Alonizing: case studies
Case Study One: ALON™ DIFFUSION COATING PROTECTS TUBES IN SOUR GAS PLANT WASTE-HEAT BOILERS
Situation: The Shell Caroline gas plant, one of the largest sour natural gas processing facilities in the world, is designed to process 8533 x 103 m3/d of raw gas and gas equivalent of liquid hydrocarbons. The overall sulphur recovery for the facility is 99.8% of the total sulphur intake, and sulphur production is approximately 4000 tonne/day. This facility began operating in late 1992.
As is the case with most western Canadian hydrocarbon sources, the gas in this plant is contaminated with a quantity of sulfur species. The concentration of H2S in the feed to the plant is approximately 35%.
During operation, the total raw gas and liquid hydrocarbons from the field are processed in the inlet feed facilities. The overhead sour gas from the feed fractionator after being stripped of heavy hydrocarbons, is sent to the gas sweetening facilities where the acid gases are removed by physical and chemical absorption. The acid gas is then sent to the Claus sulphur recovery unit.
In the Claus process, H2S in the acid gas stream is partially oxidized in the reaction furnace. This reaction is highly exothermic. Among existing gas plants, gas temperatures are reported to exit the reaction furnace at a temperature between 1400F and 2300F (760 C and 1260 C) with most plants operating in the range of 1700 F/2200 F (927 C / 1204 C). The Shell Caroline plant is designed around an exit gas temperature between 2200 / 2300 F (1204 / 1260C).
This gas plant contains two sulphur recovery units, each having two reaction furnaces and waste-heat boilers operating in parallel. The steam generating capacity of each boiler is approximately 100 t/hr at 3650 kPa. Each boiler contains 1010 tubes, 2-1/4” (57.1mm) diameter, approximately 12 meters in length. These tubes can experience high-temperature sulfidation, which can result in inefficient operation, potential for catastrophic failure and premature shut down for maintenance or replacement of tubes.
Action: All of these carbon steel tubes were Alonized™ to eliminate high-temperature sulphidation.
Alonized™ tubing is specified in many gas plants to protect the tubes in the waste-heat boilers and condensers. Additionally, a wide variety of metallurgies and configurations are Alonized for use in many other types of gas-processing plants.
Although the hot gases pass through the inside of the tube, both surfaces of the tube were Alonized™. Care was taken to eliminate the presence of aluminum alloy in the area of the tube that was welded into the tube sheets during assembly. In accordance with standard practice, the tubes were processed through Endurance’s high-temperature vapor diffusion treatment, then re-rounded by passing them through a roller straightener to bring them back to ASTM A450 tolerances. The tubes were trimmed to final length, ends dressed, and protective plugs inserted in the ends. NOTE: The heat of the Alonizing™ process may cause a slight change in tube length. For this reason, tubes or pipes with a finish length longer than about 16’ (4900 mm) are typically ordered 1% over length and then trim cut to final length after Alonizing™.
Result: After over 20 years these tubes are still in service, allowing the gas plant to operate efficiently, with minimal risk of failure, and no lost production time due to tube replacement.
Case Study Two: ALON™ DIFFUSION-COATED SO2 / SO3 HEAT EXCHANGER TUBES OPERATING EFFICIENTLY SINCE 1966
Situation: In 1966, Fenco Engineering Company built a sulfuric acid plant for SO2 abatement purposes using Alonized™ tubes for a non-ferrous mining company with a smelter operation in Ontario. In 1988(22 years later) this customer asked Fenco to conduct a study to see if the plant could last another five years, after which time it would be replaced with a more modern, larger facility which would handle all of the off-gas streams in the property.
Action: A comprehensive survey was conducted and found that the AlonizedTM tubes in the heat exchanger, in service for 22 years, were essentially in “day one” condition. Fenco assured the plant operator that there should be no difficulty in achieving another five years of operation.
Ultimately, conditions elsewhere in the facility dictated the earlier replacement of this plant, and the new larger unit was put in service in the early 1990s.
Result: Alonized™ tubes are now in service in 38 countries throughout the world. To Endurance’s knowledge, there has not been a single failure in the over 1000 miles of tubing now in service occasioned by improper Alonizing™ or a failure of the Alonized™ diffusion zone to properly perform its task of protecting the tube against sulfur-based scaling and corrosion.
Case Study Three: ACID GAS REMOVAL IMPROVED CLAUS SULPHUR RECOVERY
Situation: Recently, 26,000 ASTM A192 carbon steel heat exchanger tubes were AlonTM-diffusion-coated for ADNOC, Abu Dhabi National Oil Company, to provide scaling and corrosion resistance to acid gases. These gases, generated during the treatment of natural gas or hydrocarbon streams, start out in the form of hot exhaust gases. They then are desulfurized by a Claus process to yield either sulfuric acid or elemental sulfur, thereby converting and recovering 94 - 99% of the sulfur compounds and, in some cases, as much as 99.9% of these compounds.
The ADNOC acid gas project used a traditional modified Claus process design in the front end of the acid gas removal (i.e. furnace, waste heat boiler, and first catalytic conversion cycle). They then used the CBA (Cold Bed Adsorption) process for the back end of the process, which requires only minor modifications to the conventional Claus design practice.
This improved configuration of a single Claus reactor, followed by two sub-dew-point CBA reactors, has the same corrosion and scaling material challenges as the traditional Claus designs. Bare carbon steel tubes last only 4 ½ - 6 years in high-temperature or immediate-temperature interpass exchangers. This life cycle corresponds to the Claus first and second-pass exchangers and the CBA first and second-pass condensers.
The corrosion challenge involves utilizing the most economical and available carbon steel for the material of construction to achieve the corrosion resistance of a high alloy, without the expense. The high-temperature corrosion mechanism that gives carbon steel such a short life is the sulfidation of the iron constituent in the steel. The H2S in the acid gas stream is partially oxidized in the reaction furnace. From that point on, the corrosive nature of the sulfur compounds levy their effects in the form of sulfidation and oxidation. Acid gas process streams range from 5 to 80+% H2S and COS combined. The affected process equipment sees temperatures from 300oF to 700oF. As the initial acid gas is burned, there is a conversion of the H2S and COS to SO2, SO3, and resultant H2O and CO2. The presence of the highly oxidizing CO2 only accelerates the sulfidation process.
Action: Carbon steel is Alonized™ by a diffusion process that drives aluminum into the surface via a controlled retort, forming a new alloy, iron-aluminide, which is almost totally corrosion resistant to sulfidation. The mechanical strength of the tube material is documented after processing per ASME material specifications. Endurance also provides fabrication procedures to assist the engineering design firm and fabricator, including tube-to-tubesheet rolling requirements.
Ultimately, conditions elsewhere in the facility dictated the earlier replacement of this plant, and the new larger unit was put in service in the early 1990s.
Result: While benefits of Alonizing begin immediately with minimal corrosion and limited possibility of catastrophic failure; the most significant payback occurs over the long term due to less down time for tube replacement.
Endurance’s diffusion process has been proven in over 250 sulphur processing and recovery facilities worldwide. Using the less expensive and readily available carbon steel has allowed industry to design with a more desirable heat transfer coefficient than that of austenitic stainless steels of higher alloys. Designers can optimize equipment sizing using Alonizing™ and keep overall operating and maintenance costs down. The advantages of long uninterrupted service continue to make AlonTM-diffusion-coated material the preferred choice in the sulfur industry today.
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