Chapter 1: General Concepts in Sustainable Chemical Processes

The topics contained in this book represent the nexus between chemical engineering and chemistry. Covering many sustainable process technologies available, whilst outlining the chemistry that underpins them, this book differs from others which often focus on a subset of the processes or the chemistry, or on a specialist topic (e.g. process intensification). Process design equations and process modelling, scale up, process and energy integration, and process control are often neglected. Therefore, we aim to provide the first comprehensive coverage of sustainable chemical processes, the one-stop-shop for the area. This book therefore focuses on the chemical technologies (unit operations) from both a chemical engineering and chemistry perspective. In this chapter we discuss the overarching principles and concepts addressed in the individual chapters.

The subject of this book is sustainable chemical processes. But what are these? For us, they are processes (i.e. a set of linked unit operations^† that take in raw materials and energy, and ultimately convert these into chemicals, biochemicals, materials or products) in a resource efficient manner that allows preparation of the desired product with minimal production of waste. Sustainable chemical processes or clean technologies are those that help us meet the goals of sustainability and sustainable development in our process systems.

The essence of sustainability and sustainable development is to ensure that our use of the planet's limited material, energy and ecological resources will be such that we sustain the current standard of living for the increasing human population so that future generations are able live life at the current, or even better, standard of living. The latter is particularly applicable to the large proportion of the global human population that do not yet have access to clean water, adequate food, life-saving medication and other essentials for a happy, healthy life. Thus, sustainability and sustainable development has been defined as:¹ 

It is always worth reminding oneself that this widely quoted definition of sustainability arose from a commission asked to formulate ‘a global agenda for change’ and to recall Gro Harlem Brundtland's words from her foreword to the report:¹ 

The commission called for development and this has been reiterated in numerous reports since, for example:² 

A simple way of determining if a process or technology meets the goals of sustainability and sustainable development is to probe whether or not it meets all three components of the triple bottom line (also sometimes called the Three Pillars of Sustainability): people (social bottom line); planet (environmental bottom line); and profit (economic bottom line). There are various measures of these, which include:³ 

• People (social bottom line): worker happiness, industrial safety, benefits based on payroll expense, promotion rate, ‘loss time accident frequency’, ‘expenditure on illness and accident prevention/payroll expense’, ‘number of complaints per unit value added’, etc.

• Planet (environmental bottom line): life cycle assessment (see Section 1.4); environmental impacts such as acidification, global warming, human health, ozone depletion, photochemical ozone, wastes – hazardous and non-hazardous, and ecological health; and resource usage such as energy use, material use, water use and land use.

• Profit (economic bottom line): capital and operating costs, wealth created, value added per unit value of sales, value added per direct employee, and R&D expenditure as a percentage of sales.

When all three components of the bottom line are met (i.e. at the overlap of the three lobes, Figure 1.1), we have sustainability. The social and economic components are less directly related to the chemistry and chemical engineering emphasis of this book and therefore will not be our main focus (but they will be touched on where relevant). We will concentrate on ensuring that our sustainable chemical technologies meet the environmental bottom line. A systematic method for process choice that facilitates the selection of different technologies to meet this aim is to use the waste management hierarchy combined with the principles of green chemistry and green engineering.

First set out by Paul Anastas and John Warner in their seminal work published in 1998,⁴  it is clear that these 12 principles were defined by chemists with synthesis in mind and, as with all sets of rules or guidelines, should always be implemented in combination with careful analysis and intelligent critical thought. Nonetheless, even in cases where the process developer might decide that a particular principle is not formulated in a manner that applies to their process, these provide an excellent checklist. For example, it is certainly not always best to conduct a synthetic chemical process at ambient temperature and pressure, as described in principle 6. There are a plethora of examples where an exothermic reaction is best conducted at elevated temperature, as the process is rapid and the evolved heat is used to maintain the reaction temperature (usually after an initial short heating stage to initiate reaction). If put in the context of energy efficiency, it is easy to determine what the optimum temperature should be: that at which the process proceeds at a reasonable rate and consumes the least energy (cooling costs energy too), while remaining safe.

For completeness, below we reproduce the 12 principles of green chemistry from Anastas and Warner,

In some cases the 12 principles of green chemistry are difficult to apply directly in engineering applications, in particular those that do not involve chemicals or reactions. Consequently a further set of 12 principles were developed by Paul Anastas and Julie Zimmerman and published in 2003⁵  as a tool to systematically apply the principles of sustainability and to achieve sustainable goals within engineering and in particular as a way of identifying and applying sustainability within engineering design (and as such these can be used as performance criteria). The principles are very general since they are intended to apply at all scales and apply to all engineering disciplines (chemical, electrical, civil, environmental, mechanical, systems, etc.). For engineers, these principles can be used as additional criteria that, in a complex system, can be optimised in addition to the normal parameters used in design to define and optimise a system.⁶ 

Since the goals of achieving sustainability are shared, the 12 principles have some commonality with the 12 principles of green chemistry. For completeness, the 12 principles are reproduced below.

There are also other sets of green engineering principles including the Sandestin Green Engineering Principles⁷  and the Hannover Principles.⁸ 

For examples of how to apply the 12 principles of green engineering see ‘Design Through the Twelve Principles of Green Chemistry’⁵  and ‘EcoWorx, Green Engineering principles in Practice’.⁹ 

Waste minimisation is essentially embodied in the second principle of green engineering: ‘It is better to prevent waste than to treat or clean up waste after it is formed’. In order to put this principle into practice, either to choose a method/technology/process for dealing with a waste stream, or to choose between alternative technologies when designing a new process or process option, so that the minimal amount of waste/environmental impact is produced, the hierarchy of waste management should be followed (Table 1.1).

