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Process and apparatus for steam cracking hydrocarbon feedstocks

Process and apparatus for steam cracking hydrocarbon feedstocks description/claims

This application claims benefit of and priority to International Patent Application Serial Number PCT/US2007/018486, filed Aug. 21, 2007.

This disclosure pertains to a method for the manufacture of light olefins in a steam cracking furnace or a pyrolysis furnace, more particularly to a process of steam cracking a hydrocarbon feedstock containing at least 0.01 wt. % low-volatile compounds.

Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a steam cracking furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the steam cracking furnace for further downstream processing.

Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as natural gas liquids, (NGL's), gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, condensates, which is an associated oil occurring in small quantities with the production of gas from gas fields, crude oils, atmospheric resids, also known as atmospheric pipestill bottoms, and vacuum gas oils, crude oil, vacuum gas oil and atmospheric resid contain high molecular weight, low-volatile components with boiling points in excess of 590° C. and/or sometimes coke precursors with boiling points in excess of 760° C. The low-volatile components and/or coke precursors of these feedstocks would lay down as coke in the convection section of conventional steam cracking furnaces as the lighter components were vaporized. Only very low levels of low-volatile components and coke precursors can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized because the coke deposition normally fouls tubes in convection section which lowers the heat transfer efficiency and increase the pressure drop in the tubes.

Additionally, some naphthas are contaminated with heavy crude oil containing low-volatile components and coke precursors. Conventional steam cracking furnaces do not have the flexibility to process residues, crude oils, or many residue- or crude-contaminated gas oils or naphthas which are contaminated with low-volatile components and coke precursors.

To address coking problems, U.S. Pat. No. 3,617,493, incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. The vapors are cracked in the steam cracking furnace into olefins, and the separated liquids from the second flash tank are removed, stripped with steam, and used as fuel.

U.S. Pat. No. 3,718,709, incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.

U.S. Pat. No. 5,190,634, incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.

U.S. Pat. No. 5,580,443, incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a vapor-liquid separator to separate and remove a required proportion of the low-volatile components and coke precursors as liquid from the separator. The separated vapor from the vapor-liquid separator is returned to the pyrolysis furnace for heating and cracking.

U.S. Pat. No. 6,632,351, incorporated herein by reference, discloses a process for pyrolyzing a crude oil feedstock or crude oil fractions containing pitch feedstock, and a pyrolysis furnace, comprising feeding the crude oil or crude oil fractions containing pitch feedstock to a first stage preheater within a convection zone, wherein said crude oil or crude oil fractions containing pitch feedstock is heated within the first stage preheater to an exit temperature of at least 375° C. to produce a heated vapor-liquid mixture, withdrawing from first stage preheater the vapor-liquid mixture to a vapor-liquid separator, separating and removing the gas from the liquid in the vapor-liquid separator, and feeding the removed gas to a second preheater provided in the convection zone, further heating the temperature of said gas to a temperature above the temperature of the gas exiting the vapor-liquid separator, introducing the preheated gas into a radiant zone within the pyrolysis furnace, and pyrolyzing the gas to olefins and associated by-products.

U.S. Pat. No. 7,097,758, incorporated herein by reference, discloses a process to increase the non-volatile removal efficiency in a flash drum in the steam cracking system. The gas flow from the convection section is converted from mist flow to annular flow before entering the flash drum to increase the removal efficiency. The conversion of gas flow from mist flow to annular flow is accomplished by subjecting the gas flow first to at least one expander and then to bends of various degrees and forcing the flow to change directions at least once. The change of gas flow from mist to annular helps coalesce fine liquid droplets and thus increases the efficiency with which they are removed from the vapor phase.

U.S. Pat. No. 7,138,047, incorporated herein by reference, discloses a process for feeding or cracking hydrocarbon feedstock containing non-volatile hydrocarbons comprising: heating the hydrocarbon feedstock, mixing the hydrocarbon feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase, and varying the amount of the fluid and/or the primary dilution steam stream mixed with the hydrocarbon feedstock in accordance with at least one selected operating parameter of the process, such as the temperature of the flash stream before entering the flash drum.

U.S. patent application Ser. No. 11/068,615, filed Feb. 28, 2005, incorporated herein by reference, describes a process for cracking hydrocarbon feedstock which mixes hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, and the product effluent cooled in a transfer line exchanger, wherein the amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed.

U.S. application Ser. No. 10/851,434, filed May 21, 2004, incorporated herein by reference, and U.S. Provisional Application Ser. No. 60/573,474, filed May 21, 2004, incorporated herein by reference, describe a process to increase the non-volatile removal efficiency in a flash drum used in a steam cracking system, the flash drum having a lower boot comprising an inlet for introducing stripping steam, a ring distributor for recycle quench oil, anti-swirl baffles, and a grate.

There is therefore a need for a novel and energy efficient process for steam cracking hydrocarbon feedstocks with low level of coke formation. The present inventors surprisingly find that the coke formation in the first preheating section of the first preheating zone is negligible when the feedstock is fed at an inlet linear velocity below a threshold value and the feedstock is preheated to a temperature below a threshold value. Furthermore, the coke formation in the first preheating zone is minimized so long as at least 1 wt. % of the hydrocarbon feedstock exits the first preheating zone in liquid phase. This disclosure therefore offers a steam cracking process capable of processing hydrocarbon feedstock with minimum coke formation in the preheating zone and low pressure drop for the feedstock to flow through the convection section by optimizing the linear velocity of the feedstock entering the preheating section of a steam cracking furnace.

In some embodiments, the present disclosure provides a process for treating a hydrocarbon feedstock comprising:

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