Meysam Akbarian Shourkaei,Abbas Rashidi,*,Javad Karimi-Sabet

1Faculty of Engineering,University of Mazandaran,Babolsar,Iran

2Material and Nuclear Fuel Research School(MNFRS),Nuclear Science and Technology Research Institute,Tehran,Iran

Keywords:Life cycle assessment Oxygen-18 production Cryogenic distillation Isotope separation

A B S T R A C T In this study,life cycle assessment of oxygen-18 by using cryogenic distillation of oxygen is performed using SimaPro 8.3 software.Life cycle assessment is performed to understand the environmental profile and hotspots of this process in order to be used in design and development.Simulation of oxygen-18 process is executed by Hysys software,and the required inputs and outputs for inventory of life cycle were acquired.By doing life cycle assessment and considering achieved results after characterization and normalization of inventory data it has been investigated that in the majority of environmental impacts electricity consumption has a huge contribution relative to other parts of the system like liquefied oxygen production from air separation unit,required facilities for air separation and oxygen-18 units,and needed transportation.Also,among 17 impact categories investigated in ReCiPe impact assessment method,fossil depletion,climate change(human health),particulate matter formation,climate change(ecosystem),human toxicity,and metal depletion have the most contribution in entire environmental loads respectively.Furthermore,sensitivity analysis showed that changing life cycle impact assessment method from ReCiPe to IMPACT 2002+has no significant effect on acquired results and results are confident.In addition,assumption of market for depleted oxygen from heavy isotopes which is withdrawn from top of distillation columns showed some positive effects compared to first case and environmental impacts resulted from liquefied oxygen production(feed)reduced but because of huge contribution of electricity consumption compared to other sections,this positive effect has no remarkable influence on entire environmental loads of product system.

1.Introduction

Oxygen-18 is one of the most applicable isotopes in various fields of science and technology.One of its important utilities is in medicine and namely in Positron Emission Tomography(PET)in which oxygen-18 is used a precursor of its main raw material[1–3].

PET is an imaging technique which is used widely in the diagnosis of many diseases in areas such as oncology,neurology,and cardiology.PET is also useful for a complete examination of patients because it can produce pictures of functions of the human body which is unobtainable by other imaging techniques such as conventional X-rays,CT or MRI scans[4].

Nowadays,medicine imaging by PET is known as one of the safest,most accurate and most precise methods in cancer diagnosis in human's body in all over the world.Since PET diagnostics method is rapid,reliable and even painless to patients,it's now widely used in many countries,and as a result,its raw material demand is remarkably increasing[5].

Different methods are used for producing oxygen-18 such as distillation[6–10],thermal diffusion[11,12],chemical exchange[13–15]and membrane diffusion[16,17].Although there are various methods for oxygen-18production,some drawbacks like low purity or low production rate made researchers to develop new methods or new routes for satisfying the required production rate and purity.Among above mentioned methods,distillation is more applicable in industrial scale,and it can produce high purity product. Distillation of water(H2O)[6,7]and nitric oxide(NO)[8,9]are well-known and old methods used in industry.One of the most recent and efficient methods for oxygen-18 production is a cryogenic distillation of molecular oxygen(O2)[18]which has some suitable benefits compared with water and NO distillation and made it a new promising method.For example,NO is a corrosive and toxic gas which is dangerous in handling and safety issues while molecular oxygen is a safe material compared to NO raw material.Huge latent heat needed for water distillation is one of the important drawbacks of this method.In contrast,molecular oxygen distillation latent heat is lower compared with water distillation,and as a result,it is possible to reduce the size of distillation columns,heat exchangers and the like.Therefore,operational and apparatus cost will be reduced. By the way, both NO and water distillation contain other elements in their structure which make it difficult to produce a high purity oxygen-18,but molecular oxygen only contains oxygen molecules which make it possible to obtain high purity product. In this regard,molecular oxygen cryogenic distillation is introduced as a new promising method.

Nowadays,environmental analysis alongside with technical and economic analysis is of great importance in processes and produced products.In this study,environmental analysis of this new route to oxygen-18 production is assessed using life cycle assessment(LCA).LCA is one of the most efficient and famous methods of environmental evaluation of products,processes and services which is used widely in environmental analysis of chemical processes[19–30].LCA is known as a powerful tool for assessing the environmental profile of a process,product or service and is used widely to provide a suitable and complete understanding of the entire system.

2.Material and Methods

2.1.Cryogenic distillation of molecular oxygen

Natural oxygen is a mixture of three stable isotopes of16O,17O and18O.Therefore,natural oxygen is a six-component system of diatomic molecules of the three above mentioned isotopes.

Three oxygen isotopes of16O16O,16O17O and16O18O have a considerable composition,and other isotopes of oxygen have a negligible amount which can be ignored.Also,16O18O is the main source of oxygen-18 compounds while18O18O has a very small amount[5].

