Page 71 - Applied Process Design For Chemical And Petrochemical Plants Volume II

00 Applied Process Design for Chemical and Petrochemical Plants

separate the volatile organic heavies from the tarry poly- (z)
merized residue (heavy liquid). Steam is to be injected and - (Fraction retained)
=
into the insulated tank containing heating coils. The sys-
tem is to operate at 200 mm Hg absolute pressure and
250°F with no condensation of the steam. The organic substitute and solve for BT~.
volatile heavies contain:

Vapor Pressure Mols (8-117)
Component @ 250°F a=Pi/PA Q, &,/ai
~~
A 35 mm Hg 1.0 45 43 Knowing the relation for M, can be solved to deter-
B 20 0.57 40 70
C 6 0.171 2 152 mine mols of steam to reduce initial material to percent-
111 267 age of a compound in the remaining bottoms. If steam
mols condenses, the requirement for steam increases by this
amount.
x Steam Distillation-Continuous Flash, Two Liquid
Phases, Multicomponent and Binary

-- -- 2oo [267]-1 Because water will be present in this system, and is
Ms
111 33(111) assumed immiscible with the other components, it will exert
its own vapor pressure. This situation is similar to many sys-
= 13.75 - 1 = 12.15 tems where the liquid to be flashed enters below its dew
M, = 1,417 total mols steam required for 111 mols mixture point, and hence requires the use of steam to heat (sensible
Mols steam/mol of mixture organic volatiles = 1,417/111 = 12.8
+ latent) as well as steam for the partial pressure effect.
Mols steam in vapor phase only:
Steam Distillation-Continuous Differential,
Multicomponent or Binary
- assumed flash temperature)
M (vapor) = P, E 2; (at
The results of the differential distillation end the same
as the flash distillation, although the mechanism is some-
what different. This is a batch type operation distilling dif- where Ps = vapor pressure of steam
ferentially. All sensible and latent heat are supplied sepa- Pi = vapor pressure of each component at the flash
rately from the steam or by superheat in the steam. Steam temperature
acts as an inert in the vapor phase, and quantity will vary
as the distillation proceeds, while temperature and pres- Mols steam to heat is sum of sensible plus latent.
sure are maintained. Total mols steam is sum of M, (vapor) plus heating steam.
System total pressure:

x= Ms + BTo , absolute (8 - 118)
PBio

(21-O BT0 = mols (total) volatile material at start
If all the volatile materials are distilled : -

Open Live Steam Distillation-With Fractionation
This relation is handled very similar to the flash steam Trays, Binary
separation.
If all of the material is not to be removed as overhead Open or direct injection of steam into a distillation sys-
vapors from the still, leave a percentage of a particular tem at the bottom may be used to heat the mixture as well
compound in the bottoms, then select the particular com- as to reduce the effective partial pressure of the other
pound as the reference material “b” for a determinations. materials. In general, if steam is used to replace a reboiler,
one tray is added to replace the reboiler stage, and from
Bb = (Fraction retained) @bo) one-third to one or more trays may be needed to offset the