When natural gas and crude oil is extracted from the ground it contains some percentage of sulfur. Gas or oil with a high level of sulfur is said to be sour. Gas or oil with a low level of sulfur is said to be sweet. This sulfur is removed from the gas stream by a chemical reaction. After this process, the sulfur is removed from the gas or oil and is instead contained in amine gas and sour water gas. The sulfur is then removed from the amine gas and sour water gas in a thermal reactor called an SRU (Sulfur Reactor Unit). This process is known as the Claus process. The Claus reaction that removes these gasses from the process stream is exothermal, meaning that it creates heat once the process has begun. As a result, there are several reasons to monitor the temperature of the thermal reaction.
The thermal reaction is more efficient when run at higher temperatures, and the gas stream may be run through more quickly; therefore, there is tremendous incentive to run the process hot. Many plants inject oxygen in an effort to raise operating temperatures and to increase process capacity. However, the refractory walls of the vessel degrade at excessive temperatures, making it essential that the process temperature be closely monitored to balance the need to be efficient at extracting the sulfur while at the same time extending the life of the refractory. These gasses are highly toxic, and the safe operation of the process is highly critical.
So while temperature is a critical process control parameter in the sulfur recovery process, there can be a number of complications when trying to get an accurate measurement. There are two generally accepted methods of measuring temperature inside of a Claus reactor – thermocouples and infrared pyrometers. This article gives a very comprehensive overview of the benefits and drawbacks of each measurement technique. However, the following are the most common issues and difficulties associated with the SRU temperature measurement and how to address them.
1. Constant replacement of thermocouples
One of the most common ways to measure the temperature inside the furnace is with a contact thermocouple device. However, the inside of the reactor is a hot, nasty, and corrosive environment and thermocouples will often fail and need to be replaced. Thermocouples tend to be troublesome because of their unreliability and require a lot of maintenance work to replace them. It can be difficult to consistently run a process to the same temperature off of thermocouples alone and another means of measurement is typically required. Not to mention, that the constant replacement of these thermocouple devices can be expensive over time.
Solution: Non-contact temperature devices are often used instead or in addition to thermocouple measurements. Non-contact measurement devices such as infrared pyrometers are mounted outside of the furnace and look through a window to measure the temperature inside the furnace. Because the non-contact device is not exposed to the corrosive environment inside the thermal reactor, these devices do not have to be replaced as often as the thermocouples and are much more reliable sensors. Although more expensive than a thermocouple, infrared pyrometers tend to cost less in the long run as they do not need frequent replacement.
2. Build-up of sulfur on viewing port
When using an infrared pyrometer to measure the temperature inside an SRU, you need to have a clear viewing path into the thermal reactor. If you do not have a clear viewing path, then the temperature output from the infrared sensor will be much lower than the actual temperature inside the SRU. The most common cause of any optical interference would be a buildup of sulfur that is deposited on the viewing window or on the inside of the flange tube. The buildup is usually a result of the warm sulfur gas that is deposited on to a cooler flange tube or window.
Solution: One solution is to manually clean the window and clear the viewport during scheduled maintenance periods. While this is a good routine maintenance procedure to minimize additional buildup between runs, it does not prevent any interference from buildup during operation. The other solution is to have a warm flange to prevent the buildup of sulfur on the flange window or along the viewing tube. Because the sulfur deposits on cooler surfaces, using a warm flange helps prevent the buildup of sulfur and keeps the viewing path clear for any pyrometer in order to keep a consistent and steady reading.
3. Pyrometer overheating
While warm flanges keep excess sulfur from depositing on the window or flange tube, it also means that it creates elevated temperatures along the flange mount and thus ambient temperatures nearby. As with any electronic device, the circuit boards need to remain under 65°C/150°F to function properly. With temperatures inside the SRU reaching 1315°C/2400°F, the flange tubes can easily reach elevated temperatures without any active cooling. For any infrared pyrometer mounted directly at the end of these flange tubes, it can result in the circuit boards overheating and thus a malfunctioning sensor. Any time the circuit boards are overheated, they need to be replaced and sent the entire unit needs to be sent back for service. One way to solve this problem is to provide active cooling to the infrared pyrometer so that the electronics stay cool, but as we discussed in the previous section, this would result in the deposition of sulfur onto the cooler flange tube or window.
Solution: Use a fiber-optic pyrometer to keep the electronics a safe distance away from the warm flange while the optical components are still mounted at the flange to clearly view into the reactor. The fiber-optic cable can withstand ambient temperatures of 200°C/400°F and allows you to have a warm flange. This means that it is much less likely for any sulfur to deposit along the flange viewing tube or on the flange window and thus a more reliable temperature measurement throughout the process. It is also less likely you would need to manually clear the viewing tube for excess sulfur deposits.
While no temperature measurement solution is perfect, there are a few ways in which you can minimize measurement error. As temperature is a critical process control component to the sulfur recovery thermal reactor, it is important to have a temperature measurement system in place that is reliable. These three issues are the most common for the SRU process. Other sources of error are further explained in this helpful article. If there are other questions that you have that we can help with please feel free to connect with one of our temperature measurement experts.