Sustainable engineering becomes a fast growing field of research and education.It aims at designing and operating systems of various scales such that they can use energy and resources in a sustainablemanner.Needless to say,this is one of the most challenging engineering problem types that needs scientists,researchers,engineers,and practitioners to collaboratively work for solutions,especially due to the complexity appeared in sustainability research and practice that seeks a simultaneous achievement of economic prosperity,environmental clearness,and social justice.

This special issue includes 11 invited papers that share the current knowledge on and describe state-of-the-art research in a few challenging areas of sustainable chemical systems engineering.These papers cover various topics ranging from process design and integration,process control and optimization,energy and renewable energy,to environmental protection,where sustainability concerns and issues are addressed,and views and solution approaches are presented.

Industrial practice has shown that process integration is an effective,holistic approach to process design and optimization that exploits the interactions among process units in order to utilize resources effectively and minimize cost.As Tang and Daoutidis stated,process systems arise fromprocess integration as a result of sustainabilitymotivations,specifically economic efficiency,energy recycle,and carbon and water footprint reduction.Recognizing the high complexity of such network systems,they indicate that distributed control and optimization is a critical decision making paradigm for process operation.They reviewed a number of methods for determining a network decomposition and those of performing optimization,and presented an excellent case study for methodological illustration.

Among various types of process networks,heat exchanger networks(HENs)have been widely integrated in the process industry that has led to very significant improvement of energy efficiency in plants.To ensure the robustness of HENs in operation,the flexibility and controllability of HENs have drawn continuous attention.Du et al.contribute their study on the simultaneous achievement of flexibility and controllability in network synthesis.The methodological efficacy is demonstrated through a case study,where quantitative measures and dynamic simulation are provided.Focusing on the uncertainty issues appeared in HEN design and operation,Kang and Liu provide a review on the flexibility issue.In their paper,the methods for sensitivity analysis,resilience analysis,flexibility analysis and multi-period synthesis techniques are critically reviewed and some thoughts on future research directions are shared.The research work by Zhang et al.describe a HEN design problem associated with a single-column air separation system.In their study,exergy analysis is conducted,which facilitates the identification of economically optimal column-HEN system configurations and operating conductions.Ma et al.discuss design and control challenges known in extractive distillation that is used to separate azeotropic mixture.In their view,establishment of quantitative structure-property relationship between the solvent and themixture should be critical for reducing energy efficiency consumption and capital cost.

As an alternative to fossil fuel,renewable energy has its share of total energy consumption increased continuously and significantly.This issue includes two papers in this area.Ibarra-Gonzales and Rong provide a comprehensive review of the known process technologies,involving feedstock selection,physical pretreatment,production of biooil,upgrading of bio-oil to transportation fuels and recovery of valueadded products.Wang et al.presented their research on the conversion of some primary by product from biofuel manufacturing to various C3 chemicals in a sustainable way.

This issue also contains four very interesting papers that address some broader sustainability issues,including those related to industrial-sector-based energy sustainability,material development for improving CO2capture,regional solid waste management,and urban haze.In a paper by Man et al.,the papermaking industry is taken as an example,which is a high-energy demanding industry involving a long supply chain.The authors discussed the importance of performance life cycle analysis (LCA) in order to identify energy consumption reduction throughout the whole life cycle steps of the papermaking industry.It seems,however,data availability is still a challenge.As to CO2capture,Yuan et al.provide a detailed review of firstprinciples-based modeling (density functional theory) of materials.The review provides valuable thoughts that may stimulate novel materials designwith a specific target of CO2capture and promote the industrial sustainability of fossil fuel combustion technologies.Joshi and Seay studied the municipal solid waste management in developing regions.In their research,Locally Managed Decentralized Circular Economy (LMDCE) principles are implemented for building momentum for sustainable behavior.They describe the role behavioral economics played in the choices made by producers and consumers.Another very challenging sustainable development problem is associated with urban air quality.The paper contributed by Huang et al.indicate haze weather characterized by PM2.5 in major cities in China.They used the fault tree analysis technique to identify the causation mechanism of urban haze,which should be valuable for deriving effective controlling strategies.

Engineering sustainability is a very active,promising area of research today.Taken collectively,the papers in this special issue are filled with closely knitted and mutually interacting reviews and research.We hope the readers will appreciate the authors efforts,which have added considerable values in the fields of sustainable engineering.Researchers in both academic and industrial organizations should benefit from this collection of papers.Producing this special issue requires significant efforts from authors,reviewers,and the publication team.They are all gratefully acknowledged.