Research article
Membrane engineering for a sustainable production of ethylene

https://doi.org/10.1016/j.fuproc.2020.106624Get rights and content

Highlights

  • The existing processes producing light olefins are highly energy intensive, requiring innovative approaches.

  • Membrane engineering has already demonstrated in different fields to contribute in the shift towards sustainable productions.

  • The ‘shale gas’ exploitation calls for sustainable and delocalized choices to avoid underutilization or waste of methane.

  • Membrane Reactors, modular and automated, can accomplish the methane conversion to ethylene.

  • Membrane Gas Separation systems can be integrated with Membrane Condensers as pre-treatment in the downstream processing.

Abstract

The existing processes producing light olefins are highly energy intensive, requiring innovative approaches. Membrane engineering, as already demonstrated in different fields, can have an interesting share in the shift towards sustainable productions. The current abundant availability of ‘shale gas’ requires sustainable choices and a delocalized approach that avoids underutilization or waste of methane. This scenario is highly favourable for new technologies such as the Membrane Reactors that are advanced reactors in the Process Intensification logic. Membrane Reactors can accomplish the methane conversion to ethylene, valorizing these resources. In addition, being modular and automated, they are particularly suited for dislocated/remote locations. At the same time, Membrane Reactors can exploit the potential of CO2 as feedstock through a recycle strategy. Different conversion options and the most promising membrane-based alternatives are described, proposing their integration for a sustainable ethylene production.

Introduction

Process Intensification (PI) aims at radically improve the manufacturing industry via process and equipment design [1], attaining process and chain efficiency but also reducing wastes, capital and operating expenses, resulting in products of better quality and in enhanced process safety as recalled in the European Roadmap on Process Intensification [2]. The PI shift has been not completed yet [3] and would be very valuable in the case of energy intensive processes such as those currently applied for the production of olefins at large scale.

Ethylene and propylene, the two primary olefins, constitute the main building blocks for the chemical industry [4]. Ethylene has a worldwide demand in excess of 150 million tons/year [5]. The Steam Cracking of hydrocarbons is the leading technology for producing these olefins, a thermal process carried out in large plants, with huge energy consumption, space requirements and carbon footprint. Indeed, in the manufacturing sector the ethylene production is one of the main contributors to greenhouse gas emissions [6].

The Steam Cracking (SC) of hydrocarbons is a well-established process, articulated in the sections shown in the simplified scheme of Fig. 1. Despite the optimization carried out for a long period on this cycle [7], the following are severe issues for the SC process:

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    Endothermicity of the conversion reactions (the gas mixture reaches temperatures as high as 750–875 °C);

  • -

    Coke deposition in the tubes of the furnaces;

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    Fuel and steam requirement;

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    Complex separation and purification section requiring cryogenic separations, particularly in the case of naphtha-fed plants.

Indeed, this production belongs to the main chemical processes characterized by high exergy losses [8]. The numbers reported in Fig. 1 for each block are taken from a study that analysed an ethane-fed plant [9]. The exergy analysis evidences as the two sections requiring further improvements are the reaction block, owing to the necessity of pre-heating the feed stream, heating the furnace tubes and providing the necessary steam, and the refrigeration unit that is required by the subsequent cryogenic distillation columns for the separation and purification. On the other hand, the separation train accounts for 60–70% of the capital investment, while 30–40% is associated with the cracking furnaces [10].

The significant environmental footprint of this process requires new approaches capable of implementing the Process Intensification strategy to attain a sustainable production [7]. In this respect, Membrane systems are particularly appealing to combine environmental and economic sustainability. Membrane technology has demonstrated many advantages over other conventional technologies, including lower capital and operating costs, low maintenance, low space requirements, flexibility and ease of installation and operation [11]. A representative example is water desalination, in which membranes have replaced thermal operations owing to a 10 times higher energetic efficiency [12]. Membrane Engineering designs optimal schemes in which membrane operations replace or are integrated with conventional separation techniques (e.g., distillation, absorption, adsorption and extraction). Their features, like modularity, low space footprint, low energy intensity, no moving parts and no need for solvents, meet the requirements of green process engineering [13], allowing a sustainable industrial development. The integration of different membrane operations in the same industrial cycle brings further important benefits such as product quality, reducing the overall environmental impact and energy use [14,15].

In previous studies, we proposed a redesigned plant in the case of a naphtha Steam Cracking cycle [16]. We have considered the integration of membrane operations commercially available, evidencing their convenience via an exergetic analysis. Membrane contactors were considered for removing hydrocarbons from waste water and acid gases from furnace effluents; cross-flow microfiltration was indicated for purifying raw water from coke. Gas separation systems were included for separating the light gases (hydrogen and methane), thus reducing the compression/separation load.

Following that approach, experimental evaluations were recently completed: ceramic membranes were successfully applied in the treatment of coke-contaminated petrochemical wastewater streams [17,18].

