Introduction

1 Waste Minimisation

Waste minimisation techniques can be grouped into four categories:

• Inventory management and improved operations

• Equipment modification

• Changes in the production processes

• Recovery, recycling and reuse

The waste minimisation approaches as largely developed by the Environmental Protection Agency (EPA) are given in Table 1.1. They can be applied across a wide range of industries including chemicals manufacturing.

2 Clean Synthesis

The hierarchy of waste management techniques has prevention as the most desirable option ahead of minimisation, recycling and, as the least desirable option, disposal. The term cleaner production embraces principles and goals that fall comfortably within the waste prevention–minimisation range. It has been described within the United Nations Environmental Programme as:

The continuous application of an integrated preventative environmental strategy to processes and products to reduce risks to humans and the environment. For production processes, cleaner production includes conserving raw materials and energy, eliminating toxic raw materials, and reducing the quantity and toxicity of all emissions and wastes before they leave a process.

Cleaner processes fall under the umbrella of waste reduction at source and along with retrofitting, can be considered to be one of the two principal relevant technological changes. Waste reduction at source also covers good housekeeping, input material changes and product changes. Within chemistry and the handling of chemicals the term green chemistry has become associated with the methods of waste reduction at source and more generally with reducing the environmental impact of chemicals and chemical processes.

Within the context of cleaner production, terms such as environmentally benign chemical synthesis and clean(er) synthesis have often proven popular to help define the scope of national or trans-national programmes on waste minimisation. There is no widely accepted definition of clean synthesis but there is reasonable international agreement that the cleaner synthesis of chemicals, i.e. that involving a reduction in the toxicity and quantity of emissions and waste through changes to the process, is likely to be achieved through:

• better use of catalysis

• alternative synthesis routes that avoid the need to use toxic solvents and feedstocks

• reduction in the number of synthetic steps

• elimination of the need to store or transport toxic intermediates or reagent

• novel energy efficient methods

It should be noted that catalysis features very highly on any list of preferred/relevant technologies to help achieve a reduction in waste from chemical processes through the use of cleaner synthetic methods.

3 Catalysts and Catalysis

Catalysts are species that are capable of directing and accelerating thermo-dynamically feasible reactions while remaining unaltered at the end of the reaction. They cannot change the thermodynamic equilibrium of reactions.

The performance of a catalyst is largely measured in terms of its effects on the reaction kinetics. The catalytic activity is a way of indicating the effect the catalyst has on the rate of reaction and can be expressed in terms of the rate of the catalytic reaction, the relative rate of a chemical reaction (i.e. in comparison to the rate of the uncatalysed reaction) or via another parameter, such as the temperature required to achieve a certain conversion after a particular time period under specified conditions. Catalysts may also be evaluated in terms of their effect on the selectivity of reaction, specifically on their ability to give one particular reaction product. In some cases, catalysts may be used primarily to give high reaction selectivity rather than high activity. Stability is another important catalyst property since catalysts can be expected to lose activity and selectivity with prolonged use. This then opens the way to regenerability which is a measure of the catalyst's ability to have its activity and/or selectivity restored through some regeneration process.

Catalytic processes are the application of catalysts in chemical reactions. In chemicals manufacture, catalysis is used to make an enormous range of products: heavy chemicals, commodity chemicals and fine chemicals. Catalytic processes are used throughout fuels processing, in petroleum refining, in synthesis gas (CO + H2) conversion, and in coal conversion. More recently some aspect of clean technology or environment protection has driven most of the new developments. Many emission abatement processes are catalytic. An increasing number of catalytic processes employ biocatalysis. Most of these are fermentations classically carried out in stirred reactors using enzyme catalysts, which are present in living organisms such as yeast. Immobilised enzymes processes are becoming more common.

Catalysis is described as homogeneous when the catalyst is soluble in the reaction medium and heterogeneous when the catalyst exists in a phase distinctly different from the reaction phase of the reaction medium.

Almost all homogeneous catalytic processes are liquid phase and operate at moderate temperatures (< 150 °C) and pressures (< 20 atm). Corrosion of reaction vessels by catalyst solutions, and difficult and expensive separation processes are common problems. Traditionally the most commonly employed homogeneous catalysts are inexpensive mineral acids, notably H2SO4, and bases such as KOH in aqueous solution. The chemistry and the associated technology is well established and to a large extent well understood. Many other acidic catalysts such as AlCl3 and BF3 are widely used in commodity and fine chemicals manufacture via classical organic reactions such as esterifications, rearrangements, alkylations, acylations, hydrations, dehydrations and condensations. More recently there have been significant scientific and technological innovations through the use of organometallic catalysts.

Normally, heterogeneous catalysis involves a solid catalyst that is brought into contact with a gaseous phase or liquid phase reactant medium in which it is insoluble. This has led to the expression contact catalysis sometimes used as an alternative designation for heterogeneous catalysis. The situation can be rather more complicated with phase transfer catalysis (PTC) systems. Here the reactants themselves are present in mutually distinct phases, typically water and a non-aqueous phase (usually a hydrocarbon or halogenated hydrocarbon which has a very low solubility in water). The catalyst, which is normally a quaternary ammonium or phosphonium compound or a cation complexing agent such as a crown ether, is believed to operate at the interfacial region and strictly need not be soluble in either the aqueous or non-aqueous phases. This is demonstrated by the activity of immobilised...