Nov-2022

High efficiency contaminant removal - Improving new and existing acid gas sweetening processes

Gas sweetening processes using amine solvents are often challenged to meet productivity and reliability targets due to the impact of solid and liquid contamination. Some commonly seen issues are foaming, fouling and corrosion which lead to production loss from unscheduled downtime, reduced flowrate capability, regeneration issues, and increased maintenance labor and equipment costs.

Robert James & Dr Ali Arshad
Pall Corporation

Article Summary

This paper discusses how the installation of high efficiency liquid/gas (LG) coalescers before contactors, rich side liquid/liquid (LL) coalescers and lean and rich side particulate filters are recommended to remove liquids and solids - related process variability and the benefits that can be seen in both new and existing sweetening systems. The ultimate result: optimise and improved plant productivity and reliability with economically viable CAPEX and OPEX investments.

Acid Gas Removal Unit Performance & Optimisation
It’s well known that the acid gas removal unit (AGRU) performs an important industrial process to sweeten sour gas in gas plants, LNG facilities, refineries and chemical processes and is critical in achieving profitable economics. In fact, unreliable operations can have significant financial impact — often to the tune of millions of dollars. Consider a few examples:
- A 100 MMSCFD gas plant running 10% below capacity due to AGRU issues, with natural gas selling at $2.50/ MMBTU, experiences a daily gas revenue loss
of $25,800.
- In a refinery, the inability of the AGRU to treat acidic refinery fuel gas (RFG), coming from unit operations, may reduce the ability of that unit to run at capacity, thereby cutting back on refinery output of final products such as gasoline and diesel. That being said, if a 50,000 bbl. /day fluid catalytic cracker is forced to produce 10% below capacity due to AGRU issues, the impact to the refinery gasoline output is a reduction by 5000 bbl. /day. And, if gasoline is selling at $2/gallon, the total revenue loss is then in the range of $420,000/day.

With this financial leverage on so many AGRU users, it’s no wonder that there is strong motivation to optimise the AGRU operation. However, the complexities of this unit operation have shown that this can be a difficult challenge. In this paper, we will summarise a number of these key challenges such as foaming, fouling and corrosion and the impact that high efficiency contaminant removal can have on improving both old and new sweetening processes.

Plant Needs from AGRU Operation
All sites with acid gas treatment units share similar needs for productivity, reliability, low operating costs, safety and environmental protection. However, different plants have specific needs based on the type of facility, gas being treated and its end use.

Gas/LNG Plant Needs
- Achieve or exceed natural gas production quotas via reliable treatment of acid gases
- Maintain process reliability for production consistency and minimisation of downtime
- Provide consistent sales gas specification quality for Hâ‚‚S and CO2 levels
- Minimise off gas emissions through effective acid contaminant removal from the sour gas
- Minimise operating and maintenance costs from fouling, corrosion and filtration

Refinery Needs
- Achieve refined products unit production quotas via reliable treatment of acid gases
- Provide consistent refinery fuel gas quality to ensure reliable operation of all burner operations
- Minimise refinery off gas emissions through effective acid contaminant removal from the RFG
- Minimise operating and maintenance costs from fouling, corrosion and filtration

Challenges with AGRU Operation
Across all plants and facility types, the most common issues with AGRU operation include foaming, corrosion, fouling, the corrosion/fouling cycle, and the impact these have on productivity, reliability, emissions control and operating costs.

Foaming
Foaming in the contactor can result in huge amine losses, reduced operating capacity, and off-spec product. Amine carried over into the sweet gas can result in fouling of downstream equipment including compressors and burners. The AGRU requires efficient protection to prevent the ingression of liquid and solid contaminants into the contactor with the sour gas, as they negatively impact foaming tendency and foam stability, as measured by foam height and foam stability measurements. Liquid hydrocarbons are typically non-polar, low surface tension fluids that are strong foaming promoters, as they lower the surface tension of the polar, high surface tension amine, making easier bubble formation. Foam stability is impacted when the normally elastic, rapidly stretching and rupturing bubble wall that breaks foams readily becomes a gelatinous, stronger wall that resists rupturing. Hydrocarbons contribute to the formation of a gelatinous layer, and so they also contribute to foam stability. Particles further reinforce bubble skins by increasing viscosity of the fluid in the bubble wall and therefore reducing its ability to drain and rupture, increasing the foam stability of the amine and contributing to the build-up of a thicker foam layer. Even very small contaminants with sizes in the micron range can be extremely detrimental as they can cause severe increases in foam stability leading to foaming in the absorber, reduced flow capacity, loss of amine due to carryover, excessive use of anti-foams, and process upsets in the sulfur plant. Consequently, it is critical to effectively remove them down to very low levels prior to entering the contactor.

Corrosion, Fouling, and the Corrosion/Fouling Cycle
Most amine units contain large amounts of carbon steel internal surfaces in contact with the amine solution. These are prone to attack from the acidic environment created by the sour gases, leading to the formation of fine iron sulfide and iron oxide particulates, often less than 10 microns in size. Ideally, the hydrogen sulfide present in the sour gas reacts with the carbon steel to form an iron sulfide protective layer on the carbon steel interior surfaces. This layer resists further rapid chemical attack from the acid gases. If total suspended solids (TSS) levels in the amine solution can increase, the combination of high velocity of the recirculating amine, coupled with the abrasive nature of the hard iron sulfide and iron oxide particulates moving over the protective iron sulfide layer, can cause erosion of the protective layer. This continuous exposing of bare carbon steel to the corrosive environment of the acid gases creates rapid generation of new solids which then increases TSS levels in the amine, leading to further erosion and generation of new solids. A ‘corrosion/fouling’ cycle establishes when higher TSS leads to increased erosion of the iron sulfide protective layer, creating higher TSS.