In the last years, the development in the membrane technology field was steadily progressing, novel applications were addressed, even novel operations were designed. A representative example is the Membrane Condenser (MCo) technology that was successfully tested for water recovery [19], demonstrating its effectiveness and flexibility in contaminants (e.g., NH3, HF, SO2) removal and control from waste gaseous streams [20]. Among the novel fields covered by membrane operations, the biogas upgrading performed by gas separation membranes is a noteworthy example with a large number of installed plants in a few years has. In the last years, China is very competitive in the development and exportation of new technologies, particularly in the case of chemical processes [21]. A closer look at these projects demonstrates as these achievements are largely related to the advanced research on membrane science and engineering carried out in China. Representative case studies include the Organic Solvent Nanofiltration (OSN) technology [22] and the Membrane bioreactors (MBR) for wastewater treatment [23]. At the same time, a significant evolution has been registered in the materials science, with novel materials (e.g., polymers of intrinsic microporosity) synthetized ad hoc for membrane applications [24]. Advanced membranes (Mixed Matrix Membranes, MMMs) combining nanosized materials within a polymeric matrix were developed [25,26], exploiting the appealing 2D materials (e.g., graphene [27,28]). On the other hand, an intense research was devoted to the optimization of the preparation methods in order to decrease their costs and complexity as in the case of ceramic membranes [29,30].

Simultaneously, the social communities and the stakeholders have got an increased ecological awareness. Thus, social responsibility is more and more required to the manufacturers. In Europe, it was demonstrated that economic growth and low-carbon transition are compatible [31]. EU has funded many research projects to modernise the economy according to an ecological transition. More demanding environmental regulations are driving the development of strategies and technologies capable of radically reduced emissions of greenhouse gases.

In a situation of diminishing natural resources, the so-called ‘shale revolution’ in the North America is inducing a transition towards a decarbonized energy scenario [32]. Undoubtedly, it has an environmental cost, as for example in terms of wastewater generation [33]. In this respect, membrane operations are recognized as eco-friendly alternatives for a Zero-Liquid-Discharge desalination of shale gas wastewater [34]. Shale gas is globally distributed and other countries are going to exploit analogous resources. Very recently, Saudi Aramco has launched a large project aimed at obtaining shale gas in Jafurah (Saudi Arabia), introducing a new technology that exploits sea water, after a desalination treatment, for the fracking process [35]. The wide accessibility of natural gas (i.e., methane) related to the shale gas exploitation has changed the fuel market, impacting the production industry. At the same time, natural gas liquids (NGLs, C2–C5 alkanes) are coproduced and available at a low cost.

To fully exploit these resources, a delocalized strategy for the conversion of methane from small-scale sources is particularly appealing. The important achievable outcome is the mitigation of their underutilization and/or flaring. Indeed, flaring wastes important fractions of the world's natural gas [36]. Satellite observations have demonstrated as this practise is also taking place at the shale oil wells, owing to the lack of distributed infrastructures for the gas transport (pipelines) or when the prices are too low. Flares avoid the direct emission in the atmosphere of methane that has a larger greenhouse effect than carbon dioxide (CO2). However, its incineration in the flares involves important CO2 emissions [37], while some unconverted methane is released as well.

A holistic perspective to redesign industrial productions has to address these aspects. On the other hand, in a reshaped industrial production, CO2 represents a valuable resource (e.g., for MeOH production) [38] within to the Capture and Utilization of CO2 (CCU) strategy [39]. The CO2 reuse (CO2 separated from NG or captured from fuel gas) will contribute to reduce harmful emissions in the atmosphere, at the same time producing other fuels.

The present work provides an overview of the most interesting catalytic processes investigated to produce a ‘green ethylene’ and the benefits achievable by properly adopting the membrane technology. The most relevant examples of Membrane Reactors for the production of olefins via alternative reactions are described. The improvements that can be accomplished by integrating different membrane operations, considering the most recent advances in the field are highlighted.

Section snippets

Reaction section

A typical Steam Cracking (SC) process can be schematized in four main sections as depicted in Fig. 1 and the cracking block is the most inefficient unit. In a naphtha-fed plant the pyrolysis section accounts for ca. 75% of the total exergy losses [40], while its percentage exergy loss is 45% in an ethane-fed plant [9]. Indeed, the endothermic (not catalytic) conversion in the furnaces has a high energy requirement, both as fuel for the furnaces and as steam for the cracking reaction.

Usually,

Separation section

The cracking of hydrocarbon feedstocks results in a mixture of hydrogen, methane, ethylene, ethane and heavier components. Particularly in naphtha-based SC plants, the recovery of heavier products is mandatory for economic reasons, thus a complex distillation train is adopted.

Separation of light olefins from their corresponding alkanes owing to the close physic-chemical properties of these molecules, is typically achieved in tall cryogenic distillation columns, adopting high reflux ratios.

Final remarks

On the basis of the overview provided in the previous paragraphs, it is evident as today there are no technological barriers to reach greener productions. Membrane technologies, particularly the MR devices, can play a remarkable role to complete a shift towards a sustainable scenario.

Distributed solutions are required to fully exploit the methane conversion, even for small and delocalized sources. In this respect, the modular design of membrane separation units, combined to the possibility of

Conclusions

Membrane technology, as already proved in other fields (e.g., water desalination), provides reliable tools to implement the Process Intensification, even in very energy intensive processes such as the olefin production. Latest research and investigations constitute a strong basis for making more sustainable choices.

The advent of shale resources represents an opportunity for a holistic approach to the redesign of the production of petrochemical building blocks, starting from nonoil resources.

Declaration of competing interest

The authors declare no conflict of interest.

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