The process of evaporating sheet water on the paper machine is a key point in papermaking. Now an intelligent supervision system enables Industry 4.0 steam management, providing paper mills with benefits that can be summarized in two main aspects: higher performance and increased machine profitability.
In the paper machine, one of the most complex steps constituting the manufacture of the paper sheet is the process of evaporation of water from the sheet, as it lends itself to be influenced by countless factors. Some of these factors-such as the nature of the fibers, ash content, refining-can be controlled; other parameters, on the other hand-such as the condition of the felts, dryer cloths, and press felts-are random and constitute variables that complicate the management of the production cycle. In order to control them, action must be taken at the steam plant level. Graziano Buttiglione, technical director of Kadant Johnson Systems International www.kadant.com, spoke at Miac 2023 about this and the possibility of intelligent management of the same, with a view to Industry 4.0.
Buttiglione explains, “The ever-increasing needs of papermakers have led in recent years to the need to develop paper machine dryers capable of very high levels of flexibility, making them capable of producing different types of paper, with characteristics that are also very different from one another. These requirements prompted designers to develop increasingly complex steam systems to handle waterfalls intelligently and flexibly. It is precisely to meet these demands that Kadant developed DMS, which stands for dryer management system.
Dryer management system
It is a supervisory system, a kind of automated operator capable of calculating, managing and optimizing all the parameters involved during paper drying. That is why, the director says, its users call it a “super operator”.
It is based on a very simple operating principle: “It acquires process data from the DCS (distributed control system) the distributed control system of the paper mill or from its PLC (programmable logic controller) the programmable logic controller, in which the steam plant controllers reside. In addition to these it also acquires data from the quality control system or QCS (quality control system), which may be related to speed, grammage, size, moisture at pope, size press. It processes it all, according to carefully designed algorithms, and returns external set-points to the control system”.
The benefits to the paper mill are several, one of the main ones being improved heat transfer efficiency. “It is well known that drying efficiency depends both on factors external to the dryer cylinder and on internal factors,” Buttiglione continues. “The external ones concern environmental drying conditions and can be, for example, the tension of the drying cloths, rather than the humidity in the pockets. Internal variables, on the other hand, are, for example, the presence of incondensable gases inside the cylinder and above all, the most penalizing factor, the presence of condensation.” The automated system can overcome this drawback. “To optimize the drainage of the dryer cylinder, the characteristic curves of the condensate extraction system installed inside the dryer are entered into the DMS. The system is thus able to calculate with absolute accuracy the differential pressure required to evacuate the condensate and the amount of steam needed.”
Another advantage of using DMS is improved energy efficiency. The energy required by the dryer to dry the sheet is able to be fully utilized without any residual energy, “that is, without having the need to send excess amounts of steam to the water condenser. The DMS, in managing waterfalls, is able to direct steam from the higher-pressure unit to lower-pressure units, so that energy reuse is optimized and made more efficient. In addition, energy efficiency is also managed very accurately in the sheet breaking stages, during which the system is able to calculate and minimize the energy to be sent to condensation so as to ensure perfect drainage of condensate from the dryer, under all conditions”.
In essence, the DMS, through consumption reduction, intersects the need to reduce production costs with the need to improve environmental sustainability. “Through modifications and reconstructions of steam plants performed in Europe such as Kadant,” Buttiglione says in this regard, “we have calculated, in 2022, a reduction in CO2 emissions of about 25 thousand tons.” This is an interesting figure, all the more so considering that these types of projects also allow access to the white certificate system.
The drying curves
Different types of paper obviously require different drying. Each type of product has its own, very specific drying curve that depends on factors such as the fibers used, ash content within the pulp, pulp refining, etc., variations that, Buttiglione points out, can affect dryer performance. Again, the DMS is able to store different drying and pressure curves, which can be associated with product codes and recalled whenever the mill needs to produce a specific type of paper.
The system is also capable of reducing idle production steps, specifically paper breaks. Through its algorithms, the DMS calculates the surface temperature of the cylinder both with paper and without. During paper breakage stages, the goal is to prevent the paper from overheating or, on the contrary, overcooling when the paper is not on the dryer cylinder.
While normally such a situation is handled by operators who, based on experience and knowledge of machine reactions, rather than entirely subjective assessments, set a percentage reduction in the set-point of the main steam system unit, the DMS allows for an accurate calculation of this reduction. “The system has the great advantage of calculating and predicting the temperature at the cylinder in the absence of the foil and then keeping this temperature constant even when the foil is not present on the cylinder. In this way it will make it possible to maintain essentially the same conditions as when the machine is in production, thus avoiding variations in pull that could even tear the sheet.”
