Pingxiang Daier Separation Tech Aug 26, 2026

Can Different Sizes or Types of Random Packing Be Mixed in the Same Tower Bed?

Introduction

Different random packing sizes, geometries or materials should not normally be mixed unintentionally within the same packed bed. Mixing changes the bed structure, local void fraction, pressure drop, liquid distribution and hydraulic behavior, which can make tower performance difficult to predict.

This question often appears during:

partial packing replacement;
emergency maintenance;
supplier changes;
tower revamps;
reuse of old packing.

Typical questions include:

Can 25 mm and 50 mm Pall Rings be mixed?
Can old and new packing be installed together?
Can Pall Rings and Raschig Rings share one bed?
Can different suppliers' Pall Rings be mixed?
Can PP and PVDF packing be mixed?
Can the upper and lower sections of one tower use different random packing?

The correct answer depends on whether engineers are discussing:

uncontrolled mixing inside one continuous bed

or:

intentionally different packing installed in separate engineered beds.

These are not the same design.

The central engineering question is:

When is mixing random packing technically acceptable, and when should a packing bed remain uniform or be completely replaced?

1. Why a Random Packed Bed Is Normally Designed as One Defined Packing System

A random packed bed is usually characterized by:

packing type;
nominal size;
material;
bulk density;
specific surface area;
void fraction;
packing factor.

Hydraulic calculations are based on those defined properties.

If multiple different packings are randomly mixed together, the actual bed may no longer correspond to either supplier's published data.

This makes prediction of:

pressure drop;
flooding capacity;
liquid holdup;
mass-transfer performance

much more uncertain.

2. Random Does Not Mean “Any Mixture Is Acceptable”

The word random refers to the orientation of individual packing pieces.

It does not mean engineers can randomly combine:

different sizes;
different geometries;
different materials.

For example:

A bed of 25 mm Pall Rings may be randomly oriented, but it is still a defined 25 mm Pall Ring bed.

That is different from a bed containing:

25 mm Pall Rings;
50 mm Pall Rings;
Raschig Rings;
broken old packing

all mixed together.

3. Why Different Packing Sizes Can Be Problematic

Different packing sizes create different:

void dimensions;
surface area;
hydraulic resistance.

Smaller packing tends to provide:

higher specific surface area;
smaller flow passages;
higher pressure drop.

Larger packing generally provides:

larger flow passages;
lower pressure drop;
lower specific surface area.

When mixed randomly, these characteristics no longer remain clearly separated.

4. Smaller Packing Can Fill Voids Between Larger Packing

This is one of the main risks of uncontrolled size mixing.

Suppose:

50 mm packing forms large open voids;
25 mm packing is then added.

The smaller packing may occupy spaces between larger pieces.

This can create locally denser regions with:

lower void fraction;
greater hydraulic resistance.

The resulting bed may have a pressure-drop behavior very different from either pure packing size.

5. Mixing Sizes Can Create Local Pressure-Drop Differences

If one part of the bed contains more small packing while another contains more large packing, local resistance can vary.

Gas may then preferentially flow through:

lower-resistance regions.

Liquid may follow different pathways.

The result can be:

gas maldistribution;
liquid maldistribution;
localized flooding;
underutilized packing zones.

6. Segregation Can Occur During Loading

Different packing sizes or weights may not remain uniformly mixed.

During loading and operation, they may segregate because of:

gravity;
vibration;
fluid forces;
differences in shape.

Smaller pieces may move into lower voids while larger pieces remain elsewhere.

This means even an initially “well mixed” bed may not stay well mixed.

7. Can 25 mm and 50 mm Pall Rings Be Mixed?

They should not normally be randomly mixed in one continuous bed unless the arrangement has been specifically engineered.

The two sizes have different:

hydraulic capacity;
surface area;
pressure drop characteristics.

If a tower originally uses 25 mm Pall Rings, topping up missing volume with 50 mm Pall Rings simply because they are available is generally not a good engineering replacement strategy.

8. Can Two Different Random Packing Geometries Be Mixed?

For example:

Pall Ring + Raschig Ring;
Pall Ring + Intalox Saddle;
Cascade Mini Ring + conventional ring.

Uncontrolled mixing is generally not preferred.

Each geometry creates a different packed-bed structure.

The mixed bed may have uncertain:

packing factor;
surface area;
liquid holdup;
capacity.

If two packing types are both technically useful, it is usually better to keep them in clearly defined sections rather than randomly mix them.