Fig.1 shows the flow sheet of oxygen-18 production using cryogenic oxygen distillation which is simulated by Hysys 8.6 software.

The embodiment includes three distillation columns COL1, COL2 andCOL3which other in which the bottom section of distillation column COL1 is connected to the center position of distillation column COL2. Furthermore, the bottom section of column COL2 is connected to the center of column COL3.Also,the vapor withdrawn from the top of the second column is returned to a position of the first column between the feed and bottom sections.

Molecular oxygen starting material FEED1 enters the first column COL1 and distillation conducts within the column and then heavy isotopes of oxygen concentrate at the bottom of the column.These concentrated heavy isotopes vaporize in the reboiler of the first column,and a part of thereof enters the center of column COL2.The vapor introduced into the second column distillates and column bottom liquid which is concentrated by heavy isotopes vaporizes in the reboiler of the second column and passes through into column COL3where further enrichment occurs.Furthermore,vapor withdrawn from the top of column COL2 is pressurized in blower COMP1 to improve the efficiency of column COL1 via increasing the recovery rate.The same procedure occurs in column COL3, and concentrated heavy isotope(16O18O)passes the reboiler of the third column and is withdrawn as a product vapor PROD.The vapor OUT3 which is depleted from the desired product is withdrawn from the top of the third column COL3.

2.2.Life cycle assessment

2.2.1.Goal and scope

The goal of this study is to investigate the environmental impacts associated with oxygen-18 production by cryogenic oxygen distillation to provide a complete picture of the environmental performance of the proposed system and to identify hotspots of the process to be used in process design and development.

A cradle to gate life cycle assessment approach is used considering raw material acquisition,transport,infrastructure,operational material and energy needed for the production of16O18O as a product.An oxygen-18 production plant using cryogenic oxygen distillation in Iran is assumed in this study.

The functional unit of producing 8.1 ton of16O18O in a year is applied as a basis for doing life cycle assessment.The system boundary includes an air separation unit(ASU)for producing liquefied oxygen as a feed stream for the cryogenic oxygen-18 production unit and manufacturing phase for oxygen-18 production.Use and disposal phases of oxygen-18 are not included in this study.

2.2.2.Life cycle inventory(LCI)

The required inventory data is acquired from process simulation which is indicated in Fig.1.By simulation of oxygen-18 production plant using Hysys 8.6 the needed foreground data was obtained.

Background data for producing liquefied oxygen from the air separation unit,transportation from the air separation unit to oxygen-18 production unit,electricity production needed for manufacturing,and infrastructure for both units is obtained from Ecoinvent 3.3 database.Also,the required facility for the oxygen-18 production unit is assumed similar to the air separation unit due to their similar basic structure,and therefore the air separation facility construction which existed in Ecoinvent database is used instead.

Table 1 shows a summary of inventory of current product system with 8.1 ton oxygen-18 production as a functional unit.

2.2.3.Life cycle impact assessment(LCIA)

Fig.1.Process flow diagram of oxygen-18 production using cryogenic oxygen distillation.

Table 1Summary of life cycle inventory of 8.1 ton oxygen-18 production using cryogenic oxygen distillation

SimaPro 8.3 is used together with ReCiPe impact assessment method to calculate the environmental impacts of inventory data.ReCiPe is an impact assessment method which considers environmental loads both from midpoint and endpoint perspectives.It includes 17 midpoint impact categories as follows: climate change (human health), ozone depletion,human toxicity,photochemical oxidant formation,particulate matter formation,ionizing radiation,climate change(ecosystems),terrestrial acidification,fresh water eutrophication, terrestrial ecotoxicity,fresh water ecotoxicity,marine ecotoxicity,agricultural land occupation,urban land occupation,natural land transformation,metal depletion,and fossil depletion.These 17 impact categories are summarized in three endpoint damage impact categories including human health,ecosystems,and resources.

2.2.4.Interpretation

According to ISO standards 14040[31]and 14044[32],interpretation is the last step in life cycle assessment.ISO standards suggest a sensitivity analysis on obtained results considering changes on assumptions.In this study,two changes on the assumption were considered.First,the LCIA method is changed from ReCiPe method to IMPACT 2002+[33]method.IMPACT 2002+is a method comprising four LCIA methods including IMPACT 2002,Eco-indicator 99,CML,and IPCC.Secondly,oxygen which is withdrawn from the top of the distillation columns and is depleted from heavy isotopes of oxygen is assumed to be sold in the market instead of emitting to air as a polluting emission.Results and discussion and sensitivity analysis of the study are presented in the next section, and after that interpretation is made.

3.Results and Discussion

Fig.2 shows the characterization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe midpoint impact assessment method.

