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1.
《Drying Technology》2013,31(9):2119-2134
Abstract

A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40–80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.  相似文献   

2.
Agitated contact drying experiments were carried out in a cylindrical lab-scale paddle dryer to study the drying kinetics of sewage sludge under partial vacuum conditions. To explore the drying mechanisms, the penetration model for vacuum contact drying of agitated particulate materials, developed by Schlünder and Mollekopf, was applied to simulate the experimental results. The influence of the drying parameters (system pressure, heating temperature, stirrer speed, and dryer load) on the drying kinetics was studied both experimentally and theoretically. The results indicate that pressure and temperature significantly influence the drying rates of sewage sludge, whereas stirring speed and dryer load have no obvious effects on drying rates in the examined range. The experimental and simulated drying rates were in acceptable agreement as long as the temperature difference between dryer and sludge was small. A crust is likely to form on the dryer surface, however, when the temperature difference becomes large; this crust leads to markedly lower drying rates than the calculated ones. Furthermore, a comparison between partial vacuum contact drying and atmospheric contact drying is presented. The results indicate that the mass transfer resistance increases with rising drying potential in the examined experimental range.  相似文献   

3.
Vacuum contact drying kinetics of a model system consisting of nonporous glass beads and water has been experimentally measured on a laboratory scale. A methodology for determination of drying curves from experimental data in a statistically robust way has been developed. The effects of jacket temperature, head-space pressure, particle bed depth, vessel diameter, and particle size on drying rate during constant and falling rate periods have been studied. It was found that in the range of parameters investigated, drying rate does not depend on the means of realization of the driving force (by temperature or pressure); drying rate in the constant-rate period decreases with increasing bed depth while the overall heat-transfer rate increases due to increased surface area. A very strong dependence of drying rate and regime on particle size was observed; the constant-rate period disappeared for small particles.  相似文献   

4.
Abstract

Vacuum contact drying kinetics of a model system consisting of nonporous glass beads and water has been experimentally measured on a laboratory scale. A methodology for determination of drying curves from experimental data in a statistically robust way has been developed. The effects of jacket temperature, head-space pressure, particle bed depth, vessel diameter, and particle size on drying rate during constant and falling rate periods have been studied. It was found that in the range of parameters investigated, drying rate does not depend on the means of realization of the driving force (by temperature or pressure); drying rate in the constant-rate period decreases with increasing bed depth while the overall heat-transfer rate increases due to increased surface area. A very strong dependence of drying rate and regime on particle size was observed; the constant-rate period disappeared for small particles.  相似文献   

5.
Effective sludge management is increasingly critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks for options such as sludge landspreading and composting. Sludge dewatering challenges are exacerbated at many mills due to improved in-plant fiber recovery coupled with increased production of secondary sludge, leading to a mixed sludge with a high proportion of biological matter that is difficult to dewater. Various drying technologies have emerged to address this challenge of sludge management, whose objective is to increase the dryness of mixed sludge to above critical levels (≈42% dryness) for efficient and economic combustion in the boiler for steam generation. The advantages and disadvantages of these technologies are reviewed in this article, and it is found that many have significant technical uncertainties and/or questionable economics. A biodrying process, enhanced by biological heat generation under forced aeration, is introduced that has significant promise. A techno-economic analysis of the batch biodrying process at a case study mill showed an annual operating cost savings of about $2 million, including the elimination of landfilling practices and supplemental fuel requirements in the boiler. It was shown that if a biodrying residence time of less than 4 days can be achieved, payback periods of 2 years or less can result in many mills. The potential for the development of a continuous biodrying reactor and the fundamentals of its mathematical modeling are thus presented. Compared to the batch reactor configuration, it is expected that the continuous process would result in improved process flexibility and controllability, lower investment and operating costs due to shorter residence times, and an improved potential to fit into the crowed pulp and paper mill site.  相似文献   

6.
Effective sludge management is increasingly critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks for options such as sludge landspreading and composting. Sludge dewatering challenges are exacerbated at many mills due to improved in-plant fiber recovery coupled with increased production of secondary sludge, leading to a mixed sludge with a high proportion of biological matter that is difficult to dewater. Various drying technologies have emerged to address this challenge of sludge management, whose objective is to increase the dryness of mixed sludge to above critical levels (≈42% dryness) for efficient and economic combustion in the boiler for steam generation. The advantages and disadvantages of these technologies are reviewed in this article, and it is found that many have significant technical uncertainties and/or questionable economics. A biodrying process, enhanced by biological heat generation under forced aeration, is introduced that has significant promise. A techno-economic analysis of the batch biodrying process at a case study mill showed an annual operating cost savings of about $2 million, including the elimination of landfilling practices and supplemental fuel requirements in the boiler. It was shown that if a biodrying residence time of less than 4 days can be achieved, payback periods of 2 years or less can result in many mills. The potential for the development of a continuous biodrying reactor and the fundamentals of its mathematical modeling are thus presented. Compared to the batch reactor configuration, it is expected that the continuous process would result in improved process flexibility and controllability, lower investment and operating costs due to shorter residence times, and an improved potential to fit into the crowed pulp and paper mill site.  相似文献   

