High Intensity Drying of M.G. Paper

ABSTRACT

The technological properties of paper are strongly influenced by the drying conditions. This is the case in particular with M.G. paper and machine performance is limited largely for quality reasons. Therefore, the question as to which extent the drying conditions influence the process of drying and the paper quality is of major importance for industry.

Unlike the results already published, tests were carried out with a laboratory machine operating on a continuous basis, with high drying rates, short retention time and application of combined contact/ convectional drying. The kinds of stock used were waste paper containing wood, bleached long-fiber magnefite pulp and unbleached long-fiber sulphate pulp with a basis weight ranging between 37 and 75 g/m2.     

一种新的纸机干燥部蒸汽冷凝水热泵控制系统

简要介绍纸幅干燥原理和热泵节能机理,设计一套基于S7—300PLC的五叠网涂布白板纸机蒸汽冷凝水热力控制系统,阐述具体的控制方案和采用的控制算法,其中主要包括热泵组的压力分段控制和热泵开度的低端选择控制、湿端低温段烘缸组压力回路分段控制、热烘箱二次蒸汽回收压力回路的分程控制和成纸水分的串级控制。实际应用效果表明该控制系统方案是可行的,同时也是有效的。     

CONDEBELT BOARD AND PAPER DRYING

Abstract

The history of development of the Condebelt process is outlined. The functioning principle of the various types of existing or proposed Condebelt dryers is clarified

Condebelt drying, being a type of press drying, imparts certain quality characteristics, due to the simultaneous high z-pressure and high temperature in the presence of moisture, to the web. Some typical quality characteristics of Condebelt-dried sheets are presented for some usual paper and board grades. The web strength values of densified Condebelt-dried sheets are usually much higher than those of conventionally dried sheets. This is mostly due to the much enlarged contact areas between flattened fibers, and to the flow of surface hemicellulose and lignin under the Condebelt drying conditions. In particular, wet strength values, as well as dimensional stability under changing environmental humidity conditions, are very much improved     

Drying paper by impinging jets of superheated steam. Part 1: Constant drying rate in superheated steam

Over a wide range of operating conditions, the drying of paper by impinging jets of superheated steam proceeds by a constant rate period followed by a falling rate period. The constant drying rate, investigated here in the jet temperature range 150 ≤ T_j/ ≤ 465°C and over jet Reynolds numbers of 1000 ≤ Re_j ≤ 12000, is predicted within + 12% by a heat transfer expression using Martin's (1977) correlation for the heat transfer coefficient corrected for mass transfer by the Couette flow approximation factor, and a property ratio to account for the large temperature difference between the jet and the paper.     

A Cross-Flow Model for Continuous Plug Flow Fluidized-Bed Cross-Flow Dryers

Plug flow fluidized bed cross-flow dryers have been used in drying of particulate solids such as paddy and other grains for many years. However, simulation of the performance of any particular design of the dryer has always been problematic due to the inadequate overall empirical models used that are too inflexible and too specific to the particular design. In addition, previous theoretical models of the plug flow fluidized bed cross-flow dryer did not model the gas cross flow properly and had difficulty in modeling the moving solid bed. A new steady-state cross-flow model of the dryer that models the gas cross-flow is proposed. The profiles for the solids and air moisture contents and temperatures were found to be dependent on the gas-solid flow ratio, G/F, the specific heat demand, C_PY(T_I - T_A)/(Y_E - Y_I), the total number of a transfer units, N_T = Gε/KφaSL and the specific drying load, (X_I - X_P)/ (Y_E - Y_I). The model was validated by comparing the simulated data with experimental data that were obtained by drying paddy in a plug flow fluidized bed cross-flow dryer pilot plant. The model was found to estimate very well the solids moisture content and temperature, the gas moisture content and temperature profiles, and the driving force profile.     

PAPER DRYING: A STRATEGY FOR HIGHER MACHINE SPEED. II. IMPINGEMENT AIR DRYING FOR HYBRID DRYER SECTIONS

For printing and heavier grades, combining cylinder and impingement air drying into a multiple technique dryer section can enable higher productivity through higher machine speed. The large differences in local moisture content and temperature across the sheet which develop quickly under high intensity impingement drying provide the potential for reducing drying time by sheet reversal between impingement drying cylinders. This advantage was determined experimentally under low intensity impingement drying conditions. Use of the micro-scale based McGill dryer simulator for determining the advantage from sheet reversal between impingement drying cylinders was demonstrated for both laboratory and industrial impingement drying intensities. For completing the drying of 205 g/m² linerboard from 0.3 to 0.5 kg/kg dry under 400°C air jets of Re 20000, drying time is about 30% less with sheet reversal between two impingement drying cylinders than for a single, larger diameter cylinder. This extensively validated dryer simulator enables determining advantageous design specifications and operating conditions for hybrid dryer sections involving combinations of cylinder and impingement air drying, a concept with potential to become common industrial practice.     

