Introduction
Mini Gas Refineries are designed as an efficient and cost-effective solution for processing natural gas extracted from small gas wells. These refineries, with a typical capacity of less than 20 million standard cubic feet per day (MMSCFD), are used to purify sour gas, remove impurities, and produce high-value products such as dry natural gas, stabilized condensate, liquid sulfur, and liquefied petroleum gas (LPG). This article provides a comprehensive overview of the structure, process units, and significance of mini gas refineries for small gas wells.
What is a Mini Gas Refinery?
A mini gas refinery is a set of process units that receive raw (sour or impure) natural gas from small gas wells and convert it into usable products for domestic, industrial, or export purposes. Unlike large-scale refineries designed for massive gas fields, mini refineries are ideal for small, scattered wells due to their low capacity, reduced capital costs, and high flexibility. These refineries are typically built near the gas wells to minimize transportation costs.
Importance of Mini Gas Refineries
- Reducing Gas Wastage: Due to the lack of processing infrastructure, gas is flared or vented in many small wells. Mini refineries prevent this waste.
- Cost-Effectiveness: The cost of building these refineries is significantly lower than that of large refineries.
- Meeting Local Needs: They can supply fuel to remote areas or neighboring countries.
- Flexibility: Their modular design allows for relocation and use in different locations.
- Environmental Benefits: Refineries help reduce pollution by treating acidic gases and converting sulfur into a marketable product.
Typical Process Units in a Mini Gas Refinery
Mini gas refineries consist of various process and utility units designed to purify gas and produce high-quality products. Below is a detailed breakdown of the main units:
1. Initial Separation Unit (Separator Unit)
Raw gas from the well typically contains a mixture of gas, condensate, water, and glycol. In the initial separation unit, this three-phase stream is separated by reducing pressure:
- Gas: Sent to the sweetening unit.
- Condensate: Directed to the condensate stabilization unit.
- Water and Glycol: Routed to recovery or wastewater treatment units.
2. Gas Sweetening Unit
Sour gas contains acidic gases like hydrogen sulfide (H2S) and carbon dioxide (CO2), which must be removed. This unit uses absorbent solvents (such as methyl diethanolamine or MDEA) to strip out acidic gases. The used amine is then regenerated in a recovery system. The sweetened gas is ready for further processing.
3. Condensate Stabilization Unit
Gas condensates (liquid hydrocarbons) separated from raw gas contain volatile components that need stabilization. In this unit, the condensate is heated in a stabilization tower, separating light components (like methane and ethane). The resulting stabilized condensate can be used as feedstock for oil refineries or for export.
4. Dew Pointing Unit
To prevent liquid formation in gas transmission pipelines, the gas dew point must be adjusted. This unit lowers the temperature (often using propane refrigeration systems) to remove water and heavy hydrocarbons. The dried gas is then ready for pipeline transport.
5. Sulfur Recovery Unit (SRU)
Acidic gases separated in the sweetening unit (mainly H2S) are sent to the sulfur recovery unit. Using the Claus process, hydrogen sulfide is converted into liquid sulfur. The produced sulfur, with a purity of around 95%, is a marketable by-product.
6. Sulfur Solidification Unit (SSU)
Liquid sulfur from the sulfur recovery unit is transferred to the solidification unit, where it is converted into powder or granules. This form is safer for transportation and sale.
7. Sour Water Stripper (SWS)
Acidic wastewater produced in various units (containing H2S and ammonia) is treated in this unit. The stripped acidic gases are sent to the sulfur recovery unit, and the treated water is prepared for reuse or disposal.
8. LPG Recovery Unit
This unit separates heavier hydrocarbons (propane and butane) from the natural gas to produce liquefied petroleum gas (LPG). LPG is used for domestic or industrial purposes. This section includes ethane, propane, and butane removal processes.
9. Gas Compression Unit
The purified gas is compressed in this unit to reach the appropriate pressure for transmission through pipelines. Compressors in this unit are typically powered by gas turbines or electric motors.
10. Refrigeration Unit
This unit provides the cooling required for separation processes (such as dew point or mercaptan removal). It typically uses propane in a closed-loop system to achieve low temperatures (down to -35°C).
11. Utility Units
These units are essential for supporting refinery operations and include:
- Power Generation: Using gas turbines or diesel engines (e.g., four 30 MW gas turbines and one 19 MW steam turbine).
- Nitrogen Generation: For daily operations and instrumentation.
- Fresh Water Production: Through seawater desalination.
- Diesel Fuel Supply: For starting diesel engines and fire pumps.
- Water Cooling: For equipment cooling.
- Compressed Air: For instrumentation and general services.
12. Storage and Tankage Area
Tanks in this area are used to store raw gas, condensates, final products (like LPG and sulfur), and raw materials needed for the refinery. These tanks may have fixed roofs, floating roofs, or be pressurized.
13. Fire Fighting Unit
This unit includes fire pumps, water tanks, and fire suppression systems, which are critical for refinery safety.
Advantages and Challenges of Mini Gas Refineries
Advantages:
- Low Cost: Lower investment compared to large refineries.
- Quick Construction: Shorter build times due to modular design.
- Geographic Flexibility: Can be built in remote areas.
- Reduced Fuel Smuggling: Provides standardized fuel for local regions.
Challenges:
- Economic Viability: Requires detailed techno-economic studies for justification.
- Environmental Standards: Must comply with stringent regulations to reduce pollution.
- Competition with Large Refineries: Projects like the Siraf Refinery (eight 60,000-barrel refineries) may reduce their appeal.
- Management of Wastewater and Acidic Gases: Requires advanced technology for treatment.
Conclusion
Mini gas refineries offer an effective solution for utilizing small and scattered gas wells. With diverse process units, they transform sour gas into valuable products like dry natural gas, LPG, stabilized condensate, and sulfur. Despite challenges such as economic feasibility and environmental compliance, these refineries play a vital role in supplying energy to remote areas, reducing gas wastage, and fostering economic development. Their modular design and low cost make them an attractive option for investors and policymakers in the energy sector.
References
- Center for Oil and Gas Value Chain Studies
- Mini Refinery Feasibility Study
- How Gas Refineries Operate
- Natural Gas Refinery, Wikipedia
- Complete Refinery Process, Engineering Pathway
Editor: Meysam Azizi Kouchaksaraei