Jul-2002

Amine purification system to increase crude processing

Bringing amine system contaminants down to low levels has reduced operational problems.Refiners experience increased throughput of sour crude by continuously maintaining low concentrations of heat stable salts, hydrocarbons and solids, utilising an entirely onsite-regenerable amine purification system.

Alfred E Keller, ConocoPhillips
Arthur L Cummings and Dennis K Nelsen, MPR Services

Article Summary

This continuous purification has been responsible for reducing the frequency and magnitude of amine system operational problems, reducing maintenance costs, and facilitating the increased throughput of sour crude and hydrocarbon products.

Current refinery economics dictate that opportunity crudes become a larger portion of the refinery’s diet. In most cases, these opportunity crudes have higher levels of sulphur. With the added burden of reducing sulphur levels in finished products such as diesel and gasoline, the refiner may need to invest substantially to process these crudes.

Most processes remove sulphur by converting it to hydrogen sulphide, followed by removal of the hydrogen sulphide with an alkanolamine (amine) treater. The refineries utilise vapour-liquid and liquid-liquid contactors to absorb the hydrogen sulphide into the amine. Many have “primary” amine systems to remove acid gases from refinery process streams (hydrogen) and products as well as tailgas treaters following their sulphur plant.

The stress on the amine acid gas scrubbing systems is increasing for these refiners. Compounding the stress is accumulation of contaminants such as heat stable salts (HSS), hydrocarbons and solids in the amine. Unfortunately this accumulation can lead to numerous and frequent problems including inefficient acid gas removal, process upsets, amine losses, hydrocarbon carry-over and higher costs.

While it is easy to identify the cost for amine replacement, filters, activated carbon and labour for replacing and disposing of used filters, the larger impact of reduced refinery throughput caused by poor amine system performance is often discounted until the problems become chronic. The amine system, which often gets little attention as a “utility,” draws considerable negative attention when its effect on throughput is finally recognised. In contrast, proper attention to amine system impurities control keeps the system performing as a dependable utility, dramatically improving profitability.

Documented correlations between operational problems and accumulations of impurities such as HSS, hydrocarbons and solids in the amine solution in refinery operations has led to the development of the proprietary Amine Shield Slip Stream Unit (SSU) that can keep all these impurities at levels which keep the solution substantially out of the range of the levels that lead to upsets and loss of treating efficiency and loss of refinery unit throughput. Using the MPR SSU, operators report not only overall smoother amine system operations, but also fewer upsets. One refinery has reported a 31% increase in sulphur handling capacity while reducing the number of fuel gas and product off-spec incidents.

With this amine purification unit in place, the same refiner doubled the amine capacity of the plant in anticipation of future opportunity crude purchases without adding filtration equipment or a new flash drum to the system. Improved amine system operations for this refiner were valued at $2.6 million/year. Capital savings of $3 million for the amine system expansion were made possible by eliminating filters and the flash drum.

Other benefits attested to are reduced corrosion, filtration media and solid waste, filtration maintenance costs, better amine/hydrocarbon separation, and less worry about meeting sulphur specs. Generally, as refiners take measures to reduce amine losses, contamination accumulation increases rapidly; this generally leads to upsets with greater impact on hydrocarbon throughput. Using the SSU allows the refiner to cut amine use costs without the problems of increased contamination buildup. All three processes – HSS removal (the HSSX process), hydrocarbon removal (the HCX process), and solids removal (the SSX process) are regenerable on-site with water-based regenerants. The regeneration waste is compatible with refinery liquid waste handling systems, including the waste-water treatment plant. The respective absorption media absorb the impurities from the amine solution, and release them to the aqueous regenerant solution.

Contaminant problems
Hydrocarbons (with gasoline and higher boiling ranges) enter the amine system due to their natural solubility by entrainment, condensation into cooler amine solution and by upset conditions in contactors. The foaming, upsets and poor treating that result from hydrocarbon contamination are well known to amine system operators.

While it is well known that foaming is related to hydrocarbons, the relationship has not really been well understood. Some of the important factors to consider are surface tension, foam and emulsions, heavy liquid hydrocarbon effects, anti-foam, ammonia, iron sulphide, HSS and amino acids.

Surface tension
The function of amine contactors is to generate surface area between the liquid amine phase and the stream being treated to facilitate mass transport of acid gas components across the interface surface. Contactors can do this either by providing a large surface area for one phase to cling to while the other phase passes by the surface (packing, proprietary FiberFilm etc), or by generating the surface area by agitation with the use of trays, or by creating droplets that pass through the other phase (spray towers).

Regardless of the device, energy is expended in the formation of the surface area. The amount of energy required per unit of surface area generated is proportional to the surface tension of the liquid. Therefore as the surface tension of the fluid decreases, the greater the amount of liquid surface area can be generated by doing work on the system.

Foam and emulsions
Foam is simply a structure of expanded liquid surface area containing the gas that was agitated or entrained in the liquid. The energy from agitation generates the surface area in the liquid, the low surface tension of the liquid makes the energy more efficient in the generation of the surface area and the liquid surface stays stable because it cannot drain effectively from the structure. The properties of the soap or detergent normally employed to generate stable foam are those of lowering surface tension and increasing the liquid viscosity in the bubble film.