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Mitigating fouling in the caustic tower

Comprehensive analysis of foulants in an ethylene plant’s caustic tower led to the identification of an effective treatment programme

HUA MO and DAVID DIXON, Baker Hughes
LOWELL SYKES, Westlake Vinyls
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Article Summary
An ethylene plant experienced severe fouling in its caustic tower. The degraded performance of the caustic tower threatened ethylene production. The progression of fouling in the caustic tower indicated that the treatment programme at that time could not control the fouling.

To reduce caustic system fouling and extend unit run length, extensive testing was conducted to identify all fouling mechanisms prior to initiation of a new treatment programme. Identification of fouling mechanisms provides a clear understanding of the root causes of fouling. It also helps to identify the right chemistries to apply for a successful treatment programme.

The Baker Hughes treatment programme, based on the identified fouling mechanisms, improved the performance of the caustic tower. System degradation dramatically slowed and the run length was extended. This article reviews the ways to identify fouling mechanisms and the impact of modifying a treatment programme based upon the identified fouling mechanisms.

Description of unit
The caustic tower in the ethylene plant is used to remove acid gases from cracked gas. A general flow diagram of the caustic tower is shown in Figure 1.

There are typically four sections in a caustic tower: weak section, intermediate section, strong section and water wash section. The caustic solutions are circulated in the strong and weak sections. Boiler feed water is circulated in the water wash section, and the cracked gas from the compressor feeds into the caustic tower weak section. When the cracked gas contacts caustic solution, the acid in the cracked gas is removed by an acid-base reaction. Some hydrocarbons are also captured by caustic solution at the same time. After the acid removal, the cracked gas leaves the tower overhead and feeds the next stage of compression.

Fouling mechanisms
Polymeric hydrocarbon precursors present in the caustic solution can form various polymeric materials by different reaction mechanisms. These polymers can deposit and agglomerate in the caustic column, causing fouling, reduced throughput and decreased tower efficiency. There are three major organic 
fouling mechanisms: aldol condensation polymerisation, free-radical polymerisation 
and Diels-Alder reaction polymerisation.

Aldol condensation
The aldol condensation reaction mechanism is shown in Figure 2. As an anionic reaction, it only occurs in the base condition; the condensation reaction initiates and propagates in the circulating caustic solution. The precursors for the aldol condensation reaction include various aldehydes, ketones and unsaturated esters. The two most commonly identified aldol condensation precursors identified in olefin cracked gas streams are acetaldehyde and vinyl acetate.

Free-radical polymerisation
The free-radical reaction mechanism is shown in Figure 3. The presence of free-radical initiators, such as peroxides, may initiate the reaction. In addition, monomers such as styrene will readily self-initiate free-radical polymerisation reactions at typical caustic tower operating temperatures. The presence of olefins, such as conjugated olefin, styrene and indene, provides the monomers for chain propagation. The polymer chain length will depend on temperature, stability of the initiator or monomer concentration.
Diels-Alder reaction
The mechanism of a Diels-Alder reaction is shown in Figure 4. Diels- Alder reactions are self-initiated reactions and may occur at low temperatures. The precursors for Diels-Alder reactions are conjugated dienes, such as cyclopentadiene derivatives.
Although many mechanisms may co-exist in the caustic tower, one or more reaction mechanisms cannot significantly contribute to the fouling. Different caustic tower systems will have different fouling mechanisms. To ensure successful treatment, each fouling mechanism or combination of fouling mechanisms must be identified for each system.

Fouling control
Aldol inhibitor, free-radical inhibitor and dispersant are used to control the fouling.

Aldol inhibitor (Polyfree 305C)
The aldol inhibitor from Baker Hughes is used to quickly convert carbonyl in aldol to another non-reactive functional group. After the reaction, the aldol condensation is completely inhibited; there is no side reaction of this inhibitor.

Free-radical inhibitor (Polyfree 300R1)
The active component in the free- radical inhibitor reacts with the free-radical, either carbon or oxygen centred, in the caustic tower. After the free-radical reacts with the free-radical inhibitor, free-radical polymerisation is inhibited.

Dispersant is used to disperse foulants that could not be inhibited by the aldol inhibitor and free-radical inhibitor, such as products of a Diels-Alder reaction. Dispersant could prevent foulant from 
precipitation. The dispersed foulant will be carried out of the caustic tower by the spent caustic system.

Identification of fouling mechanis Methodology
Fouling in the caustic tower is influenced by two factors: the accumulation of existing foulant and the formation of new foulant. The accumulation of existing foulant can be evaluated and monitored with filterable solids analysis and soluble polymers analysis. The potential to form new foulants is influenced by many factors, including reaction mechanisms, fouling precursors, process temperatures and initiators. Although the existing foulant was investigated, this article will primarily focus on identifying and controlling foulant-forming reaction mechanism(s) to reduce and/or eliminate the accumulation of new foulant material.
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