A chemical company, located in Gent/ Belgium, produces as part of their production processes highly concentrated sulfuric acid. This process involves a high temperature operated incineration. The new in-situ measurement replaces a conventional measuring technology, which was subject to extensive maintenance and created high operation costs.
Roughly spoken, the process is as follows: The incineration of polluted sulfuric acid (H2SO4) leads to the formation of SO3, which again is dissolved in water to achieve highly concentrated sulfuric acid. The incineration process runs at a temperature of 1 000 °C. The composition of the incineration gas varies but is highly corrosive in any case and thus affects the measuring device.
The challenge - Rough conditions for the measuring system
The customer states: “We use an oven installation in which the sulfuric acid is evaporated. In order to control the incineration process and to keep the process conditions constant we need to continuously monitor the oxygen level of the gas mixture. After the incineration we are injecting water to achieve the final product. It is not exaggerated to say that as a result of the incineration process we obtain nearly the entire Mendeleev’s periodic system of chemical elements. Not long ago we used the classical Zirconium based probes for the oxygen measurement. The measurement operates around the dew point of the gas, which is highly corrosive to the measuring probe. Even a hastelloy protective housing for the probe was destroyed after a short time. Maintenance required a significant number of man hours. In total we speak about a value exceeding EUR 100 000 per year to keep the three measuring points running.”
The oxygen concentration in the process is the critical parameter for the incineration: The lower the concentration, the more effective the incineration. On the other side, falling below a critical concentration limit results in post-combustion with the risk of an explosion.
After the evaluation of some alternatives, the customer decided to use the Siemens LDS 6 laser diode spectrometer. This allows a measurement of the oxygen concentration in which the sensors (sender and receiver) are not in contact with the corrosive gas to be measured.
The customer: “The decision was for the Siemens system, as its design with the fiber optic connection between the sensors and the central unit allows placing the central unit with the electronics at a safe distance from the oven.
A surprising fact of the solution achieved by the Laser technology was the speed in which the measurements are obtained. The system turned out to be too fast to achieve a stable process control. Only after a subsequent delay of the measurements a stable control could be achieved.
The requirement for reduced maintenance was met fully. The maintenance is confined to an occasional cleaning of the sensor tubes, indicated by the transmission signal of the system. This job is done by the operators directly.
The process engineer: “What we get is an output signal in mA which is processed by the DCS system of the factory and which additionally goes to the safety system. Additional confidence is granted by the Ethernet connection, used for remote service option by a direct link to Siemens.
LDS 6 improves an incineration process and provides a positive environmental impact – Case Study