PVDF Bubble Cap Trays for Corrosive Distillation and Absorption Columns

Metal tower trays are widely used in distillation, absorption, stripping, and other gas-liquid contacting processes. However, some process environments create a different engineering challenge: the vapor and liquid phases may be sufficiently corrosive that conventional carbon steel or even common stainless steels become difficult to justify.

In such cases, polymer tower internals may be considered.

Among available engineering plastics, PVDF — polyvinylidene fluoride — can be used for customized bubble cap trays where the chemical environment, operating temperature, mechanical loading, and process requirements are suitable.

A PVDF bubble cap tray is not simply a metal bubble cap tray reproduced in plastic.

The lower stiffness, higher thermal expansion, creep behavior, connection method, support arrangement, and fabrication limitations of PVDF must all be considered during engineering.

This makes PVDF tray projects fundamentally different from ordinary stainless-steel tray fabrication.

Why Use PVDF for Bubble Cap Trays?

The main reason is usually corrosion resistance.

Some columns handle aggressive liquids or vapors where metallic materials face problems such as:

• rapid general corrosion;
• localized corrosion;
• chloride-related attack;
• contamination from metal ions;
• short replacement intervals.

PVDF provides a different chemical-resistance profile from conventional metals and is therefore considered in selected corrosive chemical applications.

Typical project discussions may involve:

• acid processing;
• corrosive absorption systems;
• halogen-containing streams;
• specialty chemical processing;
• high-purity chemical service;
• corrosive scrubbing systems.

However, PVDF should never be selected simply because a process is described as “corrosive.”

Actual compatibility depends on the complete chemical environment, concentration, temperature, contaminants, and duration of exposure.

Material compatibility must therefore be verified against the real process conditions before final design.

Why Use a Bubble Cap Tray Instead of a Sieve Tray?

Bubble cap trays are mechanically more complex than sieve trays, but they offer an important operating characteristic:

vapor must pass through the riser and bubble cap before entering the liquid layer.

This allows the tray to maintain gas-liquid contacting over a relatively broad operating range.

A typical bubble cap tray contains:

• tray deck;
• risers;
• bubble caps;
• slots or vapor discharge openings;
• downcomers;
• outlet weirs;
• support members;
• tray-to-shell connections.

The vapor rises through the riser, changes direction beneath the cap, and exits through openings into the liquid retained on the tray deck.

This configuration can be attractive where stable contacting at reduced vapor rates is more important than achieving the simplest possible tray geometry.

For a PVDF tray, however, each of these components must be evaluated as part of a polymer structure rather than copied directly from a steel design.

Mechanical Stiffness Becomes a Major Design Issue

PVDF does not have the same elastic modulus as stainless steel.

This means a PVDF plate of the same thickness and span as a stainless-steel tray deck will not necessarily show the same deflection.

Large unsupported areas may deform under:

• liquid weight;
• tray hardware weight;
• maintenance loads where applicable;
• differential pressure;
• thermal exposure.

The engineering solution may involve:

• increased plate thickness;
• shorter unsupported spans;
• additional support beams;
• reinforced tray sections;
• modified segment geometry.

Therefore, a PVDF tray should not be designed by simply replacing “SS316L” with “PVDF” on an existing metal tray drawing.

The structural behavior changes with the material.

Creep Must Be Considered for Long-Term Service

Metals and thermoplastics behave differently under sustained load.

A polymer component that appears sufficiently rigid during initial installation may gradually deform when subjected to continuous mechanical stress, particularly as operating temperature increases.

This time-dependent deformation is known as creep.

For a bubble cap tray, excessive long-term deformation could affect:

• tray levelness;
• liquid depth;
• weir height;
• seal geometry;
• downcomer clearance;
• bubble cap position.

This makes long-term support conditions particularly important.

A design that is mechanically acceptable for short-term handling may not automatically be acceptable for years of operating service.

Thermal Expansion Is Higher Than for Metal Trays

PVDF also has a significantly different thermal-expansion behavior from steel.

When tower temperature changes between fabrication, shutdown, startup, and normal operation, polymer components can expand or contract.

If the tray is restrained too rigidly, thermal movement can create:

• distortion;
• local stress;
• buckling;
• connection damage.

If excessive clearance is provided, however, the tray may no longer fit or seal correctly.

The mechanical arrangement therefore needs to accommodate expected thermal movement without losing the required installed geometry.

This becomes increasingly important as tray diameter increases.

Bubble Caps and Risers Require Dimensional Control

Bubble-cap performance depends on geometry.

Important dimensions can include:

• riser diameter;
• riser height;
• cap clearance;
• cap slot dimensions;
• cap-to-riser relationship;
• tray liquid depth.

Polymer fabrication methods may produce dimensional behavior different from machined or welded metal components.

The design must therefore consider how PVDF components will actually be:

• machined;
• formed;
• welded;
• assembled;
• supported.

A theoretically correct hydraulic geometry is of little value if the manufactured polymer part cannot maintain that geometry reliably during operation.

Plastic Welding Is Different From Metal Welding

PVDF internals may use thermoplastic welding methods rather than conventional metal welding.

