Alonizing: case studies
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.
Questions about Alonizing? Contact us
