Control of a cracking furnace

A method for controlling a cracking furnace in which a feed stream is cracked to produce a gaseous mixture, wherein the crackable components of said feed stream consist essentially of ethane and propane, wherein ethane is the principal component of said feed stream, and wherein said gaseous mixture contains ethane, propane, ethylene and propylene, said method comprising the steps of:

• (a) analyzing said feed stream to determine the concentration (X2) of ethane in said feed stream;
• (b) analyzing said feed stream to determine the concentration (X3( of propane in said feed stream;
• (c) analyzing said gaseous mixture to determine the concentration (Z2) of ethane in said gaseous mixture;
• (d) estimating the conversion (Y2) of ethane in said cracking furnace;
• (e) calculating the conversion (Y3) of propane in said cracking furnace based on the value of Y2 and simultaneous conversion data;
• (f) calculating the expansion of ethane in said cracking furnace based on the value of Y2 and expansion versus conversion data;
• (g) calculating the expansion (A3) of propane in said cracking furnace based on the value of Y3 and expansion versus conversion data;
• (h) calculating the expansion (A) of said feed stream in said cracking furnace based on the value of X2, X3, A2 and A3;
• (i) calculating a conversion (Y2E) of ethane in said cracking furnace based on the value of A, Z2 and X2;
• (j) calculating a new estimated conversion of ethane in said cracking furnace based on the value of Y2 and Y2E and establishing a first signal representative of the new estimated conversion;
• (k) establishing a second signal representative of the desired conversion for ethane;
• (1) comparing said first signal and said second signal and establishing a third signal which is responsive to the difference between said first signal and said second signal;
• and
• (m) manipulating the heat supplied to said cracking furnace in response to said third signal to thereby maintain said first signal substantially equal to said second signal.

Apparatus comprising:

• a cracking furnace means;
• means for supplying a feed stream to said cracking furnace means;
• means for removing a gaseous mixture, containing the cracked and uncracked components of said feed stream, from said cracking furnace means;
• means for analyzing said feed stream and for establishing a first signal representative of the concentration of a first component in said feed stream;
• means for analyzing said gaseous mixture and for establishing a second signal representative of the concentration of said first component in said gaseous mixture:
• means for establishing a third signal which is representative of the conversion of said first component in said cracking furnace means based on the value of said first signal, the value of said second signal, an estimate of the conversion of said first component in said cracking furnace means, and a calculation of the expansion of said feed stream in said cracking furnace means;
• means for establishing a fourth signal representative of the desired conversion of said first component;
• means for comparing said third signal and said fourth signal and for establishing a fifth signal which is responsive to the difference between said third signal and said fourth signal;
• and
• means for manipulating the heat supplied to said cracking furnace means in response to said fifth signal to thereby maintain said third signal substantially equal to said fourth signal.

Apparatus comprising:

• a cracking furnace means;
• means for supplying a feed stream to said cracking furnace means, wherein the crackable components of said feed stream consist essentially of ethane and propane and wherein ethane is the principal component of said feed stream;
• means for removing a gaseous mixture, containing ethane, propane, ethylene and propylene, from said cracking furnace means;
• means for analyzing said feed stream and for establishing a first signal representative of the concentration of ethane in said feed stream and a second signal representative of the concentration of propane in said feed stream;
• means for analyzing said gaseous mixture and for establishing a third signal representative of the concentration of ethane in said gaseous mixture;
• means for establishing a fourth signal which is representative of the conversion of ethane in said cracking furnace means based on the value of said first signal, the value of said second signal, the value of said third signal, an estimate of the conversion of ethane in said cracking furnace means, and a calculation of the expansion of said feed stream in said cracking furnace means;
• means for establishing a fifth signal representative of the desired conversion of ethane;
• means for comparing said fourth signal and said fifth signal and for establishing a sixth signal which is responsive to the difference between said fourth signal and said fifth signal;
• and
• means for manipulating the heat supplied to said cracking furnace means in response to said sixth signal to thereby maintain said fourth signal substantially equal to said fifth signal.