Chapter 5A

5.A Bubblepoint, dewpoint, and flash calculations (SRK EOS) Back to Main

##### A. & B. Bubble point at known pressure or known temperature

The bubblepoint may be defined as that condition at which the system is all liquid with one (infinitesimally small) bubble of vapor present. The amount of vapor is specified as a matter of convenience so that the composition of the liquid is essentially equal to the composition of the total system.

By definition, the sum of the mol fraction of all components must add up to 1 in both vapor and liquid.

It follows that (Equation 5.9)

Where: Ki xi = K value times "x" the mol fraction for each component in the mixture

The procedure is as follows for a system of known composition:

1. Assume a temperature of the known pressure (or assume pressure if temperature is known).

2. Find Ki at pressure and temperature known and assumed using graphs on page 133-142. In GCAP, the Ki value is calculated by using the SRK equation of state.

3. Multiply Ki from Step 2 by corresponding xi

4. If the summation of values from Step 3 is 1.0, you assumed the right pressure or temperature. If not, repeat step 1 – 3 unit S ( Ki xi ) = S yi = 1.0, within accuracy limits prescribed.

##### C. And D. Dewpoint at known pressure or temperature

The dewpoint is defined as an all vapor system except for one small droplet of liquid. (Equation 5.10)

1. Assume a temperature of the known pressure (or assume pressure if temperature is known).

2. Find Ki at pressure and temperature known and assumed using graphs on page 133-142. In GCAP, it is calculated by using the SRK equation of State.

3. Divide yi by the corresponding Ki found by Step 2

4. If the summation of values from Step 3 is 1.0, you assumed the right pressure or temperature. If not, repeat step 1 – 3 unit S (yi / Ki ) = 1.0, within accuracy limits prescribed.

##### E. Flash at specified pressure and temperature

The purpose of the flash calculation on a two-phase system is to establish the amounts of vapor and liquid, and the analysis of each.

For each component (5.11)

Where:

F = mols of total feed entering separation process

V = mols of gas leaving system for F mols of feed

L = mols of liquid leaving system for F mols of feed

zi = mols of component "i" in feed steam per mol total feed

yi = mol fraction of component "i" in gas stream (V)

xi = mol fraction of component "i" in liquid stream (L)

Let F = 1 mol and the sum of the entire mol fraction in any stream must equal 1. Therefore, algebraically

(5.12)

Therefore, a mean of solution is offered, through it is trial and error. By assuming an L or V, one may solve interactively the equations above.

The basic calculation procedure for a flash is as follows:

1. Find K at the pressure and temperature of the two-phase system (separation conditions).

2. Assume V or L.

3. Solve flash equation (link)

4. If objective function is satisfied, calculation is complete. If not, repeat Steps 2 – 4.

5. Calculate xi and yi

##### How do I carry out this calculation?

##### A. Bubblepoint at known pressure

##### B. Bubblepoint at known temperature

##### C. Dewpoint at known pressure

##### D. Dewpoint at known temperature

##### E. Flash at specified pressure and temperature

##### Definitions of the variables and where to obtain the unknown data

The dewpoint is defined as an all vapor system except for one small droplet of liquid.

The bubblepoint may be defined as that condition at which the system is all liquid with one (infinitesimally small) bubble of vapor present.

Specified temperature is the known temperature of the feed. Its unit is Celsius and Kelvin in SI units or Fahrenheit and Rankin in English units.

° C = 0.556 (° F - 32), K = ° C + 273, ° F = (1.8)(° C) + 32, ° R = ° F + 460

Estimated temperature is the estimated temperature of the feed, at the specified pressure, in SI units, Celsius and Kelvin or in English units, Fahrenheit and Rankin. If you try to find the bubble point temperature of a mixture, you need to give GCAP a guessed value of the bubble point temperature. The better your guess is, the faster GCAP will give you the answer.

° C = 0.556 (° F - 32), K = ° C + 273, ° F = (1.8)(° C) + 32, ° R = ° F + 460

Specified pressure is the known pressure of the feed.

Estimated pressure is the estimated pressure of the feed. If you try to find the bubble point pressure of a mixture, you need to give GCAP a guessed value of the bubble point pressure. The better your guess is, the faster GCAP will give you the answer.

Estimated liquid fraction (L/V): an estimate value of liquid fraction. It should be any value between 0 and 1. The default value in GCAP is zero.

The component table is a list of the possible compounds along with their symbols to be used with this program. In the far right column titled Mole % use the mouse to choose the available cell and enter the mole fraction of the component in the mixture. Use the Up and Down arrow keys to move from cell to cell. The total for the Mole % should be 100.00.

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