Robots In-House Robots Keep the BASF Plant Running
BASF’s new acetylene plant can produce 90,000 ton of the versatile chemical annually. A byproduct of the process is what is known as acetylene coke. To prevent the coke from clogging the combustion chamber, it is removed by so-called poking robots, specially developed in-house by BASF.
The flares of the new acetylene plant in the northern part of the BASF site, known internally as W148, blazed brightly into the night from time to time. Concerned residents of Ludwigshafen,Germany called the world’s largest chemical company to complain about additional noise pollution. But BASF experts were quickly able to reassure their neighbours. The operation of the three flares over the last few months is a sign that the start-up of the plant is continuing according to plan. Across the plant, more and more components, control elements and technical equipment are being tested for the best possible interaction across the many individual steps of the process. When the acetylene plant, the most modern of its kind in the world, goes into normal operation, flaring will no longer be necessary.
The state-of-the-art acetylene plant has a capacity of 90,000 tons per year and operates the world’s most efficient production process. The new plant covers an area of around 40,000 m2 – the equivalent of almost eight football pitches. It replaces and old plant that had been in operation since 1964 before it was decommissioned. Acetylene is a highly reactive gas that makes up a particularly valuable feedstock used by about 20 BASF plants in Ludwigshafen to manufacture chemical products. Examples include pharmaceuticals, vitamins and fragrances, crop protection agents, plastics, elastomer textile fibres, electronic chemicals and solvents. “In this sense, acetylene production is the backbone of an entire value-added network of our site here,” emphasizes Dr Uwe Liebelt, plant manager at Ludwigshafen.
Acetylene would decompose again without quenching
Acetylene is produced by the deliberately incomplete combustion of natural gas with oxygen at temperatures up to 1400 °C. In addition to acetylene and hydrogen, this process also yields carbon monoxide, carbon dioxide, water, methane and soot. A few thousandths of a second after the reaction, the acetylene-rich cracked gas must be quenched – an abrupt cooling step that prevents the acetylene from decomposing back into its chemical constituents.
The new acetylene plant at the Ludwigshafen site has a total of ten reactors, and water in a closed-loop process is used for the quenching step. Compared to the oil quenching used in the past, the closed-loop water quench reduces atmospheric emissions of volatile organic substances.
Next, the solvent N-methylpyrrolidone (NMP) is added to separate the cracked gas into its two main components: acetylene and synthesis gas, a mixture that is rich in carbon monoxide and hydrogen. After purification, the acetylene and synthesis gas are fed through pipelines to the plants where they will be used as raw materials.
Combustion chamber must be cleaned regularly
The formation of coking products is unavoidable in the production of acetylene. The soot consolidates to form lumps and layers of varying sizes, which in the long term would severely clog the combustion chambers. It therefore has to be removed regularly. For a long time, this hard and monotonous task was done by trained employees who used long poking rods to push off the coke at frequent intervals.
In the old plant in Ludwigshafen, two workers were employed for decades to clean each reactor at least once an hour. Sometimes the deposits were so thick that the rods could hardly be moved. In any case, 'poking in the dark' was a great strain, especially on the back and joints.
To eliminate this unpleasant job, BASF had already experimented with a first generation of poking robots on the old plant. For the new plant, the company has built on this experience to improve performance, with ten new poking robots that draw on the best available technology developed by BASF’s 'Robotics & Mechatronics' team.
Each of the ten reactors has its own robot, which is electromagnetically coupled to the existing manual poking rod. Three electric actuators then transmit the required motion sequences to the poking rod. During the cleaning process, seven cleaning cycles run in succession. Each cycle takes no longer than 15 seconds, otherwise the poking rod could be damaged by the extreme temperature or even burn up in the combustion chamber. A cycle begins approximately every two minutes, and after about 20 minutes all seven cycles have been completed.
The robots move the poking rods in spiral patterns that avoid any contact between the rod and the burner plate or the reactor walls. “The use of robots is particularly effective here because this Sisyphean task not only has to be repeated every two minutes, but there are also very short periods of time available,” says Ralf Bihl, Automation Manager with BASF Robotics & Mechatronics.
The forces and turning moments generated by the robot system were designed to match human strength. If an employee can successfully remove coke manually with the poking rod, the system is set up correctly for the robot to take over. If human strength is not sufficient, the mechanical setup – especially the stuffing box through which the rod enters the reactor -- is incorrect and applying further force risks damage to the robot, the poking rod or the combustion chamber. The magnetic coupling between the robot and the poking rod acts as a weak link that protects the whole system against irreversible damage due to overload.
Robot solution developed in-house
In developing the new solution, BASF engineers compared off-the-shelf industrial robots with their own in-house variant. They chose to go with their custom development for several reasons, notably its compactness. Since this robot does not need to be versatile, the engineers were able to tailor the kinematics of the system directly to the demands of the job at hand. As a result, compared to a general-purpose six-axis robot, the BASF system is compact and lightweight. Its components can be replaced manually if necessary, without the need for a crane, and the system’s relatively low moments of inertia simplifies the design of the foundations.
In addition, the robot can be pushed away by hand even when de-energized. This allows the poking rods to be operated by hand in the event of a power failure, and ensures good accessibility for burner inspections. The in-house solution is based on proven components, and its modular design aids quick servicing and component replacement to ensure that the robot system maintains high availability. In line with the 'old-school' design philosophy, the control system is built up from discrete components that can be replaced individually as needed. With a few exceptions, all the spare parts are standard items that are easy and quick to obtain.
In line with acetylene’s extreme flammability, the whole plant is a potentially explosive area classified as EEx zone 2 T2, where T2 specifies a maximum surface temperature of 300 °C to match acetylene’s autoignition temperature of 305 °C, The robots are designed accordingly, and the control systems are housed separately in pressurized enclosures: three for the main electronics and ten control panels for the reactors.
As expected, initial operating results from the new plant continue to show significant soot formation on the walls of the combustion chambers, so the robots are definitely necessary. “Removing coke by hand is monotonous, strenuous physical work for human employees,” says Ralf Bihl. “In any case, the size of the plant and the frequency with which this has to be done means that it wouldn’t be justifiable economically as well as in health terms. So our robots pay off in any case,” he adds happily.
So the 'journeymen of steel' are at work 24 hours a day, 365 days a year, without complaint. The robots only take a break during maintenance work on the burners. Thanks to water quenching, BASF assumes that overhauls will only have to take place at longer intervals than on the old plant. Nevertheless, each reactor will be shut down for a few days a year for legally prescribed safety checks – and the robots will get a brief rest from poking in the dark.