the reactions. The heat generated by a large scale reaction may exceed the
cooling capacity of the existing equipment with disastrous results. Conse-
quently, if they are not known, the heats of reaction for each of the stages
used in the process must be accurately determined and taken into account when
assessing the safety of the plant and its operatives.
Waste disposal and its environmental impact are becoming increasingly
important and must now be planned for as part of the development process. In
the past, solid wastes were buried in special landfill sites but now because of
potential water pollution it is regarded as undesirable unless alternative methods,
such as incineration, are not suitable. Aqueous liquids may be discharged into the
sewage system, rivers and the sea. However, this requires removal of high concen-
trations of impurities, especially metals, such as copper, zinc, mercury and cad-
mium, and organic halogen compounds, such as chloroform, which are highly
toxic to many forms of aquatic life. Solvents are usually recovered by distillation
but this is not always a practical proposition for some solvents such as dimethyl
sulphoxide and N,N-dimethylformamide. Gases can often be prevented from
entering the atmosphere by reaction with suitable absorbents in absorbent towers.
However, gases such as nitric oxide that cannot be disposed of in this manner
require special treatment, which can be very expensive. In all cases the disposal of
highly toxic waste is to be avoided, as it can require expensive special treatment.
An idea of the impact of a process on the environment may be determined
from its effluent load factor (ELF). This is defined as
ELF¼
mass of all ingredients of all the stagesmass of the product
mass of the product
(11:1)
The ELF is the amount of waste produced by the process per unit mass of the
product. It can refer to individual stages in the synthesis or the whole process. In
the ideal situation, where all the ingredients are converted into the product, the
ELF value will be zero. This does not happen in synthetic processes. In pharma-
ceutical processes, the ELF is usually of the order of 100. The process design
chemists will aim to minimize the ELF in order to reduce expense by selection of
the reactions and the operation of the plant.
11.2.3 Synthesis quality control
The efficiency of drug production will depend on being able to identify and
assess the chemical purity of the drug and also that of the intermediate com-
pounds involved at each step in the synthesis. This means that it is normally
CHEMICAL DEVELOPMENT 227