Elsevier

Energy

Volume 203, 15 July 2020, 117788
Energy

Rethinking energy use in distillation processes for a more sustainable chemical industry

https://doi.org/10.1016/j.energy.2020.117788Get rights and content

Highlights

  • Addresses the energy use in the chemical industry and identifies the main issues.

  • Proposes effective ways to reduce the energy use in distillation for better sustainability.

  • Case study on complex column sequencing for a sub-ambient distillation system.

Abstract

The need for greater sustainability for the production of fuels and chemicals has spurred significant research to rethink energy use in the chemical industry, and eventually substitute fossil fuel sources by renewable sources. Nowadays, the chemical industry is responsible for about one third of the total energy used - and the associated CO2 emissions - in the industrial sector. Among the energy intensive operations, distillation alone is responsible for about 40% of the energy used in the chemical industry, but there is clearly much room for improvement. This paper aims to provide an informative perspective on the current energy use in the chemical industry, with a focus on distillation – the undisputed king of industrial separation processes – and potential improvements for a more sustainable future. There is clearly an increasing need and scope for advanced distillation technologies (e.g. reactive distillation, dividing-wall columns, thermal coupling, cyclic distillation, heat pump assisted distillation, and heat integrated distillation columns) that can significantly reduce the energy usage and the carbon footprint of modern chemical plants. However, these advanced distillation technologies must be considered, along with conventional distillation operations, in the context of the process as a whole. Based on the overview provided, several challenges and opportunities for research directions are highlighted towards rethinking the energy use in distillation processes, for a more sustainable chemical industry.

Introduction

The current production of energy (electricity and heat) is still largely based on fossil-fuels (coal, oil, and natural gas), and this has a strong impact on the environment. It is undisputed that fossil fuels do more harm than renewable energy sources by most measures (e.g. air and water pollution, damage to public health, wildlife and habitat loss, water and land use, global warming emissions, etc). The chemical sector accounts for almost 30% of the energy used in all industrial sectors [1]. Fig. 1 shows a breakdown of industrial energy use with the bulk chemicals sector being the largest energy user. Fig. 1 also shows the global greenhouse gas (GHG) emissions by economic sector. To get the true picture of GHG emissions requires the emissions from electricity and heat production to be allocated to the appropriate economic sector. These figures are broadly representative of other countries worldwide.

In addition, an overview of the key figures in the chemical industry in the top European Union (EU) countries is provided in Table 1. In the European Union (EU28) the chemical sector consists of 30,000 companies employing 1,171,000 workers, with a capital spending of 22.8 bln €, R&D investment of 10 bln €, and a turnover of 565 bln € [2]. In the case of the United Kingdom in particular, the chemicals & pharmaceuticals sector is the second largest industry with 55 bln £ revenues and 19.2 bln £ value added in 2018 [2].

During the past decades, the EU chemical industry has made a significant effort to improve energy efficiency by reducing its energy use per unit of production. Remarkably, the energy intensity (energy used per unit of production) in the chemical industry (including pharma) fell by 59.7% during 1990–2015. During that 25 year period, the production grew by 85%, while the fuel and power consumption reduced by 26%, which is equivalent to a total of 51.8 × 106 tonnes of oil [2]. The 60% reduction in energy intensity is greater than the cuts achieved in the whole of manufacturing industry, where the energy intensity fell by 39% during the same period. These achievements are commendable, but there is still room for improving the energy use in the chemical industry by tackling increased heat recovery (by process integration), low-grade waste heat upgrading (by heat pumps), and particularly distillation processes, which are responsible for almost half of the energy used in the chemical industry [4]. In fact, the United States Department of Energy (DoE) estimated that distillation columns use over 2.87 million TJ (equivalent to a power consumption of 91 GW).

Distillation is among the oldest operations in the chemical industry and it is still the separations workhorse, being widely used for its simplicity and effectiveness. However, distillation also has important drawbacks such as high capital (equipment) costs and large operating costs (due to its low energy efficiency). In fact, the thermodynamic efficiency of distillation is low, ranging from 18% in air separation, to 12% in crude oil separation, and just 5% for C2 and C3 splitting [5]. The key issue is that distillation makes use of expensive high quality energy (usually steam) that is rejected in the condenser, at a low temperature and thus at an exergy loss. For this reason, higher efficiency and thus lower costs are crucial targets for all players in the chemical industry. However, there are no unique answers regarding energy efficiency, as the required minimum energy and the potential saving depend on operating pressures and temperature span, while the utilities available on a chemical site also have an impact on selecting the most suitable technology and its operation. In this respect, the literature reveals many key studies that focus on the synthesis of distillation-based separation systems [6] and distillation configurations [7], taking into account multicomponent feed and product streams [9],[10] and various complex distillation columns [11] such as thermally coupled distillation columns and dividing-wall columns [12]. In addition, novel distillation technologies based on process intensification principles have also been explored and shown to be very promising, e.g. cyclic distillation and reactive distillation [5]. It is thus clear that improving distillation processes has received great attention.

Specifically invited for the special issue of Energy dedicated to the 22nd Conference on Process Integration, Modelling and Optimization for Energy Saving and Pollution Reduction, this paper aims to focus on improving distillation processes by various means, such as process optimization of various complex distillation configurations, heat pumping, and using process intensification technologies (such as dividing-wall columns or reactive distillation) that can greatly reduce the energy usage and the carbon footprint of the modern chemical plants.

Section snippets

Problem statement and motivation

Distillation is recognized as overall the most energy intensive operation in the chemical industry, accounting for over 40% of the energy used [13]. As such, tackling the energy efficiency of distillation holds the promise of the largest energy savings potential in the chemical industry, having the single largest impact. The issue is that distillation works as a heat engine with poor efficiency [14]. Although it imports a considerable amount of heat, this heat is also (for the most part)

Advanced distillation processes

Process Intensification (PI) is a set of new innovative principles applied in process and equipment design, which can bring major benefits in terms of process efficiency, lower capital and operating costs, higher quality of products, less waste and improved process safety [19]. Basically, PI helps chemical engineers to achieve more (higher performance and efficiency) with less or smaller equipment and lower energy use.

Although distillation accounts for over 40% of the energy used in the

Challenges and opportunities

There is a need for major changes in the way the process industry uses energy in the future. In the past when power was generated by burning fossil fuels in centralised power stations, the resulting economics would dictate that power would be somewhere between 3 and 5 times more expensive than heat. However, as sources of energy change from fossil to renewable sources it is likely that the price differentials between power and heat will change significantly. It is possible that power and heat

Conclusions

Rethinking the energy use in the chemical process industry (particularly related to distillation) is really essential for improving the sustainability of the chemical sector. Several concluding remarks can be drawn based on the overview provided in this study:

  • Despite the maturity of distillation, it still presents an enormous scope to reduce its energy consumption by significantly improving its low thermodynamic efficiency.

  • Innovative technologies should not be considered in isolation as

Author statement

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication.

CRediT authorship contribution statement

Anton A. Kiss: Conceptualization, Methodology, Formal analysis, Investigation, Validation, Resources, Visualization, Supervision, Project administration, Writing - original draft, Writing - review & editing. Robin Smith: Conceptualization, Methodology, Formal analysis, Investigation, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

AAK gratefully acknowledges the Royal Society Wolfson Research Merit Award (No. WM170003). The authors are thankful to Stephen Doyle (CPI, The University of Manchester) for the outstanding technical support on COLSEQ software.

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