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
I t is extraordinary to find a chemical process that does not require either a preliminary purification of raw mate - rials or a final separation of products from byproducts. In the oil and gas industry, hydrogen sulphide (H2 S) 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 commer - cial 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 gen - erated 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 ade - quate 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 insuffi - cient 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-dis - persive 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 increas - ing 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 regula - tions 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 solu - tions as the treating reagent to remove acid gases, mer - captan 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 intro - duced 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 continu - ally 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-wet - ted 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 magni - tude 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 separa - tion 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
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PTQ Q1 2025
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