Separation units often use membranes, absorbers, and other devices to reduce the mole fraction of selected constituents in gaseous mixtures. Consider a mixture of hydrocarbons that consists of 62 percent (by volume) methane, 28 percent ethane, and 10 percent propane. After passing through a separator, the mole fraction of the propane is reduced to 1 percent. The mixture pressure before and after the separation is 100 kPa. Determine the change in the partial pressures of all the constituents in the mixture.

See explanation

Explanation:

We first obtain the mole fraction of propane at the start and end;

0.01 = x/0.62 + 0.28 + x

0.01[0.62 + 0.28 + x] = x

0.01[0.9 + x] =x

9 * 10^-3 + 0.0.1x = x

9 * 10^-3 = x - 0.0.1x

9 * 10^-3 = 0.99x

x = 9 * 10^-3/0.99

x = 9.09 * 10^-3

After

For methane

0.62/ 0.28 + 0.62 + 9.09 * 10^-3     = 0.68

For ethane

0.28/0.28 + 0.62 + 9.09 * 10^-3       =0.31

Change in partial pressure of methane = (0.68 - 0.62) * 100kPa = 6 kPa

Change in partial pressure of ethane = (0.31 - 0.28) * 100kPa = 3 kPa

Change in partial pressure of propane= (0.01 - 0.1) * 100kPa = -9 kPa