As it is obvious from this figure,electricity which is used in reboilers and condensers of the first to third columns has a huge contribution in all impact categories except agricultural land occupation and fresh water eutrophication.In these two impact categories,“Natural Oxygen(feed)”which is obtained from the air separation unit is dominant against other impact categories.Furthermore,“Natural Oxygen(feed)”has a comparable contribution in metal depletion,urban land occupation,ionizing radiation and human toxicity.

Fig.3.Characterization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe endpoint impact assessment method.

Fig.3 shows the characterization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe endpoint impact assessment method.

In Fig.3 which shows aggregated endpoint impact categories of ReCiPe method,electricity contributes hugely in all three damage impact categories of human health,ecosystems,and resources.

To compare the amount of current product system impact categories with equivalent impact categories in the world,a normalization diagram per each impact category(midpoint)is illustrated in Fig.4.

Fig.2.Characterization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe midpoint impact assessment method.

Fig.4.Normalization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe midpoint impact assessment method.

Fig.4 shows that among these 17 impact categories,only six impact categories including fossil depletion,climate change(human health),particulate matter formation,climate change(ecosystems),human toxicity,and metal depletion have significant impacts and should be focused more on process design and development to minimize these impacts.

Fig.5.Normalization diagram of oxygen-18 production using cryogenic distillation of oxygen with ReCiPe endpoint impact assessment method.

Fig. 5 shows these normalized impacts in endpoint impact categories perspective.

This figure illustrates the significance of“Resources”,“Human health”and “Ecosystems”endpoint impact categories respectively.The normalized diagram shows that the oxygen-18 production life cycle has the biggest impacts on resources and then human health.Ecosystem impact is negligible compared to impacts mentioned above.

To understand the source of these six important impact categories,the contribution of materials and energy affecting each impact category is presented and discussed here. Tables S1 to S6 in Supplementary Material indicate the normalized inventory results of these six impact categories in brief.

3.1.Climate change(human health)

According to Table S1,carbon dio xide(to air)is the most important material affecting climate change(human health)in which 94%of total environmental loads belongs to this impact category.Electricity has the highest contribution in producing carbon dioxide compared to other sections such as transportation,raw material(natural oxygen),and oxygen-18 separation facility.This contribution is reasonable because electricity mix used in Iran is highly dependent on fossil fuels which produce a huge amount of carbon dioxide and methane.

3.2.Human toxicity

Table S2 shows that manganese(to water)contributes about 40%in human toxicity impact category.Barium(to water)and vanadium(to air)are in subsequent positions causing human toxicity in oxygen-18 production using cryogenic distillation of oxygen.Raw material production(natural oxygen),electricity,and oxygen-18 separation facility have the most contribution in manganese,respectively.

3.3.Particulate matter formation

Sulfur dioxide(to air)contributes about 50%of total loads in this impact category as it is shown in Table S3.Nitrogen dioxide(to air)is the second contributor to particulate matter formation.Electricity is the main source of sulfur dioxide and nitrogen dioxide production due to the high share of fossil fuels in electricity mix of Iran.

3.4.Climate change(ecosystems)

As it is obvious from Table S4,similar to climate change(human health)impact category,carbon dioxide has the dominant effect on climate change(ecosystems)impact category.Methane and dinitrogen monoxide have a subsequent effect on this impact category.Electricity has almost the entire contribution compared to other sections of oxygen-18 production.

3.5.Metal depletion

Chromium,iron,and manganese(raw materials),are the most important materials affecting metal depletion impact category.According to Table S5,natural oxygen production(including air separation facility)and oxygen-18separation facility have the highest contribution in this impact category.This is significantly due to air separation and oxygen-18 facility construction in which metal is the key material used in their structure.Electricity,natural oxygen production,and oxygen-18 separation facility are important contributors to iron and manganese,respectively.

3.6.Fossil depletion

As it is expected,electricity has the highest share in fossil depletion impact category.As Table S6 indicates the shares of different substances,gas,and oil are highly effective on this environmental impact category.It's reasonable because the primary source of energy in Iran is fossil fuels in which gas and oil consumption has the highest share in electricity production.Therefore,fossil depletion has a straightforward relationship with gas and oil consumption for electricity production in Iran.

To do a sensitivity analysis and investigate the effect of life cycle impact assessment method on LCA results, IMPACT 2002+method is used instead of ReCiPe method.IMPACT 2002+method midpoint impact categories include carcinogens,non-carcinogens,respiratory in organics,ionizing radiation,respiratory organics,ozone layer depletion,aquatic ecotoxicity,terrestrial ecotoxicity,terrestrial acidification,land occupation,aquatic acidification,aquatic eutrophication,global warming,nonrenewable energy,and mineral extraction.Similar to ReCiPe method,IMPACT 2002+aggregates midpoint impact categories into four endpoint impact categories as follows:human health,ecosystem quality,climate change,and resources.

