Feature
The latest in sulfuric acid plant process gas dewpoint/moisture leak detection By: Daniel T. Menniti, Director of Business Development, Mississippi Lime/Breen
Since the mid-1980s the sulfuric acid manufacturing industry has desired a maintenance-free, early indication of moisture ingress (most likely from boiler or economizer tube leaks) into their sulfuric acid manufacturing process. It was understood that if there was moisture present in the process gas, that the acid dew point temperature of the gas would increase. So, it was decided that if one could measure the acid dew point continuously, producers could monitor the trends and an increase of acid dew point would indicate a leak, or moisture ingress into the process gas. This desire brought about decades of frustration, while the industry trial and errored an acid dew point meter technology through various phases of commercial design. Finally, after multiple attempts, the industry gave up and the acid dew point meter manufacturer discontinued their product. In 2016, the industry discovered that there was a second manufacturer of acid dew point technology who had designed a robust system for permanent installation in traditional combustion applications. Breen, part of Mississippi Lime Company, was approached for a solution. The criteria given were “simple”—survive in a standard acid plant process gas condition, without maintenance or failures for a year, and the industry would regard the product suitable for use in their applications. Responding to the request from the industry, Breen and the sulfuric acid industry collaborated to develop a commercially available permanently installed sulfuric “acid dew point” monitor.
point with the Breen probe had similar short term operating life and considerable maintenance requirements that made the use of traditional continuous acid dew point monitoring a failure in the first three months of development. After discussion with industry experts, Breen determined that to keep the sensor free of process condensables for the long term and provide moisture leak detection, the probe would have to operate “above dew point cycle.” The above dew point cycle would allow the probe to operate in the duct at a temperature higher than the process gas dew point, but low enough to detect an increase, or step change, in process gas dew point. The commercial design for the Breen-SA Probe can be seen in Fig. 1.
The problem
Fig. 1: Breen-SA Probe for sulfuric acid manufacturing plants.
The gas laden with SO3 in the process is kept moisture free and the only time moisture gets introduced in the gas is when there is a process upset. Moisture in the process gas combines with the SO3 which will condense on the process equipment surfaces, and is generally indicative of one of the following conditions, among others: • Drying tower malfunction • Contamination in sulfur feed • Waste heat boiler tube leaks • Economizer tube leaks • Superheater tube leaks • Sulfur gun steam jacket leaks The presence of moisture, and therefore weak acid vapor, in the gas stream is detrimental to equipment health and can cause extreme corrosion. This is a highly undesirable condition and can be potentially hazardous. A secondary impact of moisture in the process gas can be the formation of hydrogen gas, which can create an explosion hazard [1]. • SO3+H2O H2SO4 • Fe+H2SO4 FeSO4+H2 The H2 formed in the previous equations can create an explosion hazard in the presence of O2 and an ignition source.
The solution
Since the industry had coined the term “acid dew point” monitor for a leak detection instrument, Breen attempted a probe with the same design criteria as the prior attempt. In short, the continuous measurement of acid dew PAGE 28
Fig. 3: Breen-SA Probe normal cycle and check cycle.
Fig. 4: Leak detection simulation.
Moisture leak detection commercial design
In the Fall of 2018, Breen installed the commercial design into a sulfuric acid plant in California (Fig. 2). The probe system was allowed to run at the above dew
assumed that the air pressure was not powerful enough to push sufficient air into the duct. Subsequently, the process engineer put water into the pressure point line and used instrument air to push that water into the duct. The amount of water added was approximately a gallon. At 13:15 we can see the probe immediately responded to the event. It was concluded, with a fairly strong certainty, that the probe is very responsive to even small amounts of moisture. There were also other minor responses in the previous weeks which were believed to have been caused when other pressure point lines were cleaned out.
Further evidence of response to moisture ingress and leaks
In July 2019, a second and third system were installed in another acid plant in the Western United States. One SA-Probe was installed at the waste heat boiler outlet and one probe at the final economizer exit (Fig. 5).
Fig. 2: Breen SA Probe installation at J.R. Simplot Lathrop plant.
Fig. 5: Breen-SA Probe installation points.
point mode, with weekly “Check Cycle” cooling the sensor to the process gas dew point. The probe cycle and check cycles are shown in Fig. 3. On January 23, 2019, the plant performed a test to verify that the system would indeed detect a moisture leak. At approximately 10:45 (Fig. 4) we can see a probe response when a pressure point line was cleared out with instrument air. A compressor was then hooked up to the line to pump ambient air into the duct; no response was seen, and it was
During start-up operations, this plant experienced a small leak. In Fig. 6, we can see that the Breen-SA Probe detected the leak when it was still very small at about 05:00 on June 27th. The first plant DCS indication that there was an issue was much later, at about 13:30 on the 27th, when the dilution water flow starts to drop indicating the leak is large enough that less water is needed to maintain the acid concentration balance in the plant. Clearly, the probe provided almost eight hours advance notice compared to the traditional method of leak detection at the plant. Continued on page 30
Sulfuric Acid Today • Fall/Winter 2021
