Temperature measurement for sulfur recovery
Can you depend on an operator’s eyeball evaluation when optimizing critical combustion? In-plant tests verify infrared can be more precise and help improve process performance.
David Ducharme, LumaSense Technologies
Hydrogen sulfide (H2S) is a noxious, toxic gas found in crude oils and natural gas. At all temperatures, it is corrosive to carbon steel and most engineering alloys. In the refining process of products like crude oil or raw natural gas, gaseous H2S is the by-product of physical and chemical treatments used. It must be removed from finished petroleum products and pipeline quality natural gas typically using the Claus process, which is common in most refining environments. This converts the useless H2S into recoverable elemental sulfur.
Generally, the Claus process takes place in a sulfur recovery unit (SRU) where H2S is converted into sulfur through thermal and catalytic processes. Due to the complex reactions involved, accurate temperature control is required, particularly in the reaction furnace to prevent damage to the refractory lining. Given the high temperatures and corrosive nature of the process, this is a challenge for temperature instrumentation.
There are two ways to look at temperature measurement and control in the SRU. One is typical in a gas processing plant where the concentration of H2S is known and is constant. Constant flow with sufficient air to burn one-third of the H2S will produce a constant reproducible temperature, and so long as the mixture ratio remains constant, there is no need to know the specific temperature.
The other method is used in a refinery or gas plant that receives its feedstock from a variety of sources. The operator has to monitor the composition of the feed gas constantly, adjusting mixture ratios accordingly, while watching for refractory-damaging temperature excursions resulting from unexpected loads of ammonia or hydrocarbons. In these plants, accurate temperature measurement is essential for several reasons:
- Helps avoid refractory damage;
- Provides data for combustion control; and
- Provides data for mixture ratio feed-forward control.
Given the importance of the data, it is rare to find a plant that doesn’t monitor temperature at least to some extent.
Measuring temperature in a Claus furnace
Temperature control of the reaction furnace in the SRU has taken on a new importance with the advent of processes like oxygen enrichment. Operating much closer to the refractory temperature limit requires not only accurate temperature measurement but early warning of excessive gas temperatures. As such, accurately monitoring temperature fluctuations is of the utmost importance.
There are several methods of temperature determination practiced in the SRU, but three of the most common include the “eyeball” method, thermocouples, and infrared thermometers.
The most common method of measuring temperature is through a visual (eyeball) assessment by a trained operator, who simply looks into a viewport and observes the color of the combustion process. Surprisingly, some operators can estimate temperatures to within 50 °C. Where a plant lacks such a trained eye, color/temperature pocket cards exist which allow novices to estimate the temperature. These can be accurate within 100 to 200 °C. Though you can argue for the accuracy and reliability of this method under favorable circumstances, using an appropriate measuring device is a much more effective way of determining temperature. Depending on an operator also precludes the possibility of creating automated control.
When it comes to instrumentation in the SRU, thermocouples and infrared thermometers (IR-Ts) are probably the two most widely used approaches. Each has its own strengths and weaknesses, and preferences vary according to the operator.
Given the temperature levels, thermocouples are effectively the only choice for a conventional sensor. However, the harsh Claus environment is very hard on any type of temperature probe. Highly corrosive H2S at 1,315 °C (2,400 °F) combined with combustion vibration and thermal shock can shorten a thermocouple’s service life. In order to improve measurement reliability, multiple thermowells and purges are typically used. However, these protections make it difficult for the probe to reflect the actual gas temperature. On the other hand, well-positioned thermocouples embedded in the refractory material provide accurate refractory temperature measurements at any point in the furnace, and are helpful when used as part of a high-temperature shutdown system.
IR-Ts can be mounted outside of the combustion process read through a viewport into the SRU, effectively solving the problem of the harsh environment. Like thermocouples, IR-Ts can measure localized refractory temperatures, so they can also have a part in high-temperature shutdown systems. Measuring gas temperatures, though, is the main advantage that some IR-Ts have over thermocouples. Because gas temperatures lead refractory temperatures during the Claus process, IR-Ts that detect gas allow process operators to detect a high-temperature event much earlier. Some IR-T devices are capable of taking only one type of reading, so separate devices are required to read gas and refractory temperatures. However, for a complete control system, the need for simultaneous gas and refractory measurement is required.
Measuring gas and refractory temperatures
Being able to measure both gas and refractory temperatures simultaneously with a single IR-T is important for SRU operators and process engineers. Eliminating the need for separate installations for gas and refractory measurement systems was seen as a natural next requirement for the evolution of the IR-T system. A single IR-T can differentiate between the gas and refractory surface by using multiple wavelengths. The device can measure anything it can see, so it can reach points in the furnace that are impractical to reach with a thermocouple.
BOC hosted a series of tests at its SRU pilot plant in Manchester, U.K., to determine the relative reliability of IR-T measurements compared with thermocouples in thermowells and embedded in the refractory material.
Working with BOC, LumaSense Technologies evaluated IR-Ts using six specific wavelengths, some absorptive and some transmissive, under a variety of operating parameters. Test results on the pilot plant under closely controlled conditions indicated that an absorptive wavelength of the gas temperature presented the highest gas temperature under varying amounts of ammonia, bypass, and percents of oxygen added. The results also revealed that thermocouple placement had a significant effect on the reading. For instance, a thermocouple buried just 5 mm below the surface of the refractory read 140 °C lower than the infrared thermometer measuring the surface of the refractory. This illustrated that elements of the environment prevented the refractory thermocouple from reporting the true surface temperature, a problem that the IR-T was able to overcome.
Given the difficult conditions involved in an SRU, any information that can be gathered from outside of the process presents distinct advantages. The ability to measure both the refractory and gas temperatures simultaneously without being exposed to the heat and corrosive gas makes for a higher level of precision and reliability than conventional temperature probes. Additionally, IR-Ts allow for greater effective temperature spans, are easily interchangeable, and provide operators with more advance warning and quicker response time in the event of an abnormal situation.
For SRU operators, the main advantage of continuous real-time gas temperature measurement is the ability to provide early warning when a potentially hazardous temperature is developing in the furnace. Rapid response time is essential since refractory failure accelerates with increasing temperatures. For example, a sudden gas temperature surge due to an unexpected slug of hydrocarbon can be alarmed in 20 milliseconds, alerting the operator for response. The simultaneous refractory temperature output provides a signal for your high-temperature shutdown/alarm systems.
For instrumentation engineers, a key advantage of the installation is that only one port is required for both maximum gas temperature and refractory temperature. This represents a significant saving over single-wavelength measurement thermocouple and infrared installations in new installations. The design of a system compatible with existing product installation hardware also reduces the cost when upgrading client’s current installed systems.
When it comes to instrumentation in a sulfur recovery unit, IR-Ts carry an advantage in terms of reliability, cost-effectiveness, and ease of use. In addition, they also carry a safety advantage with the ability to warn operators of hazardous operating conditions. Further, with complete accuracy necessary to prevent high-temperature damage during the Claus process, the ability to simultaneously measure both gas and refractory temperatures is paramount, especially when employing oxygen enrichment. When considering temperature instrumentation in harsh environments like sulfur recovery, infrared thermometers provide reason for consideration.
David Ducharme is the Mikron products manager for LumaSense Technologies. He has extensive IR-T field and application experience and has worked with sulfur recovery unit instrumentation for over 15 years.