Through Drying of Paper

Through drying of paper of basis weight 25 to 50 g/m² was studied through 89 experiments with 22 to 90°C air at mass flow rates from 0.125 to 1.45 kg/m²s. The complete drying rate curves are represented with five quantitative parameters:moisture contents at the end of the increasing and constant rate periods, the constant drying rate, and exponents for the increasing and falling rate period relations. The constant drying rate period disappears at higher drying intensities leaving the increasing rate period accounting for as much as 37% of the drying. The constant drying rate may be controlled by thermodynamics or by transport phenomena. The critical moisture content depends on effects from the intrinsic local nonuniformity of through drying as well as on sheet average conditions. A universal set of relations was obtained for representation of the complete drying rate curve and was verified for prediction of drying rate and drying time. Through drying rates cannot be increased significantly by providing the drying air as an impingement flow.     

Condebelt Drying of Paper and Paper board for Optiemizing Quality and Production for Many Cpades

ABSTRACT

The development history of the Condebelt paper and paperboard drying method is briefly described. The process conditions experienced by the web during Condebelt drying are essentially similar to those prevailing in the so–called press drying experiments. In fact, condebelt drying produces. qualitywise, results quite similar to those reported by press drying researchers from many laboratories. condebelt drying has been studied over a span of many years by means of several static units, as well as a pilot stage dynamic device. Based on these experiences, as well as several theoretical studies, designs have been developed for doing Condebelt drying as a production process. Condebelt drying is versatile, in that the process conditions can be altered to produce optimum quality for most paper and board grades. In this capability the Condebelt process is much superior to canventional cylinder drying. The very high drying rates characteristic of Condebelt dryers are discussed, along with the Consequences as far as machine room space saving is concerned. The good energy consumption prospects of the Condebelt process are briefly discussed.     

Powder Technology Handbook edited by Keishi Gotoh,Hiroaki Masuda and Ko Higashitani Marcel Dekker,Inc., Cimarron Rd., Monticello,NY 12701 Second Edition. 1997, 944 pages.

Abstract:

In 2000, the U.K. government's Energy Efficiency Best Practice Programme commissioned a survey to determine the energy consumption of spray dryers within the chemicals, foods, and ceramics industries. The results of this survey, which included dryers having evaporation rates ranging from 0.1 to 12 t/h, revealed values of the specific energy consumption E _s varying from around 3 to 20 GJ/t water evaporated. The average for all dryers included in the survey was 4.87 GJ/t. The fuel-to-electricity consumption ratio averaged around 27. The data obtained in the survey were interpreted with the aid of a newly developed model that enabled the performance of a particular dryer to be compared with that of its ideal adiabatic counterpart. Using the model, it was estimated that around 29% of the energy supplied to the dryers included in the survey was being wasted.     

THE EFFECT OF DRYING TEMPERATURE ON THE QUALITY OF PAPER

ABSTRACT

The effect of drying temperature on the optical properties of various papers was investigated. For paper from mechanical pulp, it was found that as drying temperature increased, light scattering coefficient decreased leading to lower brightness. The decrease in light scattering was associated with improved strength properties even at constant sheet density, and was attributed to increased bonding. These effects were not observed for chemical pulp furnishes.     

THE EFFECT OF DRYING TEMPERATURE ON THE QUALITY OF PAPER

The effect of drying temperature on the optical properties of various papers was investigated. For paper from mechanical pulp, it was found that as drying temperature increased, light scattering coefficient decreased leading to lower brightness. The decrease in light scattering was associated with improved strength properties even at constant sheet density, and was attributed to increased bonding. These effects were not observed for chemical pulp furnishes.     

污泥间接干燥过程的实验研究

污泥间接干燥是以热传导为传热方式的干燥过程。采用楔形桨叶式干燥机进行污泥干燥实验,研究了污泥的间接干燥过程,测得了污泥在干燥机不同位置的温度及其相应的含水率,验证了污泥的间接干燥过程遵循预热阶段、恒速干燥阶段和降速干燥阶段的一般规律,获得了不同转速下污泥干燥过程的平均传热系数,且平均传热系数随着搅拌转速的提高而增大。