Mixing time halved by modifying Nauta mixers

Proving modification can work just as well as replacement

At Ashlands production plant in Zwijndrecht, the Netherlands, two Nauta mixers dating from the 1980s had been underperforming and needed to be replaced. Considering the lack of space available for new machinery, the company decided to opt for modifications rather than replacement. John Bervaes, Reliability Specialist, and Ron Jongeneel, Process Engineer, explain how they solved the performance issues with modifications, only to be confronted by a whole different challenge.

Ashland halves mixing time by modifying its Nauta mixers

Asking Bervaes for the reason behind this ambitious operation, he replies: "We are continually trying to optimise our production processes and maximise asset utilisation. In other words: trying to get the most out of our processes in as short a time as possible. Every minute we save on total production time is a bonus and to achieve this, we keep a keen eye out for bottlenecks in the various processes and do our best to find ways of solving them." "In this specific case we are talking about the Natrosol© mixing process (Natrosol© being a water-soluble cellulose powder, Hydroxyethyl cellulose (HEC)), whereby a variety of viscosities are mixed using different combinations of ingredients. The bottleneck in question relates to the time it took the Nauta mixers to complete a cycle. With a capacity of 7 m³ in each mixer, one cycle would take as long as 30 minutes. It was our aim to reduce this by half: an ambitious goal to say the least."

Hercules approached Hosokawa to find a solution. "We thought it only fitting that our supplier should help us fix the bottle neck," the process engineer continues. "No one is more familiar with the machinery than they are, so we asked them if they had the answer. After carrying out the necessary research in their test centre, Hosokawa provided us with a detailed proposal of their recommended solution. We then formed a special multidisciplinary project group to implement and manage the modifications. Multidisciplined is our preferred way of working here at Hercules."

From milling to filling

John Bervaes is Reliability Specialist and part of the special multidisciplinary project group: "We used to have a maintenance supervisor who was solely responsible for maintenance work. My remit is much broader and includes not only diagnosis but also optimising our equipment's performance. This involves everything from maintenance to manuals, from operating the machinery to best practices. Moreover, I am responsible for testing new ways of packaging our products. We are a very horizontal organisation and most of us have been working here for a good number of years. The atmosphere is great and we as a team, have a considerable amount of experience to share with our customers."

In contrast to Bervaes, Ron Jongeneel, Process Engineer, is a relative newcomer. He has been at Hercules (Ashland) for a year and a half. "My specialism is milling, mixing, filling and packaging," comments Jongeneel, "and this process here in Zwijndrecht is made up of a number of stages," he explains. "It starts with the delivery of the rolls of cellulose derived from wood and cotton, which is subsequently shredded. Then it is chemically treated to extract the salts. The salts are flushed out, we dry the remaining substance, mill it into a powder, ensuring we meet the customers' specified particle sizes, and add the required additives. Then we send it to the filling station where it is poured into Big Bags.

Finding the solution

Hosokawa's proposal was to refit the two mixers, used in this process since the 1980s, with new rotating screws. Alternative tests carried out in the supplier's test centre included dual screws and increased rotor speeds, but these proved less effective.

Finally, Hosokawa made the following recommendations:

  • Increasing the rotorspeed by adjusting the transmission ratios
  • Replacing the mixing screw by a re-engineered one
  • Placing a ball head bearing at the bottom of the mixer

This, in order to reduce mixing time by half. The new screw was engineered using Duplex stainless steel as explicitly requested by John Bervaes (see sidebar). "I'm a big fan of Duplex," comments Bervaes. "The old screws were made of Duplex and were indestructible. The new screws are also made of Duplex but have been engineered differently, using a bigger outside and a smaller inside diameter, the pitch was also altered."

The mixing vessel was still in very good condition and did not need any modifications except for an extra opening in the wall of the vessel for a new automatic product sampler. According to Jochem Venderbosch from Hosokawa, Nauta mixers are often modified and adjusted to suit their applications: "The vessel lasts a very long time but the ingredients tend to change over the years. Modifications can improve performance considerably as we can seen in this case."

Pleasing results

Jongeneel is pleased with the results: "Mixing time has been reduced on both mixers from 30 minutes down to 12, which is a significant achievement indeed. In our calculations, we have adopted a 15-minute mixing time. Yet the reduction we have actually achieved is more than our initial goal of 50%. We have doubled our output and exceeded all of our original expectations." "Moreover," Jongeneel continues, "we have simplified the sampling process and extended the bearing's lifecycle from one year to four using the new bearing temperature monitoring system."

The mystery continues

All is well that ends well you might think, but for Hercules this was not the case. Whilst both of the vessels were identical when purchased, and had been modified in exactly the same way, the company is still having problems with one of the mixers. This problem causes the motor to fail intermittently when the vessel is loaded to maximum capacity. "This has been a problem right from the very start," comments Bervaes. "We have spent a lot of time and energy - as have Hosokawa - trying to find the cause, but it remains a mystery to all of us.

We have investigated everything we can think of: is it the motor? Is it the controls? Is something effecting the rotary motion of the screw? And so far there is only one thing it could possibly still be: the gear box." "We have saved the most expensive modification until last," adds Jongeneel, "the gear box!" "One of our other very reliable suppliers has overhauled the gear boxes for us, yet we are still unable to conclude whether or not this is the root cause of the problem. Having said that, I'm sure we'll find the answer sooner or later."

Reliability

"So how is your relationship with Hosokawa now?" we asked Bervaes. "Very good," he replied. "The first time this problem occurred was on a Friday evening - this kind of thing always happens at the weekend! Hosokawa were at the end of the line, ready to give us the support we needed, right away." "We really value the kind of long-term and good-working relationship we have with our suppliers. When we can combine our knowhow with theirs, we can achieve much more. And this is just as important in the delivery and installation stage as in the stages that follow. A specialist purchaser familiar with the ins and outs of our business, someone who doesn't just look for a quick fix, but tries to find the best possible package deal: that's what we value."

"In the end, it all boils down to optimal reliability," Jongeneel adds. "It is always crucial for your suppliers to work with you to solve any problems you may have. That was the case here. It wasn't about allocating blame. It was about working together to identify the cause and find the right solution, and we have managed to achieve fantastic results: a 100% improvement in output. Now all we need to do is work together to find the solution for this next mystery!"

www.ashland.com