Automatic retention aid control improves process and quality levels at Mochenwangen Papier

Transcription

1 Automatic retention aid control improves process and quality levels at Mochenwangen Papier Peter M. Raser, Application Specialist, BTG Mütek GmbH Udo Habnitt, Technology, Mochenwangen Papier GmbH The Mochenwangen mill together with the Golzern and Hainsberg papermills, Kartonagen Schwarzenberg and Grünperga Papier GmbH are part of the Golzern Holding GmbH. The Mochenwangen location runs three paper machines with a total capacity of 110,000 yeartons. A staff of 190 are engaged in the manufacture of book papers, web offset and sheet offset papers. PM 2 produces 60,000 t of offset, speciality and letterpress papers in the basis weight range of g/m 2. The maximum machine speed is currently at 960 m/min. Over the past three years, the product portfolio of PM 2 has been continually extended. For the Mochenwangen Papier GmbH papermaker, this strategy has involved a higher number of grade changes with longer transition periods, less stable production and increased broke volumes. The plant operator thus had to meet several challenges in terms of quicker and more stable grade changes reduced broke volumes optimized consumption of retention aids stable ash retention and simplified handling of retention aid dosages The schematic in Fig. 2 shows the approach flow of PM 2. The required raw materials are conveyed to the machine chest via a mixing chest. Downstream of the machine chest, the stock-water mix is diluted twice. Equipped with two mixing pumps, this system decouples the cleaners from the headbox so as to keep constant the flow rates and pressures during production. To enable the plant operator to stabilize retention and thus overall production conditions, BTG installed the following devices: whitewater sensor with separate pump circuit headbox sensor control unit for direct control of retention aid pump PM 2

2 Figure 2 Wet end of PM 2 Measuring technology For measuring purposes, the Mütek RET-20 Retention Inline of BTG was used whose measuring principle is based on the patented Peak method. Applying transmitted light, this method utilizes the presence of particles of different sizes in stock suspensions. Fibres are normally present in the form of large particles, whilst fillers and fines come as small particles. The investigation of a suspension sample reveals that it contains a high number of small particles that show a comparatively constant and homogeneous distribution over time. The number of large particles by comparison is relatively small and varies significantly over time. The large particles form a comparatively transparent network in which the small particles float around. A narrow light beam transmitted through the suspension is affected by both large and small particles. Since the RET-20 measures 200,000 signals per second, a sufficient number of time periods is detected during which the light beam is only damped by small particles. Since the transmitted light intensity is highest in this case, it is called peak value. During the rest of the time, the beam is affected by large and small particles (Fig. 3). In addition, a Mütek RET-20 Retention Controller was employed. Apart from incorporating two PI controllers for the retention aid pumps and for calculations, it is additionally provided with analog outputs for total and ash retention. Parameterization of the controller is done via Windows software. Besides, all available data is saved and visualized (Fig. 4).

3 Figure 3 The Peak method and the RET-20 Retention Inline Sensor Figure 4 RET-20 Controller and visualization Figure 5 Control schematic

4 Control Strategy Retention aid control is based on the parameters of oven-dry production on the one hand and whitewater consistency on the other. From the production and a corresponding predetermined retention aid dosage per ton, a fictitious setpoint is calculated. Taking the deviation of the whitewater consistency from the setpoint into account, this setpoint is controlled to within 0.5 times to 1.5 times its value (Fig. 5). Figure 6 Block diagram A detailed schematic is shown in the block diagram of Fig. 6. The first PI controller serves as a flow controller for the retention chemical. It operates on an external setpoint that is calculated from the o.d. production, the chemical concentration, the theoretical kg/ t ratio and the output signal of the 2nd PI controller. Comparing setpoint and actual value of the whitewater consistency, this second PI controller produces a corresponding output signal. For safety reasons, both controllers are provided with a manual/ automatic function and min/max limitations. An additional timer element facilitates machine startup after standstills.

5 Results The present chapter covers: whitewater stability consumption of retention aid stability of ash In addition, examples of grade changes and a comparison between controlled vs. manual operation will be presented. Fig. 7 illustrates the development of whitewater consistency and retention aid additions at different production stages. In the period covered, production was constant. In the first section of the diagram, automatic control was activated and the whitewater stabilized after a comparatively short time period. Figure 7 Comparison of manual vs. automatic mode At about 09:30, automatic control was switched off because of grade change problems. Until about 10:15, the mill staff tried to keep the whitewater consistency reasonably stable via manual adjustments. Afterwards, they started to add a constant dosage of retention aid, but whitewater consistency increased only slowly, because chemical dosages were obviously too low. At 12:15, automatic control was reactivated at a new setpoint and the whitewater stabilized relatively quickly. When looking at another time period (Fig. 8), we see an 8-hour trend with automatic control activated. Retention i.e. total and ash retention and the whitewater consistency are stable, whereas retention aid dosages vary under automatic control within a range of l/h. Generally speaking, if constant additions of retention chemical had been applied during that production stage, fluctuations would have occurred in the whitewater and thus in retention, too.

6 Figure 8 Stable whitewater consistency Figure 9 Control vs. manual intervention Fig. 9 shows a comparison between automatic control of retention aid dosages and manual control i.e. fixed chemical additions. Under automatic control of retention aid dosages, the signal variation for the whitewater consistency is reduced by almost 50 %. Naturally, this favourably impacts the stability of the trend parameters and thus the overall process.

7 Figure 10 Grade change 1 Figure 11 Grade change 2 Figs. 10 & 11 illustrate a grade change from 42g/m 2 to 50g/m 2. In this context, the headbox consistency was increased. The control concept automatically provides for increased additions of retention aid, with the controller keeping the whitewater consistency constant. As a result, total retention and ash retention were somewhat higher compared to lower basis weights. As a decisive benefit, the process stabilized very quickly on a new level after the grade change.

8 Stabilization of Ash At Mochenwangen Papier, 8 % CaCO 3 related to the thick stock volume are added. In order to attain the target ash in the paper, kaolin is additionally dosed manually. Accordingly, kaolin dosages have to be continually adjusted to meet fluctuations in ash retention. Figure 12 Stable ash content Fig. 12 illustrates the evolution of ash retention and ash content in the whitewater under automatic control. At about 13:00, automatic control was switched to manual mode with a fixed setpoint. This caused a rise in whitewater ash, which is indicative of poorer ash retention and thus of a lower ash content in the paper product. To counteract this development, mill staff adopted manual dosage, which slightly improved ash retention but failed to eliminate ash variation. This means that, once retention and particularly that of ash has stabilized, any further adjustment of kaolin additions becomes superfluous. Summary For PM 2 at Mochenwangen Papier, all the objectives were met and, over and beyond that, expectations were exceeded. The retention chemicals were optimized to allow approx % savings in consumption. After grade changes, processes stabilized within a short time period. During the actual grade changes, the PM speed was successfully maintained and the trend parameters for the paper machine proved to be very stable. Owing to a stable total and ash retention, kaolin dosages need hardly ever to be adjusted. The ash content of the paper is near to constant. Moreover, thanks to constant retention levels, constant flow conditions are obtained in the inflow to the fibre recovery of whitewater II (saveall). This has enabled a follow-up optimization of the saveall and of the controlled application of retention chemicals and the resulting system stabilization have made it possible to increase the PM speed from 910 m/min to 950 m/min with peaks of 960 m/min.