LumaSense’s optical pyrometers help keep sulphur recovery units at the right temperature
The main reason refineries produce sulphur is simple – they have to. Regulations on sulphide gas emissions and sulphur content in fuels makes removing sulphur compounds necessary. Regulation has led where engineering already operated – sulphur wrecks process plant and pipework.
Sulphur recovery is usually a back-end process in a refinery. Removing sulphur compounds from sour oil and gas needs a carefully controlled band of temperatures – too cold and the reaction becomes inefficient or stops altogether; too hot and the reactor could be damaged. That makes sulphur recovery units expensive to install (around US$2-5 million) and difficult to run.
Around 90-95% of refineries use the Claus process to make sulphur. Hydrogen sulphide is burned with oxygen over a pre-heated catalyst bed (generally activated alumina) to produce water and sulphur dioxide. Then, more hydrogen sulphide is added to end up with water and sulphur. The Claus reaction is an equilibrium reaction, so the feed hydrogen sulphide is cycled through additional reactors, converters and condenser beds two, three or more times, until no more can be economically reacted – depending on the plant leaving between 0-5% of the gas. The first reactor in the step does the most work, removing 60-70%.
The burning process begins at reactor temperatures up to 925°C for flame stability, but optimisation comes at around 1,300°C. At each stage liquid sulphur must be condensed for extraction, for which the gas stream needs to fall to 140°C. Going outside these ideal operator ranges has serious consequences for the reactor, the refinery and the environment.
If the temperature gets too high it can damage the reactor, reduce its lifespan or in extreme cases cause the unit to leak hydrogen sulphide, which can be deadly at high concentrations. In low temperature cases the catalyst bed can be damaged, reducing the efficiency of the process to the point where unconverted hydrogen sulphide is vented through the stack. Since the reaction is exothermic this can happen if the mixture of gas and oxygen is wrong and the reaction gets out of control. The operator naturally wants to run the process at the highest safe level of throughput and constant accurate temperature measurement is the key to that.
For most refineries the sulphur recovery unit is an essential part of the overall flow process. If it stops, the refinery stops. Refineries commonly install a second sulphur recovery unit and additional hydrogen sulphide tanks, but this increases the investment needed.
If temperatures fall too low, ammonia and other contaminants will not be destroyed and will damage the reactor and catalysts. If the temperature falls below 925°C the reaction become unstable and can stop. If the temperature gets out of control on the upside it is not easy to correct quickly – heating or cooling rates above 50°C per hour can cause thermal shock. Cooling the reactor too fast can damage it (like pouring cold water on a hot plate). Ammonia, benzene, toluene, and xylene isomers found in the feed gas need temperatures of at least 1,250°C to be destroyed. While every sulphur reactor works to different tolerances, a safe temperature range is surprisingly narrow – 50°C either side of about 1,250°C. One approach to monitoring is to measure gas concentrations at each end of the process, but this does not give data on the inside of the furnace. Furnace temperature is the key measurement to optimising the reaction.
However, the hot and caustic atmosphere inside a reactor makes it hard to gather data. In theory physical thermocouples can provide data, but are quickly consumed and replacing them requires the reactor to be powered down.
LumaSense has developed a range of optical pyrometers which avoid contact with the environment they are measuring as they operate by analysing the infrared light radiation emitted inside the reactor by the incandescent gas. Two independent infrared detectors measure different wavelengths on each sensor. The reactor chamber is a volatile environment of multiple gases (superheated steam, hydrogen sulphide, oxygen, nitrogen, sulphur dioxide and those pesky trace contaminants), all of which can interfere with an accurate temperature reading. By measuring two wavelengths a compensation calculation can be applied to balance out the emitted light and get an accurate reading.
LumaSense’s pyrometer also measures temperature gradients in parts of the furnace, such as the secondary combustion zone at the back of the furnace, where combustion products exit the reactor. The temperature of the reactor varies by much more than the target combustion range. For example the reactor wall is one of the cooler parts of the chamber – around 200°C cooler than the hottest part. Using infrared energy provides more data on heat distribution in the reactor. The reaction core is usually 1.5-2 metres into the furnace (depending on the size of the furnace and the burner design) and is the key area to measure. LumaSense gives the operator a full thermal image of the reactor, to support better decision-making in managing the process.
Anomalies can be detected in as few as 20 milliseconds, but in order to avoid burying the operator in readings the system provides a reading every second. It is accurate to 0.3% (4-5°C). When the pyrometer fails it can be removed and fixed without shutting down the reactor (they are usually installed in pairs) to support continuous operation.
Lumasense’s pyrometers are aimed at allowing operators to run at higher throughputs and with more uptime, while keeping these high-energy systems safe.