2 replies to this topic

Dear all,
Recovery of natural gas liquids (NGL) from natural gas is a common procedure in the gas processing industry. Basically, in NGL plant the following three columns will be modeled:
D

• e-Methanizer,
• De-Ethanizer and
• De-Propanizer

My question, how to know how many stages in the Reboiled Absorber/Distillation column is used to get the maximum product that we want?

[Zauberberg]

Normally each of these products (whether as C2+ or, more common, as separate C2, C3, C4, and C5+ streams) has its own specification. For Ethane it is - maximum Methane content, for Propane it is - maximum C2- content, etc. In other words, mole fraction of each key component is known in the each stage of separation.

Now, the task becomes very much straightforward. You can use famous Fenske-Underwood-Gilliland correlations for minimum number of stages and minimum reflux ratio, and also for minimum number of theoretical plates at given reflux ratio, i.e.:

a. Identify light and heavy key components for each column/system;
b. Guess splits of the non-key components and compositions of the distillate and bottoms products;
c. Calculate relative volatility of light and heavy component(s);
d. Use Fenske equation to find N,min;
e. Use Underwood method to find R,min;
g. Use Gilliland correlation to find actual number of ideal stages at given operating reflux;
h. Use Kirkbride equation to locate the feed stage.

Of course, design of real De-C1, De-C2, and De-C3 towers has plenty of "know-how" which makes the situation somewhat different than described here, but these are the rules of thumb that should give you results in the same ball park. Also, in reality, when you want to design a fractionator tower, consulting tray/packing vendors is very much desirable since they will verify your initial design and provide you with the optimum solution.

[Zilpa Varindravada]

Use the shortcut to preliminary know the preliminary theoritical stages. Number of stages should be optimized by multistage flash simulation.

Regards,