Page 52 - Chemical and process design handbook

Speight_Part 1_N&O 11/7/01 3:02 PM Page 1.38

1.38 REACTION TYPES
tion zone, with suitable means for adding reactants rapidly or slowly as
may be required and for removing the product, and provided with adequate
jackets or coils through which heating or cooling means may be circulated
as required.
In the case of liquid-phase reactions in which oxidation is secured by
means of atmospheric oxygen—for example, the oxidation of liquid
hydrocarbons to fatty acids—special means must be provided to secure
adequate mixing and contact of the two immiscible phases of gaseous oxi-
dizing agent and the liquid being oxidized. Although temperature must be
controlled and heat removed, the requirements are not severe, since the
temperatures are generally low and the rate of heat generation controllable
by regulation of the rate of air admission.
Heat may be removed and temperature controlled by circulation of
either the liquid being oxidized or a special cooling fluid through the reac-
tion zone and then through an external heat exchanger. Mixing may be
obtained by the use of special distributor inlets for the air, designed to
spread the air throughout the liquid and constructed of materials capable of
withstanding temperatures that may be considerably higher at these inlet
ports than in the main body of the liquid. With materials that are sensitive
to overoxidation and under conditions where good contact must be used
partly to offset the retarding effect of necessarily low temperatures, thor-
ough mixing may be provided by the use of mechanical stirring or frothing
of the liquid.
By their very nature, the vapor-phase oxidation processes result in the
concentration of reaction heat in the catalyst zone, from which it must be
removed in large quantities at high-temperature levels. Removal of heat is
essential to prevent destruction of apparatus, catalyst, or raw material, and
maintenance of temperature at the proper level is necessary to ensure the
correct rate and degree of oxidation. With plant-scale operation and with
reactions involving deep-seated oxidation, removal of heat constitutes a
major problem. With limited oxidation, however, it may become necessary
to supply heat even to oxidations conducted on a plant scale.
In the case of vapor-phase oxidation of aliphatic substances such as
methanol and the lower molecular weight aliphatic hydrocarbons, the ratio
of reacting oxygen is generally lower than in the case of the aromatic
hydrocarbons for the formation of the desired products, and for this reason
heat removal is simpler. Furthermore, in the case of the hydrocarbons, the
proportion of oxygen in the reaction mixture is generally low, resulting in
low per-pass conversions and, in some instances, necessitating preliminary
heating of the reactants to reaction temperature.