safety for the processes were evaluated using the process route index, PRI, and the streams representing a higher risk to each process were identified through the values of the process stream index, PSI. The economic aspects of the processes where estimated via return on investment calculations, and to assess the environmental impact, CO2 equivalents emissions were calculated for each process.

3. PROCESS ROUTES DESCRIPTION Some processes have been recently reported as options for the transformation of shale gas into value-added chemicals. Such works have been promoted by the discovery of recent shale gas reservoirs that offer methane or ethane feedstocks at low prices.20 Some of such developments include the analysis of gas-to-liquid schemes,21 and methanol and ethylene production alternatives.22,23 We consider in this work two ethylene routes and four methanol processes for which basic flowsheets have been developed and compare the process alternatives from safety, economic and environmental considerations. A description of the processes is given below. 3.1 Ethylene Technologies The main route to produce ethylene is based on steam cracking of naphtha or ethane, although alternatives from methane in natural gas have been considered. Two alternatives for ethylene production that have received particular interest are the oxidative coupling of methane (OCM) and the methanol to olefins (MTO) route. 3.1.1 Oxidative Coupling of Methane (OCM) The oxidative coupling of methane is a direct process where methane, or natural gas in this case, is fed to a catalytic reactor along with oxygen. The outlet stream of the reactor contains ethylene, CO2, H2, CO, water, ethane and unreacted methane. Before sending the outlet stream to a distillation train, it is chilled and compressed with a multistage compressor in order to remove water and prepare the stream for a CO2 absorber, where methyl diethanolamine is used as solvent. After water and CO2 have been removed, the stream is sent to a cryogenic distillation train, where a de-methanizer column is used to recover the unreacted methane, and a C2-spplitter carries out the purification of ethylene.23,24 3.1.2 Methanol to Olefins (MTO) The MTO process can be divided into three main sections. In the first section, the reforming of the natural gas is carried out. In this stage, steam and natural gas are first fed to a pre-reformer


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reactor in order to crack the heavier hydrocarbons and avoid coking in the steam reformer reactor, where syngas is produced. The outlet stream of the reformer is sent to a flash unit, where water is removed, and then CO2 is separated and recycled. The ratio of H2 to CO for the production of methanol must be 2:1; therefore, before sending the syngas stream to the methanol production reactor, the ratio of hydrogen to carbon monoxide has to be adjusted. The stream is sent to the methanol reactor, after which the outlet stream containing mainly methanol is cooled and sent to a series of flash units to recover the unreacted syngas and to obtain crude methanol. The MTO process does not require a high purity of methanol, which avoids the need for an additional purification process. The crude methanol stream is sent to the MTO reactor, where the main products are ethylene and propylene. The outlet stream is chilled, after which the removal of CO2 and water is done. Finally, a distillation train is used to obtain the purified ethylene and other byproducts, such as propylene and other low-weight olefins.23 3.2 Methanol Technologies For the methanol processes, the production routes begin with a reforming stage to obtain syngas, which is converted into methanol with a catalytic reactor, and finally purified in a distillation train. Different alternatives for the reforming stage affect the process flowsheet obtained for methanol production. Four reforming alternatives for the production of methanol can be considered, namely, steam reforming (SR), partial oxidation (POX), autothermal reforming (ATR) and combined reforming (CR). A brief description of each reforming alternative and of the methanol production section is given next; a detailed information is available in Julián-Durán et al.22 3.2.1 Steam Reforming (SR) The steam reforming alternative starts with a pre-reformer reactor, where high-pressure steam and natural gas are fed; the pre-reformer is used to crack higher hydrocarbons and avoid the formation of coke in the reforming reactor. The outlet stream is then heated and sent to the steam reforming reactor, where syngas is produced. The outlet stream is mainly composed of syngas and waste water, and the mixture is separated using a flash unit. The gas stream is sent to a CO2 removal unit, where CO2 is recovered for recycle. The CO to H2 ratio is adjusted, and the syngas is compressed before it is sent to the methanol synthesis reactor. 3.2.2 Partial Oxidation (POX) For the partial oxidation alternative, oxygen and natural gas are fed to a POX reactor. The syngas produced by the POX reactor contains a lower H2 to CO ratio than that required for methanol production, so the outlet stream is sent to a water gas shift (WGS) reactor in order to increase the amount of H2. After that step, water is removed using a flash unit, and CO2 is removed with an amine absorber. Finally, the syngas is compressed and sent to the methanol synthesis reactor. 3.3.3 Autothermal Reforming (ATR)