Best practice is to start at the top of the hierarchy (i.e. eliminating the waste) and if this is not possible, to systematically test options working downwards towards the bottom (least preferable). That is, try to eliminate, reduce or recycle the emission streams from a process first by looking at the source of the emissions. Failing that, try to recover energy from the waste stream, then treat the waste stream to lessen the risk and environmental impact, and finally discharge the resulting stream to the environment. Disposal with no treatment is the least preferable option. This hierarchy has been made law in many countries and regions, including the European Union.

The nearer a chemical process and/or chemical technology is to the top of the hierarchy of waste management practice, the more sustainable it is (especially in terms of the environmental bottom line).

To determine the environmental impacts of a particular set of processes, new material or product, or to compare processes and products, one requires rigorous metrics that can be objectively applied to assist decision making. Life cycle assessment (LCA) is widely accepted as an appropriate technique to provide the data for evaluation of environmental impacts, through all stages of the lifetime of a product from raw materials extraction to final disposal or recycling (cradle to grave). LCA is defined by a set of international standards: ISO 14040: LCA – Principles and Framework and ISO 14044: LCA – Requirements and Guidelines (see ref. 11 for a review).

The briefest possible overview of LCA is, once again, provided by consideration of the principles embodied therein (now included in ISO 14040). These are reproduced below.¹¹ 

LCA is a discipline requiring great expertise and there are numerous reference works and texts on the topic. Some more approachable texts for the non-expert include Green Chemistry Metrics: Measuring and Monitoring Sustainable Processes¹²  and Inherently Safer Chemical Processes: A Life Cycle Approach.¹³  While the process is, by necessity, very detailed and requires the collection of significant quantities of data, failure to adopt a full life cycle approach can lead to rejection of processes that are indeed less environmentally hazardous. This is a particular challenge when comparing a new (sometimes not optimised) technology, or process, with a well-established one.

For example, if a life cycle approach is not taken when choosing the optimal degree of abatement for a sustainable chemical technology used to recycle a component from a waste stream (e.g. a pervaporation membrane process used to remove and recycle a volatile organic compound from a wastewater stream), then there is the possibility that the environmental impact of the energy consumption and raw material use (amongst other items that have an impact beyond the process itself) will be larger than the reduction in environmental impact that removal of the pollutant has.¹⁴  Going beyond the optimal degree of abatement by not considering the impacts from a life cycle point of view therefore does more harm than good, which is not the intention of developing or using a sustainable chemical technology.

Two major processing approaches are commonly used to improve the sustainability of chemical processes and therefore to produce more sustainable chemical processes. These are: the use of continuous, or flow, processes; and the integration of different unit operations for process intensification. As these two approaches will feature in many of the chapters of this book, they are described here.

Processes can be classified as batch, continuous or semi-batch. In a batch process, the feed is introduced at the beginning of the process and products are removed at the end. The process change occurs over time. Figure 1.2 shows a typical batch stirred tank reactor. This could be used, for example, in beer fermentation where yeast and wort are added to the batch fermentation vessel, and beer (the product) and spent yeast (the waste) are removed at the end of fermentation. No flow occurs through the vessel over time—there is a loading before the start of processing and an unloading at the end of the processing.

In contrast, in a continuous, or flow, process, there are continuous inputs and outputs and the process changes over the volume and length of the vessel. Since this allows a smaller inventory of material in the process over time, the use of continuous processes typically results in smaller equipment, higher throughput and better economics. However, the control of the process is usually more complicated, some operations in batch are difficult to achieve in flow (e.g. crystallisation) and flow equipment is not as flexible or amenable to short and changing production campaigns, as batch processes are. Examples of typical continuous reactors are shown in Figure 1.3. An example of a continuous process is the spray drying of milk powder, where milk concentrate and air are fed constantly to the spray dryer, and the products—powder and humidified air—are constantly removed. Continuous processes can be more sustainable, since the smaller reactor size can result in a reduction in energy use and the continuous throughput can help overcome limitations of equilibrium and product inhibition, making the process more efficient.

For completeness, a further type of process is the semi-batch process, which is any process that is neither batch nor continuous. Two typical process industry examples are powder dissolution, which involves the slow addition of a powder to a solvent in a mixing vessel without removing any material, and combustion, which involves the continuous addition of air onto batch solid reactant.

Stankiewicz and Moulin¹⁵  defined process intensification as:

To put this in a shorter form: process intensification is any chemical engineering development that leads to a substantially smaller, cleaner and more energy efficient technology. More commonly, a process intensification technology is one that combines several different unit operations into one (e.g. a membrane reactor, which combines reactions and separations), or a technology that overcomes rate limiting steps in a significant way (such as spinning disc and spinning mesh disc type reactors, where the centrifugal force and micromixing overcomes the inherent mass transfer limitations of many conventional chemical reactors). For further information on process intensification and a comprehensive treatment of the engineering of these process intensification technologies see Process Intensification Technologies for Green Chemistry: Engineering Solutions for Sustainable Chemical Processing.¹⁶ 

The structure of the book is roughly divided into sections that follow the typical flow through a process plant starting from the raw materials and ending in products (both wanted and waste). This covers the most important overall unit operations and the new developments that have and can make these into sustainable chemical technologies and processes:

• Chemical Transformations

• Biochemical Transformations

• Separations and Purifications

• Process Integration

The concepts outlined in this chapter form part of the discussion in many of these chapters. As with any finite volume, not all unit operations and processes can be covered. However, the application of the concepts in sustainability, outlined in this chapter, to any process is a good starting point in achieving the triple bottom line benefits to these, and indeed any processes, in order to produce more sustainable processes for the future.