Fouling in hydrogen recycle gas compressors
Hydrogen recycle gas compressors are one of the most significant rotating equipment in oil refineries, petrochemical plants and upgraders.
Hydrogen (H2) recycle gas compressors are one of the most significant rotating equipment in oil refineries, petrochemical plants and upgraders. The compressor is usually a centrifugal gas compressor between bearings. A single-shaft centrifugal compressor design is based on the API Standard 617.1 Without the compressor, a plant’s production will be stopped. FIG. 1 shows a typical H2 recycle gas compressor in a hydrotreating unit.
FIG. 1. Simplified hydrotreating process.
H2 recycle compressors are used with oil refinery processing equipment, including hydrocrackers, hydrotreaters, platformers and isomerization units. The hydrotreating process is similar in oil refineries and petrochemical plants like aromatic plants and upgraders. Depending on the licensor, the number and configuration of the equipment may be different; however, the process is very similar.
The risk of fouling is high in recycle gas compressors due to chemical injection and reactions in the reactor. Stress corrosion cracking, possibly combined with high cycle fatigue, causes impeller and rotor failures.
This article will explain the hydrotreating process and address compressor fouling, which impacts the recycle gas compressor’s performance and reliability, based on the author’s experience working in large oil refineries, petrochemical plants and upgrader plants.
The hydrotreating process
The hydrotreating process reduces mercaptan, sulfur, metal content, nitrogen, oxygen, organic halides (chlorides and bromides), olefins and aromatic components of the feedstock while enhancing the cetane/octane number in oil refining and the density and smoke point of the product. Some hydrocracking and thermal cracking reactions may take place in the hydrotreating process. FIG. 1 shows a typical sketch of the hydrotreating process in oil refineries, upgraders and petrochemical plants. The primary duty of the recycle gas compressor is to move the feedstock (Point A) through a fired heater, reactor, reactor feed effluent exchanger, air and trim cooler, separator and finally, the compressor’s suction. Make-up gas (Point C) supplied by a separate reciprocating gas compressor replaces the H2 consumed. The large quantity of H2 gas recycled from the separator to the reactor has the following purposes:
- Absorbing some of the heat of reaction, minimizing the catalyst bed temperatures.
- Lowering the heater and feed effluent exchanger tube wall temperatures by increasing the flow through the equipment.
- Preventing coke formation as the feedstock is heated to the reaction temperature.
- Ensuring the reactions are carried out completely.
FIG. 1 shows a liquid feedstock sent to the heater by a charge pump, normally a centrifugal pump. Consider the pump shutoff pressure when designing the compressor to prevent surging. Typically, two check valves are placed close to the injection point to prevent reverse liquid flow towards the pump. Another check valve is required downstream of the anti-surge valve.
H2 gas
H2 gas is colorless, odorless and highly flammable. H2 gas forms explosive mixtures with air at concentrations from 4%–74% and with chlorine at 5%–95%. A spark, heat or sunlight can trigger explosive reactions. The H2 auto-ignition temperature—the temperature of spontaneous ignition in the air—is 500°C (932°F). Pure H2-oxygen flames emit ultraviolet light, and high-oxygen mixes are nearly invisible to the naked eye. Detecting a burning H2 leak may require a flame detector, and a leak can be hazardous. Working around H2 compressors, especially reciprocating gas compressors, requires knowledge, understanding and experience.
H2 recycle gas compressors
The compressor is typically a centrifugal type based on the API 617 design and consists of a vertically split main casing, which houses the rotor, inlet guide vanes, diaphragms and intake/discharge walls. The balancing piston seal, shaft end and inter-stage seals are also contained in the main casing. By pulling the barrel out of the compressor casing and lifting off the top half of the inner barrel, all compressor internals are accessible for inspection.
The compressor gas is mainly H2 with a low molecular weight. Low molecular weights are difficult to compress and have little pressure rise or volume reduction per stage. Due to this, the H2 recycle compressors normally have 6–12 stages, run at high speeds like a steam turbine, or both. The compressor location is generally at the mezzanine and driven by a steam turbine. The compressor seal is a dry gas seal tandem. The purpose of the recycle gas compressors is to circulate H2 gas to the reactor circuit to provide H2 for reaction and quench gas for reaction temperature control.
Compressor fouling-reliability improvement
Compressor gas composition shall be specified in detail in the datasheet and clarify to the various prospective vendor’s the amount of condensable liquid, water, hydrogen sulfides (H2S), chlorides and various amines. Within the reactor, in the presence of a catalyst, sulfur compounds are decomposed to form hydrocarbon and H2S. Oxygen compounds convert to hydrocarbon and water, and nitrogen compounds convert to hydrocarbon and ammonia (NH3). The sulfur and nitrogen contained in the feed convert to H2S and NH3 in the reactor.
These two reaction products combine to form ammonium salts that can solidify and precipitate as the reactor effluent is cooled. Likewise, ammonium chloride may form if there is any chloride in the system. The purpose of the water is to dissolve these salts before they precipitate. Water is injected in a vertical run of pipe through an injection quill that facilitates mixing the water with the reactor effluent.
Chloride and ammonium chloride build up on casing diffusers and impellers reduce a compressor’s efficiency. The build-up can reduce the seal gas supply flow from the compressor casing port to dry the gas seal console, and it can build up on the orifices and the seal gas filter casing. The following is recommended to solve compressor fouling:
- Carefully monitor all heat exchanger differential pressure trends upstream of the compressor with the proper process software.
- Carefully monitor compressor efficiency and plan to shutdown the compressor to perform internal cleaning. Consult with a dry gas seal manufacturer for online cleaning provisions. Depending on the process, loss of efficiency is expected to occur roughly every 1 yr–2 yr.
- Install a coalescer filter to remove entrained water within the process stream.
- Place the wash water injection point properly to minimize the entrained water.
- Run higher suction inlet temperatures to sublimate the chloride solids back into the gas stream.
- Consider an engineered and sufficiently sized recycle gas scrubber upstream of the compressor. The recycle compressor suction line exiting the scrubber should be heat traced and insulated to prevent condensation, which causes recycle compressor damage.
- Refer to API 5712 for more detail. HP
NOTE
The recommendations outlined in this article are based on personal experience and not related to any company.
LITERATURE CITED
1 API 617-8th Ed., “Axial and centrifugal compressors and expander-compressors for petroleum, chemical and gas industry services,” September 2014.
2 API RP 571-4th Ed, “Damage mechanisms affecting fixed equipment in the refining industry, March 2020.
The Author
Zardynezhad, S. - TurboTech Consulting Corp., Calgary, Alberta, Canada
Shahab Zardynezhad is a registered Senior Mechanical Engineer in Alberta with more than 30 yr of experience working on many of the world’s largest oil, gas and petrochemical projects. He has experience in engineering, procurement services, manufacturing, shop/field inspection, installation, commissioning, startup, reliability, and the maintenance and operation of pumps, compressors and turbines. Mr. Zardynezhad holds a BS degree in mechanical engineering from the University of Petroleum of Iran, an MS degree in industrial engineering from I.U.S.T-Iran, an MS degree in project management and an MS degree in mechanical engineering from the University of Calgary. He is a certified API inspector for rotating equipment.
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