7.
The total effluent load of a paper mill can be significantly decreased by recycling of purified clear filtrate (CF) back to paper-making process. The CF treated with membranes can be reused, for instance, as wire section shower water and in the dilution of chemicals. The main requirements for a membrane in CF treatment are high filtration capacity, high retention of turbidity and low fouling tendency. Previous studies have shown that the regenerated cellulose (RC) ultrafiltration (UF) membrane C30F (current trade name UC030T) is especially suitable for the treatment of paper mill process waters. Every paper-making process is, however, different. Thus, filtration experiments are required in order to find the most optimal membrane for the treatment of a certain process water. In this study the best membrane for the treatment of acidic clear filtrate (ACF) was searched. The performance of the C30F membrane was compared with five UF and three microfiltration (MF) membranes. The results revealed that in addition to the C30F membrane, also some other membranes produced high filtration capacity with ACF (approximately 200 L/(m2h bar)). All the tested membranes also retained over 90% of turbidity. The extremely hydrophilic C30F membrane had, however, lower fouling tendency compared to the other tested membranes. Therefore, it was concluded that the C30F membranes were the best possible membrane for the ACF treatment.  相似文献   

8.
This article theoretically investigates the drying of uncoated paper with gas-fired infrared (IR) emitters installed within the drying section of a given commercial paper machine. Specifically, it presents the ideal location of two opposing IR emitters within the drying section. The ideal location of the two opposing IR emitters corresponds to the highest machine speed with specified average moisture content of paper sheet at the end of the drying section. This article also presents the average evaporation rate, temperature, and moisture content of the paper sheet as it travels through the drying section with the two opposing IR emitters operating at the optimum location. In addition, the temperature and moisture profiles in the sheet thickness direction, before and after the IR emitters, are provided. This article is not concerned with the potential effects of drying with IR emitters on paper sheet properties and machine runnability issues.  相似文献   

9.
Vacuum‐fluidized bed drying experiments were carried out with porous as well as compact particles, employing distinct operating pressures in two periods of drying, such that the combination of the effects of decrease in the transport capacity of the exterior medium and the enhancement of the internal diffusivity results in favour of the drying process. In a variety of operating conditions, it was observed that in the case of porous particles, increasing the operating pressure during the decreasing drying rate period, after applying a vacuum pressure in the constant drying rate period results in a lower final particle humidity.  相似文献   

10.
Energy consumption optimization belongs to the main agendas in sustainable industries. Here, cost-saving opportunities in the pulp and paper industry as one of the largest industrial energy consumers worldwide are in the focus. Energy-saving measures are proposed employing a large-scale mathematical model of a paper machine based on mass and energy balances supported by real-time experiments in paper machine operation. The presented measures are based on the optimization of paper stock pumping and a drying section operation. The total energy-saving effect of the four proposed measures accounted for a reduction in specific heat and electricity consumption by more than 2 %. The proposed procedures require no or low investment costs with the overall simple rate of return of less than one year.  相似文献   

11.
间歇循环延时曝气活性污泥法处理城市污水的试验   总被引:1,自引:1,他引:1  
对间歇循环延时曝气(ICEAS)工艺处理模拟城市污水进行了研究,结果表明,在运行周期为4.5h,污泥负荷在0.07~0.09kg[BOD5]/(kg[MLSS]·d)的条件下,出水COD的质量浓度为15.2~42.2mg/L,COD去除率达80%~88%;BOD5的质量浓度为5.12~16.9mg/L,去除率可达85%~93%;氨氮的质量浓度为4.5~7.5mg/L,去除率为60%~70%,磷的去除率为60%~75%。说明ICEAS工艺处理南方地区城市污水是可行的。  相似文献   

12.
以城市污水处理厂污泥为原材料,采用热处理-机械活化工艺研制了一种火山灰质掺合料,确定了适用于该掺合料制备的方法及工艺条件,并利用XRD方法分析其对水泥基材料的增强机理.实验结果表明,城市污水污泥在800℃下烧结2.5h,快速冷却后粉磨60 min,制得了火山灰活性高的污泥焚烧灰,其可提高水泥砂浆的强度,最优掺量达20%.  相似文献   

13.
E. Rodier  A. Rizzo 《Drying Technology》2003,21(8):1527-1548
Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.  相似文献   