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This reforming process is a more energy-efficient option, given by the combination of the endothermic nature of the SR technology and the exothermic characteristic of POX. First, natural gas and high-pressure steam are fed to a pre-reformer, after which the outlet stream is fed along with oxygen to the ATR reactor, where both steam reforming and partial oxidation reactions take place. The outlet stream containing syngas and water is cooled and water is removed. CO2 is separated in another unit, and the syngas is compressed to provide the feed for the methanol synthesis reactor. 3.3.4 Combined Reforming (CR) In the combined reforming alternative ATR is used together with SR in order to get a better energy usage with respect to the individual use of each option. In this process, natural gas and high-pressure steam are fed directly to an SR reactor, and the outlet stream reacts with oxygen in the ATR reactor. The product stream is sent to a flash unit to remove water, followed by CO2 removal. The syngas is then compressed, and the stream is sent to the methanol synthesis reactor. 3.3.5 Methanol Synthesis and Purification The methanol synthesis stage consists of a catalytic reactor operating in a pressure range of 35 to 55 bar and temperatures around 200 to 300 °C. The outlet stream is cooled, and the unreacted syngas is recovered with a flash unit and recycled. Crude methanol is sent to a distillation column, where methanol with high purity is obtained. For the SR alternative, a second flash unit is needed.

4. SAFETY ASSESSMENT Out of the several indices that have been proposed to measure safety, the Process Route Index (PRI) and Process Stream Index (PSI) were selected due to their suitable application at early stages of design, and for their convenient adaptation to the information extracted from process simulators.5 A detailed description of both indexes is given below. 4.1 Process Route Index (PRI) The basis for the PRI index is a set of process parameters related to the potential damage that can cause an explosion. Based on those parameters, equation (1) is used to calculate the maximum distance of the damage caused by explosion.9

= =

(1)

where Ri is the distance of affectation in meters, is a constant, is an efficiency factor, is a mechanical factor, is the total mass of flammable material (kg), and is the lower limit of ACS Paragon Plus Environment

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the heat of combustion (kJ/kg). The type of data required for PRI estimation therefore is in terms of mass and energy data, which are process parameters, and combustibility, which can be obtained from the difference between flammability limits of the mixture. To accommodate better the use of data extracted from the process simulator, mass can be substituted by density and pressure, based on fluid mechanic principles. Therefore, PRI can be expressed as, = , , , !

(2)

Flammability can be affected by temperature, and equations 3 and 4 show such a relationship,

"#"$ = "#"%& '1 −

0.75. − 25 2 ∆1

(3)

3#"$ = 3#"%& '1 +

0.75. − 25 2 ∆1

(4)

where "#"$ and 3#"$ stand for lower and upper flammability limits at a given temperature ., "#"%& and 3#"%& are the lower and upper flammability limits at 25 °C, and ∆1 is the heat of combustion. The flammability limits of a mixture can be calculated as follows,

"#"5 =

3#"5 =

1

∑=>? 7

? 7

89

:;:9

89

9

where "#"5 and 3#"5 are the lower and upper flammability limits of the mixture, is the mole fraction of component , and "#" and 3#" are the lower and upper flammability limits of component .



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Once flammability values have been obtained, average values of the parameters can be used to calculate the PRI metric for the process using equation 7. BCB B BCB BCB BCB ! A7 ℎB CB! < ∗ G H ∗ 7 < ∗ 7 ∆#"

Including Inherent Safety in the Design of Chemical Processes

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