Carbon dioxide fire extinguishers are metal canisters containing CO2 under pressure higher than 1 at — Physical Chemistry Chemistry Question
Carbon dioxide
Carbon dioxide fire extinguishers are metal canisters containing CO2 under pressure higher than 1 atm. Given are [NIST Webbook of Chemistry, CRC]: Critical point pc = 73.75 bar, Tc = 304.14 K; Triple point p3 = 5.1850 bar, T3 = 216.58 K. Assume room temperature is 25 ºC. Choose the correct answers.
What is the required pressure in the fire extinguisher for the coexistence of solid and liquid CO2?
A) 2 bar, B) 5.185 bar, C) 20 bar, D) 73.8 bar, E) not possible under any pressure.
Model Answer
A rather accurate phase diagram for CO2 is shown here using data from Landolt – Brönstein New Series IV/20B, p. 22 which has been based on Dykyi, J., Repas, M.: Saturated Vapor Pressure of Organic Compounds, Bratislava Slovakian Academy of Sciences, 1979.
A qualitative one can be drawn based on the triple and the critical points. Since the room temperature is well above the triple point, there is no way there can be any solid CO2 in the fire extinguisher.
What is the required pressure in the fire extinguisher for the coexistence of liquid and gaseous CO2?
A) about 2 bar, B) 5.1850 bar, C) about 20 bar, D) about 63 bar, E) 73.8 bar, F) about 100 bar, G) not possible under any pressure.
Model Answer
At 298 K, the CO2 vapor pressure is 63.1 bar. This value can be estimated by drawing a straight line between the triple and the critical points. It can be calculated also based on the empirical Antoine equation, viz.,
log P = A - B / (T + C)
with A = 6.46212, B = 748.28 and C = – 16.9.