Alumina Forming Bimetallic Tube And Method Of Making And Using

PATENT
Alumina Forming Bimetallic Tube And Method Of Making And Using
United States Patent Application 20120097289
Inventors:
Chun, Changmin (Annandale, NJ, US)
Deutsch, Samuel D. (Flemington, NJ, US)
Feather, James E. (Fairfax, VA, US)
Jones, Jeffrey P. (Houston, TX, US)
Spicer, David B. (Houston, TX, US)
Application Number: 13/271808
Publication Date: 04/26/2012
Assignee: EXXONMOBIL RESEARCH AND ENGINEERING COMPANY (Annandale, NJ, US)
Abstract:
Provided is a bimetallic tube for transport of hydrocarbon feedstocks in a petrochemical process unit and/or refinery process unit, including: i) an outer tube layer being formed from a steam cracker alloy including at least 18.0 wt. % Cr and at least 10.0 wt. % Ni; ii) an inner tube layer being formed from an alumina forming bulk alloy including 5.0 to 10.0 wt. % of Al, 18.0 wt. % to 25.0 wt. % Cr, less than 0.5 wt. % Si, and at least 35.0 wt. % Fe with the balance being Ni, wherein the inner tube layer is formed plasma powder welding the alumina forming bulk alloy on the inner surface of the outer tube layer; and iii) an oxide layer formed on the surface of the inner tube layer, wherein the oxide layer is substantially comprised of alumina, chromia, silica, mullite, spinels, or mixtures thereof.

FIELD
The present disclosure provides for the composition of, methods of making and methods of using bimetallic tubes for fired heater tubes and/or transfer line exchangers for the transport of hydrocarbon feedstocks in petrochemical process and/or refinery process units in order to reduce corrosion, coking and fouling.

BACKGROUND
Petrochemical Processes

In petrochemical processes, ethylene is the lightest olefinic hydrocarbon and represents the largest building block for a variety of petrochemical products such as plastics, resins, fibers, solvents, etc. Ethylene does not occur freely in nature and is produced primarily from the thermal cracking of hydrocarbon feedstocks derived from natural gas and crude oil. The conventional hydrocarbon feedstocks used for the production of ethylene include ethane, propane, butane, pentanes and naphthas. Naphtha cracking represents about 45% of world production capacity, whereas nearly 35% of capacity is produced from ethane cracking. Other possible feeds include refinery offgas, natural gasoline liquids, wide-boiling condensate fractions, atmospheric and vacuum gas oils, and hydrotreated or hydrocracked vacuum gas oils.

The thermal cracking of hydrocarbon feedstocks, which is the main route to ethylene production, is carried out in tubular coils located in the radiant zone of fired heaters. Steam is added to reduce the partial pressure of the hydrocarbons in the radiant coils. The reactions that result in the transformation of mostly saturated hydrocarbons to olefins are highly endothermic and require reactor temperature in the range of 750 to 1050° C. depending on the feedstock and design of the reactor coils. Thermal cracking reactions also produce valuable by-products, including propylene, butadiene, benzene, gasoline, and hydrogen. The on-stream availability of the thermal cracking reactor is it determined by coking of either the cracking coils or the cracked effluent transfer line exchangers (TLEs). Coke is produced from aromatic feed component as a side product of thermal cracking and deposits on the radiant coil walls and inside the tubes of TLEs. This limits the heat transfer and increases the pressure drop, thus reducing the olefin selectivity. The run length is normally determined by the tube metal temperature increase of the radiant coil, the outlet temperature of the TLE, or the increased pressure drop.

Coke is believed to be formed by two mechanisms—catalytic and polymerization. The metal surface of the cracker coil catalyzes the growth of a filamentary type of coke and contains metal particles. The second type of coke is formed by condensation, polymerization, and/or agglomeration of heavies in the gaseous phase. The coking behavior of various feedstocks differs in cracking coils and TLEs, and these can be influenced by contaminants in the feed, dilution steam, and coil surface. In addition to general coking in cracking coils and TLEs, ethane and propane cracking plants experience coking at the inlet (on the tubesheet) of the TLEs due to gas-phase reactions at higher inlet temperatures and to the discontinuities in flow distribution. This inlet coking normally results in high-pressure drop, limiting the run length. Liquid-feed cracking results in coking on the TLE tubes near the outlet. This is caused by the condensation of tarry materials, which form a thin, oily layer that gradually polymerizes.

Steam cracking heaters are a very important part of an ethylene plant. Thermal cracking of hydrocarbons takes place in tubular coils placed in the center of a fired radiant box. The cracked effluents leave the radiant coils at a temperature of 750 to 1050° C., depending on feedstock, cracking severity, and selectivity. In order to maintain the overall process efficiency, it is required to efficiently recover the heat in the cracked effluents. The cracked effluents also need to be quenched quickly to stop secondary reactions that result in yield degradation. This is achieved by the TLEs, which cool the furnace effluents to nearly 350 to 450° C. at clean conditions, and this heat is used to generate very high pressure steam (˜125 bar). The higher steam pressure results in higher tube metal temperature and therefore minimizes condensation of tarry materials.

The typical steam cracking heater consists of a convection section in the upper offset arrangement and a radiant section at the lower end. The vertical radiant coils are located close to the center plane of the radiant box and are suspended on a hanging system from the top of the radiant box. The hanging system allows the radiant coils to expand without causing any additional stresses on the radiant coils. The radiant coils are centrifugally cast from 25Cr/35Ni or 35Cr/45Ni alloys for their carburization and creep resistance. These materials have a maximum service temperature of up to 1150° C. The typical composition of radiant coil materials is shown in Table 1. The convection section recovers the flue-gas heat by preheating the hydrocarbon feedstocks and dilution steam. In addition, heat is recovered through boiler feed water preheating and superheating of very high pressure steam. The high temperature coil or part thereof is bare due to excessive tube metal temperatures, and normally all the other convection coils have fins to improve the heat-transfer coefficient.
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