9. Can Different Brands of the Same Pall Ring Be Mixed?

Possibly—but not automatically.

Two suppliers may both call a product:

25 mm Pall Ring

while differing in:

actual dimensions;
wall thickness;
opening pattern;
bulk density;
specific surface area.

Before mixing old and new packing from different suppliers, compare:

geometry;
dimensions;
material;
physical data.

The nominal product name alone is not enough.

10. Same Nominal Size Does Not Guarantee Equivalent Packing

A 25 mm packing from one supplier may not be identical to a 25 mm packing from another.

This becomes especially important for:

metal Pall Rings;
high-performance random packing.

If the replacement geometry is significantly different, partial mixing may create local hydraulic differences.

11. Can Old and New Packing Be Mixed?

Sometimes.

If the old packing is:

the same type;
same nominal size;
same material;
mechanically intact;
clean enough for reuse,

adding technically equivalent new packing may be possible.

But engineers should first inspect the old bed.

Do not mix new packing with old packing that is:

damaged;
deformed;
heavily fouled;
brittle;
corroded.

12. Old Packing Should Be Evaluated Before Partial Reuse

Before partial replacement, inspect for:

broken pieces;
deformation;
fouling;
corrosion;
chemical degradation;
loss of bed height.

If a significant portion of the old bed has lost its original geometry, topping it up with new material may not restore predictable tower performance.

13. Partial Replacement Is Not Always Economically Better

At first, replacing only damaged packing appears cheaper.

But partial replacement may require:

sorting old packing;
cleaning;
identifying reusable material;
additional labor.

If tower shutdown time is expensive, complete replacement may sometimes be more economical and technically cleaner.

14. When Partial Replacement Makes More Sense

Partial replacement can be reasonable when:

damage is localized;
remaining packing is clearly healthy;
equivalent replacement packing is available;
bed properties can remain substantially consistent.

Examples might include:

a damaged top layer caused by maintenance;
limited packing loss from an abnormal event.

The decision should be based on condition—not only replacement cost.

15. When Full-Bed Replacement Makes More Sense

Complete replacement deserves consideration when:

packing condition varies widely;
geometry has changed significantly;
fouling is severe;
material degradation is widespread;
exact original packing is unavailable;
tower performance is already unstable.

A uniform new bed provides a more predictable hydraulic baseline.

16. Can Different Packing Materials Be Mixed?

This requires more caution.

Examples include:

PP + PVDF;
SS304 + SS316L;
ceramic + plastic.

Even when packing shape and size are similar, material differences can affect:

density;
mechanical behavior;
chemical compatibility;
thermal expansion.

Material mixing should not be assumed acceptable simply because the pieces physically fit together.

17. PP and PVDF Packing

PP and PVDF have different:

density;
temperature capability;
chemical compatibility;
cost.

If mixed in one bed, they may also behave differently mechanically.

If PVDF was specified because PP is not sufficiently compatible with the process, mixing PP into that bed would undermine the material-selection basis.

18. SS304 and SS316L Packing

Mixing stainless grades may sometimes be mechanically possible.

But the key question is corrosion.

If SS316L was selected because SS304 is not sufficiently resistant to the process, adding SS304 may create:

premature localized corrosion;
inconsistent service life.

The weakest material can become the limiting factor.

19. Mixing Metal and Plastic Packing

This is generally a more substantial change.

Metal and plastic differ greatly in:

density;
mechanical strength;
surface characteristics;
temperature capability.

Randomly mixing them within one bed can create an especially non-uniform packing structure.

If both materials are needed, separate engineered beds are usually more logical.

20. Mixing Ceramic with Other Packing Materials

Ceramic packing is much heavier and more brittle than plastic packing.

Uncontrolled mixing can create:

mechanical mismatch;
segregation;
unpredictable bed density.

It should not be treated as a simple substitute for missing plastic or metal packing volume.

21. Different Packing Can Be Used in Separate Beds

This is an important distinction.

A tower may intentionally use:

Upper Bed → Packing Type A

then:

Collector / Redistributor / Support

then:

Lower Bed → Packing Type B

This can be completely valid.

Each bed is then:

hydraulically defined;
mechanically supported;
separately distributed.

22. Separate Beds Are Not the Same as Mixing

Consider:

Configuration A — Uncontrolled Mixing

25 mm + 50 mm Pall Rings randomly combined in one continuous bed.

This creates uncertain hydraulic properties.

