Process and Apparatus for Cooling Liquid Bottoms from Vapor-Liquid Separator by Heat Exchange with Feedstock During Steam Cracking of Hydrocarbon Feedstocks

Abstract

A process and apparatus for cracking liquid hydrocarbon feedstocks utilizes a vapor-liquid separator to treat heated vapor-liquid mixtures to provide an overhead of reduced residue content. Hot liquid bottoms from the separator are heat exchanged with cool hydrocarbon feedstocks for cracking to provide cooled liquid bottoms and preheated feedstock. At least a portion of the preheated feedstock is directed to a convection section of a pyrolysis furnace for additional heating and subsequent cracking.

1 . A process for cooling liquid bottoms from a vapor-liquid separator used in separating heated hydrocarbon feedstock that is to be cracked, which comprises:
 i) indirectly heat exchanging hot liquid bottoms with a cool hydrocarbon feedstock to provide cooled liquid bottoms and preheated hydrocarbon feedstock; and   ii) directing at least a portion of the preheated hydrocarbon feedstock to a convection section inlet of a pyrolysis furnace comprising a) a convection section having at least one convection zone, and b) a radiant section.   
   
   
       2 . The process of  claim 1  wherein the hot liquid bottoms range from 200° C. to 500° C., the cool hydrocarbon feedstock contains resid and ranges from 20° C. to 200° C., the cooled liquid bottoms range from 50° C. to 350° C., and the preheated hydrocarbon feedstock ranges from 50° C. to 300° C. 
   
   
       3 . The process of  claim 1  wherein the hot liquid bottoms range from 250° C. to 350° C., the cool hydrocarbon feedstock ranges from 50° C. to 150° C., the cooled liquid bottoms range from 70° C. to 300° C., and the preheated hydrocarbon feedstock ranges from 70° C. to 200° C. 
   
   
       4 . The process of  claim 1  which further comprises:
 iii) introducing diluent steam to the preheated hydrocarbon feedstock; and, optionally,   iv) introducing a liquid diluent to the preheated hydrocarbon feedstock.   
   
   
       5 . The process of  claim 4  wherein the introduced liquid diluent is reduced as a function of the extent to which the hydrocarbon feedstock is preheated. 
   
   
       6 . The process of  claim 5  wherein the liquid diluent comprises water. 
   
   
       7 . The process of  claim 6  wherein the introduced liquid diluent is substituted for the steam diluent in an amount sufficient to maintain a substantially constant temperature in the vapor-liquid separator. 
   
   
       8 . The process of  claim 7  wherein the hydrocarbon feedstock includes a nominal final boiling point of at least 315° C. (600° F.). 
   
   
       9 . A process for cracking a hydrocarbon feedstock in a pyrolysis furnace comprising a convection section having at least one convection zone, and a radiant section, the process comprising:
 (a) introducing a preheated hydrocarbon feedstock for further heating in a convection zone;   (b) mixing the preheated hydrocarbon feedstock with dilution steam and, optionally, a dilution liquid to form a mixture stream;   (c) separating the mixture stream in a vapor-liquid separator to form I) a vapor phase and II) a liquid phase, relative to the heated mixture stream;   (d) removing the vapor phase as overhead and the liquid phase as hot bottoms from the vapor-liquid separator;   (e) cooling the hot bottoms in a heat exchanger outside the pyrolysis furnace by indirect heat exchange with a hydrocarbon feedstock to provide at least a portion of the preheated hydrocarbon feedstock;   (f) cracking the vapor phase in the radiant section of a pyrolysis furnace to produce an effluent comprising olefins; and   (g) recovering cracked product from the effluent comprising olefins.   
   
   
       10 . The process of  claim 9  wherein the mixture stream is heated in an additional convection zone prior to its introduction to the vapor-liquid separator. 
   
   
       11 . The process of  claim 9  wherein the vapor phase is heated in an additional convection zone prior to cracking. 
   
   
       12 . The process of  claim 9  wherein the at least one convection zone comprises a tube bank heat exchanger. 
   
   
       13 . The process of  claim 9  which further comprises quenching the effluent comprising olefins from cracking, prior to recovering cracked product. 
   
   
       14 . The process of  claim 9  which further comprises recycling a portion of the cooled liquid bottoms to the vapor-liquid separator. 
   
   
       15 . The process of  claim 9  wherein the dilution liquid is water. 
   
   
       16 . The process of  claim 9 , wherein the hydrocarbon feedstock contains resid and is selected from the group consisting of steam cracked gas oil/residue admixtures, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, Fischer-Tropsch gases, natural gasoline, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric residue, heavy residue, hydrocarbon gas/residue admixtures, hydrogen/residue admixtures, C 4 's/residue admixtures, naphtha/residue admixtures and gas oil/residue admixtures. 
   
