Structured Packing in EDC Purification: Heavy-End Removal, Vacuum Operation and Fouling Control

Ethylene dichloride purification is an important part of the vinyl chloride monomer process because the EDC entering the cracking furnace should not carry an uncontrolled mixture of light contaminants, heavy chlorinated byproducts and accumulated recycle impurities.

A typical VCM plant produces EDC through direct chlorination and oxychlorination. After purification, EDC is thermally cracked to vinyl chloride monomer and hydrogen chloride. Unconverted EDC is recovered and returned to the purification loop rather than being discarded.

This recycle structure means that EDC impurities cannot simply be allowed to accumulate.

Distillation is therefore normally used to remove both light ends and heavy ends before the EDC returns to the cracking section. In the heavy-end service, structured packing can be attractive because it provides fractionation while reducing the hydraulic resistance of the column.

The main engineering objective is not “high efficiency at any cost.”

It is:

recover clean EDC while minimizing heavy-residue buildup, unnecessary reboiler temperature and loss of valuable EDC into the purge.

Why EDC Has to Be Purified Before Cracking

The basic VCM route contains a recycle loop.

EDC is produced, purified and sent to a cracking furnace. The furnace converts part of it to VCM and HCl, while some EDC remains unreacted. After the cracking products are separated, that unconverted EDC comes back into the process.

This is economically attractive because EDC is recovered.

But recycle also concentrates impurities if the purification system does not remove them.

EDC purification therefore normally includes several different duties.

A light-ends column removes compounds more volatile than EDC and may also contribute to water removal.

A heavy-ends column recovers purified EDC overhead while concentrating higher-boiling material in the bottom stream.

Some flowsheets then use an additional heavy-end concentration step to recover more EDC before the final heavy purge is discarded or treated.

These towers should not all be treated as one generic “EDC distillation” application.

Their feed compositions and hydraulic problems are different.

The Heavy-End Column Is Really an EDC Recovery Column

The heavy components entering the bottom of the EDC purification train are unwanted, but the stream containing them still contains valuable EDC.

If the plant simply purges a large amount of this liquid, raw-material yield falls.

The heavy-end column therefore needs to achieve two competing objectives:

• keep heavy contaminants out of purified EDC;
• keep EDC loss in the final heavy purge as low as economically practical.

Adding separation stages can improve recovery, but that also increases:

• column height
• pressure drop
• reflux or reboiler requirements
• capital cost

Structured packing becomes useful when enough effective contacting can be obtained without creating excessive hydraulic resistance.

A published EDC purification process specifically identifies both random packing and structured sheet metal packing as alternatives to trays in the heavy-end distillation section.

Why Pressure Drop Matters in This Column

In many distillation services, pressure drop is mainly discussed as a capacity or energy issue.

Here it can also affect the thermal environment of the heavy liquid.

Every contacting device creates resistance as vapor travels upward.

If the column top operates at a fixed pressure, additional internal pressure drop raises the pressure experienced toward the lower part of the tower.

The reboiler then has to operate at a correspondingly different boiling condition.

A structured packed bed can reduce this resistance compared with a tall conventional tray section.

The advantage becomes even more valuable when the heavy-end column is integrated with vapor recompression or another energy-recovery arrangement, because lower column pressure drop reduces the pressure increase that the vapor-recompression system must provide. This relationship is explicitly discussed in published EDC purification designs.

So “low pressure drop” has a direct process consequence.

It can affect both the column itself and the energy-integration system around it.

Vacuum Can Help Reduce Reboiler Fouling

Heavy-end streams are rarely as clean as the purified EDC product.

They contain higher-boiling chlorinated species and accumulated process byproducts.

These materials remain in the hottest part of the purification train.

Published EDC purification technology specifically notes that the heavy-end column may operate either under pressure or under vacuum, with vacuum operation potentially preferred to reduce fouling of the main bottom reboiler.

The logic is straightforward.

Reducing absolute pressure allows vaporization at a lower temperature.

Lower thermal exposure can be valuable when the bottom stream contains material that tends to:

• decompose
• polymerize
• form tar
• create reboiler deposits

Structured packing complements this strategy because a low-pressure-drop contacting bed preserves more of the vacuum benefit through the full tower height.

A vacuum system cannot protect the reboiler effectively if most of the pressure advantage is lost across restrictive column internals.

Fouling Does Not Start and End Inside the Packing

If an EDC heavy-end column begins to show rising pressure drop, replacing the packing immediately may be the wrong response.

The source may be elsewhere.

Possible contributors include:

• heavy byproduct formation upstream
• thermal degradation in the reboiler
• solids or corrosion products
• poorly drained internal areas
• contaminated recycle EDC

If foulant is generated continuously in the bottom system, a new fine structured packing may eventually experience the same contamination.

For a retrofit, removed packing and reboiler deposits should be examined together.

Their location can help identify the mechanism.

Deposits concentrated near the lowest packing section suggest a different problem from deposits appearing uniformly through several beds.

This is also where packing openness matters.

A very fine high-area geometry may give strong theoretical efficiency, but a somewhat more open geometry can sometimes provide a better long-term operating margin if heavy contamination is expected.

Light Ends, Heavy Ends and VCM Purification Should Not Be Mixed Together

A VCM plant contains several distillation duties whose names can sound similar in an RFQ.