Fig.6 shows the characterization diagram of oxygen-18 production system using cryogenic oxygen distillation by endpoint IMPACT 2002+LCIA method respectively.

Fig.6.Characterization diagram of oxygen-18 production using cryogenic distillation of oxygen with IMPACT 2002+endpoint impact assessment method.

As it is obvious from Fig.6,no significant change occurred,and electricity hugely contributes to most of the impact categories of the entire system.

The normalized impact categories resulted from IMPACT 2002+LCIA method are shown in Fig.7.

According to Fig.7,similar to ReCiPe LCIA method,human health and resources have more impacts against ecosystem quality.Furthermore,climate change has a significant impact in which it was considered both in human health and ecosystems impact categories in ReCiPe method.

Therefore,changing the LCIA method didn't affect the results of LCA study in the current product system,and therefore the results are confident.A similar conclusion is made by Renou et al.[34]where the LCIA method had no significant effect.

Fig.7.Normalization diagram of oxygen-18 production using cryogenic distillation of oxygen with IMPACT 2002+endpoint impact assessment method.

The second assumption which is changed in this study is a market for depleted oxygen.According to streams OUT1 and OUT3 in Fig.1,depleted oxygen from heavy isotopes of oxygen is emitted to the air.Here,it is assumed that this oxygen is further sold in the market instead of being emitted.In this regard,characterization and normalization results of impact assessment of study by ReCiPe method are illustrated in Figs.8 and 9.

Fig. 9 shows negative environmental loads (environmental benefits)for“Natural Oxygen(Feed)”section in some impact categories,especially in particulate matter formation.Comparing Fig.9 with Fig.2 indicates that the contribution of“Natural Oxygen(Feed)”is reduced in some impact categories.It means that market for oxygen instead of emission to air will improve the environmental loads but because of huge contribution of electricity consumption it can't affect the entire system,and no significant changes will occur.Fig.9 shows a similar behavior as Fig.5.

Therefore,electricity consumption is the biggest hotspot of oxygen-18 production using cryogenic distillation of oxygen.In this regard,for process design and development improvements,minimizing electricity consumption or changing the electricity consumption mix from fossil resources to more renewable resources should be in priority to reduce environmental impacts of a product.Using renewable energy sources as wind,biomass and solar energy can be useful to reduce the energy impacts imposed by the system.Changes in reboiler and condenser structure and design to reduce the required energy are a secondary suggestion which could be applied.

4.Conclusions

In this study,life cycle assessment of oxygen-18 production by cryogenic distillation of oxygen was performed according to ISO 14040 and 14044 standards.The goal of the study was to investigate the environmental profile and hotspots of producing oxygen-18 to be used in design and development of this new process.

The implementation of LCA was done via SimaPro 8.3 software.Inventory data was obtained by process simulation and Ecoinvent 3.3 database.ReCiPe impact assessment method was applied to translate inventory data into environmental loads and impact assessment categories.

Fig.8.Characterization diagram of oxygen-18 production using cryogenic distillation of oxygen by ReCiPe midpoint impact assessment method with a market for oxygen.

By life cycle assessment execution and considering achieved results after characterization and normalization of inventory data,it has been investigated that electricity consumption is the most important hotspot of the entire product system.Electricity consumption contributed significantly in the majority of impact categories relative to other parts of system like liquefied oxygen(feed)production from the air separation unit,required facilities for air separation and oxygen-18 units, and needed transportation.Also,among 17 impact categories investigated in ReCiPe impact assessment method,fossil depletion,climate change(human health),particulate matter formation,climate change(ecosystem),human toxicity,and metal depletion had the most contribution in the entire environmental loads respectively.

Furthermore,sensitivity analysis showed that changing life cycle impact assessment method from ReCiPe to IMPACT 2002+had no significant effect on acquired results and therefore results were confident.Also, finding a market for depleted oxygen isotopes withdrawn from the top of distillation columns indicated some positive effects on oxygen-18 production but because of huge contribution of electricity consumption compared to other sections,this positive effect had no remarkable influence on the entire environmental loads of the product system.

Fig.9.Normalization diagram of oxygen-18 production using cryogenic distillation of oxygen by ReCiPe endpoint impact assessment method with a market for oxygen.

In this regard,for improving the environmental performance of oxygen-18 production,particularly electricity must be considered and a more efficient mechanism must be applied.Using renewable energy sources as wind,biomass and solar energy can be useful to reduce the energy impacts imposed by the system. Changes in reboiler and condenser structure and design to reduce the required energy are a secondary option which could be applied.

Acknowledgments

The authors wish to thank to Pré Consultant Institute which provided the appropriate software and to the department of chemical engineering,University of Mazandaran.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2017.12.008.