Configuration B — Engineered Multi-Bed Tower

Upper bed: 25 mm packingRedistributor/supportLower bed: 50 mm packing

This is a defined design.

The difference is fundamental.

23. Why Engineers May Intentionally Use Different Packing in Different Beds

Possible reasons include:

different mass-transfer requirements;
different fouling tendency;
different hydraulic loading;
capacity optimization;
process-section differences.

For example, one section may prioritize:

efficiency;

while another prioritizes:

low pressure drop;
fouling tolerance.

The design must still be evaluated as a complete tower system.

24. Intentional Graded Packing

Some engineered systems may use different packing sizes in distinct layers or sections.

This can sometimes be used to optimize:

hydraulic capacity;
distribution;
fouling tolerance.

But this is different from casually mixing leftover packing.

A graded arrangement should have:

defined layer boundaries;
known packing specifications;
an engineering reason.

25. Do Not Create an Accidental Graded Bed During Maintenance

Maintenance personnel may think:

“We will put the larger rings at the bottom and the smaller ones on top.”

That becomes a design change.

Before doing this, engineers should review:

pressure drop;
mass-transfer duty;
distributor arrangement;
support requirements.

Field convenience should not replace process design.

26. Different Packing Sizes May Require Different Support Considerations

Smaller packing may require:

smaller support openings;
retaining screens;
different support geometry.

A support designed for 50 mm packing may not retain 16 mm packing safely.

Therefore, switching packing size affects more than the bed itself.

27. Mixing Can Cause Packing Loss Through the Support

If smaller packing is added to a bed supported by openings designed for larger packing, some pieces may:

pass through;
become trapped;
block gas passages.

Support compatibility should always be checked before size changes.

28. Hold-Down Requirements May Change

Lightweight packing may require greater attention to:

bed movement;
hold-down.

If an existing bed is partially replaced with a lighter packing material or geometry, the original restraint design may no longer behave the same way.

29. Bulk Density Changes Matter

Two packing types can have very different bulk densities.

Mixing them changes:

total bed weight;
support loading;
shipping quantity calculations.

For heavy materials such as ceramic or metal, support capacity may become important.

30. Mixing Can Make Quantity Verification Difficult

A uniform bed can be estimated from:

Bed Volume × Packing Bulk Density

A mixed bed may not have one reliable bulk-density value.

This makes:

quantity estimation;
weight checks;
future replacement planning

more difficult.

31. Published Hydraulic Data May No Longer Apply

Supplier data are usually based on a defined packing product.

If a bed contains an uncontrolled mixture, neither supplier's:

pressure-drop curve;
capacity data;
packing factor

may accurately describe the actual bed.

This creates uncertainty during:

design review;
troubleshooting;
capacity calculation.

32. HETP Data May Also Become Uncertain

For distillation service, HETP depends on:

packing;
system;
hydraulic load;
distribution.

If two packings are randomly mixed, using the HETP of either individual product may not accurately predict the mixed bed.

This is another reason mixing should be engineered rather than improvised.

33. Mixed Packing Can Complicate Troubleshooting

Suppose tower performance deteriorates.

If the bed is uniform, engineers can compare operating data with known packing characteristics.

If the bed is an undocumented mixture, questions appear:

What packing dominates the pressure drop?
Did the small packing segregate?
Which material is failing?
What is the real packing factor?

Uncontrolled mixing increases diagnostic uncertainty.

34. Document Any Partial Replacement

If old and new packing are combined, record:

old packing specification;
new packing specification;
approximate quantity replaced;
installation date;
bed elevation affected.

This documentation becomes valuable during future maintenance.

35. Do Not Mix Damaged Packing Back into the Bed

During a shutdown, removed packing may include:

intact pieces;
cracked pieces;
crushed pieces;
fragments.

Do not simply clean everything and reinstall it.

Separate packing that has lost its intended geometry.

Broken material can reduce:

void fraction;
drainage.

36. Ceramic Packing Requires Special Sorting

For ceramic packing, remove:

fragments;
severely chipped pieces;
broken rings/saddles.

Returning large quantities of fragments to the tower can create a locally dense bed.

Reuse should focus on packing that remains mechanically suitable.

37. Metal Packing Should Be Checked for Deformation

Before mixing reused metal packing with new packing, inspect for:

flattened rings;
collapsed openings;
severe corrosion.

A clean but deformed metal ring is not equivalent to a new packing element.

