How to Estimate Plastic Packing’s Bed Height under Higher Temperatures

Figure 1 The yield strength-to-temperature curve of PFA provided by Solvay (Solvay Specialty Polymers, 2014). It shows how rapidly the strength drops as temperature increases.

When a process engineer specifies the bed height for a packed distillation column, it could be advantageous to go for the maximum height possible. Doing so would save the column space and minimize the number of accessory internals, such as liquid distributors and collectors). The usual engineering guideline recommends a maximum packed bed height of 15 theoretical stages (Górak & Olujić, 2014), which translates to about 6 m or greater for a 250 m²/m³standard structured packing. Other common practices limit the bed height to 10 stages, which corresponds to a bed height of 4-6 m for common industrial packing types, whether structural or random. However, these figures are intended for metal packings.

Care must be taken when the material of construction (MoC) is plastic. Before using the 6 m bed height for plastic packing, engineers must ensure that the plastic material will not become too “soft” at the working temperature. This detail is sometimes forgotten even by experienced engineers, especially when they recall prior examples of high plastic bed heights, such as extraction columns with 6-m PVDF 1” Pall Ring random packing beds or gas absorption towers with 7-m PFA structured packing beds. Those bed heights present no problems because they operate at low temperatures, usually near 30 °C. Higher temperatures dramatically weaken the yield strength of plastics (see Fig. 1), causing them to “soften.” Note that this does not mean the material is no longer viable under increased temperatures. I have encountered multiple instances where even experienced plant personnel confused the permitted working temperatures of non-load-bearing fluoroplastics, such as PTFE gaskets and PFA linings, with those of load-bearing applications. Modern high-end PTFEs and PFAs generally exhibit good thermal stability above 200 °C. However, thermal stability simply means the material does not melt or degrade easily. It does not prevent the plastic packing at the bottom of a packed bed from collapsing due to the accumulated weight as the temperature rises. At 150 °C, the mechanically permitted bed height for many plastics could be just over 1 meter, which is usually considered too short for distillation, even though the PTFE gaskets and the PFA vessel wall linings perform flawlessly at temperatures above 200 °C.。

So, how to “sanity-check” the permitted mechanical height for a fluoroplastic bed as a process engineer (so the mechanical colleague would not laugh at you as you pass down the design)? The first thought might be to consult your mechanical engineer friend. The truth is, even if your mechanical engineer colleague would like to help, he or she probably does not have the packing-specific strength-vs-temperature curve that the plastic packing vendor has extensively tested and certified. For sure, the packing vendor will not publish this precious internal knowledge. However, you can and should ask a plastic packing vendor for the exact permitted height at your process conditions before issuing a final design to the mechanical engineering department. But, since an accurate answer is almost guaranteed to require mechanical calculation and evaluation work on the vendor’s side, a quick estimate by yourself can be helpful to avoid egregious preliminary designs if the project is not there for serious buying inquiries yet.

The starting point is to establish the reference bed heights for the common plastic packing types. Table 1 lists the approximate maximum bed heights for a couple of common plastic packings based on the author’s past project experience.

Material	Packing Type	Packing Density [kg/m³]	Max Bed Height [mm]	Temperture [°C]
PVDF	250 m²/m³	170	6000	90
PFA	250 m²/m³	240	6000	50

Table 1 The Maximum bed heights for select plastic packing types. The data are based on the author’s project experience and were good for the author’s applications. Different plastic packing vendors might provide different values.

As temperature increases, the yield strength ( σᵧ,the stress at which deformation becomes permanent) of a plastic invariably decreases. We can compare the yield strength at the target temperature (σᵧᵀ) to the yield strength at the reference temperature (σᵧᴿ) to estimate how much the plastic material has weakened. Since the packing’s density is uniform, the packing’s height directly represents the load it supports. The packing reaches its maximum mechanical height when the load equals its yield strength. Therefore, the ratio of the yield strengths at the two temperatures, r = σᵧᵀ / σᵧᴿ, can be linked to the ratio of the maximum bed heights at the respective temperatures:

r = σᵧᵀ / σᵧᴿ = Hᵀ / Hᴿ     —— (eq. 1)

其中：

σᵧᵀ：MPa; the yield strength of the plastic at the target temperature;

σᵧᴿ：MPa; the yield strength of the plastic at the reference temperature

Hᵀ：mm; the maximum mechanical packing height at the target temperature;

Hᴿ：mm; the maximum mechanical packing height at the reference temperature

For a quick working example: suppose you want to apply a 250 m²/m³PFA packing to a distillation column with a maximum working temperature of 180 °C. According to Figure 1, at this target temperature, the yield strength (σᵧᵀ) for a Solvay Hyflon M series PFA is about 3.5 MPa. The yield strength (σᵧᴿ) at the reference temperature of 50 °C is about 12 MPa. Therefore, the ratio is r = σᵧᵀ/σᵧᴿ= 0.29, suggesting that at the target temperature, the strength has weakened to about 29% of the reference value. Based on the reference value, the maximum mechanical height (Hᴿ) at 50 °C is 6,000 mm. Thus, the estimated maximum mechanical height at the target temperature of 180 °C, according to eq.1,   Hᵀ = Hᴿ ×r=1740mm

A few important notes regarding this estimation method:

1. Use the equipment design temperature of the column, rather than the operating temperature used in process simulations, for an inherently safe design. Note, however, that using the design temperature will further reduce the allowed mechanical plastic bed height.

2. This method serves only as a preliminary estimate, as the mechanical properties of plastics have been greatly simplified. A plastic packing vendor’s rating is required for the final design.

3. The method described here uses the tensile yield curve published by a popular raw material vendor. The compressive yield curve should theoretically be used for better approximation, but compressive yield curves are not readily available in open literature. The author finds that using a tensile yield curve is good for most preliminary estimates. 

4. The exact mechanical modeling used by plastic packing vendors is much more involved. Plastics behave differently from metals and ceramics. Their load-bearing abilities are also affected by time and pressure, a property termed “compressive long-term creep strength.” In short, traditional tensile or compressive yield strengths are tested under short-term loads as dictated by ASTM protocols (ASTM D638 for tensile and ASTM D695 for compressive), but for plastics, the duration of the applied load also matters. Continuously applied loads keep weakening plastics further, especially under high stress. Therefore, a rigorous height calculation for plastics depends on the design life of the packing, the load, and the temperature. A vendor’s accurate evaluation is mandatory for the final design.

5. Do not forget that in the final load calculations, the packing not only supports its own weight but also supports liquid holdups and potentially a filled distributor on top.

Modern fluoropolymers such as PFA (perfluoroalkoxy alkane) and PTFE (polytetrafluoroethylene) have excellent corrosion resistance and moldability. They are well suited for manufacturing the complex corrugated structures needed for modern industrial packings, and they perform well in highly corrosive environments involving chemicals like HF, HCl, Sulfuric acid. However, close attention must be paid to the mechanically allowed bed height. If the process temperature restricts the bed height too severely, other packing options, such as silicon carbide, are available for equivalent or better performance.

[1] Górak, A., & Olujić, Ž. (Eds.). (2014). Distillation: Equipment and processes. Academic Press. https://doi.org/10.1016/C2010-0-67014-0

[2] Solvay Specialty Polymers. (2014). Hyflon® PFA design & processing guide (Version 2.2). https://www.nevicolor.it/prodotti/ricerca-polimero/solvay/hyflon-pfamfa-food/documenti/hyflon-pfa-design-and-processing-guide-en.pdf