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    Home > Chemicals Industry > Chemical Technology > Gas production reaction (1)

    Gas production reaction (1)

    • Last Update: 2022-01-27
    • Source: Internet
    • Author: User
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    Reactions (including post-processing) often produce gas and generate positive pressure inside the container
    Ordinary ordinary glass reaction flask, whether it is made of 95 material or 17 material borosilicate glass, its internal pressure is generally not more than 10psi (the hydrogenation bottle processed by special material and special process can withstand 60psi pressure, which is about 0.
    4MPa Or 4atm)

    The larger the reaction flask of the same thickness and the same material, the lower the pressure; the higher the temperature, the lower the strength of the glass reaction flask

    The gas continuously produced in the reaction needs to be discharged and led away in time.
    If the gas pressure in the bottle exceeds the pressure of the container, flushing or physical explosion will occur

    There are many cases of such accidents .
    In addition, if the fume hood where the reaction flask is located, if there are many dangerous items, a physical explosion often causes a chain of secondary accidents

    Therefore, the most taboo of the glass reaction bottle is the over-standard positive pressure under the rigid airtight


    For pressure reaction vessels such as steel autoclaves and stainless steel PTFE tanks, even if they are equipped with safety rupture discs, the gas that exceeds the discharge capacity of the vessel cannot be released immediately and may easily cause the vessel to explode.
    It needs to be known in advance.
    :Whether it belongs to a gas production reaction, what kind of gas is produced, the total amount of gas produced, the maximum gas production rate, and when the gas is released

    Only by fully knowing these can corresponding measures be taken to prevent accidents

    If the generated gas is toxic or corrosive, exhaust gas absorption is required, and the concentration of quencher in the absorption liquid is always in the effective range


    Safety measures


    For reactions and post-treatments that are not taboo against humidity and air, and do not require pressure, they can generally be operated with open ports, so that the generated gas can be discharged at any time

    Use nitrogen flow

    For reactions and post-treatments that are taboo against humidity and air and do not require pressure, nitrogen flow can be used.
    The nitrogen flow is connected to the outlet.
    This not only prevents moisture and avoids air, but also maintains a constant pressure and continuously dilutes the emitted gas, such as hydrogen.
    Make it impossible to form an explosion limit and reduce the hazard of the gas produced


    Soft airtight

    1) Balloon buffer form

    If the gas generated during the reaction or post-treatment is 2 to 3L, air balloons can be used cautiously as a buffer
    Generally, the filling capacity of commercially available balloons is about 3L.
    If the gas generated exceeds the filling capacity of the balloon, it may be dangerous


    2) Liquid seal

    In order to prevent the reaction from directly contacting the air, a liquid sealing device can be used to smoothly lead out the positive pressure gas generated
    The liquid seal generally needs to be equipped with a safety bottle to prevent the liquid seal medium from sucking back

    Generally, the pressure difference between the inside and outside of the liquid seal that is suitable for the seal is not a great response


    It is necessary to consider whether the liquid sealing medium reacts with the sealed gas.
    For toxic or corrosive gases, use a liquid sealing medium that can absorb these gases

    If there is no special requirement for the liquid sealing medium, water or oil can be used


    Prevent sudden outbreaks of reactions

    If gas is generated during the reaction, it is necessary to strictly control the smooth progress of the reaction by adjusting the temperature (not necessarily the lower the better), adjusting the dripping rate, and leaving enough space
    If there is a large amount of response, a small test should be done in advance, and the reaction phenomenon of the small test should be carefully observed

    The following are some typical accident cases


    Case 1

    According to the literature, a researcher responded as follows:

    At 0°C, sodium hydride (70.
    1 g, 1.
    75 mol) was suspended in anhydrous tetrahydrofuran (5 L), and then a solution of compound A (400 g, 1.
    46 mol) in anhydrous
    tetrahydrofuran (1 L) was added dropwise
    The mixture was refluxed and reacted for 5 hours


    After the feeding is completed, there is no obvious sign of exothermic reaction, and then heating to reflux, that is, a large number of bubbles are generated, and the reaction intensifies suddenly and the upper part of the condenser tube catches fire
    As a result, the lamp glass on the top of the fume hood was broken, and the 10L reaction flask was damaged by the broken glass that fell, causing a bigger fire


    Reason analysis: heating to reflux, the reaction erupts instantly, a large amount of hydrogen produced overflows through the outlet of the condenser tube, and encounters the electric spark in the mechanical stirring motor and causes a fire

    Case 2

    A R&D personnel reacted as follows:

    Under stirring, NaH (100g, 4eq.
    ) was slowly added to the THF reaction solution of A (150g) and B (100g), cooled with an ice bath, during which a large amount of solid appeared in the bottle, and the system became very viscous.
    Magnetic stirring Unable to stir, the R&D personnel assisted in shaking the reaction flask by hand to make the reaction system uniform

    When all the NaH is added, the reactants A and B all form sodium salts, the system becomes thinner, and the magnetic stirrer suddenly rotates.
    The system produces a large amount of gas (hydrogen) foam, causing flushing, and the reaction liquid enters the ice-water bath, catching fire and causing Burn the solvent THF in the reaction flask and the organic solvent hanging on the cabinet wall to wash the flask, and the fire increases

    The R&D personnel closed the fume hood as soon as possible, and extinguished the fire with yellow sand and dry powder fire extinguishers with the assistance of everyone

    The accident photo is shown in Figure 24-31


    Figure 24-31 The scene after the accident

    Reason analysis: The two substrates of the reaction have three active hydrogens.
    The amount of NaH-requiring substances is large, and the amount is large, so more gas is produced

    Because the initial reaction system is very viscous, as the temperature rises slowly, the reaction liquid becomes thinner, and the reaction suddenly erupts, resulting in instantaneous production of a large amount of hydrogen and flushing

    The reaction liquid containing excessive NaH enters the ice-water bath and catches fire

    Lesson: Large batches of NaH should be fed in batches, and the reaction system should be stirred evenly.
    Make sure that every addition of NaH has to be basically reacted before it can continue to be added in small amounts


    Related Links: Decomposition and explosion of polynitrogen compounds



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