Study On The Mathematical Model Of Heat And Mass Transfer In Paper-drying Process

With a focus on energy saving and emissions reduction, this study presents an overviewof the current energy related research status for both domestic and international paper industry,outlines commercialized and emerging energy-efficiency technologies. It also discusses thecommonly applied energy-saving potential assessment methods, e.g. energy-efficiencybenchmarking and energy auditing. As an application, a plant-wide energy audit is conductedfor a paper mill, the results identified14%total energy-saving potentials. From above analysis,we found the paper drying process dehydrated the smallest amount of water with the largestamount of energy use and cost. It is also found that paper drying process has the highestenergy-saving opportunities among the papermaking processes. In addition, it has asignificant implication on the overall energy efficiency of the whole papermaking process.Therefore, we finally choose the paper drying process as the main research object for thefollowing studies.Initially, with the purpose of having a full picture about the current operational status andthe drying performance of paper machine, we conducted a diagnostic analysis on the energysystem of a corrugated paper machine dryer section. A completed test and analysis methodsthat focused on the key operation parameters is also presented. The comprehensive analysis isfound to be a useful assistant tool for help understanding the current drying performance. Themain energy-saving opportunities is recommended based on the dominant issues that affectedthe drying energy consumption. As part of the dryer section, the heat recovery system is alsoanalyzed combined energy and exergy analysis methods. Based on the results of a coatedpaper machine, a new waste heat integrated proposal is expected which could increase theenergy utilization by7.3%. The proposed energy diagnosis method of dryer section and thecombined analysis methods of heat recovery systems, as well as the corresponding casestudies, could have a significant meanings for reducing the energy use of paper machine dryersection.Next, a static energy model and related modeling system of the paper drying process isdeveloped using sequential modular approach based on the principle of mass and energy conservation. The developed static model is composed of eight basic functional blocks, whichalso considering the relationship between each subsystems. As the case study, the static modeland system of a newsprint machine is then constructed according to its specific drying processand conditions. The MATLAB simulation result shows that it could be used to simulate theactual newsprint drying process correctly. A series of simulations are also performed toinvestigate the influences of some key operating parameters on the drying performance, e.g.initial paper dryness and its temperature, supply air temperature, exhaust air humidity, andambient conditions. The simulated results indicate that the consistency of the static modelwith experimental results and experience is reasonable. Some commonly important results arepresented in this chapter. Not only could the static model simulate the mass and energy flowinformation of the drying process at a macro level, it could also be used to analyze howoperating parameters affected the drying performance. The static model could also helpdeepen awareness and understanding of the whole paper drying process thoroughly. Inaddition, it could provides some constructive advice to guide energy saving and processoptimization for the practical paper drying process.Subsequently, this study focus on the various water existing status within paper web andrelated evaporation mechanisms. The basic mass transfer equations of free water andhygroscopic water are presented at the beginning. To investigate the performance of contactpaper drying process, we established a mathematical model on the basis of heat and masstransfer theory, then the drying kinetics for this process was studied with numericaltechnology. The simulation result shows that the predicted paper temperature, moisturecontent and evaporation rate, varying with drying time, is basically fit in with the qualitativedescriptions about the theoretical paper drying process cited in the literatures. Additionally,the initial paper dryness, hot wall temperature, and air velocity were quantitatively examinedto study their impacts on drying rate and energy consumption. The results from this studypresented that increasing initial sheet dryness could reduce drying time and energy use.Increasing heat-wall temperature could improve drying efficiency besides reducing dryingtime and energy use. Increasing air velocity could also help to reduce the drying time, butmay influence drying efficiency negatively at some ranges. It was found the contact paperdrying model and the numerical research methods for theoretical paper drying process could be acted as a qualitative supplement to the current available theoretical paper drying curves.This research also has some instructive significance for further developing some moreaccurate paper drying mathematical model for paper machine drying process.Finally, we developed a original and innovative paper drying model based on the heatand mass transfer theories with the intention of conducting some exploratory studies andpreliminary simulations for paper machine dryer section according to its drying state of the art.This model coverers the holistic understanding of the dryer section reflected by the staticenergy model and the quantitative description of the drying mechanism revealed by abovecontact drying model. It was constructed from the viewpoint of heat and mass transferaccording to the relationship of the local and the entire drying process. The basic paperelement and coupled cylinder element were chosen as the basic modeling unit, respectively.Then the governing equations for paper moisture content, paper temperature and cylindertemperature were deduced step by step, including the detailed description of the relatedboundary conditions under different circumstances and the physical properties of materials formodeling the actual paper drying process. The free-term presented in the paper and cylindergoverning equations were paid more attention to describe considering the different heat andmass transfer mechanisms between the contact drying zone and the convective drying zone.Besides, the influence of the fiber saturation point on drying process was also always kept inmind. The paper drying mathematical model was ultimately presented as an ordinarydifferential equations initial value problem coupled with a partial differential equationsboundary value problem. Next, we solving this coupled problem using finite differencemethod based on the distributed computational grids and the differential governing equationsas well as its boundary conditions. After that, all the distributed algebraic equations alone thewhole calculated domains for each node were automatically solved using iterative methodswith MATLAB. The modeling results imply that the predicted drying curves are basically inaccordance with the theoretical drying process and practical engineering experience. However,the developed drying model and the simulation process, particularly the involved heat andmass transfer coefficients and evaporation mechanism in the transition drying zone, still need further study to improve the predicted results in order to make it more closely represent theactual paper drying process.