Theoretical Estimation of Evaporation Heat in Paper Drying Process Based on Drying Curve

At present, the theoretical estimation of paper web’s evaporation heat is based on sorption isothermals. The measuring conditions are harsh, and the test speed is slow. This paper attempts to explore a theoretical method which can quickly determine the evaporation heat of paper web. During the new method, based on the measurement of the paper drying curve, the theoretical estimation model of paper evaporation heat was obtained by deducing the mechanism of heat and mass transfer. Compared with the traditional method based on sorption isothermals, the new model based on the drying curve has some advantages in measurement speed and easy access to basic data. Finally, the paper verifies the reliability of the model from two application scenarios of the laboratory and production line.

Energy saving in papermaking by application of hybrid calcium carbonate

Hybrid calcium carbonate (HCC) is prepared by pre-flocculating calcium oxide and ground calcium carbonate (GCC) with ionic polymers, and by injecting carbon dioxide to the pre-flocculated flocs until termination of the reaction at neutral pH. When used in papermaking, HCC gave higher bulk, higher stiffness, and higher tensile strength than GCC did. Furthermore, when higher wet pressing pressure was applied to the paper containing HCC, higher solid content was achieved with still better stiffness, smoothness, and tensile strength in comparison with the paper containing GCC. Actually, very high bulk of the HCC containing paper was compromisingly reduced to obtain high solid content by applying higher wet pressing pressure. Higher solid content achieved by combination of HCC technology and higher wet pressing pressure resulted in paper drying energy savings of more than 10%, while a high quality of all the essential printing paper properties was maintained. Drying energy savings may contribute to greenhouse gas (GHG) reduction and lowering of the paper production cost.

Spraying starch on the Fourdrinier— An option between wet end starch and the size press

Technology to apply suspensions of starch grains to the wet surface of paper, during the dewatering process, is reviewed. Though the technology is not new, it continues to attract the attention of papermakers as a means to increase bonding strength. Starch grains that are sprayed onto the wet-web of paper can be retained at levels exceeding what can be effectively added to the fiber suspension at the wet end. Unlike adding a starch solution at a size press, no additional drying capacity is required on the paper machine. To be effective, the starch needs to be able to swell and develop bonding during the paper drying process. Paperboard applications with recycled fibers appear to be a good fit. There is potential to increase bonding by processes that favor fuller gelatinization of the starch grains by the time the paper becomes dry.

Deformations of drying cylinders during paper creping and methods of their elimination

