The different process technologies like paste-extrusion, RAM extrusion or isostatic pressing use different raw materials. For example, the difference in permeation and diffusion rates are a ratio of 1 to 5 for paste-extruded PTFE versus RAM-extruded PTFE, with the same lining-thickness. Permeation is the transport of chemical molecules through a barrier material by absorption and adsorption. On the inside of the PTFE-lined equipment, molecules diffuse through the PTFE moving in a direction of higher to lower concentrations. Permeation and diffusion rates limit the lifetime, because of corrosion on the steel shell and blistering which leads to leakage at the later stage. Therefore, permeation and diffusion rates are responsible, besides other reasons, for unexpected failures, shutdowns and maintenance costs. Furthermore, permeation and delusion are responsible for pollution and injuries of chemical plant personnel in the case of a dangerous chemicals.

          The permeation and diffusion rates are dependent on the composition of the lining, wall thickness, chemical service, temperature and pressure. The service, temperature and pressure are defined by the process; improvements can be made only with the material selection and the lining thickness. Different PTFE processes provide different types of PTFE, each with their own advantages and disadvantages. Paste-extruded PTFE provides the lowest permeation and diffusion rates, compared to other types of PTFE with the same wall thickness (see Table). As already mentioned above the paste extrusion process uses PTFE fine powder for its raw material that produces a homogeneous material with a defined and compact molecular structure. Beside the liner material, the lining thickness has a direct impact on the permeation and delusion rates, according to (Figure), it is not a linear relationship. This means in addition to aforementioned advantages of a paste-extruded PTFE lining, the availability of liner thicknesses up to 5/16” (8mm) further reduces the permeation and design rates.

LIMITS OF DIMENSIONS FOR PASTE-EXTRUDED PTFE-LINED COLUMNS AND VESSELS :

          Paste-extruded PTFE lined carbon steel or stainless-steel columns and vessels are limited in diameter, wall-thickness and section length, because of the limited extruder-volume for example. The actual availability is shown in (Table). The basis was the maximum available wall thickness for the paste-extruded PTFE liner.

TEMPERATURE AND PRESSURE LIMITS:

          The temperature limit for the PTFE itself is given by the raw material suppliers and is usually around 500”F (260”C). The PTFE lining needs a certain strength to stay in operation, therefore many PTFE processors for PTFE lining material restrict the maximum temperature to generally around 395°F (200”C). The pressure limit is based on the flange connections and the creeping effect of PTFE. Generally, the pressure limit is at 150psig (10 barg) for standard designed paste-extruded PTFE lined equipment. If a large amount of temperature/pressure cycles is required, springs for the flange bolts have to be installed on each flange connection even for processes below 150psig (10 barg). Special applications with a higher operating pressure are possible under certain circumstances, like no pressure peaks and low temperatures. Vacuum operation is possible with a vacuum support system.

TYPICAL DESIGN OF PASTE-EXTRUDED PTFE LINED COLUMNS AND VESSELS:

          The paste-extrusion process can extrude straight PTFE pipes only for columns and vessels. Therefore, dished heads are not possible, on the other hand dished heads are very often not really necessary. Columns with a diameter of 48” (1200mrn) and 190 psig (13 barg)at360”F(180”C) for example are manufactured according to the ASME pressure vessel code with a flat bottom and head. The costs for additional carbon steel needed for thicker bottoms and heads are very low compared to the total costs of such a column.

          Internals have to be clamped between column flanges, because welding to the PTFE liner is not possible. Suppliers, who use the paste-extrusion process with a homogeneous material, use a special nozzle design to avoid welding seams. The nozzle design eliminates welding seams (and a possible source of leakage). Furthermore, if leakage occurs at the nozzle, only re-tightening of the bolts is required thus avoiding costly shut-down. It is recommended that PTFE-lined equipment be insulated, if a condensable medium is in a gas phase inside. This keeps the dewpoint outside the PTFE lining and reduces blistering.

TYPICAL CHEMICAL PROCESSES WHICH REQUIRE PASTE-EXTRUDED PTFE:

          Generally, all processes with highly permeable chemicals require paste-extruded PTFE, especially with high temperatures and high pressures. The advantage of the above-mentioned PTFE is the low permeation and diffusion rates, which increases the lifetime dramatically. There is no sharp boarder, where we can decide it is absolutely necessary to use paste-extruded PTFE. However, the usage of paste-extruded PTFE lined steel equipment for the following processes has resulted in a longer operational lifetime and therefore a greater commercial success can be assured.