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Unformatted text preview: around a leaching unit.
Given: Feed rate to extractor of 6.375 lb/h with 10.67 wt% moisture and 0.2675 g oil per g dry,
oil-free flakes. Solvent rate to extractor of 10.844 lb/h. Extract of 7.313 lb/h with 15.35 wt%
oil. Leached solids with 0.0151 g oil per g dry, oil-free flakes.
Assumptions: Solvent is n-hexane. Extract contains no solids. Leached solids contain all of the
moisture in the feed and are wet with solvent.
Find: Check on mass balances around the leaching unit for oil and hexane. Mass ratio of oil to
flakes in leached solids.
Analysis: Make calculations with AE units, noting that g/g is the same as lb/lb. Calculate the
flow rates of components in the feed:
Moisture = 0.1067(6.375) = 0.680 lb/h moisture
Dry flakes = 6.375 – 0.680 = 5.695 lb/h dry flakes
Oil = [0.2675/(0.2675 + 1.00)] 5.695 = 1.202 lb/h oil
Dry, oil-free flakes = 5.695 – 1.202 = 4.493 lb/h dry, oil-free flakes
Calculate flow rates in the leached solids, assuming all dry, oil-free flakes are in the leached
Oil = 0.0151(4.493) = 0.068 lb/h oil
Therefore, by mass balance around the leaching unit,
Oil in the extract = 1.202 – 0.068 = 1.134 lb/h oil
Compare this to data given for the extract:
Oil in the extract = 0.1535(7.313) = 1.123 lb/h oil
This number is 99% of the mass balance number, which is an excellent check.
A similar check cannot be made for the hexane, but we can calculate the amount of hexane
contained in the wet leached solids.
Hexane = 10.844 – (7.313 – 1.123) = 4.654 lb/h hexane in the wet leached solids.
Mass ratio of oil to wet leached solids = 0.068/(0.068 + 4.493 + 0.680 + 4.654) = 0.0069
Summary of mass balance in lb/h:
Dry, oil and
solvent-free flakes Wet, leached solids
0.000 Exercise 16.2
Subject: Manufacture of insoluble barium carbonate by precipitation of barium sulfide in an
aqueous solution with sodium carbonate, followed by washing with water to remove sodium
Given: Specifications given below for a 5-stage countercurrent flow system, where precipitation
occurs in the first stage and washing is done in four stages of thickeners.
Assumptions: No solids in the overflows. Liquid in the underflow and liquid in the overflow
leaving a stage have the same composition. Stoichiometric amounts for the precipitation
Find: (a) A schematic diagram of the process. (b) The flow rates of sodium carbonate and wash
water required. The flow rates of barium carbonate and sodium sulfide produced. (c) The wt%
of sodium sulfide leaving in the liquid in the underflow from each stage. The flow rate of
sodium sulfide in the barium carbonate after drying.
Analysis: Because the underflow liquid is given as a mass ratio of Na2S-free water to the solid
BaCO3 in the underflow, use Na2S compositions in the liquid as mass ratios, X, to water.
(a) Schematic diagram of the process with given information...
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