Graphite’s outstanding thermal conductivity makes it suitable for the construction of heat exchangers and other process equipment. Increases in delivery times and the prices of nickel alloys and titanium have led to interest in graphite process equipment as a competitive material. Impervious graphite resists most concentrated acids such as hydrochloric, hydrofluoric, phosphoric and sulfuric acids; pickling liquors; and aqueous solutions containing halides, nitrates or sulfates. Impervious graphite material is noted for its high corrosion resistance at temperatures between -76 and 392°F (-60 and 200°C). Maximum permissible material temperatures are 392°F (200°C), depending on the type and concentration of the process medium. In addition, impervious graphite withstands nearly all common organic solvents.
One impervious graphite material on the market consists mainly of polycrystalline graphite. Manufactured graphite is porous, having an average of 10 to 20 percent porosity, depending on grade. Open pores are impregnated with a phenolic resin, converting it into a material impervious to gas and liquid. Among the few media to which graphite is only partially resistant are those with a powerful oxidizing action such as nitric acid, highly concentrated sulfuric acid (oleum) and chromic acid.
The impregnation process involves precisely controlled time and temperature cycles to ensure a quality finished product, and the impregnation of the phenolic resin is critical to the lifecycle of graphite process equipment. Proper impregnation increases the strength of the graphite/resin composite material. An improper impregnation can lead to the material becoming porous and then mechanically failing prematurely.
Graphite materials used in chemical process equipment applications differ in proportion of crystalline carbon, pore volume, pore-size distribution and synthetic resin used for impregnation. The resulting differences in mechanical and thermal properties allow the graphite grades to be ideally matched to particular applications.
Standard Grade. This impermeable, synthetic-resin-impregnated grade of graphite for process equipment has a highly homogeneous material structure. The material is used for the production of heat exchangers, hydrogen chloride synthesis plants, and other pressure- and temperature-stressed components such as pumps, distillation columns, scrubbers and vacuum jet internals.
Advanced Grade. This impermeable synthetic-resin-impregnated grade of graphite has a low pore volume combined with reduced pore size and greater mechanical strength. It is suitable for components exposed to elevated mechanical stress or extremely corrosive media and solvents.
PTFE Impregnated. This grade of graphite has a highly homogeneous material structure and is used for process equipment such as heat exchanger blocks exposed to highly oxidizing, alkaline or aggressive solvent media.
Fluoroplastic Bonded. A specialized grade with an extremely high graphite content mixed with PVDF, this material has a highly homogeneous structure and is used in some plate-and-frame heat exchanger applications.
Heat Exchanger DesignsThree common designs of graphite exchangers are used in the process industries: shell-and-tube, cross-drilled block and plate-and-frame exchangers.
Shell-and-tube exchangers are constructed with all wetted process surfaces made of impervious graphite. Channel covers, tubesheets and tubes are graphite. The tubes are cemented to the tubesheets using graphite and phenolic-resin-based material. The tubes can be fiber reinforced using a high-strength carbon-filament winding around the tube. This improves durability, especially in difficult applications and rough operation.
Baffles commonly are constructed of graphite but other material choices are possible. The exchanger shell may be constructed of almost any material, including carbon steel, stainless steel, fiberglass-reinforced plastic (FRP) or lined steel systems. A floating-end design similar to TEMA’s AEP designation is used to compensate for the difference in thermal expansion of graphite and the shell material. Shell-and-tube exchangers may be configured in single or multipass configurations. Applications include heaters, coolers, condensers, evaporators or absorbers, as they often have heat transfer areas up to 20,000 ft2 in a single unit. Standard design pressure typically is 100 psig although special designs may handle up to 225 psig.
Cross-drilled block exchangers have three configuration options: cylindrical block, cubic block and monolithic block.
The most common design uses stacked cylindrical blocks that are drilled with vertical holes for the process side and horizontal holes for service fluids. Heat is transferred through the ligament between the drilled holes. Manufactured in several standard diameters and drilling configurations, the blocks are stacked upon each other to obtain the proper surface area for the application. Soft PTFE gaskets are used between the blocks to seal the process side.
Similar to shell-and-tube exchangers, many shell construction materials can be used; a floating-end arrangement is used; and the exchanger may be installed in numerous process applications. The baffle assembly is in the annulus between the graphite blocks and the inner diameter of the shell. Surface areas of up to 8,500 ft2 are available per unit. Also like shell-and-tube designs, design pressure typically is 100 psig although special designs may handle up to 225 psig.
Cubic block exchangers are constructed in a similar method to cylindrical block exchangers. The primary difference is that lateral plates are used to collect the service-side fluid. The service fluid then flows from block to block through a series of interconnecting piping elbows. Standard designs have surface areas up to 2,500 ft2 and normal design pressure of 100 psig.
Monolithic block exchangers are constructed from a single graphite block. Process- and service-side holes are drilled horizontally and heat transfer takes place through the ligament. Multiple passes for the process and service sides are integrated into the block by milling. The exchanger’s sides are covered by steel panels that are normally PTFE-lined for corrosion resistance. Heat transfer areas of 500 ft2 and standard design pressures of 150 psig are available. Block exchangers are used as heaters, coolers, condensers and evaporators. Monolithic block units also are used commonly as interchangers due to their corrosion resistance on both sides and ability to have true globally countercurrent flow.
Plate-and-frame exchangers consist of multiple thin (0.26 to 0.39") impervious graphite plates stacked together in between two thick steel plates. The steel cover plates are PTFE lined for corrosion resistance. Identical to flow paths through metallic plate-and-frame exchangers, the fluids flow within alternate channels between the plates. A specially developed PTFE seal is utilized between the plates to ensure leak-free operation. High turbulence within the exchanger increases the overall heat transfer cooefficent and normally results in the lowest heat transfer area solution for an application. The compact design of the plate-and-frame unit allows for a large amount of heat transfer area to fit into a small space within a plant. These advantages make the plate heat exchanger a design that continues to grow in popularity. Heat transfer areas of 500 ft2 and a design pressure of 125 psig are standard. A somewhat higher design pressure also can be attained. Plate-and-frame exchangers are used as heaters, coolers, interchangers, condensers and evaporators. Units are completely corrosion resistant on both process and service sides.
Although some European regulations exist, graphite is not currently an ASME code material. Manufacturers use conservative safety factors regarding allowable stresses, and some states require a special design review by a state-registered professional engineer. Over the past several years, leading graphite manufacturers have been working closely with the ASME to develop an ASME code standard regulating the design and manufacture of exchangers made of graphite.
The steel portion of graphite exchangers (all designs) commonly receives a code stamp. However, the impervious graphite material cannot currently be certified in this manner. Significant progress has been made in the last two years. Graphite manufacturers are optimistic that a code addendum will be approved and a standard will go into effect by 2010. This important advancement will ensure equipment owners and suppliers are working from a common basis regarding design safety factors and that equipment integrity is ensured.
Graphite process equipment, especially heat exchangers, continue to be of strong interest to the chemical processing, pharmaceutical, steel pickling, fertilizer and environmental industries. Manufacturers have numerous material choices and exchanger configuration options available that allow for an effective heat transfer solution. As demand for alloys and titanium continues to be strong, impervious graphite is being safely applied for corrosive applications in countries around the world.