Fractionating column control apparatus and methods

Apparatus for separating a mixture of hydrocarbons into discrete products, comprising:

• a fractionating column;
• means for providing heat to said fractionating column;
• means for feeding a stream of said mixture of hydrocarbons to said fractionating column whereby said stream is fractionated into an overhead product stream and a bottoms product stream;
• and
• means for controlling the Reid Vapor Pressure of said overhead product stream, including:
• means for establishing a first signal representative of the actual temperature of said overhead product stream;
• means for establishing a second signal representative of the actual vapor pressure of said overhead product stream;
• means for establishing a third signal representative of the actual rate of flow of said overhead product stream;
• means for establishing a fourth signal representative of the actual rate of flow of said bottoms product stream;
• means for establishing a fifth signal representative of the actual Reid Vapor Pressure of said overhead product stream in response to said first, second, third and fourth signals without an analysis of said overhead product stream based on the following relationships:
• Reid Vapor Pressure=A+B (X.sup.E -C(Y))+D/Z wherein:
• X equals the temperature of said overhead product stream as represented by said first signal;
• Y equals the vapor pressure of said overhead product stream as represented by said second signal;
• Z equals the rate of flow of said bottoms product stream as represented by said fourth signal divided by the rate of flow of said overhead product stream as represented by said third signal;
• and
• A, B, C, D and E are equal fitted constants;
• means for establishing a sixth signal representative of the desired Reid Vapor Pressure of said overhead product stream;
• means for comparing said fifth signal and said sixth signal and for establishing a seventh signal responsive to the difference between said fifth signal and said sixth signal;
• and
• means responsive to said seventh signal for varying the rate at which heat is provided to said fractionating column to maintain said actual Reid Vapor Pressure of said overhead product stream substantially equal to said desired Reid Vapor Pressure of said overhead product stream.

A method for determining the Reid Vapor Pressure of an overhead product stream of a fractionating column used for fractionating a multi-component hydrocarbon feed stream into an overhead product stream and a bottoms product stream, comprising:

• establishing a first signal representative of the temperature of said overhead product stream;
• establishing a second signal representative of the vapor pressure of said overhead product stream;
• establishing a third signal representative of the rate of flow of said overhead product stream;
• establishing a fourth signal representative of the rate of flow of said bottoms product stream;
• determining said Reid Vapor Pressure of said overhead product stream in response to said first, second, third and fourth signals without an analysis of said particular product stream based on the following relationship:
• Reid Vapor Pressure=A+B (X.sup.E -C(Y)) D/Z
• wherein:
• X equals the temperature of said overhead product stream as represented by said first signal;
• Y equals the vapor pressure of said overhead product stream as represented by said second signal;
• Z equals the rate of flow of said bottoms product stream as represented by said fourth signal divided by the rate of flow of said overhead product stream as represented by said third signal;
• and
• A, B, C, D and E are equal fitted constants.

In a method of separating different products from a multi-component hydrocarbon stream whereby said stream is fed into a fractionating column, heat is provided to said column and said stream is fractionated into an overhead product stream, a side draw product stream and a bottoms product stream, wherein the improvement comprises controlling the Reid Vapor Pressure and quality of said side draw product stream comprising:

• establishing a first signal representative of the temperature of said side draw product stream;
• establishing a second signal representative of the vapor pressure of said side draw product stream;
• establishing a third signal representative of the rate of flow of said bottoms product stream;
• establishing a fourth signal representative of the rate of flow of said overhead product stream;
• establishing a fifth signal representative of the rate of flow of said side draw product stream;
• establishing a sixth signal representative of the actual Reid Vapor Pressure of said side draw product stream in response to said first, second, third, fourth and fifth signals based on the following relationship:
• Reid Vapor Pressure=A+B (X.sup.E -C(Y))+D/Z+E/U+F/V
• wherein:
• X equals the temperature of said side draw product stream as represented by said first signal;
• Y equals the vapor pressure of said side draw product stream as represented by said second signal;
• Z equals the rate of flow of said overhead product stream as represented by said fourth signal divided by the rate of flow of said side draw product stream as represented by said fifth signal;
• and
• U equals the rate of flow of said bottoms product stream as represented by said third signal divided by the rate of flow of said side draw product stream as represented by said fifth signal;
• V equals the sum of the rate of flows represented by said third, fourth, and fifth signals divided by the rate of flow of said side draw product stream as represented by said fifth signal;
• and
• A, B, C, D, E, F and G are equal fitted constants;
• establishing a seventh signal representative of the desired Reid Vapor Pressure of said side draw product stream;
• comparing said sixth signal and said seventh signal and establishing a eighth signal responsive to the difference between said sixth signal and said seventh signal;
• and
• adjusting the rate of flow at which the side draw product is drawn from said fractionating column in response to said eight signal to maintain the actual Reid Vapor Pressure of said side draw product stream substantially equal to the desired Reid Vapor Pressure of said side draw product stream.