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FILTRATION & SEPARATION


THE SWEET AND


contactor are therefore those carried by or added to the solution, and those formed within the solution. Controlling contamination is achieved either by limiting ingress or by direct removal.


Foam formation is critical to effective amine contactor operation and the process of stripping acid contaminants from natural gas. As the gas rises through the amine inside the contactor, it disperses in the form of bubbles and forms a froth or foam (see Figure 2). In a good quality amine, the foam will quickly dissipate. Problems begin when this foam becomes stabilised due to increases in surface tension caused by the presence of contaminants.


Cleanova’s Dollinger range of GP-198 high-efficiency coalescers


Colm Joy, Global Engineering Director at Cleanova, explains how well-designed and maintained filtration systems can permanently reduce foaming to improve product quality and lower costs in amine sweetening operations


as plants and refineries must meet strict specifications on the quality of their produced sales gas. This raises ongoing challenges in terms of process and control. Engineers are tasked daily with improving plant efficiencies and minimising downtime, and often find themselves fighting the visible impacts of contaminants in their process: namely excessive contactor foaming, solvent losses due to degradation, and mechanical corrosion of system hardware.


G


While antifoam agents can temporarily reduce excessive foaming, only filtration technology can permanently prevent the contamination that leads to foaming and


Figure 1: AGRU


associated operational problems, as this article will demonstrate.


Gas sweetening process Many natural gas reservoirs produce ‘sweet’ gas, a product that has a good heat value and is relatively simple to process. With other reservoirs, the underground natural gas is contaminated and is termed ‘sour’ gas. This sour gas requires cleaning, so only sweet gas is transferred through pipelines to downstream processes.


40 MAY 2024 | PROCESS & CONTROL


The two sour gas components most associated with oil & gas production are hydrogen sulfide (H2S) and carbon dioxide (CO2). These contaminants occur in a variety of concentrations depending on the reservoir and are corrosive to both piping and equipment. Removing CO2 also improves the heating value of the sales gas and pipeline transmission. The process of removing these contaminants is called gas sweetening.


An Acid Gas Removal Unit (AGRU) is generally used in the gas sweetening operation (See Figure 1). While complex in terms of control and operation, an AGRU basically consists of two primary processes operating in tandem:


i. The absorption process: whereby gas and amine mix in the contactor, stripping the acidic contaminants from the gas to produce sweet gas at the contactor outlet. ii. The amine regeneration process: which enables the stripped acidic contaminants to be removed so the amine can be recycled. This is a closed loop cycle, so anything that enters the amine solution – acids, particulates, antifoaming agents, etc – will remain within the loop until action is taken to remove them. The two categories of contaminants that directly contribute to foaming in an amine


Antifoam does not eliminate foaming: it merely reduces its severity. Once antifoam injection ceases, the system becomes vulnerable to foaming again. This allows contaminant build-up over time, leading to severe problems such as amine degradation, solvent losses, and corrosion. In fact, excessive antifoam dosing can turn the antifoaming agent into a foam promoter.


Filtration solutions The correct application of filtration systems to both the incoming gas and amine loop has proven to be the most effective means of maintaining an AGRU system. This approach permanently removes contaminants such as liquid hydrocarbons, solid particulate and heat stable salts (HSS) before they enter the process. Figure 3 demonstrates the application of different filtration methods in the gas sweetening process.


Combining upstream condensation of heavy hydrocarbons through dewpoint control with higher efficiency gas coalescers as a final stage filter before the sour gas enters the amine contactor has clear advantages. Where gas separators are used without downstream high-efficiency coalescers, heavy hydrocarbons (C5 – C12) can pass downstream of the inlet gas separator, often as aerosols too small (<5µ) to be effectively captured by separator filters intended for that very purpose. Even minute amounts of liquid hydrocarbon in either the feed gas or the lean amine will change the surface tension of the amine, causing foaming. Minimising or eliminating them from the contactor inlet gas feed and the circulating amine should be the primary goal for AGRU plant operators. Cleanova’s Dollinger range of GP-198 high-efficiency coalescers (see above) have been successfully deployed at this final gas feed stage to ensure that the minimum volume of hydrocarbon liquid reaches the contactor trays.


Particulates in the feed gas only represent part of the solids problem. Analysis of total


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