Feb-2025

Mass transfer solutions: Selecting the optimal solution

Utilising low differential pressure, high surface area mass transfer devices reduce treating unit size while meeting more stringent regulations and improving performance.

Mark Knobloch
Merichem Technologies

Article Summary

It is extraordinary to find a chemical process that does not require either a preliminary purification of raw materials or a final separation of products from byproducts. In the oil and gas industry, hydrogen sulphide (H2S) and other mercaptans present in crudes must be removed from intermediate or final products for performance, economic, environmental, and health reasons. The goal is to find the best way to do this, and, in many cases, mass transfer is the root of the solution.

Mass transfer
There is a library full of technical literature on commercial processes used for impurity removal. However, it has become clear that not all treatments are equal, and not all treatment offerings using mass transfer as the solution can be counted on to work the same way.

Selecting the optimum treatment for removing impurities from hydrocarbon streams has been a challenging task for industries around the world. Conventional dispersion and phase separation methods are subject to numerous shortcomings. The conventional method of contacting two immiscible liquids is to disperse one liquid thoroughly into the other as small droplets. Where small droplets are generated using high differential pressure mix valves, larger droplets are formed using trays. Impurities pass between the two phases at the surface of the droplet.

Even when the dispersion-based system provides adequate treatment, separating the two phases is usually extremely inefficient. The smaller the droplet size, the more separation time is required. Since the separation drum has a fixed volume, increasing the quantity of smaller, micron-sized droplets to improve mass transfer results in insufficient time to separate fully. Thus, the micro-droplets ‘carry over’ with the treated hydrocarbon phase, causing minor to moderate contamination in downstream equipment. If the shear force is set too high across the mixing device to force more mass transfer, stable emulsions can form, resulting in massive carryover out of the separator vessel.

Mass transfer can only be improved by creating more numerous and smaller droplets to increase the surface area. This is more effectively accomplished using non-dispersive hydrocarbon treating processes for caustic, amine, and acid.

Mass transfer market
Research firm Market Research reported late last year that the mass transfer market is expected to grow substantially over the next few years. It attributes this to an increasing demand for purified substances in various industries, including petrochemicals and refining, where there is a need to separate and purify crude and gas. The analyst firm also calls out aggressive research and development for efficient and cost-effective distillation systems along with innovative techniques that are expected to revolutionise the mass transfer industry. Continuously evolving regulations and standards related to the use of environmentally friendly and energy-efficient distillation systems on federal and state levels are also driving market growth.

Technology solution
There is a highly adaptable, non-dispersive mass transfer device that utilises caustic, amine, and other aqueous solutions as the treating reagent to remove acid gases, mercaptan compounds, and other aqueous-soluble impurities from liquid and gas hydrocarbon streams. It consists of a vertical cylinder packed with thousands of metallic fibres, also known as a ‘fibre bundle’. The hydrocarbon needing treatment and the aqueous treating solution are both introduced to the top of the bundle and flow cocurrently through the bundle (see Figure 1).

As both phases flow down the bundle, the aqueous phase adheres to, or wets, the metal fibres and is continually renewed as it flows down the length of the fibre via a combination of gravity and interfacial drag between the two immiscible phases. The hydrocarbon phase flows through the cylinder concurrently and between the aqueous-wetted fibres. The large surface area and tight packing of the metal fibres bring ultra-thin falling films of the aqueous phase into intimate contact with the hydrocarbon phase. The interfacial surface area produced is orders of magnitude larger than in conventional droplet dispersion devices, allowing impurities to diffuse easily between phases.

After the mass transfer is completed, both phases enter a separator, which quickly allows complete phase separation using their density difference. Due to the lack of micro droplets, bulk separation takes only a few minutes.

This technology has many benefits. Its large interfacial surface area, microscopic diffusion distance, and continuous renewal of the aqueous phase combine to yield mass transfer efficiencies far greater than possible with conventional dispersive treatments. Since it is highly customisable, fibre bundle geometry allows the treatment of a wide range of hydrocarbon types with different physical properties. The inherent low differential pressure drop across the fibre bundle and the fast phase separation time facilitate the retrofit of this treatment process into existing systems as a debottlenecking solution.

The cocurrent flow is more forgiving during hydrocarbon flow upsets, allowing treatment even under sub- optimal conditions. Carryover is virtually eliminated due to the avoidance of droplet formation as the aqueous phase adheres to the fibres in the contactor rather than being dispersed into the hydrocarbon phase. Emulsion formation is also effectively eliminated. Since efficient phase contact occurs without dispersion, stable emulsions rarely form in the unit.

Similarly, the system does not depend on gravity settling for micro droplets or emulsion coalescence, which greatly reduces separation time. Processing vessels can be much smaller. In most cases, expensive downstream coalescers and other clean-up equipment are not required. However, coalescers can further enhance the separation, taking the ultimate separation down to almost non-detectable levels of carryover of aqueous treating liquids in the hydrocarbon product. The smaller size separation requirement translates to less expensive equipment cost, and with fewer pieces of smaller equipment, plant space is more efficiently utilised. Most importantly, this method of mass transfer achieves maximum removal of impurities from the hydrocarbon to meet today’s stringent standards as set forth by the US Environmental Protection Agency’s Clean Air Act.  

Challenges of other solutions
Conventional caustic treating processes with dispersive mixing devices and phase separation were once the only option available to the industry. However, conventional dispersion and phase separation methods are subject to numerous shortcomings, including lack of turndown capability, plugging, flooding, channelling, unpredictable treating results, long settling times, aqueous phase carryover, generation of dilute aqueous wastes, lower service factor, hydrocarbon losses, larger plot space, product contamination, and additional processing steps and equipment needed to separate phases.

Even when the dispersion-based system provides adequate treatment, separating the two phases can be problematic. The mixture must remain in the phase separator until the caustic droplets settle out by gravity, which can take hours. As the treating requirement becomes more stringent, mixing energy is increased to maximise interfacial surface area. This results in a greater dispersion of the aqueous phase, requiring exponentially longer separation times.