   
       17 . The process of  claim 9  which further comprises at least one of:
 (h) heat exchanging high pressure boiler feed water with hot flue gas in a convection zone to provide preheated high pressure boiler feed water;   (i) heat exchanging dilution steam with hot flue gas in a convection zone to provide heated dilution steam;   (j) optionally adding desuperheater water to the heated dilution steam;   (k) adding the heated dilution steam and optional desuperheater water to the mixture stream of (b);   (l) heat exchanging superheated high pressure steam with hot flue gas in the convection section to provide heated superheated high pressure steam;   (m) adding desuperheater water to the heated superheated high pressure steam to provide a stream of superheated high pressure steam at reduced temperature; and   (n) heat exchanging the stream of (m) with hot flue gas in a convection zone to provide superheated high pressure steam at a temperature suitable for export to a header.   
   
   
       18 . The process of  claim 9  wherein the liquid bottoms from the vapor-liquid separation apparatus range from 250° C. to 350° C., the cooled liquid bottoms range from 90° C. to 260° C., the hydrocarbon feedstock ranges from 70° C. to 150° C., and the preheated hydrocarbon feedstock ranges from 90° C. to 200° C. 
   
   
       19 . The process of  claim 9  wherein the dilution liquid used to form the mixture stream of (b) is reduced as a function of the extent to which the hydrocarbon feed is preheated. 
   
   
       20 . The process of  claim 9  wherein the dilution liquid is substituted for the dilution steam in an amount sufficient to maintain a substantially constant temperature in the vapor-liquid separator. 
   
   
       21 . The process of  claim 9  wherein the hydrocarbon feedstock includes a nominal final boiling point of at least 315° C. (600° F.). 
   
   
       22 . An apparatus for cracking a preheated hydrocarbon feedstock in a pyrolysis furnace comprising a) a convection section having at least one convection zone and b) a radiant section, the apparatus comprising:
 (1) an inlet for receiving preheated hydrocarbon feedstock;   (2) a convection zone for heating the preheated hydrocarbon feedstock;   (3) at least one inlet for introducing steam, or steam and water, to the heated hydrocarbon feedstock to form a mixture stream;   (4) a vapor-liquid separator for treating the mixture stream to form I) a vapor phase and II) a liquid phase, the separator further comprising an overhead outlet for substantially removing the vapor phase as overhead and a liquid outlet for substantially removing the liquid phase as heated bottoms from the vapor-liquid separator;   (5) a heat exchanger, external to the pyrolysis furnace, for cooling the vapor-liquid separator bottoms by indirect heat exchange, comprising an inlet for receiving heated bottoms from the separator, an outlet for withdrawing cooled bottoms, an inlet for receiving cool hydrocarbon feedstock as a heat exchange medium to the heat exchanger, and an outlet for withdrawing preheated hydrocarbon feedstock;   (6) a line for directing at least a portion of the withdrawn preheated hydrocarbon feedstock to the inlet for receiving preheated hydrocarbon feedstock; and   (7) a radiant section for cracking the heated vapor phase to produce an effluent comprising olefins.   
   
   
       23 . The apparatus of  claim 22  which further comprises at least one of:
 (8) a convection zone for heating the mixture stream;   (9) a convection zone for heating the overhead to provide a heated vapor phase;   (10) a quench exchanger for cooling the effluent comprising olefins; and   (11) a recovery train for recovering light olefins from the effluent comprising olefins.   
   
   
       24 . The apparatus of  claim 22  wherein the at least one convection zone comprises at least one heat exchanger tube bank. 
   
   
       25 . The apparatus of  claim 22  which further comprises at least one of:
 (12) a heat exchanger for preheating high pressure boiler feed water with hot flue gas in a convection zone to provide preheated high pressure boiler feed water;   (13) a heat exchanger for heating dilution steam with hot flue gas in a convection zone to provide heated dilution steam;   (14) an inlet for adding desuperheater water to the heated dilution steam;   (15) an inlet for adding the heated dilution steam to the mixture stream of (3);   (16) an inlet for adding desuperheater water to the mixture stream of (3);   (17) a heat exchanger for superheating high pressure steam with hot flue gas in a convection zone to provide heated superheated high pressure steam;   (18) an inlet for adding desuperheater water to the heated superheated high pressure steam to provide a stream of superheated high pressure steam at reduced temperature; and   (19) a heat exchanger for heating the mixture of heated superheated high pressure steam and desuperheater water with hot flue gas in a convection zone to provide superheated high pressure steam at a temperature suitable for export to a header.