Flow streamlining device for transfer line heat exchanges

#This invention relates to novel heat exchangers and to chemical processes involving their use. More particularly, this invention relates to new and improved indirect shell-and-tube heat exchangers of the type known as transfer line heat exchangers (TLEs), and to improved processes of quenching or recovering heat from high temperature fluids, and particularly high temperature gases, which involve their use. These novel TLEs are modified at their inlet ends in two respects in comparison to conventional TLEs by means which, taken together, can be characterized as a flow streamlining device. These modifications minimize or prevent inlet end fouling, which commonly occurs in conventional TLEs due to coke buildup resulting from condensation or precipitation, and then decomposition at high temperature, of tars, high polymers or other high molecular weight materials during processing. Their use also reduces the overall down time required to clean the inlet ends of the heat transfer tubes should inlet end fouling eventually occur.

#Transfer line heat exchangers are in widespread use in commercial chemical processing. In general, they operate to cool hot gases by passing these gases through a bundle of tubes in heat exchange relationship with a cooling fluid, such as water, passing around the outside, or shell side, of the tubes and contained within a defined area along the tubes by means of a pair of tubesheets which are generally perpendicular to the tubes contained within them. In certain processes, the heat removed from the process gas is sufficient to vaporize the fluid on the shell side. In such cases if water is used as the cooling fluid the heat exchanger also becomes a steam generator.

#TLEs are commonly used to cool very hot process gases. For example, they are used in processes for producing ammonia such as that disclosed in U.S. Pat. No. 3,442,613, issued May 6, 1969 to Grotz, to cool the approximately 850.degree. F. ammonia-containing gas exiting a syngas converter. They are also used in olefin plants and in other hydrocarbon cracking operations to recover usable heat from reactor gases, e.g., gases exiting pyrolysis furnace coils at temperatures above 1500.degree. F. To avoid secondary reactions leading to less valuable or useless products, the residence time spent by the exiting gases between the furnace coil outlet and the TLE inlet should be minimized. The pressure drop across the TLE should also be minimized, since cracking selectivity towards more useful products in the furnace is directly dependent on cracking-coil outlet pressure, and ordinarily a pressure rise of no more than a few p.s.i. at the furnace outlet is all that can be tolerated if process stability is to be maintained. A discussion of va ious TLE designs is found in Albright et al, "Pyrolysis Theory and Industrial Practice (New York: Academic Press, 1983), Chapter 18.

#FIG. 1 is an end view of a portion of the inlet end of a typical TLE, showing the ends of four tubes and the tube sheet impact area between them perpendicular to the direction of gas flow.

#FIG. 2 is an end view of a portion of the inlet end of a TLE drawn to the same dimensions as the TLE of FIG. 1 but partially modified in accordance with the invention, showing four tubes containing flared end means having no peaked gas guide means between them so as to illustrate the reduced tube sheet impact area between the inlets of the tubes.

#FIG. 3 is a plan view of a portion of the inlet end of a TLE modified in accordance with the invention, showing four tubes containing flared end means having a peaked gas guide means between them.

#I do not wish to be bound by any particular mechanism or theory advanced to explain the mode of operation of my novel flow streamlining device, the advantages obtainable therefrom, or the mechanism(s) of chemical reactions, physical phenomena or combinations thereof occurring in and around this device as positioned in accordance with the invention at the inlet end of a TLE situated at or near the exit of a chemical reactor generating a stream of high temperature gas. I believe, however, that TLE inlet end fouling by coke deposit formation is chiefly due to at least one and possibly three distinct mechanisms, each of which can contribute to slow cooling at and in the vicinity of the TLE inlet end, a condition believed to be conducive to coke deposition.

#First, solid coke particles entrained in the entering gases can impact on TLE surfaces, particularly surfaces perpendicular to the direction of the gas flow, and progressively build up deposits on these surfaces. Ultimately, such deposits can block the inlet ends of the TLE tubes by "scaffolding" or cantilevering across the tube openings.

#Second, nonideal gas flow distribution in the TLE at its inlet and beyond, and on the hot tubesheet, can cause turbulent eddies and backmixing of the gases present, cooling them to also result in increased fouling.