При крепировании бумаги на сушильных цилиндрах (СЦ) больших диаметров происходит износ лезвий шаберов и поверхности СЦ. Износ от трения лезвия шабера по поверхность корпуса СЦ неравномерен. По краям СЦ по боковой поверхности происходит ускоренная выработка канавок, а на лезвии шабера – ускоренный локальный износ лезвия. Цель работы – исследовать деформацию СЦ и определить влияние деформации на процесс крепирования бумаги и износ шабера и СЦ. Методология основана вначале на исследовании раздельной деформации корпуса СЦ и торцевых крышек как деталей, а затем совместной деформации СЦ как сборочной единицы от факторов технологического процесса и износа СЦ в период эксплуатации. В качестве основных факторов технологического процесса, вызывающих деформацию деталей по отдельности и СЦ в целом, приняты тепловые процессы сушки бумаги и внутреннее давление в СЦ. При исследованиях экспериментально измерена температура корпуса СЦ и торцевых крышек с лицевой и приводной сторон. Измерения выполнены пирометром TemPro 300. Действие процессов рабочего режима на корпус СЦ и крышки как отдельных деталей приводит к существенным отличиям в изменении их сопрягаемых размеров. В рассматриваемом примере корпус СЦ по диаметру сопряжения с крышками имеет в три раза большее приращение, чем крышка. Как сборочная единица СЦ деформируется с образованием изгиба на образующей с уклоном к оси цилиндра в направлении к крышкам. Деформация образующей СЦ локальна, снижается от крышек к центру СЦ и на расстоянии в 2,5–3 толщины стенки крышки затухает. На участках изгиба происходит ускоренный износ лезвия шабера и корпуса СЦ от прижима шабера. На участках изгиба СЦ имеет меньший диаметр, чем в средней части, что приводит к образованию по краям бумаги крепа с более длинным шагом. When creping paper on drying cylinders (DC) of large diameters, the blades of scrapers and the surface of the DC wear out. Wear from friction of the scraper blade on the surface of the housing of the DC is not uniform. Along the edges of the DC on the side surface, there is an accelerated development of grooves, and on the scraper blade – accelerated local wear of the blade. The purpose of the work is to study the deformation of the DC and determine the effect of deformation on the process of crepe paper and wear of the scraper and DC. The methodology is based first on the study of separate deformation of the DC body and end caps as parts, and then joint deformation of the DC as an Assembly unit, due to factors of the technological process and wear of the DC during operation. As the main factors of the technological process that cause the deformation of parts individually and the DC as a whole, the thermal processes of paper drying and internal pressure in the DC are accepted. During the research, the temperature of the DC housing and end caps on the front and drive sides was measured experimentally. Measurements were made with a TemPro 300 pyrometer. The action of the operating mode processes on the housing of the DC and the cover, as separate parts, leads to significant differences in changing their mating dimensions. In this example, the housing of the DC has a three-times greater increment in the diameter of the interface with the covers than the cover. As an Assembly unit, the DC is deformed with the formation of a bend on the generator with a slope to the axis of the cylinder in the direction of the covers. The deformation of the forming DC is local, it decreases from the covers to the center of the DC and, at a distance of 2.5–3 of the thickness of the cover wall, it fades. At the bend sections, the scraper blade and the DC housing are accelerated by the scraper's pressure. In the bending sections, the DC has a smaller diameter than in the middle part, which leads to the formation of crepe along the edges of the paper with a longer step.

Investigation of Flashing Flow in a Siphon to Extract Condensate in Paper Dryer Application

The paper industry uses steam to dry paper web through cylinder dryers. As steam condenses inside the dryer, the condensate is removed by means of either a stationary or a rotary siphon. However, during the siphoning process for transporting the condensate, flashing of the condensate occurs, which could cause backflow or discontinuity in the siphoning process. To resolve this flashing issue, two approaches have been employed: (a) increasing the amount of steam supplied to the cylinder to “blow-through- the stalled condensate-steam mixture and (b) reducing the back pressure by inducing “suction- through a thermocompressor from downstream. The objective of this study is to investigate and improve understanding of the flashing phenomena during condensate transport through the siphon and piping system in order to develop means to reduce the excessive steam consumption during the paper drying process. A computational fluid dynamics (CFD) simulation is performed that uses the Eulerian-Eulerian multiphase method. The steady-state case is first solved to obtain the flow field without flashing. Then the transient method is initiated by employing flashing and condensation models. The results show that reduction of local pressure triggers flashing; however, flashing in turn reduces local temperature, and subsequently induces condensation, resulting in an alternating flashing and condensation behavior. To maintain continuity of the siphon flow, the inlet pressure fluctuates corresponding to the variation of total vapor volume ratio inside the siphon. The results will be used to modify the current siphon system design and operating practices to reduce steam consumption.

Fundamental understanding of removal of liquid thin film trapped between fibers in the paper drying process: A microscopic approach

In the fabrication of paper, a slurry with cellulose fibers and other matter is drained, pressed, and dried. The latter step requires considerable energy consumption. In the structure of wet paper, there are two differ-ent types of water: free water and bound water. Free water can be removed most effectively. However, removing bound water consumes a large portion of energy during the process. The focus of this paper is on the intermediate stage of the drying process, from free water toward bound water where the remaining free water is present on the surfaces of the fibers in the form of a liquid film. For simplicity, the drying process considered in this study corresponds to pure convective drying through the paper sheet. The physics of removing a thin liquid film trapped between fibers in the paper drying process is explored. The film is assumed to be incompressible, viscous, and subject to evaporation, thermocapillarity, and surface tension. By using a volume of fluid (VOF) model, the effect of the previously mentioned parameters on drying behavior of the thin film is investigated.

