Getting the most from online viscosity monitoring

Paul Boughton

With its world headquarters in Melbourne, Australia, Amcor offers a broad range of plastic, fibre, metal and glass packaging products, along with packaging-related services.

The company was also Australia's first paper and cardboard recycling company, and remains a leading recycler of paper and cardboard products. For more than 120 years Amcor has provided effective, innovative and economically viable services that to date have recovered and recycled over 36 million tonnes of waste paper and cardboard.

Viscosity measurement used to be a problem at Amcor’s fibre packaging plant in Port Botany, Sydney, which receives and processes around 50 per cent of the waste paper collected in New South Wales. Since the paper is 100 per cent recycled, the base sheet is coated with starch before the final drying stage to give it the necessary strength.

Amcor uses a refined, acidified wheat starch slurry, which is cooked at 140°C and run into a service tank. From there it is pumped up to the size press and applied to the paper. The excess is pumped back into the service tank, making a closed-loop system.
 

The viscosity of the starch directly affects the degree of penetration and hence the efficiency of starch usage. For some grades the starch has to penetrate the paper, so a lower viscosity is required. Other grades are optimised by providing a surface coating to the sheet, requiring less penetration and therefore higher viscosity. The aim is to ensure uniform viscosity during production and hence the most efficient use of starch.

In the past Amcor could only check starch viscosity by conducting spot tests with a rotational viscometer. At best these tests were only done four times a week, so it was impossible to tell whether process problems were starch-related or had some other cause. There was a clear need for continuous viscosity monitoring using an online instrument.

Blade-type viscometers were tried but these proved unreliable and tended to block up. They needed constant cleaning, so bypass lines had to be installed to enable them to be cleaned on the run. The weekly maintenance required was both time-consuming and highly inconvenient.

Acting on a recommendation from another paper mill in Australia, Amcor contacted Hydramotion. After an initial consultation, a Hydramotion XL7 online viscometer was installed on the discharge side of the service tank pump to monitor the mixture of fresh and recirculated starch, in order to detect viscosity effects resulting from loss of heat, starch degradation and contaminants leaching from the paper sheet Fig. 1).

As soon as the XL7 came on line Amcor was able to measure viscosity continuously in real time without interruption. Encouraged by the successful results, the company went on to install a second XL7, this time on the fresh starch line exiting the cooker/converter in order to monitor cooking efficiency (Fig. 2).

“Both installations were very easy,” commented Paul Chappell, Amcor technical manager. “These devices are extremely easy to use. Basically just plug them in and that’s it. We run ours back to a PLC and trend starch viscosity online on each of our two paper machines.”

The XL7 was chosen for its ease of use and negligible maintenance requirements. With the benefit of continuous real-time data from the Hydramotion viscometers it has been possible to make significant improvements in process control.

“We have been able to troubleshoot starch cooking faults and overusages as a direct result of the data provided by these devices. The benefits gained are in starch uniformity and efficiency. In Australia the cost of wheat and therefore starch is rising, and these are operational costs to us. We always aim to reduce costs of manufacture where we can, and these devices have enabled us to optimise and reduce our starch consumption,” added Chappell.

By careful selection of instrument location cleaning has been reduced to a single, once-a-year operation that can be carried out alongside routine servicing of the lines. The viscometers have not needed any other maintenance.

“The performance has been excellent. We have not had any issues to date with these devices. The operators have come to trust them and use them to optimise the process.”

Other challenges

Founded in August 1974, Polyacryl Iran Corporation is the largest and most advanced man-made fibre manufacturer in Iran, producing over 95 000 tonnes of polyester and acrylic fibre each year.

Acrylic fibre is easily washed, resistant to oils and chemicals, readily dyeable and quick drying — all properties which make it very suitable for a wide range of knitted, woven and pile fabrics for clothing, blankets, upholstery, carpets, hosiery, sail covers and many other materials.

Polyacryl Iran produces acrylic fibre by dissolving a precise proportion of polyacrylonitrile powder in dimethylformamide and extruding the solution through a spinneret (die). As the polymer solution leaves the spinneret, the solvent is evaporated by heated nitrogen so the fibres solidify and can be collected on a take-up wheel. This process is called “dry spinning”.

The viscosity of the polymer solution is a direct reflection of its polyacrylonitrile content, which determines the properties of the product fibre. Viscosity can therefore be used as a control parameter to ensure that the composition of the solution is correct.

Polyacryl Iran had been having trouble with viscosity measurement. The polyacrylonitrile solution is highly viscous (up to 300 000 centipoise) and the company’s previous instrument was not very accurate. Moreover, it required constant maintenance and frequent repair, rendering it an expensive and inefficient part of the process.

By integrating two Hydramotion XL7-103 on-line viscometers into the process control system, engineers have been able to keep the percentage of polyacrylonitrile in the solution to within the required ±0.5 per cent using an automated control loop. The instrument provides a continuous, real-time viscosity signal that is fed directly to the PLC, enabling instant corrective action to be taken automatically as soon as the viscosity begins to wander off specification.

“The installation was very easy”, commented R Saghafian of Polyacryl Iran, “and the viscometer itself is very easy to use.”

The result has been a significant improvement in the efficiency of the process and a concomitant increase in productivity. Consistent product quality is assured while costly maintenance problems have become a thing of the past.

 It’s a similar story in the US, where Texas-based Champion Technologies selected an XL7 to measure the viscosity of the emulsion breakers (liquid polymers) that it uses during the dewatering of crude oil. Here, the viscometer was also modified because of the highly acidic – below pH 3 – process conditions with sensors being made out of Hastelloy C. “We can see far more in the reaction than we expected,” commented senior process engineer Jason Laws. “The XL7 viscometer has helped us determine reaction issues that needed to be overcome. It exceeded engineers’ expectations when regarding in-line viscosity.”

A new model

Building on these successful projects, in August Hydramotion announced the introduction of an upgrade to the XL7 range. The new XL7-HT2 gives continuous, real-time viscosity measurement of fluids up to 450°C without the need for any special cooling (Fig. 3).

The sensor always operates at the fluid temperature, which the company says will eliminate potential errors from localised cooling of the fluid. In addition, measurements are unaffected by flow rate or the presence of suspended solids or gases. It can also be supplied in an intrinsically safe version certified to ATEX Ex II 1G for Zone 0 Hazardous Areas (Class 1 Group 1 Div A).

The dedicated readout unit can be sited up to 1000 metres from the sensor. Outputs include a fully-configurable 4-20mA analogue viscosity signal and, optionally, a more comprehensive serial data link for recording or viscosity control.

Recent Issues