1. Material
Balance
A
number of years ago the laboratory in an old
chemical plant faced a not unusual problem.
We were noted a drop in production in the old
plant. The process had been operating out
there in an open wooden tank since about
1920.
The
process was the preparation of a zinc salt in the
presence of sodium carbonate. The zinc salt would
be drawn off in a water solution. Most of
the sodium would be deposited as a sludge
believed to be a complex mixture of zinc and
sodium salts. I asked a young chemist to take two
very simple actions. First, I asked him to
analyze the sludge for the percentage of elements
in the material. And to propose a chemical
structure for the sludge, just the way you did in
first semester General Chemistry. And second, I
asked him to write a balanced chemical equation
for the process.
He
reported back in two days. The analysis
showed the sludge was actually a rather pure
material - an insoluble mineral found in
nature - called hydrozincite. With this
knowledge he was able to write the balanced
chemical equation for the process.
But
the chemical equation revealed a disturbing
fact. The reaction evolved considerable
carbon dioxide from the solution. Carbon dioxide
emerging from the solution in the open tank was
an extremely dangerous situation - made
critically dangerous by the plans to soon enclose
the tank. Carbon dioxide, molecular weight 44D,
is heavier than air. We can picture the
gas, every day, filling the space in the tank
above the liquid. We can follow the gas
then spilling over the edges of the tank, just as
if it were a liquid. It collects along the
floor, waiting for some unfortunate employee to
bend down or kneel down to do a task -only to
become unconscious and die for the lack of oxygen.
A tragedy waiting to happen for the lack of a
balanced chemical equation!!
2. Heats of Reaction
In mid January 1981, at just after
6AM, a manufacturing plant in the Midlands of the
United Kingdom blew up. A rotating vacuum
dryer split in two. The larger half - the
size of small sedan - was launched through
the plant and landed in the parking lot.
The energy released from the
explosion knocked down the walls of the
plant. The staff, safely drinking tea in a
concrete blockhouse when the dryer
exploded, donned their gas masks and
emerged unharmed into their devastated workplace.
That night the dryer was not working
properly. Water was to be removed from the
contents of the dryer under vacuum at low
temperature. The pump evacuating the water was
faulty, the water just stayed there.
The temperature in the dryer rose
slowly. The salt in the reactor, sodium
dithionite, Na2S2O4,
reacts slowly with water and the reaction is
exothermic. On the laboratory bench, this
exothermic reaction is calm and lethargic - the
heat from the reaction is dissipated easily into
the surroundings - so little heat, so big
surroundings. But in the dryer that winter night,
the heat from the reaction could not be
dissipated away from several tons of sodium
dithionite.
Think this through. As heat is
generated and not conducted away, the temperature
rises. As the temperature rises, what
happens to the rate of reaction of the dithionite
and water? The rate increases, of
course. More heat is evolved in a runaway
chemical reaction. The explosion was a steam
explosion - the rapid heat generation boiled the
remaining water so quickly and generated so much
steam that the vessel burst from the pressure.
A dramatic steam explosion ripping
apart a modern chemical plant as the result of an
unanticipated runaway chemical reaction!!
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