Graphite vs. Silicon Carbide SiC Heat Exchangers: criteria for proper selection

Graphite heat exchangers provide excellent corrosion resistance to a wide variety of chemicals and are therefore commonly used in the following industries such as: chemical refining and processing, petrochemical, food and beverage, pharmaceutical, fertilizer, steel pickling, mining and metal finishing.

Mersen has designed and manufactured graphite shell and tube heat exchangers for over 50 years with many units in service in the most demanding processes, specially in the phosphoric acid industry with more than 200 references in North Africa and China.

Polytube®, the graphite shell and tubes heat exchanger that is based on Graphilor® 3, the only ultra-fine graphite, combines optimum thermal efficiency, mechanical durability and chemical inertness.

Mersen graphite tubes are manufactured with its trade mark exclusive graphite:Graphilor® 3, the only extruded impregrated graphite resulting from the combination between isostatic graphite and phenolic resin.

1- The diameter of the poly-tube tube-sheet is made with monolithic blocks up to 36” and with ML technology for diameters between 36” and 82”. ML technology consists in assembling several graphite blocks with resin.
2- Rigilor® is an additional option to increase the abrasion resistance of the tube sheet by 6: the graphite parts are strengthened by the treatment of a carbon fiber basal layer.

 

Key advantages of Graphilor® 3 tubes:

• Particularly high resistance to corrosion
• Very good temperature resistance
• Excellent refractory qualities and mechanical properties
• Non-contaminating properties

SiC Polybloc® heat exchangers lead to optimized performance and are the first choice for applications in the pharmaceutical and fine chemicals industry. They can be installed instead of existing graphite block heat exchanger units as a technical upgrade with no piping modification. The blocks are made from Boostec® sintered Silicon Carbide, an ideal material for corrosion-resistant heat exchangers. Boostec is a Mersen company

No particle emission - Highly resistant against abrasion, hardness close to diamond- 0 % porosity thus no impregnation - No permeation - No contamination for high purity applications - Higher fluid velocity - Extremely high thermal conductivity, close to aluminum - Higher efficiency Solution suitable for extreme environment - Resistant to temperatures of nearly 1000°C - Universal anticorrosive solution - Pressure resistance up to 40bars - Thermal shock resistance compared to graphite and glass-lined Easy maintenance - Low fouling - No preventive maintenance - Tell Tale system (leak detection) - Compatible with many cleaning methods (in-place, pyrolysis,high-pressure water-jet, heavy chemicals detergents)

Key Markets

Fine chemicals, specialty chemicals, condensers for API Enhanced process Abrasive product inside a corrosive stream Extreme Environment (Temperature, Pressure) Hydrofluoric acid Flash or forced evaporators, thermosyphon Bromine Heat recovery units - Interchanger 

SiC Polybloc® heat exchangers are assembled as a stack of single blocks which is then inserted into a metallic shell; seals are placed between the individual blocks. Due to this design, Polybloc® heat exchanger offers the opportunity to easily replace single blocks in case of a defect. During the engineering study, Polybloc® heat exchanger can be designed in a flexible way due to the SiC material properties

Comparison of the thermal conductivity of various corrosion resistant materials used in the chemical processing industry

In last years pharmaceutical parameters (mainly API) have been requiring higher standards and erosion/abrasion issues due to small particles of impregnated graphite  generated in condensers on glass lined reactors created a switch to SiC heat exchangers in these applications.  His excellent resistance to erosion (silicon carbide is 50% harder than tungsten carbide) was a key feature.

SiC Heat Exchangers comply with GMP guidelines and high purity requirements prevent cross contamination, are fully drainable and easy to clean 

In a Polybloc®, the two streams are separated by a wall of silicon carbide and gaskets. As the unit ages, the gaskets can become more brittle and prone to failure. Without any specific design feature or maintenance program, this could lead to a cross contamination between streams. 

So should we assume  Graphite Heat Exchanger life curve is close to the end?

Surely Graphite HE is a mature product (over 40 years experience with thousands of units installed) but majority of installations in chemical refining and processing, petrochemical, fertilizer, steel pickling, mining and metal finishing will be with this material even in the  future.

A minor high tech section (API, fine chemicals) moved to SiC HE's due to multipurpose use of reactors and high amount of money in intermediate/final product that can not accept any minimum defects or particles.

Therefore when you have to evaluate which material to use with corrosive fluids first criteria should be: what is value of product involved in the process? There is no need to use Sic HE for steel pickling, impregnated graphite is perfect.

But if you have to set up a new API project with large glass lined reactors with final product with value of thousands Euro/g, SiC condenser will assure you best selection and mechanical resistance. If you would like to avoid any further trouble on possible leakage from pipes/head  sealing from double O-rings don't design a SiC Shell& Tube HE, select a SiC Polybloc® heat exchanger.