Page 90 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN002E-49 May 17, 2001 20:13

54 Batch Processing

of internal pressure. Two computer programs, SAFIRE fracture is almost ten times larger in compression than in
and DEERS, were developed by the Design Institute for tension. In dealing with large strains, one must distinguish
Emergency Relief Systems (now a branch of the Center between conventional stress, which is the axial load di-
for Chemical Process Safety of the American Institute of vided by the original cross-sectional area, and true stress,
Chemical Engineers) to rate the relief area needed for safe whichistheloaddividedbytheactualcross-sectionalarea.
design of high-pressure vessels. The validity of the results A slip or glide of part of one body over the other re-
from these two simulation programs depends on the as- sults in plastic deformation. At the beginning of plastic
sumptions made for the critical flow of two-phase flow deformation, the stress produces a permanent strain on
systems through the exhaust manifold. Unfortunately, the the material. The progress of plastic deformation in those
hydrodynamics, and especially the pressure drop of gas– materials exhibiting strain hardening is marked by strain
liquid–solid systems flowing through a manifold are not hardening; each additional increase in deformation re-
well known. Also, the assignment of liquid and solid phase quires an additional increment of stress. The axial load
entrainments to the vapor phase outflowing the vented ves- reaches a maximum before the material ceases to strain-
sel may not be realistic. harden. Thereafter, testing conditions become unstable.
The stress corresponding to the maximum is the ultimate,
The simpler and most reliable approach to the use of or tensile, strength, which is intended to prevent failure
the DIERS methodology is the use of FAUSKY’s reactive by excessive plastic deformation. Strain-hardening (cold-
system screening tool (RSST). It is an experimental auto- working) is a cumulative process even if the deformation
clave which simulates actual situations that may arise in is reversed. Recovery and recrystallization of the mate-
industrial systems. The RSST runs as a differential scan- rial at almost 0.4 times its absolute melting point removes
ning calorimeter that may operate as a vent-sizing unit strain-hardening.
where data can readily be obtained and can be applied Thixotropy is the tendency of certain substances to flow
to full-scale process conditions. The unit is computerized under external stimuli (e.g., mild vibrations). A more gen-
and records plots of pressure vs. temperature, temperature eralpropertyisviscoelasticity,atime-dependenttransition
vs. time, pressure vs. time, and the rates of temperature from elastic to viscous behavior, characterized by a relax-
rise and pressure rise vs. the inverse of temperature. From ation time. When the transition is confined to small re-
these data it determines the potential for runaway reac- gions within the bulk of a solid, the substance is said to
tions and measures the rates of temperature and pressure creep. A substance which creeps is one that stretches at a
increases to allow reliable determinations of the energy time-dependent rate when subjected to constant stress and
and gas release rates. This information can be combined temperature. The approximately constant stretching rates
with simplified analytical tools to assess reactor vent size at intermediate times are used to characterize the creeping
requirements. The cost of setting up a unit of this kind is characteristics of the material.
close to $15,000.

B. Failure of Materials
X. MATERIALS
Materials used in batch processing are subjected to strin-
gent changes of operating conditions during the process-
A. Properties of Materials
ing cycle. They fail when (1) they are subjected to stresses
Elasticity of solids determines their strain response to beyond the yield point due to accidental runaway; (2) op-
stress. Small elastic changes produce proportional, re- erating conditions become more demanding than those
coverable strains. The coefficient of proportionality is the set for design, after several plant expansions; (3) unpre-
modulus of elasticity, which varies with the mode of de- dictable conditions due to side reactions or lack of heat
formation. In axial tension, E is Young’s modulus; for dissipation fail to keep intermediates in a thermally sta-
changes in shape, G is the shear modulus; for changes in ble state; (4) material properties are not as good as ex-
volume, B is the bulk modulus. For isotropic solids, the pected because of fabrication deficiencies or deterioration
three moduli are interrelated by Poisson’s ratio, the ratio by corrosion or embrittlement; (5) sudden changes occur
of traverse to longitudinal strain under axial load. in operating conditions (pressure and thermal cycling and
When solids deform almost to the breaking point, they shocking of the materials occur several times per day until
exhibit brittle behavior; the stress at fracture is several or- the materials fail by fatigue or stress shock).
ders of magnitude lower than the computed strength. The Failure may be mechanical, due to wear, abrasion and
loss is ascribed to the presence of minute cracks proba- erosion, britle fracture, surface deterioration, cyclic load-
bly formed during solidification. Compressive stress can ing, embrittlement, thermal or pressure shock, or fatigue.
induce crack propagation; the magnitude of the stress at Failure may also be chemical, in essence due to corrosion.