Diaphragm seal systems are ideal for many processes in the chemical industry as a resistant and safe measuring solution, for example, to monitor the level in a reactor using differential pressure. Precisely for this task, a manufacturer of terephthalic acid (Purified Terephthalic Acid, PTA) uses such a measuring arrangement with an integrated differential pressure transmitter. In view of the aggressive medium and the measuring location in the reactor, their reliability depends primarily on the diaphragm seal. In this case, it is a custom-made product by WIKA with an extra-long extension made of a special material.
PTA is one of the most needed chemical basic materials. It is mainly used for the production of saturated polyesters such as PET. The most common method for the production of PTA is the liquid-phase catalytic oxidation of p-xylene. This is a multi-step process including oxidation, crystallisation, crystal separation and drying.
Diaphragm seal system for level is heavily stressed
In order to obtain an optimal product, the temperature in the reactor must be between 150 °C and 200 °C at all times. Both the heat and the corrosive substances in the reaction mixture put a great deal of strain on the pressure measuring instruments used in the process. This also applies to the diaphragm seal system that measures the level.
In the case of level monitoring, however, there is another challenge: Reactors in PTA production have considerable wall thicknesses to prevent heat loss. That means: A standard diaphragm seal is out of the question.
Extension for both measuring locations from tantalum
The level in a PTA reactor can be monitored using a diaphragm seal system with a differential pressure transmitter and an extra-long tantalum extension. The photo shows a measuring arrangement with the model DPT-20 transmitter from WIKA.
WIKA therefore produced a specific version of the model 990.29 with an insertion length of up to 350 mm. At the customer’s request, all media-impacted components, i.e. from the diaphragm to the raised face, were made from tantalum. This is because both the process medium and corrosive vapour can penetrate into the space between the vessel socket and the extension.
A material combination with stainless steel, as is often used in comparable applications, would not withstand such aggression for long. The WIKA solution, on the other hand, offers excellent corrosion resistance and thus a correspondingly long service life for the measuring arrangement, which in the case of the PTA application delivers an accuracy of 0.1% of the span.
Note
On the WIKA website you will find further technical details on the model 990.29 diaphragm seal, which can be combined with the model DPT-20 differential pressure transmitter for level measurement. There you can also get a general overview of WIKA measuring solutions for the chemical sector and download a corresponding brochure. If you have any questions, your contact will gladly help you.
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You can also find out more about the turndown function of WIKA process transmitters in the following video.