Joint quality can depend on:

• surface preparation;
• welding temperature;
• equipment;
• joint geometry;
• operator technique;
• material cleanliness.

Poor welding can create weak joints, distortion, incomplete fusion, or stress concentration.

For bubble cap trays, fabricated joints may occur at:

• risers;
• tray segments;
• reinforcement pieces;
• downcomers;
• support features.

These joints should be considered during the design stage rather than treated as a workshop detail after the drawing has already been finalized.

Tower Diameter Strongly Affects the Design

Small laboratory or pilot columns and large industrial columns should not automatically use the same tray construction concept.

As tower diameter increases:

• tray span increases;
• liquid inventory increases;
• individual sections become heavier;
• manway access becomes more important;
• support requirements increase;
• thermal movement becomes larger.

A large PVDF tray may therefore require a segmented design with multiple sections assembled inside the vessel.

Segment size must consider both mechanical performance and installation access.

Manway Installation Must Be Planned Before Fabrication

Large trays cannot usually enter a column as a complete circular assembly.

The tray may need to be divided into several panels or sectors.

The engineering drawing should therefore consider:

• manway clear opening;
• maximum segment dimensions;
• segment weight;
• internal support arrangement;
• field joining;
• identification and orientation.

This is particularly important for customized replacement projects where the vessel already exists.

A tray can be hydraulically correct and still fail as a project if it cannot physically pass through the manway.

Downcomers and Weirs Still Control Tray Hydraulics

Using PVDF does not change the basic hydraulic purpose of the tray.

The system still needs to manage:

• vapor flow;
• liquid residence on the tray;
• liquid transfer to the tray below;
• froth behavior;
• downcomer capacity;
• liquid backup;
• entrainment.

Therefore, material selection cannot compensate for poor hydraulic design.

A PVDF bubble cap tray still requires appropriate evaluation of tray spacing, weir arrangement, downcomer area, operating rates, and expected turndown.

The polymer material solves a material-of-construction problem.

It does not eliminate the need for proper tray engineering.

Where PVDF May Not Be the Right Choice

PVDF has useful corrosion resistance, but it is not a universal replacement for stainless steel.

A project may favor metal construction where there are:

• high structural loads;
• very large unsupported spans;
• temperatures outside the acceptable polymer range;
• severe mechanical impact;
• very high pressure demands;
• operating conditions requiring greater stiffness.

Cost also matters.

PVDF is considerably more expensive than common PP and may not be justified where PP already provides sufficient chemical and temperature resistance.

The correct engineering question is therefore not:

“Is PVDF better than stainless steel or PP?”

It is:

“Which material provides the required chemical resistance, mechanical reliability, temperature capability, and lifecycle value for this specific tray?”

What Information Is Needed for a Custom PVDF Bubble Cap Tray?

A meaningful evaluation normally requires more than tower diameter.

Useful project data include:

• tower internal diameter;
• tray spacing;
• number of trays;
• gas or vapor flow;
• liquid flow;
• operating temperature;
• operating pressure;
• process medium;
• chemical concentration;
• required turndown;
• existing vessel drawing;
• manway dimensions;
• nozzle positions;
• support-ring arrangement;
• required tray material;
• existing tray drawings or photographs where available.

For replacement trays, actual site measurements can also be valuable.

If the customer already has an approved drawing, fabrication can be evaluated against that drawing.

If the customer does not have a complete fabrication drawing, the project may require an engineering and drawing-development stage before manufacturing.

Design and Fabrication Should Be Evaluated Together

PVDF bubble cap trays illustrate why tower-internal design and manufacturing should not be separated completely.

Hydraulic requirements determine the contacting geometry.

Mechanical engineering determines whether the tray can maintain that geometry.

Fabrication capability determines whether the proposed components and joints can actually be produced.

Installation constraints determine whether the finished parts can enter and be assembled inside the vessel.

A practical project therefore connects:

process data → hydraulic requirements → material selection → mechanical support → manufacturing drawing → fabrication → installation.

DAIER Custom Tower Tray Support

DAIER can support customized tower-internals projects based on customer drawings, existing equipment information, or project data requiring further technical evaluation.

Depending on project scope, supply can include customized:

• bubble cap trays;
• sieve trays;
• tray components;
• packing support internals;
• liquid distributors;
• collectors;
• other packed-column and tray-column internals.

For PVDF bubble cap tray projects, chemical compatibility, operating temperature, hydraulic requirements, vessel geometry, support arrangement, and fabrication method should all be confirmed before production.

Final Engineering Perspective

PVDF bubble cap trays can provide a useful option for corrosive tower applications where metallic materials are unsuitable or undesirable.

But the design cannot be approached as a simple material substitution.

PVDF introduces its own engineering requirements:

corrosion resistance must be matched with adequate structural support, creep control, thermal-expansion allowance, reliable fabrication, accurate tray geometry, and practical installation access.

When these factors are evaluated together, polymer tower trays can be engineered for demanding chemical-service applications while maintaining the required gas-liquid contacting function.

The correct solution starts with the process conditions—not with the assumption that one material is automatically superior to another.