Finally, the DMS allows the mill to guarantee itself another important factor, the repeatability of sheet quality. An aspect, says Buttiglione, that is far from trivial, “being able to memorize the drying curve of the paper, it allows to maintain the same conditions with all operators. So with the guarantee that the quality characteristics of the sheet and the way the paper is dried are always the same and constant over time.”
Structure in modules: basic and energetic
The Kadant DMS was designed and developed in modules. The starting structure consists of the basic module while the other modules can also be implemented at different times.
The basic module offers the main features of a control system, thus the graphical operator interface and historical recording of production parameters. In addition, there is provision for intelligent paper break management, with dedicated software, as well as possible interconnectivity of the system remotely.
“With the basic module then,” Buttiglione explains, “our technicians are also able to provide assistance remotely, directly from our offices.” Another feature is the automatic startup and shutdown sequence of the steam system. “This means that, when restarting from a maintenance stop, operators, thanks to the use of a simple run button, will be able to heat the plant and start the machine, relaunching it in the optimal conditions to immediately produce according to the expected quality standards.”
Among other modules, one of the most interesting for a paper mill is the energy module. It provides for the calculation of the dry outgoing from the presses, an extremely important data for the performance of a plant; “it is well known that one point of variation in dryness can lead to variations in both energy consumption and execution efficiency performance even in the order of 4 or 5 percent. Well, the DMS is able, through a very accurate thermodynamic balance, to accurately calculate the dryness coming out of the presses.” This value is recorded and made available within the plant’s trend history, also giving the ability to predict when it will be time to maintain the press section, for example by replacing its felts.
Ventilation and instrumental modules
Closely related to the energy module is the ventilation module. “Typically, hood ventilation control systems use sensors that measure the humidity in the fumes extracted from the hood and adjust the fans according to this value. The problem we have found and that our customers have reported to us,” the director explains, “is the need for frequent intervention to clean, recalibrate, and maintain these sensors. When problems arise, it happens that the operator intervenes by manually managing the humidity controller.” In contrast, the modules developed by Kadant, thanks to the calculation of the dryness coming off the presses performed by the energy module, allow the operator to know continuously exactly how much water is evaporating from the sheet.
Having this parameter available, it is possible to quickly adjust the speed of the fans, through the ventilation module. An action that has the advantage that it can be performed repetitively and consistently over time. “The ventilation module thus makes it possible to maximize the efficiency of the heat recovery units on the fumes, optimizing the flow rates of air in the extraction and supply, and to punctually adjust the temperature of the air blown inside the dryer, with an indirect energy recovery, in when the air is not heated beyond the value necessary for the type of production being managed.”
Finally, the DMS is complemented by the instrumental monitoring module, which is designed to mainly help steam plant maintenance personnel. “Basically, on new equipment we fit control valves equipped with position recopia. The DMS compares the opening expected by the controller with the actual opening achieved by the valve and, if necessary, sends warning messages that allow maintenance personnel to understand when a valve is drifting and thus determine in time when to service it”.
Slovenian case study
An interesting application case of the DMS system involved a rebuild performed by Kadant at the Mayr-Melnhof paper mill in Slovenia. The company, which produces packaging board for the pharmaceutical and food sectors, complained of frequent breaks and, as a result, production efficiency below 80 percent, with also difficulties in meeting sheet quality standards and very sudden moisture variations in both transverse and longitudinal directions. The plant also had frequent flooding of the drying cylinders and high consumption of thermal and electrical energy. This was made even more difficult by the very varied range of boards produced, which resulted in great difficulty in matching the drying requirements of the various types of product.
“It was decided, with our technicians, to organize a site visit to take measurements and an assessment of the situation of the plant,” Buttiglione recounts. Following the inspection, interventions were proposed to be carried out. “First, the condensate extraction system was replaced, fitting stationary siphons and turbulence bars. In parallel, the steam system was balanced, revising the pre-existing system, which was partly with thermocompressor. Above all, a new vacuum system was installed with a special manifold system. Finally, DMS was implemented to be able to ensure proper management of all these operating parameters.”
The innovations made at the Slovenian plant have achieved all the benefits that were projected at the contract stage, from increasing drying capacity by 20 percent to reducing the amount of steam used to produce the same paper produced before interventions-calculations speak of more than a ton of steam less-with a cost equivalent to reducing CO2 emissions by about 1,350 tons per year.
Circuit optimization and improved condensate drainage from the dryer cylinder also led to a “slight but noticeable” reduction in power consumption of about 5 kWh per ton of product. In addition, there has also been a dramatic reduction in the number of sheet breaks in the dryer and by about 50 percent in the time it takes to restore machine productivity after stops for paper breaks.
All for an investment that, it was calculated, had a payback time of only seven months.
Such was the satisfaction with the new system that Mayr-Melnhof, when it decided to carry out a major rebuild of the dryer, requested the installation of the DMS itself, to supervise the steam circuits of the steam plant.