38. Plastic Packing Should Be Checked for Material Aging

Old plastic packing may have experienced:

heat exposure;
chemical attack;
embrittlement;
creep.

New packing may therefore behave differently from aged material even if the nominal specification is identical.

Condition assessment remains necessary.

39. What If the Original Packing Is No Longer Available?

This is common in older towers.

Possible options include:

find the closest technically equivalent packing;
replace the full bed with a new design;
engineer separate replacement sections.

The safest solution is not automatically to mix any available packing into the old bed.

Compare:

geometry;
size;
material;
hydraulic characteristics.

40. Supplier Change During Partial Replacement

If the original supplier is unavailable, obtain technical data from both:

existing packing records;
proposed replacement supplier.

Compare:

dimensions;
bulk density;
surface area;
void fraction;
material.

The objective is to determine how close the new packing is to the existing bed.

41. When Mixing Different Suppliers May Be Acceptable

It may be reasonable when the new product is demonstrably close to the existing packing in:

geometry;
size;
material;
physical properties.

Even then, document the change.

For critical separations, broader process review may be appropriate before partial replacement.

42. When Mixing Should Be Avoided

Avoid uncontrolled mixing when differences are substantial in:

nominal size;
geometry;
material;
bulk density;
hydraulic performance.

Also avoid mixing when:

process performance is critical;
tower operates near flooding;
pressure-drop margin is small.

43. High-Purity Distillation Deserves More Conservative Treatment

High-purity distillation may be sensitive to:

maldistribution;
packing efficiency;
pressure drop.

An undocumented mixed bed can make performance harder to predict.

Uniformity becomes more important when separation margins are tight.

44. Fouling Service May Require a Different Decision

In dirty service, engineers may want a more open packing.

But replacing only part of a restrictive bed with larger packing may not solve the overall problem.

The remaining smaller packing can still become the hydraulic bottleneck.

Sometimes full-bed redesign provides more value.

45. Partial Upgrade vs Full Upgrade

Suppose the existing tower contains small random packing and pressure drop is too high.

Replacing only the top 20% with larger packing may provide limited benefit if:

the lower 80% remains the main restriction.

The expected improvement should be evaluated before partial upgrade.

46. Can the Bottom Bed Use Larger Packing?

Potentially, if the tower is intentionally designed that way.

But engineers should review:

support grid;
liquid redistribution;
pressure drop;
mass-transfer duty.

Do not assume larger packing at the bottom is universally better.

47. Can the Upper Bed Use Smaller Packing?

Potentially.

A smaller packing may provide higher surface area, but also higher hydraulic resistance.

The arrangement should be evaluated based on:

vapor/gas load;
liquid load;
separation requirement.

Again, separate engineered sections are different from uncontrolled mixing.

48. Distributor and Redistributor Design Matter

When different packing sections are used, the transition between beds deserves attention.

A redistributor can help:

collect liquid;
reset distribution;
separate hydraulic sections.

This helps each bed operate closer to its intended design basis.

49. Do Not Use a Redistributor Solely to Justify an Arbitrary Packing Change

A redistributor can support a multi-bed design.

But simply inserting one does not automatically make any combination of packings optimal.

The entire tower duty still needs engineering review.

50. Common Mistake 1: Filling Missing Packing Volume with Whatever Is Available

This is a maintenance shortcut.

It can create:

mixed size;
mixed geometry;
uncertain hydraulics.

Correct approach:

identify technically compatible replacement packing.

51. Common Mistake 2: Assuming Same Diameter Means Same Packing

Two 25 mm rings may have different geometry.

Compare actual technical data.

52. Common Mistake 3: Mixing Dirty Old Packing with New Packing

The old material may immediately introduce:

deposits;
broken fragments;
hydraulic restrictions

into the new bed.

Inspect and clean/reject old packing first.

53. Common Mistake 4: Mixing Materials Without Reviewing Chemistry

A weaker material may fail even though the stronger material remains healthy.

The bed's reliability can be limited by the least compatible material.

54. Common Mistake 5: Ignoring Support Openings After Changing Size

A smaller replacement packing may fall through or block the original support.

55. Common Mistake 6: Calling a Random Mixture “Graded Packing”

An engineered graded bed has:

intentional sizing;
defined layers;
design calculations.

Dumping leftover sizes together is not the same thing.

56. Common Mistake 7: Assuming Partial Replacement Always Saves Money

Sorting, cleaning and uncertain performance may outweigh the material savings.

