Company Technical articles The Better Alternative PTFE System Solutions Enhance Equipment Performance
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Article for CAV 8/2010 June 23, 2010 Authors: Dr. Michael Schlipf, Head of Development The Better Alternative
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 Figure 1: The centrally configured gas feeder and distributor system as a full-PTFE construction enables homogenous reaction conditions to be set across the entire reactor cross-section. |
The full-plastics system solutions based on PTFE and modified PTFE are nearly universally usable thanks to the special properties of these materials. They have long life cycles and protect both the chemical products produced and the environment from contamination. The long lifetimes of the equipment used and high degrees of availability are of major value to plant operators.
Stainless steels are frequently used in equipment engineering when high mechanical strengths are required in combination with high material resistance that can be achieved without special coatings. However, the advantages are offset by the materials' high price, poor machinability and the risk of delayed corrosion. Enamel coatings are suitable in the presence of primarily acidic media. However, they are sensitive to mechanical impacts and tend to crack in case of a thermal expansion of the steel body to be protected.
Undetected pores harbor the risk of back-corrosion with a sudden leakage of chemicals and they are difficult to seal in the flange area due to their surface irregularities. Glass is resistant to a very wide range of chemicals. It merely exhibits weaknesses in heavily alkaline environments and is not suitable for handling hydrofluoric acid.
A special advantage is its transparency for observing reactions and processes but the destruction by mechanical impacts frequently prevents the use of glass in large-scale chemical process operations. In addition, flange connections are difficult to seal due to the low permissible surface pressures.
The protection of non-corrosion-resistant steel by plastics, in particular fluoro-polymers, is a well-known, proven technology and is frequently used. The two materials are used both as solid composites, created by gluing for example, and as composites that are merely connected to each other in selected places. Components and systems which offer high reliability and long service life can be created this way. With regard to the thermal expansion coefficient the partners are highly heterogeneous. Due to the chemicals permeating the fluoro-plastics back-corrosion may occur.
Full-Plastics System Solution
 Figure 2: Optimal welding of modified PTFE provided, test samples taken from the welding zone and the non-welded original material exhibit an identical course of the force-strain curve far beyond the yield point. |
Full-plastics PTFE installations in high-capacity reactors, particularly feeder pipes for liquid chemicals and gases, have not been possible up to now. Therefore, enameled or PTFE-covered pipes or, in special cases, stainless steels up to solid silver constructions have been used. The use of modified PTFE in combination with a special welding technology now enables full-plastics constructions of these heavy-duty components. Even high-capacity reactors with volumes above 20,000 liters can be equipped with them. Figure 1 depicts a gas distributor system which allows efficient distribution of highly aggressive gases in the reaction medium of high-capacity reactors. The gas is fed in through the centric flange in the reactor dome into which the distributor system is hung and fixed. Through the full-PTFE pipe, which is made of DyneonTM TFMTM PTFE *), the gaseous reagent is conducted toward an area close to the reactor bottom where it is distributed across the entire reactor cross-section via the star-shaped distributor system. The usage process of the reaction component, which is distributed in the form of extremely small bubbles, occurs while it rises in the reactor. Consequently, nearly identical concentration conditions can be set in the entire reactor volume, which enables the chemical reaction to be conducted in a highly selective manner. As a direct result, the formation of by-products is suppressed, the yield of the target product is increased and the costs for purifying the reaction mixture are minimized.
 Figure 3: The permanent deformations depicted are determined under a load of 15 N/mm² for 100 hours. A highly carbon-filled compound based on modified PTFE proves to be the ideal material for the flange area due to a reduction of cold flow by a factor of four. |
High Operating Reliability due to PTFE
The size of the gas distributor system presented here requires it to be manufactured from several single components. They are joined into a total system by welding or by threaded connections.
Figure 2 depicts the results of strength tests on welded connections using the example of DyneonTM TFMTM 1700 PTFE in a comparison with the non-modified PTFE version, Dyneon™ TF 1750 PTFE. The following statements apply to modified PTFE in non-strained applications:
The identical course of the force-strain curve – in the area of the weld seam and in the area of the non-welded original material – clearly demonstrates that no strength losses compared with the original material have to be taken into account, provided that the welding process has been optimally performed. The separation conditions of the test sample, which far exceed 400 % strain, are not reached in the application. The stress exerted on the gas feeder pipe, which is subject to significant local differences, requires the use of different materials.
Heavy surface pressures occur particularly in the upper flange area. Therefore, the material used in this area has to exhibit particularly high resistance to cold flow, the PTFE-typical deformation under load. This is achieved by additional fillers in the modified PTFE used.
Figure 3 shows the deformation under load of different materials: after application of a 100-hour load of 15 N/mm² at room temperature a different, permanent deformation of the materials occurs.
By means of using modified PTFE and 25 % carbon filler the high cold flow of standard PTFE, in this case 11 %, can be reduced to less than 3 %, which amounts to a factor of four. Hence a highly filled carbon compound based on Dyneon™ TFMTM PTFE is the ideal material for the flange area.
The load situation in the central pipe of the gas distributor system is completely different. Due to the intrinsic weight, vibrations and possible pressure shocks of the reaction gas being fed into the system this equipment area is loaded with maximum tensile stress.
By using pure, unfilled PTFE in this area the maximum achievable tensile strength values are ensured and the quality of the welded connection is not impaired by the addition of fillers either.
 Figure 4: The modulus of unfilled standard PTFE of approx. 550 N/mm² allows only low torque levels when designing threaded connections. By using highly filled, modified PTFE, the modulus is increased by a factor of two and the resulting threaded connections are able to absorb higher forces accordingly. |
In the area of the distributor system, which is positioned near the reactor bottom, the nearly unlimited freedom of design of this full-plastics construction should be supported by additional joining techniques.
They also include threaded connections. As an important material parameter for strength calculation, the modulus of elasticity should be used. It describes the strength property of the material in a nearly unstrained condition.
Tailored Material Properties
Figure 4 shows the variation range of the modulus, starting with unfilled standard PTFE all the way to highly filled modified PTFE: the doubling of the modulus by chemical modification of the PTFE matrix in combination with a suitable filler enables the design of high-strength threaded connections.