Method And System For Removal Of Foulant Precursors From A Recycle Stream Of An Olefins Conversion Process

PATENT
Method And System For Removal Of Foulant Precursors From A Recycle Stream Of An Olefins Conversion Process
United States Patent Application 20120253092
Inventors:
Van Egmond, Cornelis F. (PASADENA, TX, US)
Leung, David (SUGAR LAND, TX, US)
Application Number:
13/076863
Publication Date:
10/04/2012
Filing Date:
03/31/2011
Assignee:
Stone & Webster Process Technology, Inc. (Houston, Tx, Us)
Abstract:
A post fractionation process for removing heavy hydrocarbons from the C₄⁺ olefins conversion process reactor effluent, which act as foulants when recycled to the C₄⁺ olefins conversion reactor. This simple and effective process improves the run length of the reactor by reducing catalyst fouling, which also improves yields in a C₄⁺ olefins conversion process to light olefins. Essential to present invention is the efficient recycling of a hydrocarbon stream to the reactor, utilizing well proven equipment in a novel way to separate more valuable product from less desirable components in the recycle product stream.
Description:

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a post fractionation system and process for removing reactor effluent foulant in a hydrocarbon process stream. More particularly, the invention provides a system and process for improved olefins conversion by removing foulant from an olefins conversion rector feed stream.

Hydrocarbons are feedstocks for petrochemical industries. Olefins, diolefins, and paraffins are useful for preparing a wide variety of petrochemicals, especially light hydrocarbons, for example, ethylene, propylene, C4 hydrocarbons (i.e., mixtures consisting of butanes, butylenes and butadienes) and heavier hydrocarbons, such as C5 hydrocarbons and gasolines (generally, C6+ hydrocarbons). These petrochemicals are typically produced by cracking petroleum feeds. A large number of methods described in the literature are directed to the production of olefins, such as steam pyrolytic cracking or catalytic cracking in processes such as fluid catalytic cracking (FCC) and deep catalytic cracking (DCC). With the high cost of suitable feedstocks for producing olefins, there is an increasing demand for energy efficient and lower capital production methods. Key to project economics is also reducing the loss of valuable reaction products during subsequent purification steps.

A variety of methods are available for producing light hydrocarbon streams, such as, steam cracking, fluid catalytic cracking, deep catalytic cracking, catalytic naphtha cracking and the conversion of methanol to olefins (MTO). These methods generate light hydrocarbons and heavy hydrocarbons.

Currently, C4+ olefins conversion processes are finding greater application in areas of the world where end users are trying to produce more propylene from steam cracking and/or refining operations. Industry data indicate the demand for propylene production is strong and is expected to remain strong over the next 10 years in areas, such as, South America, China and South East Asia.

The prior art is replete with conventional C4+ olefins conversion process methods to convert olefins to ethylene, propylene and the like. These methods include for example the use various types of zeolite catalyst to produce, inter alia, aromatic hydrocarbons and ethylene and propylene; methods of conversion of butene to ethylene and propylene; and methods of converting an olefin of 4-12 carbon atoms to ethylene and propylene. Included among these many methods for producing ethylene and propylene is a process for recycling a fraction comprising C4 and higher hydrocarbons. However, in these processes, the C4 and higher hydrocarbons are recycled to the reactor without removal of heavy hydrocarbons.

Previous attempts to improve ethylene and propylene yields by recycling hydrocarbon compounds of 4 or more carbon atoms, without removing heavy materials has been attempted. See, for example, U.S. Pat. Nos.: 5,026,935 and 5,043,522 to Leyshon et al.; and 7,754,934 to Takashi et al. However, adverse effects, such as, accelerated deposition of carbonaceous materials (coking) on the surface of the catalyst and equipment resulting in deterioration in catalytic activity and shorten the reactor cycle time decrease production efficiencies.

Prior art recycling of C4+ , which includes C5 and higher hydrocarbons, contains materials that act as “foulants” to the conversion process. Foulants, such as, C6+ hydrocarbon materials have a predisposition for creating coke in the feed vaporization equipment feeding the reactor and the catalyst in the reactor. As such, these materials shorten the reactor cycle time, and catalyst life as well as reducing the benefits of the downstream processing units.

Accordingly, a need still exists for an efficient and effective system and process for improving the yields in a C4+ olefins conversion process.

SUMMARY OF THE INVENTION

The present invention is directed to a process for recovering olefins from a hydrocarbon feedstream, the process comprises: (i) reacting a hydrocarbon feedstream in a reactor to produce a reactor effluent containing hydrogen and C1 to C4+ hydrocarbons; (ii) separating the reactor effluent to obtain a C3− hydrocarbons overhead stream and a C4+ hydrocarbons bottoms stream; (iii) separating the C4+ hydrocarbons bottoms stream in a separator to obtain a C4-C5 hydrocarbon overhead recycle stream and C6+ hydrocarbons bottoms stream for processing downstream; and (iv) recycling the C4-C5 overhead recycle stream to the reactor of step (i), wherein said C4-C5 overhead recycle stream is substantially free of C6+ hydrocarbons.

Further, the present invention is directed to a system for recovering olefins from hydrocarbons comprising, sequentially connected, a hydrocarbon feed, a reactor for reacting said hydrocarbon feed to produce a reactor effluent containing C1 to C4+ hydrocarbons, at least one separator for separating C3− hydrocarbons from C4+ hydrocarbons and at least one separator for simultaneously separating the C4+ hydrocarbons into a C4-C5 hydrocarbons recycle stream and a C6+ hydrocarbon stream for processing downstream, and recycling the C4-C5 hydrocarbons recycle stream to the reactor for cracking with the hydrocarbon feed, wherein said C4-C5 hydrocarbons recycle stream is substantially free of C6+ hydrocarbons.

The process and system of the present invention improves the quality of the reactor effluent stream that would otherwise shorten the reactor cycle time and catalyst life as well as reducing the benefits of the downstream processing units. The inventive process and system not only increase product yields, but extend catalyst life, lengthen the time intervals between decoking of the reactor, and minimizing equipment sizes and capital investment.
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