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Chemical
Concepts
These
chemical concepts will help you understand GE's
commercial synthesis of the first methylsilicone
polymers.
| 10. Large Scale Exothermic
Reactions |
require |
Equipment that can dissipate
heat and keep the temperature under control. |
| 11. Isolation of purified
substances |
usually
necessitates |
Specially designed
manufacturing units. |
Micro/Macro
and Symbolic
Representation
Chemical
engineers use representational language just as chemists
do.
 |
The sketches of the
"fluid bed" reactor and the
distillation units shown on the right employ
standard engineering symbols to indicate
individual equipment components. |
| Problem: The
automobile represents engineering solutions to
the problem of converting the energy from the
burning of hydrocarbons to moving a mass in a
desired direction at a convenient acceleration
and velocity.
Sit in groups of three and
discuss the engineering inherent in the chemical
reactor called the automobile engine:
1.
When we burn gasoline in an automobile engine we
carry out an exothermic process that gives heat
and energy. Gasoline and air are mixed in a
cylinder and a spark ignites the mixture.
a.
Initially in the cylinder the concentration of
gasoline and air is high, the temperature is low.
But as gasoline and air burn, the temperature
rises but the concentration of fuel and air drop.
Draw on a chart as shown below your best estimate
as to the rate of burning and mixture temperature
after the spark ignites the mixture.
b.
What do we do with the energy? Be specific.
c.
What do we do with the heat? Be specific.
d.
Why does an automobile have a radiator?
e.
How does the radiator operate?
|
|
|
Heat and Chemical
Resistant Silicone Rubber
8. High Temperature and Chemical Resistant Silicone
Rubber
 |
General Electric
soon cleared a cornfield near the Mohawk
River in New York and built on it a
manufacturing plant to make the methyl
silicones. This is what Chemical
Engineers do. |
Building a manufacturing plant might
seem to be a simple activity. Wouldn't GE
just increase the size of Rochow's equipment;
make a bigger chemistry set to make the
silicones? Let's look at the steps:
- Buy
some HCl and methyl chloride.
- Mix Si
and Cu, heat the mixture and flow the two
gases, HCl and CH3Cl over the
Si and Cu catalyst.
- Take
the product (CH3)2SiCl2 (9),
and react it with water.
- Collect
the methylsilicone polymer.
But the commercial manufacture of
the silicones required far more than Rochow's
experiments. It required the work of many
men and women called Chemical Engineers.
| Making Silicone
Polymers The reaction
of silicon with methyl chloride has a
high activation energy. This means
that a great deal of energy energy must
be suppplied -- in the form of heat -- to
achieve the activated complex that
proceeds to products.
2CH3Cl
+ Si --> (CH3)2SiCl2(9)
But
then the reaction itself is highly
exothermic and if left unattended, the
process temperature will rise, the rate
will increase and even more heat will be
generated in a "runaway"
process. Additonally, if the
temperature of the reaction mixture rises
as result of the heat generated in the
exothermic synthesis, a series of
alternate products is formed. These
are called "impurities" among
chemists. They represent synthesis
of unwanted byproducts.
So
GE's engineers faced a situation in
which the temperature must be nearly 300
degrees Celsius to start the reaction,
but heat must be removed efficiently to
operate the sysnthesis of (9) most
efficiently.
|
Figure 18.17, Chemistry:
Molecules, Matter and Change, Third
Edition Peter Atkins and Loretta
Jones, W. H. Freeman, NY, 1997.
Used with permission. |
 |
A sketch of the reactor in
which methyl chloride and Si were
contacted reveals the kind of
"units" that the GE engineers
combined into the methylsilicone
manufacturing unit. |
A "superheater" heats the
methyl chloride before it reaches the
reactor. The silicon is fed into a reactor
that is stirred or "fluidized" by
passage of hot gas throught the bed of
copper-containing silicon. A "heat
transfer coil" removes excess heat by
transferring it to a cooler, circulating
fluid. The engineers stopped silicon powder
from being carried through the reactor by
collecting it in two different types of
filters. A "condenser" cools the
gases from the reactor to a liquid that contains
all the products.
But a chemical reactor is not
sufficient equipment to isolate a usable chemical
product in the purified state from all other
substances. Experiments led to a material
balance for the preparation of (CH3)2SiCl2 (9). GE's
engineers isolated the following products in the
order of their abundance:
(CH3)2SiCl2(9) boiling
point 70.0 degrees Celsius
CH3SiCl3 boiling
point 65.7 degrees
HSiCl3
boiling
point 31.8 degrees
(CH3)HSiCl2
boiling point 40.7 degrees
(CH3)3SiCl
boiling point 57.3 degrees
SiCl4
boiling
point 56.7 degrees
Isolation
of (9) from this complicated series of
materials required a difficult
"distillation" carried out in the
system seen at left.
The
reaction products of silicon and methyl chloride
enter a series of "columns" on the
left. The columns separate the products by
boiling point, isolating pure compounds wherever
practical.
|
Compound
(9) is the highest boiling of the products and
is purified by removal of all the other, lower-boiling
materials. They achieved purity of greater than
99.9% of (9).
One of
the other products, HSiCl3 boiling point 31.8 degrees,
is purified in the process and we will see how this
compound is important in the late-century world of the
computer revolution. General Electric chemists and
engineers worked under the lash of time as they tried to
convert (9) to useful products. The lash
was the lash of wartime. It was 1943, 1944, 1945
and the new materials would be critical for the armies,
navies, air forces in Europe and Asia.
The
chemists knew that Rochow's reaction of (9)
with water:
n(9)
+ nH2O
--> n[(CH3)2SiO]n
+ nHCl
could
give a polymer, a molecule in which the number of
repeated units, n, was very high, only if (9)
was close to 100 % pure. That is, (9)
could have no other chemical impurities in the
sample. In addition they would have to deliver the
wate in small and very controlled amounts. If they
used an excess of moles of water over that of (9),
they would not get long chains. And GE's chemists
knew they must find a way to "cure" the long
chain silicone if it were to have reliable rubber
properties.
GE's
engineers solved the purification problem with the
distillation. Rochow and the other GE chemists
applied the two skills that chemists must master: hard
intellectual work and careful observation, to solve
the other two.
GE's Dr. J. Marsden installs
a silicone gasket in the turbosupercharger of a
B-29 bomber, November 15, 1944.
Silicones Under the Monogram, H. A.
Liebhafsky, John Wiley, NY, 1978, p. 171
|
Remember that
inorganic salts are often most stable as hydrate;
molecules containing water of
crystallization. One chemist realized these
small amounts of water could be easily measured
out by weighing the hydrate salt, they mixed FeCl3(H2O)6 with (9) and
supplied exactly the correct amount of
water. The resulting polymers had thousands
of units bonded in a row (n=>1000).
Another one of GE's scientists examined many
agents to cause the rubber to cure after articles
were first made. He found a compound called
benzoyl peroxide cured the rubber.
The
scientists were in the war effort; they worked
seven days a week, they made silicone rubber
gaskets for battleship searchlights that would
prevent the lenses from shattering from the
vibration when the warships loosed their 16 inch
shells.
|
In
less than sixty years manufacturing grew to more than 1
billion kg. of silicone resins made by Rochow's process.
|
