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Silicones 8a. Chemical Engineers

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    Heat and Chemical Resistant Silicone Rubber

    In the early days of this century, chemical manufacturing grew by enlarging the laboratory equipment in a quite arbitrary way. Often the laboratory process took place in an open room, sometimes in an open vat.

    Diagram Illustrating a Laboratory Steam Distillation Apparatus
    < face="Arial">
    < face="Arial" size="1">Diagram Illustrating a Laboratory Steam Distillation Apparatus
    from Organic Chemistry, W. H Perkin and
    < face="Arial" size="1">F. Stanley Kipping< face="Arial" size="1">, W. R. Chambers, London, 1911.

    But early in the 20th century, the chemists who were building these manufacturing plants recognized they stood on a slippery slope in attempting to make more and more of these new synthetic chemicals. As the size of the operation grew, so did two unanticipated problems:

    But early in the 20th century, the chemists who were building these manufacturing plants recognized they stood on a slippery slope in attempting to make more and more of these new synthetic chemicals. As the size of the operation grew, so did two unanticipated problems:

    The first was obtaining the exact material they wanted at the quality required. Most chemical reactions have alternate pathways. (Those of you who think through your understanding of kinetics and activation energy will realize why. If two reactions are possible with closely matched activation energy to reach the activated complex, these reactions will take place in competition with each other.) Manufacturers needed to know the exact amount of each starting material and each product in the process. Manufacturers needed exact, balanced chemical equations for all chemical reactions taking place.

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    The second was handling the energy of the process. Simple balanced chemical equations were not enough. For each equation someone had to assign the energy of the reaction - the endothermic or exothermic energy change for each of these equations. Someone had to define how the heat would be added to the process if it was required. And often more important was the question of removing heat for processes that were exothermic.

    The discipline called Chemical Engineering came from the need for this information. A chemist named Charles M. A. Stine at the DuPont Company understood the need for material and energy information most clearly. He was perhaps the first to articulate the need for what is now called material and energy balances when it came time for him to build a large manufacturing plant for the preparation of the high explosive TNT (1,3,5-trinitrotoluene). Stine preferred that his new plant not suffer any rapid, unexpected chemical reactions -- we would call them explosions.

    Chemical Engineers face serious issues of design and planning to insure that manufacturing if as efficient and safe as possible.

    Chemical Engineering must meet the requirements of understanding material and energy balances to prevent incidents like these.

    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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