High performance heat exchangers

#This invention relates generally to means for improving the service-life and reliability of indirect heat exchangers used in chemical reaction systems which produce high temperature, reactive effluent gasses. In particular, this invention relates to means for improving service-life of indirect heat exchangers used in the production of hydrogen cyanide.

#In many chemical processes, the reaction effluent comprises hot, reactive, and/or abrasive fluids and/or gasses. For many of these reactions, it is desirable to rapidly quench the reactor effluent to prevent decomposition of the product component(s). Quenching may be accomplished through direct contact, such as application of water sprays, or more commonly through indirect methods such as through the use of indirect heat exchangers. Because indirect heat exchangers provide the added advantage that they may be configured to recover waste heat, these are the more preferred method and have been in use for many years. Typical indirect heat exchangers used in such chemical processes consist of a shell and tube design.

#In many systems, the thermal, kinetic and reactive properties of the effluent may individually or collectively serve to corrode, crack, or otherwise degrade the materials used to form the heat exchange zone. In particular, the exchange tubes and the tube-to-tubesheet welds nearest to the reaction zone see the most severe conditions and are most susceptible to degradation. For example, the hot effluent may chemically react with the metal of the heat exchange tube itself, thereby causing erosion and/or corrosion, that is, metal dusting, carburization and the like, all of which lead to failure of the heat exchanger. The weld area of the heat exchange tubes is also susceptible to stress corrosion cracking, which leads to failure of the heat exchanger. These problems may be encountered, for example, in the production of hydrogen cyanide or acrylonitrile, in nitric acid waste heat recovery exchangers, in hydrocarbon cracking units, and in tubeside fired boilers and exchangers.

#FIG. 1a shows a schematic cross-section of a typical exchange tube to tubesheet weld used in hydrogen cyanide reactors having an exaggerated annular space.

#FIG. 1b shows a schematic cross-section of a rolled exchange tube to a tubesheet connection having an exaggerated annular space.

#FIG. 2a shows a schematic cross-section of a tubesheet and exchange tube affixed by a down-hole weld having an exaggerated annular space.

#As used throughout the specification, the following terms shall have the following meanings, unless the context clearly indicates otherwise. The terms “heat exchange units” and “heat exchange vessels” are used interchangeably. The terms “first” and “second” and “upper” and “lower” and “entry and exit”, respectively, are used interchangeably, though it is recognized that the reactor may be in a vertical, horizontal, or other configuration in which the terms “upper” and “lower” would not be apt to describe the relation of components to one another. The term “entry” refers to that portion of the heat exchange apparatus that is nearest to the reaction zone, that is, where hot process gasses enter the heat exchange apparatus. The term “exit” refers to that portion of the heat exchange apparatus that is furthest from the reaction zone, that is, where hot process gasses exit the heat exchange apparatus. The following abbreviations are used throughout the specification: C=centigrade and cm=centimeter. All ratios and amounts are by weight, unless otherwise noted. All numerical ranges are inclusive, unless otherwise noted.

#The present invention provides a solution to the problem of short service-life of shell and tube heat exchangers through the use of specific ferrules and/or welds. Preferably, the design of the ferrules of the present invention further serves to minimize the potentially damaging effects of the reactor effluent stream. In addition, the design of the heat exchanger and the use of down-hole welds in affixing the exchange tubes to the tubesheet of the apparatus simplify construction, maintenance and repair of the unit, minimizing downtime and increasing operational efficiency. The down-hole weld also eliminates the annular space between the exchange tube and the tubesheet which can serve as a crevice to concentrate corrosive agents. Thus the present invention is useful in extending the service-life of any shell and tube heat exchanger. The ferrules and welds of the present invention are particularly useful in shell and tube heat exchangers that are exposed to hot, reactive effluent gasses, and preferably where the heat exchanger is exposed to a reducing, carburizing or nitridizing environment, such as in hydrogen cyanide production.

#There are numerous heat exchanger designs suitable for use with reactors producing reactive, high temperature, high abrasion effluent streams. Particularly in connection with hydrogen cyanide production, the need for quenching the hot process gasses has long been appreciated. By quenching the hot process or effluent gasses is meant cooling the gasses sufficiently such that the products contained therein do not degrade. For example, it is well known in hydrogen cyanide production that if the hot process gasses are not cooled sufficiently, the hydrogen cyanide present will degrade.