Process for cracking low-quality feed stock and system used for said process

• US 5,580,443 A
• Filed: 05/12/1994
• Issued: 12/03/1996
• Est. Priority Date: 09/05/1988
• Status: Expired due to Term

• Current Assignee: Mitsui Chemicals Incorporated (Mitsui Chemicals)
• Original Assignee: Mitsui Petrochemical Industries, Ltd. (Mitsui Chemicals)
• Inventors: Yoshida, Masahiko, Iida, Tsukasa, Kitayama, Yutaka
• Primary Examiner(s): MYERS, HELANE ELEANORA
• Application Number:
  US08/241,676
• Time in Patent Office: 936 Days
• Field of Search: 585/648, 208/130, 208/132, 208/107
• US Class Current: 208/130
• CPC Class Codes: C10G 55/04 including at least one thermal cracking step
• CPC Class Codes: C10G 9/00 Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
• CPC Class Codes: C10G 9/14 in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
• CPC Class Codes: C10G 9/20 Tube furnaces

Abstract

In a process for thermally cracking a low-quality feed stock containing a considerable proportion of heavy fractions such as high-boiling fractions, and evaporation residual oil, and a system used therefor, in which the low-quality feed stock is withdrawn from the preheater of the thermal cracking furnace to separate and remove a required proportion of said heavy fractions and thereafter returned to subject the feed stock to further preheating and thermal cracking. The low-quality feed stock thermally cracked has a predetermined proportion of the heavy fractions removed, and coking in the thermal cracking system at various lines is avoided from occurring.

Claims

1. A process for thermally cracking a heavy natural gas liquid containing heavy fractions in a cracking furnace having a convection zone containing a first preheater and a second preheater and a radiation zone containing a thermally cracking reactor comprising the steps of:

supplying said heavy natural gas liquid to the first preheater provided in a top portion of said convection zone of the cracking furnace;

preheating the supplied heavy natural gas liquid to a temperature and pressure at which fractions unlikely to cause coking problems are fully evaporated and fractions likely to cause coking problems are maintained in a liquid phase, in the first preheater and maintaining an exit temperature thereof in the range of from 200°

to less than 300°

C. and a gas/liquid ratio of the preheated heavy natural gas liquid in the range of from 60/40 to 98/2;

withdrawing the preheated heavy natural gas liquid from said first preheater;

superheating a suitable amount of dilution steam at a temperature higher than that of the preheated heavy natural gas liquid;

mixing a required amount of superheated dilution steam with the heavy natural gas liquid from said first preheater through a connection line between said first preheater and a gas-liquid separator to obtain an adjusted gas/liquid ratio of the heavy natural gas liquid, corresponding to the properties of differing heavy natural gas liquid feedstocks, to result in a predetermined gas/liquid separation in said gas-liquid separator;

supplying said mixture of said superheated dilution steam and said heavy natural gas liquid, having an adjusted gas/liquid ratio, to said gas-liquid separator;

separating and removing the heavy fractions from the withdrawn heavy natural gas liquid in said gas-liquid separator;

mixing the heavy-fraction-removed heavy natural gas liquid from the separator with a remaining amount of the superheated dilution steam necessary for thermal cracking;

passing said heavy-fraction-removed heavy natural gas liquid from the gas-liquid separator to said second preheater provided in said convection zone of the cracking furnace;

preheating the passed heavy-fraction-removed heavy natural gas liquid to a temperature below the temperature at which cracking is promoted;

introducing the preheated heavy-fraction-removed heavy natural gas liquid into the thermally cracking reactor; and

thermally cracking the introduced heavy-fraction-removed heavy natural gas liquid in said thermally cracking reactor.

2. The process for thermally cracking a heavy natural gas liquid according to claim 1 wherein a pressure of said gas-liquid separation is in a range of from 2 to 12 kg/cm² G.

3. The process for thermally cracking a heavy natural gas liquid according to claim 1 wherein said heavy natural gas liquid is fully preheated to a temperature below the temperature at which the thermal cracking is promoted in the second preheater.

4. The process for thermally cracking a heavy natural gas liquid according to claim 1, wherein said gas/liquid ratio of the preheated heavy natural gas liquid is adjusted in a range of from 70/30 to 95/5.