For example:

EDC light-ends columnremoves lighter impurities and may also contribute to dewatering.

EDC heavy-ends columnrecovers purified EDC from less volatile contaminants.

HCl/VCM separationseparates cracking products.

VCM purification columnproduces the final purified vinyl chloride stream.

Published VCM process descriptions show VCM and HCl being separated after EDC cracking, while unconverted EDC is recovered and sent back through EDC purification.

These are four different separation duties.

So when a customer sends:

“Structured packing for VCM plant”

the equipment tag or process service is essential.

Otherwise the supplier can easily quote the correct product family for the wrong column.

Water Content Can Change More Than Product Purity

Water is another reason the exact column service matters.

Some EDC purification schemes combine removal of light components with dewatering, while the downstream HCl/VCM separation system can also have stringent water control requirements.

Water in chlorinated hydrocarbon systems can also influence corrosion behavior when acidic species are present.

That means material selection should not be simplified to:

“EDC service = stainless steel.”

The actual material specification depends on the complete stream composition, including:

• water
• HCl
• chlorinated byproducts
• temperature
• operating pressure
• corrosion history

For DAIER, the EPC or plant corrosion specification should govern final alloy selection.

Packing, support grids, distributors, fasteners and other wetted internals need to be considered as one material system.

The Distributor Still Matters in a Heavy-End Tower

Low-pressure-drop structured packing only provides its expected performance when reflux and feed liquid are distributed properly.

If one portion of the bed receives excessive liquid while another is under-irrigated:

• effective separation falls
• local hydraulic loading increases
• heavy contaminants may penetrate farther into the purified EDC stream
• useful bed capacity is reduced

For a tall column with several packed sections, collectors and redistributors also need to be reviewed.

This is particularly relevant in a retrofit where an old tray tower is converted to structured packing.

The old feed entry or liquid collection system may have been perfectly acceptable for trays but unsuitable for a structured bed.

A successful conversion is therefore an internals-system redesign, not merely removal of trays and filling the empty space with packing.

Vapor Recompression Makes Low Pressure Drop Even More Valuable

Some EDC purification concepts recover energy from the purified EDC overhead vapor.

Instead of condensing all overhead vapor conventionally and supplying unrelated reboiler heat, vapor recompression can raise the vapor pressure and temperature so that its condensation heat can be reused.

In such a system, column pressure drop directly influences how much compression is required.

A larger pressure difference between the column top and reboiling condition increases the work that must be supplied elsewhere in the heat-pump loop.

Published EDC heavy-end purification concepts specifically connect the use of packing with reduced pressure drop and easier vapor-recompression energy integration.

This creates a strong revamp case when a plant is pursuing both:

• higher throughput
• lower steam consumption

But the packing cannot be evaluated independently from the compressor, reboilers and condenser arrangement.

The entire energy balance needs to be recalculated.

Capacity Increase Can Move the Bottleneck Downstream

A high-capacity structured packing may allow more vapor through the EDC heavy-end column.

That does not guarantee that the VCM plant can increase production by the same percentage.

Additional throughput must still pass through:

• reboilers
• condensers
• vacuum equipment
• pumps
• EDC cracking furnaces
• VCM/HCl separation
• recycle systems

If the heavy-end column was not the true plant bottleneck, installing higher-capacity packing may create little production benefit.

The retrofit should therefore begin with plant operating data.

Useful questions include:

• Is the column approaching flooding?
• Is pressure drop limiting vacuum?
• Is the reboiler fouling too quickly?
• Is too much EDC being lost in heavy purge?
• Is purified EDC failing impurity specification?
• Has plant throughput increased beyond original design?

Different answers lead to different packing decisions.

What DAIER Needs for an EDC Heavy-End Column RFQ

The inquiry should identify the exact service as EDC heavy-ends purification / heavy-ends concentration, not merely “VCM plant tower.”

Useful technical information includes:

• EDC feed rate
• EDC concentration
• identified light and heavy impurities
• water and HCl content
• operating pressure
• vacuum level where applicable
• top and bottom temperatures
• vapor flow
• liquid and reflux flow
• tower inside diameter
• available packed height
• purified EDC specification
• allowable EDC loss in heavy purge
• allowable pressure drop
• current packing or tray design
• distributor and support arrangement
• fouling history
• reboiler cleaning interval
• material specification

For a retrofit, two additional records are particularly useful:

pressure drop trend over timeandphotographs or analysis of heavy-end deposits.

They help distinguish a hydraulic problem from a chemistry or reboiler problem.

The Packing Should Improve EDC Recovery Without Making the Bottom Dirtier

EDC purification is a recycle-economics problem as much as a distillation problem.

The plant wants clean EDC for the cracking furnace, but it also wants to recover as much EDC as practical before heavy impurities are purged.

Structured packing can support that objective by providing efficient fractionation with relatively low pressure drop, especially in vacuum or energy-integrated heavy-end columns.

But the right geometry must still respect the dirty nature of the bottom stream.

The strongest engineering question is therefore:

What packing can achieve the required EDC recovery and impurity rejection while keeping bottom temperature, pressure drop and fouling low enough for a practical operating run?

That is the real job of the structured packing in an EDC purification train.