Retrograde Condensation Mechanism

In organic chemistry, the behavior of hydrocarbons in a gas reservoir, at constant temperature, to transform into liquid phase from gaseous phase is known as condensation. The opposite of condensation is known as retrograde condensation, because some of the gas condenses into liquid under isothermal conditions, instead of expanding or vaporizing when the pressure is decreased.

The term retrograde condensation was first used by Kuenen at Leiden in 1892. Kuenen observed, during an experiment based on van der Waal's theory, that the binary mixture at a temperature above the critical temperature mixture was converted to liquid, followed by vaporization of the liquid with increase in pressure. Kuenen's term 'retrograde condensation' was interpreted differently, and lead to a lot of confusion.

One carbonyl partner with an alpha hydrogen atom is converted by base into its enolate ion. The enolate ion acts as a nucleophilic donor, and adds to the electrophilic carbonyl group of the acceptor partner. The neutral condensation product is obtained by protonation of the tetrahedral alkoxide ion intermediate .

Retrograde condensation occurs in petroleum gases containing heavy hydrocarbons. These hydrocarbons are found as single-phase fluids in deep reservoirs at high pressure and temperature. If the pressure is reduced at constant temperature, it may hit the dew-point curve, that leads to 'retrograde condensation'.

Hydrocarbon fluids are in two phases between the bubble point and the dew point curves. The fluids that can enter the two-phase region to the right of the critical point curve are called retrograde condensates.

The fluids that are present at temperatures higher than the cricondentherm, remain in single-phase at all pressures.

If the initial reservoir condition of temperature and pressure is above the phase envelope between critical temperature and cricondentherm, the fluid will go through a dew point, and the gas phase is converted to liquid, as the pressure in the reservoir decreases.

Bumble point calculations: ∑y_i = 1 = ∑K_i . x_i

Dew point calculations: ∑ x_i = 1 =∑y_i / K_i

Both require trial and error for the unknown T or P. The best demonstration of this condensation is the ethane-heptane system. The retrograde condensation can occur for an 88.7 mol% ethane mixture, but is not seen for a 26.5 mol% ethane mixture.

The upper and lower limits of the initial producing gas-oil ratio is approximately 3300-150,000 scf/STB for a retrograde gas. These liquids formed are light-colored, like brown, orange, or water-white. The retrograde gas condensate has an API gravity between 55 to 70° API.

• It is used to crack and give rise to smaller, lighter chains of alkanes, like methane and ethane.
• They are fractionally distilled to produce other components, like refinery gas.

Thus, the retrograde condensation mechanism is significantly recognized in the petroleum industry.