On the solubility of acetylene in the supernatant

Only acetylene is recovered from the clear night fluid, and at present, reusing the clear night fluid is the main approach.

Recycling acetylene from calcium carbide slag is done in small quantities and is difficult; it is not feasible from either economic or technical perspectives. It would be worth researching how to reuse calcium carbide slag

It is not very meaningful to recover acetylene from calcium carbide slag. If it is necessary to reduce the acetylene entrainment in calcium carbide slag, it would be better to consider dry acetylene

I’ve seen some patents that mention that when the liquid is reused, a large amount of acetylene disperses into the air during the cooling process. Can’t this acetylene be recovered? Also, I’m a bit confused: the solubility of acetylene gas in water increases as the temperature decreases. So why does the amount of acetylene in the supernatant decrease after cooling?

It seems that there are already manufacturers in China recycling acetylene gas, with some more under construction. Our factory is also considering whether to increase acetylene recovery at the moment.

That’s indeed the case; once the supernatant is cooled, the acetylene dissolved in it disappears. Our tests showed that the acetylene concentration in the cooled supernatant is between 4-10 ppm, while the clear calcium carbide slurry contains around 200 ppm of acetylene.

You can use Henry’s law to explain this problem! Henry’s law primarily describes the relationship between a gas and an absorbent! Henry’s law: At a constant temperature, the saturated concentration of a gas in a liquid is directly proportional to the equilibrium partial pressure of that gas above the liquid surface. It is an empirical law discovered by the British scientist W. Henry in 1803 based on experiments. Experiments show that this law holds true only when the solubility of the gas in the liquid is not very high; in such cases, the gas acts as a volatile solute in a dilute solution, and the gas pressure corresponds to the vapor pressure of the solute. Therefore, Henry’s law can also be expressed as follows: at a constant temperature, the vapor pressure of a solute in a dilute solution is proportional to the concentration of the solution: pB = kxB, where pB represents the vapor pressure of the solute in the dilute solution ； xB is the mole fraction of the solute ； k is the Henry constant, whose value depends on temperature, pressure, and the nature of the solute and solvent. Since various concentrations in a dilute solution are proportional to each other,xB in the above equation can also be mB (mass molar concentration) or cB (mole concentration), etc.; in such cases, the value of k will change accordingly. 　　Henry’s law applies only when the molecular states of the solute in the gas phase and the liquid phase are identical. If the solute molecules undergo dissociation, association, etc. in the solution, then xB (or mB, cB, etc.) in the above equation should refer to the concentration of the portion that is in the same molecular state as that in the gas phase ； When the total pressure is not high, if multiple gases are dissolved in the same liquid simultaneously, Henry’s law can be applied separately to each of these gases ； Generally speaking, the weaker the solution, the more accurate Henry’s law becomes; as xB→0, the solute can strictly obey the law.

In other words, in order to maintain the ratio between the acetylene concentration in the atmosphere and that in the supernatant, acetylene in the aqueous phase must continuously diffuse into the atmosphere, which ultimately results in the loss of most of the acetylene?

Given the solubility of acetylene in water, it is considered uneconomical to recover it; this is because power, condensation equipment, and precise control systems are required, and most importantly, the recovery device must have high sealing standards, otherwise there is a risk of explosion.

The recovery of acetylene gas from the acetylene supernatant is now a mature technology; the negative pressure method can be used for this purpose, but the investment costs are very high. For a PVC production scale of 200,000 tons, calculations show that less than 10 tons of calcium carbide can be saved per day, resulting in a long time required to recoup the costs.

What you said is correct – the lower the temperature, the greater the solubility of acetylene. However, in reality, the partial pressure of acetylene in the gas phase inside the generator is clearly higher than that in the supernatant. But once the mixture exits the generator, it comes into contact with the atmosphere, causing the partial pressure of acetylene in the gas phase to drop to 0. Therefore, no matter what the temperature is, as long as acetylene is released, it will definitely end up in the atmosphere. As a result, the acetylene content in the supernatant will only continue to decrease