Chemically Induced Foaming in Gas-Liquid Contactors

Contamination in feed gas streams to processing plants is one of the leading causes of lost revenue, upsets, and low throughput. More detailed testing shows that surfactants in the gas stream are one of the most damaging contaminants affecting the process.?Surfactants often pose a number of chemically induced challenges such as foaming in gas-liquid contactors and emulsification in liquid-liquid contactors. ?In fact, surfactant contaminants in gas or liquids feed streams to processes units are the predominant cause foaming and emulsions treating units leading to high H2S/CO2 in the treated stream, in addition to solvent losses and negative downstream impacts.

Surface active contaminants in feed streams should therefore be tested, sampled, analyzed, and removed in order to enable processing plants to run stable. ?This article discuss details of the presence of surfactant contaminants directly linked to solvent foaming and the associated solvent losses. Specialized methods for detecting surfactants presence such as surface tension and surface rheology evaluation will ne examined in forthcoming articles. Techniques for on-site sampling of the inlet feed stream will also be discussed.?Future articles will also discuss some measures to be implemented to remove contaminants such as surfactants from the process, how to best maneuver during foaming events and methods to eliminate downstream effects. The concepts herein related to foaming can also be translated to emulsions.

In simple words, foam is a materials formed when pockets of gas are trapped within a liquid envelope. The gas pocket or bubble is encapsulated by a liquid layer. This layer can be resilient, elastic and deformable leading to foam preservation overtime. Certain components such as surfactants (surface active materials) will populate the interface between the gas and liquid, and impart elasticity and deformability stabilizing foam. Certain solids will also have the same effect. These solids will have dual properties, interacting with both polar and non-polar media. Emulsions will follow a similar rationale compared to foam, however, emulsions are pockets of liquids trapped within another liquid. Both liquids in an emulsion need to be immiscible within each other for an emulsion to be created. Creation of foam and emulsion need energy to created. This energy is generally in the form of agitation, mixing or turbulency.

Surfactant Contaminants

Surfactants in the gaseous phase, water, or hydrocarbon dissolved and entrained in the feed gas to processing units such as amine units or dehydration units can cause a number of detrimental effects, most predominantly foaming.?Foaming can then lead to several secondary problems such as inability to meet specifications and/or solvent carry over.?One of the most common and difficult challenges in processing units such as amine units is dealing with the various forms of surfactant ingression into the system.

Surfactants are interfacial-active molecules.?They generally consist of a polar section (head) or group and a non-polar group, generally hydrocarbon chains (Figure 1 below).?The polar part of the molecule can interact with polar solvents like water and is therefore also called the hydrophilic portion.?The non-polar part, on the other hand, can interact with non-polar materials such as hydrocarbons and is therefore called the lipophilic or hydrophobic portion.

Surfactants can be classified according to the charge of their polar head group:

·??????Anionic surfactants have a negatively charged head group

·??????Cationic surfactants have a positively charged head group

·??????Zwitterionic surfactants have both positive and negative charged groups

·??????Nonionic surfactants have an uncharged polar head group

Surfactants adsorb preferably at interfaces where they find the most energetically favorable conditions, because of their two-part structure.?At a water surface, for example, the surfactants orient themselves in such a way that the head group resides in the water and the hydrocarbon chain points to the gaseous phase (Figure 2 below).?Thus, surfactants can "mediate" between two phases as they can form strong interactions with both of them.?The interfacial tension consequently decreases.?The addition of surfactants hence facilitates the mixing of non-polar and polar phases, which is used in the detergent industry for example.

The decrease of the interfacial tension caused by surfactants becomes stronger as more surfactants are located at the gas/water interface.?Once the gas/liquid interface is saturated, the addition of more surfactants will not necessarily decrease the interfacial tension further.?

It is important to mention that solids such as iron sulfides can under certain conditions also act as a surfactant because the solids’ surface can interact with both water and hydrocarbon at the same time. For the purposes of this article, solid-based surfactants (particles) will not be covered; only molecular surfactants as described previously are discussed.

Examples of some surfactants commonly found in processing feeds, such as amine unit and dehydration unit feed gas, include lubrication oils and upstream or pipeline chemical additives.?Lubrication oils from gas compressors typically contain 90+% base oil (most often heavy petroleum fractions) and additives for various functions, which often have surface-active properties.?These additives deliver reduced friction and wear, increased viscosity, improved viscosity index, as well as resistance to corrosion, oxidation, and ware, a.? Upstream and pipeline chemical additives on the other hand can be biocides, corrosion inhibitors, H2S scavengers, paraffin inhibitors to name a few. Corrosion inhibitors (filming amines or quaternary ammonium salts with alkanol segments) are an example of process additives with surface active properties. As with surfactants in general, filming amines have a hydrophobic section (long alkyl chain called tail), and a hydrophilic section (polar ionic center called head).?

To illustrate this point, Figure 4 below shows the change in surface tension of distilled water compared to distilled water when contacted with a lubrication oil from a compression system.?The decrease in surface tension from 72 mN/m (millinewtons/meter) to 46 mN/m is a clear indication of the surfactant properties of water-soluble additives present in lubrication oils.?Similar effects are observed with some upstream chemical additives. The decrease in surface tension leads to an increase in entrainments and dissolved contaminants as separation equipment loses its phase separation efficiency when foam and/or emulsions are present.?Poor phase separation efficiency leads to downstream impacts including fouling, corrosion, contamination, and foaming, in addition to number of secondary effects such as solvent losses, waste, unscheduled maintenances, and unit performance decay.

David Engel, Ph.D. | Short Biography

David has over 25 years of industrial experience in a variety of areas of chemical synthesis, pharmaceutical chemistry, sensors, light to energy conversion, membranes, nanotechnology, security technology, chemical additives, process chemistry, contaminant separation technologies and process optimization

David is the inventor in 21 United States Invention Patents on various technologies. David has executed several technical projects, troubleshooting projects, new technology development projects, and business development initiatives for Eastman Kodak, Porous Media/Pentair, General Electric and Sulphur Experts globally. David has authored 100+ papers and seminars on a variety of technical and scientific subjects. Recently David has focused on process contamination removal, enhancing process efficiency/profitability, and equipment reliability with reduced use of resources.?

Currently, David is the Managing Director of Nexo Solutions in addition to Senior Partner at Seleniumm Holdings. David is also a Member at the American Filtration & Separations Society Southwest Region, Member of Board of Directors, Expert Witness in several legal platforms and member of the of the GPA Midstream Section M.

contact: [email protected]