Page 91 - Design of Simple and Robust Process Plants

76 Chapter 4 Process Synthesis and Design Optimization
The design philosophies of inherently safer design and environmentally sound
processes set the stage for these decisions (Kletz, 1991; CCPS, 1993, 1996 and a spe-
cific developed training package of IChemE, 1995). The risk of an operating facility
and its surrounding areas must also be subjected to an extended evaluation as part
of the synthesis study. Hazardous indices such as the Fire and Explosion Index
(F&EI) and the Chemical Exposure Index (CEI) are useful as screening tools. In case
of indices which are considered too high, measures might be taken by the elimina-
tion of chemicals or reducing inventory and hazardous conditions. The most impor-
tant Safety Health Environmental (SHE) activities are to be planned during the
initial synthesis layers, reaction, and separation. At this stage, the emphasis is on
the prevention of hazardous situation, but later during the design the safety of the
installation will be increased by add-on provisions, although this will address inher-
ent safety aspects only to a limited extent.

4.1.2.3 Chemical reactivity
This requires special attention (CCPS, 1995), since it is the properties of chemical
reactivity which set constraints on the design and operation of a facility. Identifica-
tion of the hazards of chemical materials, mixtures and reaction masses is required.
The reaction involved include not only decomposition reactions but also polymeriza-
tion reactions.
The thermal stability of a component can be determined experimentally or theore-
tically, although on a theoretical basis the kinetic rates cannot be determined. A
database is available which provides this information (Bretherick, 1990). The most
frequently used software is called CHETAH, which screens organic, organo-metallic
chemicals, inorganic salts and mixtures for their potential to undergo decomposi-
tion. This program is also very useful for the estimation of thermodynamic data
such as enthalpy, entropy, heat capacity, and the Gibbs' free energy for specified
reactions. The CHETAH program classifies the energy hazard potential of a compo-
sition by a pattern recognition interpretation based on several criteria.
The most common thermal and reactivity measurements are DSC/DTA (differen-
tial scanning calorimetry/differential thermal analyzer), these test methods being
used as a screening tool. An advanced, commercially available method to obtain
reactive chemical data is the accelerating rate calorimeter (ARC) (Townsend, 1981;
Kohlbrandt, 1987). The ARC measures heat of decomposition, onset temperature,
the heat rate as function of temperature, overall reaction kinetics and the pressure
as function of temperature. It is an excellent tool for determining the design and
operational constraints of a facility, and the data are also used for the sizing of relief
devices. The ARC is ± next for pure components ± used to test all types of mixtures,
and eventually can include solids such as catalyst particles, which might have an
impact on the decomposition rate. The selection of the mixtures is based on the
local compositions of the different processing streams.
Incompatibility of substances is another reactivity factor that plays a role in selec-
tion of the constraints of a process design. Incompatibility can be screened using
DTA/DSC, but may also be identified using CHETAH. Advanced testing methods
are known as reactivity testing and flammability testing. It should be noted that