Chemical Engineering: Stoichiometry and Mass Balance

Extracted Content from Question Files:

A3. Solid calcium fluoride (CaF2) reacts with sulphuric acid to form solid
calcium sulphate (CaSO4) and gaseous hydrogen fluoride (HF). The
HF is then dissolved in water to form hydrofluoric acid. A source of
calcium fluoride is fluorite ore containing 96% w/w CaF2 and 4% w/w
SiO2.
In a typical hydrofluoric acid manufacturing process, fluorite ore is
reacted with 93% w/w aqueous sulphuric acid, supplied in 15% excess
of the stoichiometric amount. Ninety five percent (95%) of the CaF2
reacts with the acid. Some of the HF formed reacts with the dissolved
silica (SiO2) in the reaction. The unreacted HF gas exiting from the
reactor is subsequently dissolved in enough water to produce 60% w/w
aqueous hydro fluoric acid.

(a) Determine the mass of 93%w/w acid in the feed.

(b) Calculate the quantity of fluorite ore needed to produce 2500 kg
of 60% w/w acid. Assume all the SiO2 present in the ore reacts
with HF. The reaction equations are:

CaF2 (s) + H2SO4(aq) → CaSO4(s) + 2 HF(g)
6 HF(g) + SiO2(aq) → H2SiF6(s) + 2 H2O(l)

DATA
RAM
Ca = 40, F = 19, H = 1, S = 32, O = 16, Si = 28

A4. Formaldehyde (CH2O) is manufactured by the catalytic oxidation of
methanol using an excess of air. Formic acid (HCOOH) is also formed
if conditions are not properly controlled. The reaction equations are as
follows:

Main reaction

CH3OH + ½ O2 → CH2O + H2O --------- (1)

Secondary reaction

CH2O + ½ O2 → HCOOH --------- (2)

The product gases in a test run have the following composition:

Component Mol %
CH3OH 8.6
CH2O 3.1
HCOOH 0.6
H2O 3.7
O2 16.0
N2 68.0

Determine the following:

(a) The molar ratio of feed air to feed methanol

(b) The per cent excess air based on the main reaction

(c) The conversion of methanol

(d) The yield of formaldehyde

(e) The selectivity of formaldehyde
R1. The schematic diagram for the process producing pure B from pure A
is as follows:-

Recycle gas, RC
SP
W

FF1 Pure A
MF Reactor RP
Absorber
FF2 Air

P

Pure B

The stoichiometric equation for the reaction between A and air can be
represented as

2A + O2 2B

During a test run on the plant the following analysis was obtained for the
purge stream.

Component % v/v
Nitrogen 72.9
Oxygen 15.3
A 11.8

The ratio of recycled gas to purge was 3:1.

Determine
a) The ratio of A to air in the fresh feed.
b) The overall yield.
c) The conversion on a once through basis.

DATA:
N2:O2 = 79:21
B1 In the production of hydrogen, methane is reacted with steam over a
metal catalyst at 1200 K to produce a mixture of H2, CO2, CO, H2O, and
unreacted methane. Steam and pure methane are fed to the reformer
in a molar ratio of 2.5 and are at 500 K, and the exit gas from the
reformer has the following composition:

Component mol %

CH4 5.0
H2O 25.0
H2 55.0
CO2 10.0
CO 5.0

If the reformer is supplied energy at the rate of 268,370 kJ per kmol of
methane in the feed, determine the exit temperature of the reformer
product stream.

DATA

∆hθf at 25 C in kJ/kmol
Component
CH4(g) - 74,900
CO(g) - 110,600
CO2(g) - 393,700
H2O(v) - 242,200

For Cpm of CO2 at 500K use the following polynomial equation

Cp = 19.795 + 7.343 x 10-2 T – 5.601 x 10-5 T2 + 1.715 x 10-8 T3

For other mean specific heat values refer to the Cpm table.
B2 A waste gas containing 15 mol% CO2, 3 mol% CO, 12 mol% H2O,
68 mol% N2 and 2 mol% O2 leaves a furnace at a temperature of 800 K
and passes through a waste heat boiler in which it cools to 500 K and
in doing so generates saturated steam at 4.8 bara. The water fed to the
boiler enters at 20 C and 4.8 bara. The waste gas flow rate is 500
kmol h-1 and the boiler is assumed to operate adiabatically.

(a) Calculate the amount of saturated steam raised in the boiler.

(b) How much heat would be required in a super heater to raise the
saturated steam to 200 C (see diagram given below).

DATA

For Cpm values refer to the attached Cpm table.

Steam tables are provided.

Super heated steam
at 200 C and 4.8 bara

Super heater

Q

4.8 bara saturated steam

Boiler

Waste gas at 800 K waste gas, 500 K

Water at 20 C,
4.8bara