Compare total shutdown economics.

57. Common Mistake 8: Failing to Document the Mixed Bed

Future engineers may assume the tower contains one uniform packing type when it does not.

Maintain accurate records.

58. Data Required Before Mixing or Partially Replacing Packing

Existing Packing

type;
size;
material;
supplier/model if known;
age;
condition.

Proposed New Packing

type;
size;
material;
geometry;
bulk density;
hydraulic data.

Tower Data

internal diameter;
packed height;
number of beds.

Internals Data

support-grid opening;
hold-down;
distributor;
redistributor.

Operating Data

gas flow;
liquid flow;
pressure;
temperature;
pressure drop;
performance requirement.

59. Partial Replacement Decision Workflow

Step 1 — Identify Why Packing Is Being Replaced

Is the issue:

damage;
fouling;
corrosion;
missing quantity;
hydraulic limitation?

Step 2 — Inspect the Remaining Packing

Determine whether it is truly suitable for continued use.

Step 3 — Compare Old and New Packing

Check:

geometry;
size;
material;
physical properties.

Step 4 — Determine Whether the Bed Will Remain Hydraulically Predictable

Avoid large uncontrolled differences.

Step 5 — Check Support Compatibility

Especially if packing size changes.

Step 6 — Check Material Compatibility

Especially if a different material is proposed.

Step 7 — Decide Between Partial and Full Replacement

Compare:

technical risk;
downtime;
material cost;
future reliability.

Step 8 — Keep Different Packing in Separate Beds Where Appropriate

Use defined support/distribution arrangements.

Step 9 — Document the Final Configuration

Record:

quantity;
packing specification;
bed location.

Step 10 — Recommission the Tower

Establish a new baseline for:

pressure drop;
capacity;
separation/removal performance.

Frequently Asked Questions

Can I mix different Pall Ring sizes in one tower?

Different sizes should not normally be randomly mixed within one continuous bed unless the arrangement has been specifically engineered.

Can 25 mm and 50 mm Pall Rings be mixed?

Physically yes, but uncontrolled mixing can change void structure, pressure drop and distribution. Separate engineered beds are generally more predictable.

Can old and new random packing be mixed?

Potentially, if the old packing remains clean, mechanically sound and technically equivalent to the new packing.

Can different suppliers' Pall Rings be mixed?

Possibly, but compare actual geometry, dimensions, material and physical properties. The same nominal product name does not guarantee identical packing.

Can PP and PVDF random packing be mixed?

It should not be assumed acceptable. The materials have different chemical, thermal, mechanical and density characteristics. Material selection should reflect the actual process.

Can SS304 and SS316L packing be mixed?

Mechanical compatibility alone is not enough. If SS316L was selected for corrosion resistance, adding SS304 may reduce bed reliability.

Can different random packing types be used in separate tower beds?

Yes.

Different packing types or sizes may be intentionally used in separate engineered beds with appropriate support and redistribution.

Should I replace the entire bed or only damaged packing?

It depends on:

extent of damage;
remaining packing condition;
availability of equivalent replacement;
process sensitivity;
shutdown economics.

Can mixed packing increase pressure drop?

Yes.

Smaller packing can occupy voids between larger elements or create locally dense regions, increasing hydraulic resistance.

Engineering Takeaway

Random packing should be random in orientation—not random in engineering specification.

Uncontrolled mixing of different:

sizes;
geometries;
materials

within one continuous bed can create an unpredictable hydraulic structure.

The recommended decision sequence is:

Inspect Existing Packing → Define Replacement Need → Compare Old and New Packing → Check Material and Support Compatibility → Decide Partial vs Full Replacement → Separate Different Packing into Engineered Beds Where Appropriate → Recommission

The key distinction is:

Mixed Bed ≠ Multi-Bed Design

A tower can successfully use different packing types in separate engineered sections.

But casually combining different packing within one continuous bed can affect:

pressure drop;
distribution;
capacity;
mass-transfer performance;
future troubleshooting.

When in doubt, preserve a defined and documented packed-bed structure.

Need help evaluating a partial random-packing replacement or mixed old/new packing project?

Prepare:

existing packing type/size/material · proposed replacement packing · tower diameter · packed height · quantity to be replaced · support-grid details · gas/liquid rates · operating pressure/temperature · current pressure drop · reason for replacement

DAIER Tower Packing Engineering Assistant can support preliminary packing and hydraulic screening before detailed replacement or retrofit review.