Pressure Measurement & Control


The dream team for heat and condensate

August 2019 Pressure Measurement & Control

Meeting the highest safety standards in all process steps is an indispensable requirement in the chemical industry. There is a small but important building block of BASF’s highly efficient, global safety management at its Swiss site in Kaisten. It’s in the form of a guarantee that all processes are completely free of silicone oil. This is why traditional differential pressure transmitters can reach their limits when monitoring the levels during the production of additives for paints and plastics. They work by means of capillary lines filled with silicone oil, which – in the unlikely event of an accident – could come into contact with the medium. In contrast, VEGA electronic differential pressure measuring systems provide the perfect safety alternative through the innovative combination of ceramic and metallic measuring cells.

Plastics have been produced on an industrial scale for over 100 years. But anyone who spent their childhood in the 1970s or ’80s knows that the plastics of yesteryear have little in common with today’s special plastics. The coveted cowboy and Indian toy figures quickly turned yellow and some of the Lego bricks have become brittle. Plastics age due to thermo-oxidative degradation, i.e. under the influence of heat and oxygen. The fact that plastics today can be colourfast and elastic, crystal clear, high gloss, and abrasion resistant and heat-resistant, is due to a wide variety of specialist additives.

The vapour pressure of liquids plays a decisive role in the production of such additives.Their precise measurement not only contributes to BASF’s highly efficient control of the special processes involved, but also provides additional information on how process components can be optimised and plant availability increased. The following applies: the more precise the values, the clearer the processes and the higher the potential for optimisation. There are extreme conditions; aggressive substances and acids, which can adhere to vessel walls forming deposits, as well as high vacuum and steam levels. Through all this, the reliable measuring results of VEGA pressure transmitters are decisive.

No experiments, please: one system for two extremes

Actually, measurement and control engineers work according to the principle ‘never change a running system’. For processes to run smoothly, all production parameters must be precisely coordinated. It was only with the introduction of this new, oil-free pressure measurement technology, that some special features became apparent at BASF during the course of changeover. Distillation, including that of a reaction mixture under vacuum, is based on different boiling points of the liquids involved and their different vapour pressures. The desired separation effect depends on the composition of the liquids and the steam. For each differential pressure measurement, the pressure transmitters are precisely matched to the media to be monitored. In addition, special separating diaphragms are installed between the measuring cells and the process to protect the cells from high temperatures or moisture.

The highly robust ceramic Certec measuring cell in the BASF vacuum reactor is ideal for the perfectly flush application in hot, aggressive solvents in the lower reactor section. In the upper reactor area, however, completely different conditions prevail: here, the extreme steam caused condensation in the measuring cell and led to inaccuracies. Initially, the obvious solution was to use the differential pressure system based on metallic Metec measuring cells instead of ceramic ones.

Condensate and humidity no longer have any negative influence. The upper measured value remained acceptable and highly accurate in all situations. However, the high temperature of the solvent in the lower reactor area repeatedly caused drifts in the measured value.

Dream team for heat and condensate

It took expertise, willingness to experiment, and above all, hand-in-hand work between BASF’s Measuring and Control Technology department and Jürgen Feser, the key account manager at VEGA, to get everything running smoothly. The solution was to use an innovative electronic differential pressure measurement system involving a combination of ceramic and metallic measuring cells. Both pressure transmitters record their respective values separately and calculate the differential in the master sensor. This is made possible by structurally identical instrument electronics behind the two very different measuring cells. They work perfectly, even in a mixed team.

“For our application, this combination is a real dream team,” says Feser with satisfaction. “As far as I know, only VEGA offers this combination capability.” The bottom line result is ‘one’ solution for two challenges: The ceramic measuring cell easily handles the high temperatures and aggressive substances in the lower reactor area (HP side). In the upper part (LP side), the metallic measuring cell performs perfectly in the presence of condensation.

Why is silicone oil-free so important?

In the 1970s, all major car companies rigorously declared their painting lines ‘silicone oil-free zones’. What was the reason? Time and again, mysterious paint defects occurred on vehicles. In the end, the unlikely cause was determined to be the hairspray used by some employees. Nowadays, considerably less hairspray is likely being used than on those ‘interesting perms’ of past decades. However, since then, the regulations for silicone oil-free production have become a standard requirement in many industrial production areas. They cover mechanical equipment, components such as pipes, pumps, valves or hoses, and suppliers are required to provide guarantees and documentation of their silicone oil-free production. BASF prohibits silicones in many production processes and areas at the Kaisten site.

Even the most ingenious manufacturing process for additives is only as good as the equipment of the production plant. In the case of BASF’s distillation reactors in Kaisten, the sensors of pressure transmitters really get the job done, even in the most inhospitable locations: in the lower part of the vessel, under the high temperatures of the aggressive, adhesive liquids, and at the same time, in the upper part of the vessel, subject to steam and heavy condensation. The perfect solution for these vastly differing process conditions turns out to be a simple variant of electronic differential pressure measurement from VEGA – the combination of a ceramic Certec and a metallic Metec measuring cell.

For more information contact Leandi Hendrikse, VEGA Controls SA, +27 11 795 3249, leandi.hendrikse@vega.com, www.vega.com



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