Impervious graphite heat exchange tubes, chemical piping and other equipment have a recognized position in commerce. Impervious graphite engineering materials have been described in many articles such as those entitled "Impervious Graphite for Processing Equipment", by John R. Schley in the journal Chemical Engineering (Feb. 18, 1974, pages 144-150 and Mar. 18, 1974, pages 102-110) and by Dennis G. Hills (Jan. 20, 1975, pages 116-119). From such publications it is evident that the term "impervious graphite" means graphite which has been manufactured (i.e. by molding or extrusion and subsequent heat treatment) into a desired form and then impregnated with a synthetic polymeric resin to make it impervious to gases and liquids. Processes for manufacturing impervious graphite structures are well known in the art, and need not be detailed here, except to point out for purposes of illustration that any synthetic resin which can be used to manufacture conventional impervious graphite heat exchange tubes can also be used in processes for manufacturing the metal coated, partially thermally degraded impervious graphite processing structures of this invention. Examples of such synthetic resins are the phenolic resins, epoxy resins, furan resins, polyester resins and polyfluoronated resins such as polytetrafluoroethylene resins. It is preferred, however, that the resins used in making the graphite impervious be of the thermosetting or chemical setting type, such as the phenolic resins, epoxy resins and furan type resins. Other resins can also be used, as desired, to make the graphite impervious, and the selection of which is within the ordinary skill of any heat exchange specialist or engineer and will depend largely upon the fluid(s) with which the graphite structure will be in contact in its intended use. (See the articles by Schley described above for a description of how conventional impervious graphite materials can be manufactured.)
Although in conventional practice, tubes and other forms of impervious graphite have somewhat improved mechanical properties over non-impervious or porous graphite, besides being impermeable, they nevertheless are weak and very brittle materials of construction. This is stated by Mills, and by Schley in the previously mentioned articles. for example, Schley states (Chemical Engineering, Feb. 18, 1974, page 150) "A crucial consideration in the success or failure of any impervious graphite structure is notch sensitivity." (Notch sensitivity is a characteristic of brittle materials which leads to catastrophic failure.) "In flexural tests, the strengths of notched specimens will be 35 to 40 percent less than similar specimens having no notch." This weak and brittle nature of impervious graphite imposes severe limitations on the design of equipment such as heat exchangers. More so since pressure tolerance is also low for conventional impervious graphite structures, being between about 50 and 75 psig for standard, commercially available heat exchange tubes. In describing shell and tube heat exchangers, Mills (page 82) states, "These designs take up a very large amount of valuable process space and are not recommended for use in the fine chemical and pharmaceutical industries." A further design limitation resides in the fact that impervious graphite is not metallic and therefor cannot be fastened by conventional welding or soldering methods and the like.
Since impervious graphite is notch sensitive and brittle, any surface defect becomes a potential failure point, and therefor is hazardous, particularly in chemical piping applications. Here, extremely hot and corrosove materials under pressure would be extremely dangerous if an impact caused a catastrophic failure.
The corrosion resistance of impervious graphite depends basically upon the inertness of graphite itself to most chemicals, except the resins used in filling the voids of graphite, which generally amounts to about 10 to 15 percent, reduce the corrosion resistance of the impervious graphite as well as its use at higher temperatures.
The resins primarily used in the manufacture of impervious graphite are the phenolics, epoxies and furans. These impose a temperature limit of about 340.degree. F. on conventional structures which contain impervious graphite. If this temperature is exceeded in use, the resin begins to thermally degrade. These resins do not melt since they are of the thermosetting type, but instead become more brittle and harder by increased crosslinking, for example. This degradation continues with increased temperatures and time until charing occurs, and eventually the resin disintegrates. In the case of the phenolics, which are among the most widely used resins particularly for applications in which acids are present, the upper temperature limit is about 600.degree. F., but for only very short periods of time. As the resin thermally degrades it becomes increasingly stable toward chemicals and therefore has enhanced corrosion resistance compared to ordinarily processed resins. However, since the heat stabilized resin also becomes much more brittle, thermal degradation ordinarily adversely affects the mechanical properties of the impervious graphite. Conventional impervious graphite structures and materials therefor have heretofor been limited to about 340.degree. F. for ordinary use, since they are also already mechanically weak and notch sensitive.
It is therefor apparent that there has existed for some time a need in the chemical processing field for tubes, pipes, and the like having significantly improved chemical or corrosion resistance, having better impact strength (including resistance to, and freedem from, catastrophic explosive failure upon being ruptured), having better heat transfer properties, being more useful and safer in processes which involve higher pressures, having better resistance to mechanical shock, and having better resistance to attack by acids, organic solvents and other corrosive liquids and gases. It has now been discovered that these needs can be fulfilled by practicing the present invention as described hereinafter.