Page 89 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Batch Processing 53

andhealthprotectionofthepersonnelinchargeoftheplant above. These runaways may be relieved safely. Chemical
must meet OSHA (Occupational Safety and Health Ad- reactions which degenerate in deflagrations characterized
ministration) regulations. The protection of the environ- by pressure-rise rates higher than 4000 psia/sec may not
ment against chemical contamination must meet national, be relieved safely for lack of suitable industrial technology
state, and local EPA (Environmental Protection Agency) and should not be used for industrial purposes.
regulations. Protection of the capital investment in the A bursting disk is used to protect the reaction vessel. It
plant and the equipment in the case of fire or hazardous shouldbeproperlyratedtoensurethattheinternalpressure
conditions must meet the codes of the National Board of developed during the upset does not exceed the allowance
Insurance Underwriters. Technical recommendations for made by ASTM Code Section VIII (or any other perti-
the proper design of an emergency relief system (ERS) nent section) for pressure vessels. The actual disk relief
have been compiled by the Design Institute for Emer- area used is 25 to 60% larger than indicated by process
gency Relief Systems (DIERS) of the American Institute requirements (as reported by the disk manufacturer) to ac-
of Chemical Engineers. count for incomplete bursting of the disk. An emergency
Technology leading to the safe design of industrial pro- relief system (ERS) manifold should be used to collect
cesses is available. ASTM offers a computerized proce- and convey the effluent material to a train of equipment
dure, DS-51 (1974), to test the reactants, intermediates, where it is recovered or properly destroyed to avoid envi-
and products of a process early in its development. The ronmental contamination.
program CHETAH rates the components of a process as The DIERS Institute of the American Institute of Chem-
hazardous if they are likely to undergo decompositions ical Engineers has developed procedures for the safe ERS
which may result in thermal reaction runaways. The rating design of processes undergoing thermal runaways caused
is based on the energy release due to probable oxidation of by deflagrations. Runaways may be of three types:
the atoms in the molecule at higher temperatures. These
situations may result from a process upset, a fire situation, 1. Vapor systems, where boiling is reached before po-
or by an explosion occurring in the reactor. tential gaseous decomposition. The heat of reaction is re-
Molecules containing O, S, P, N, F, Cl, Br, and I are moved by vaporization of the solvent present or added on
chemically unstable and should be suspicious. If any of purpose to keep the system thermally stable.
the components of a process are rated hazardous, the pro- 2. Gassy systems, where a gaseous decomposition oc-
cess may be substituted for one which uses no such com- curs in the absence of tempering. The total pressure de-
ponents, or experimental tests for the safe design of the veloped during the upset is due to the presence of noncon-
pilot plant and the commercial process must be scheduled. densible gases.
The safe design of a process which uses hazardous 3. Hybrid systems, where gaseous decomposition oc-
chemicals requires testing the reacting mixture to mea- curs before reaching boiling, but the rate of reaction is
sure the rate of chemical decomposition during any reac- tempered by vapor stripping. The pressure developed in
tion runaway. When the components decompose in several the system is due to the vapor pressure of the volatile
stages, the experimental study must be carried out to high components and to the partial pressure of noncondensible
temperatures to ensure the complete decomposition of all gases.
the components of the system. Multistage decomposition
is typical of chemicals containing two or more of the atoms There are two approaches to ERS design. One is system
listed above, such as halogenated compounds which oxi- modeling, which identifies the cause of a pressure rise
dize one halogen atom at one temperature, the second at a from a hazard analysis. It uses approximate models—all-
higher temperature, and so forth. Completeness of the de- vapor flow, all-liquid flow, or two-phase flow—to simulate
composition must be verified via a material balance and the pressure increase of the reacting system vs. time and
chemical analysis of the products made and the residue to determine vent size. The method is complex since it
remaining in the decomposition autoclave. must identify the stoichiometry, the mechanism, and the
Explosions resulting from process upsets are detona- kinetics of the decomposition causing the pressure rise.
tions or deflagrations. Processes complicated with deto- Two pressure models are used for vent sizing:
nations cannot be used commercially since the rates of
pressure rise during the upset are too large (about 100,000 1. Low-pressure models applicable to the protection
psia/sec) to be relieved by any existing technology. Indus- of process buildings and storage tanks ruled by the API
trial deflagrations are thermal runaways where the rates codes. RUST’s low-pressure model is usually successful
of pressure rise are about 4000 psia/sec or less. They for vent design of internal or external overpressure.
are due to reactions with combustion-supporting mate- 2. High-pressure models applicable to pressure ves-
rials such as oxygen, or any of the other elements cited sels and chemical reactors subjected to more than 15 psig