The control of condensate in the dryer cylinders by their temperature gradient coolings

Дроссельные шайбы, применяемые в сушильных цилиндрах (СЦ) для выпуска конденсата, имеют нерегулируемый условный проход, что приводит при интенсификации теплового и гидравлического режимов работы к накоплению конденсата. Повышенное содержание конденсата в СЦ увеличивает термическое сопротивление тепловому потоку от конденсирующегося пара к внутренним стенкам СЦ. Разработка методов обнаружения повышенного содержания конденсата в отдельных цилиндрах актуальна для снижения расхода пара на сушку бумаги. Выполнены исследования трендов температуры СЦ (скорости изменения температуры по времени, °С/мин) за период охлаждения после отключения подачи пара. Выбраны диагностические точки контроля и обоснован режим измерения температур в этих точках СЦ. Произведено исключение грубых ошибок экспериментов. Разработана методика и алгоритм расчета допустимых уровней трендов температур. Выполнены расчеты допустимых характеристик трендов температур за время интервалов охлаждения СЦ. По трендам температур определены СЦ с повышенным содержанием конденсата. Разработанный авторами метод и алгоритм расчета допустимых трендов температур охлаждения СЦ при останове машины, основанный на различиях в скорости охлаждения СЦ с конденсатом и без конденсата, теоретически обоснован и экспериментально апробирован в производственных условиях. Обнаружение СЦ с повышенным содержанием конденсата по данному методу можно производить параллельно с другими работами по технологическому и техническому обслуживанию сушильных частей бумагоделательных машин. Throttle washers used in drying cylinders (DC) for condensate release have an unregulated conditional passage, which leads to the accumulation of condensate during the intensification of thermal and hydraulic modes. The condensate accumulates in DS due to the tolerances of the scale uneven. The increased condensate content in the DC increases the thermal resistance to the heat flow from the condensing steam to the internal walls of the DC. Studies of temperature gradients during the cooling period after the steam supply to the drying cylinders has been performed. The method and algorithm of calculation of permissible levels of temperature gradients are developed. Drying cylinders with increased condensate level were determined. The development of methods for detecting increased condensate content in individual cylinders is relevant for reducing the steam consumption for paper drying. Studies of temperature gradients of the DC during the cooling period after the steam supply was switched off in the drying part of the boom-making machine were performed. Diagnostic control points are selected and the temperature measurement mode in the DC these points is justified. The exception of gross errors of experiments on the t-criterion Student's was made. A method and algorithm for calculating the permissible levels of temperature gradients and the minimum and maximum permissible temperature deviations of the surveyed SCS has been developed. A graph is constructed for detecting and excluding rough measurement errors from the sample. Calculations of permissible characteristics of temperature gradients during cylinder cooling intervals are performed. By temperature gradients during cooling intervals, DC with an increased condensate content are determined. The method and algorithm developed by the authors for calculating the permissible levels of cooling temperature gradients of the DC, when the machine is stopped, based on differences in the cooling rate of the DC with condensate and without condensate, is theoretically justified and experimentally tested in production conditions. Detection of DC with high condensate content using this method can be performed in parallel with other works on technological and technical maintenance of drying parts of paper machines.

Intensification of Paper Drying on Drying Cylinders

One of the most energy-intensive processes is the drying process. Therefore, an important task is to reduce heat consumption for drying. Various ways to improve the drying devices of paper and cardboard machines are identified. The main disadvantages of these improvements are given. The limiting factor of heat transfer from water vapor to paper is established. The dependence of thermal resistance on the thickness of the condensate layer is given. The authors have developed a new design for condensate drainage. The developed design allows installing movable siphons without replacement of steam inlet heads of the old type (for fixed siphons) with their insignificant modernization and cutting of an annular groove inside the drying cylinder. This will ensure a minimum level of condensate and eliminate the possibility of suction of steam into the condensate return system. Additionally, this will reduce thermal resistance, heat loss, and reduce energy consumption for paper drying as a whole. Keywords: paper drying, moisture content, drying cylinder, thermal resistance, heat transfer.