Steam cracking, also referred to as pyrolysis, is used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that 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 heated and at least partially 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 chemistry primarily takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Olefin gas cracker systems are normally designed to crack ethane, propane and on occasion butane, but typically lack the flexibility to crack heavier liquid feedstocks, particularly those that produce tar in amounts greater than one percent. As gas feeds tend to produce little tar, primary, secondary, and even tertiary transfer line exchangers (TLEs) are utilized to recover energy through the generation of high pressure and medium pressure steam, as the furnace effluent cools from the furnace outlet to the quench tower inlet. The process gas is normally then fed to a quench tower wherein the process gas is further cooled by direct contacting with quench water.
Conventional steam cracking systems have also been effective for cracking high-quality liquid feedstocks which contain fully volatile hydrocarbons, such as gas oil and naphtha. Cracked effluent from furnaces processing these feeds can also be quenched in at least a primary TLE, although for heavier naphthas and all gas-oil feeds a secondary oil quench is often required downstream of the primary TLE. The process effluent from such furnaces is normally fed to a primary fractionator where heavy hydrocarbons are removed and a light hydrocarbon stream is passed to downstream units for further processing.
However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipe still bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 595° C. (1100° F.). The non-volatile components of these feedstocks gradually lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. To crack feeds containing significant amounts of non-volatile material it is necessary to pass the partially preheated feed through a vapor-liquid separator, preferably at a temperature below that at which all the volatile hydrocarbons vaporize. Furnaces employing such a vapor-liquid separator are described in U.S. Pat. No. 7,138,047 and U.S. Patent Publication No. 2005/0209495 A1.
Cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which leads to rapid coking in the radiant section and quench section of the furnace, leading to frequent feed interruptions to enable coke removal. This process is known as decoking. However, even the cracking of light gas feedstocks may result in the deposition of coke on the inside surfaces of the radiant coils and the need for periodic decoking.
Within the industry the normal method of removing coke from the radiant and quench systems of a cracking furnace is steam-air-decoking. During this process hydrocarbon feed is interrupted to the furnace and steam passes through the furnace. The furnace effluent is redirected from the recovery section of the olefins plant to a decoking system. Air is added to the steam passing through the furnace and the heated air/steam mixture removes the coke deposits by controlled combustion. While steam-air-decoking is effective at removing coke deposits from the radiant coil and quench systems of cracking furnaces, it has the drawback of requiring a complete cessation of olefins production from the furnace for the duration of the decoking process.
U.S. Pat. No. 3,365,387 proposes a process for removing coke from cracking furnace tubes by passing through one or more tubes a steam and/or water feed to decoke those tubes, while maintaining the furnace on stream. The steam and/or water feed is substituted for the hydrocarbon feed stock at the point where the hydrocarbon feedstock is introduced into the furnace. The advantage of this process over steam-air decoking is that the sections of the furnace not being decoked continue to produce olefins product and since no air or oxygen is added to the process, the furnace effluent does not need to be directed away from the recovery section of the olefins plant. This process, referred to as “on-stream decoking,” therefore has the advantage of generating less variation in the olefins production rate of a given plant furnace section and also generates a lower workload for the plant operators since redirection of the entire furnace effluent is not required.
U.S. Pat. No. 3,557,241 proposes a process for removing coke from cracking furnace tubes by passing through at least one tube or tubes a decoking feed of steam and/or water and hydrogen, while maintaining the furnace on stream and continuing the thermal cracking process in tubes that are not being decoked. The steam and/or water and hydrogen feed is substituted for the hydrocarbon feed stock at the point where the hydrocarbon feedstock is introduced into the furnace.
While the two afore-mentioned patents describe on-stream-decoking techniques that eliminate many of the disadvantages of steam-air-decoking, the substitution of the steam/water mixture at the point where the feedstock is introduced to the furnace generates some drawbacks. Because the on-stream-decoking stream is introduced at the point where feed is introduced into the furnace, it must pass through the entire convection section process heating coils or banks. If the decoking stream is steam alone, then the temperature leaving the convection section and entering the radiant section of the furnace (known as the crossover temperature) is beyond the capacity of the materials commonly used in this section of the furnace. To keep the crossover temperature within the capacity of commonly used materials, it is necessary to add water to the steam. The presence of excessive water however, can generate mechanical problems if the water stratifies and runs along the bottom section of the heated convection tubes. Such phenomena can cause the tubes to bow, which restricts their free expansion and contraction within the tube supports (also known as tubesheets) within the convection section. Additionally, the requirement to add water to the decoking steam adds to the complexity of the pipe work and control system required on the furnace. What is desired is an on-stream decoking process and furnace design that does not require the use of water to keep convection section and crossover temperatures within the limitations of commonly used materials.
Despite advances in the art, there is a need for an improved process for the on-stream decoking of a pyrolysis furnace.