14.
《Drying Technology》2013,31(8):1527-1548
Abstract

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.  相似文献   

15.
Effective sludge management is increasingly critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks for disposal options such as sludge landspreading and composting. A novel continuous biodrying process has been developed to dry mixed sludge so that it can be combusted efficiently in a biomass boiler for energy recovery. Modeling this process is important in order to better understand the transport phenomena in the biodrying reactor and for design and scale-up of the process. A one-dimensional (1D) distributed model for heat transfer coupled with mass and biological transfer phenomena is introduced in this article that shows that the temperature of the sludge matrix is a critical parameter. The model assumes lumped parameters in the gas flow direction and distributed parameters in the (vertical) solids flow direction. Bioheat as a source term and evaporative heat as a sink term are critical issues. In order to evaluate the parameters and assess the model accuracy, a series of experiments was performed. The matrix temperatures predicted by the model were found to be in reasonable agreement with the experimental results, showing that the main transport phenomena were reflected in the model. Larger discrepancies between the water removal rates predicted by the model and the experimental values were indentified at higher aerobic exothermicity, which can be attributed to the complex mechanisms governing the growth cycle of mesophilic and thermophilic bacteria. A dimensionless analysis was performed to identify key dimensionless groups as well as the most dominant transport phenomena in the biodrying process. The results confirmed that convection processes dominated heat transfer at the top of the reactor, and the exothermic aerobic bioenergy dominated at its bottom.  相似文献   

16.
ABSTRACT

In this study, electrocoagulation (EC) was used to determine the optimum conditions on the basis of maximum chemical oxygen demand (COD) and color removal. At the optimum conditions chlorophenols (CPs), biological oxygen demand and total organic carbon (TOC) were determined. The biodegradability of wastewater was increased significantly with 63% COD, 98% color, 61% TOC and overall 65.51% reductions in CPs. Further, the electro-coagulated sludge was characterized by using different analytical techniques to assist the physicochemical and elemental phases, to find-out better management option, reusability for plant growth and safe disposal. Additionally, aluminum content (70.62%) was successfully recovered from sludge.  相似文献   

17.
This article deals with the drying of the waste by-product of an activated sludge manure treatment plant. The studies concern the low temperature drying of planar plates 3 cm thick. The upper face of the sludge is subjected to a tangential air flow controlled for speed, temperature, and humidity. The lower face can be subjected to contact heating. The maximum temperature for air and the bottom of the product is 60 °C. After characterizing the main thermophysical and hygric properties of the product, the laboratory set-up specifically designed for this study is described. Experiments are performed to determine temperature, mass loss, and deformation evolutions for different boundary conditions. These experimental data are used to build a numerical one-dimensional heat and mass transfer model to predict temperature and moisture content fields. Sludge is assimilated with a hygroscopic porous medium, and shrinkage is not taken into account. To validate this model, several comparisons between simulated and measured data are made for different drying methods (hot or fresh air convective drying, contact drying, and combined drying) and various drying conditions.  相似文献   

18.
PVC干燥动力学研究   总被引:1,自引:0,他引:1  
实验研究了悬浮法PVC的干燥特性。在测定物料流化参数的基础上,采用单层筒形流休床测定干燥动力学曲线,并回归得到描述PVC干燥特性曲线的解析函数;确定了实验条件下PVC干燥的临界湿含量xc及传质系数K。由于实验方法比较接近工业干燥过程。获得的结果对于工程应用较为可靠。  相似文献   

19.
Drying behavior of green apples in a laboratory dryer was examined. Prior to drying, the apples were cut in 8 mm thick slices, which were then treated with citric acid solution and blanched hot water at 80°C. Next, they were dried at 65°C with an air velocity of 2.0 m/s. The shortest drying time (270 min) was obtained with apples pretreated with citric acid solution. The drying data were fitted with 11 mathematical models available in the literature. Selection of the best model was investigated by comparing the determination of coefficient (R 2), reduced chi-square (χ2), root means square error ( RMSE ), and mean relative percentage error (P) between the experimental and predicted values. The results showed that the Wang and Singh, logarithmic, and Verma et al. models gave the best results in describing thin-layer drying of apple slices. The effective moisture diffusivity of pretreated samples with citric acid solution was higher than the other samples.  相似文献   

20.
A. Michaud  J. Andrieu 《Drying Technology》2013,31(7-8):1163-1173
Modified literature models were successfully applied to simulate the evolution of the average solvent content (ethanol) of potassium chloride bed during vacuum contact drying with intermittent stirring. Our new modeling approach incorporated the following modifications: the introduction of a jacketed vessel heat transfer coefficient and an accumulation term for the heating wall temperature; the application of alternate static bed and stirred bed conditions; and modeling of all the three drying phases, namely the constant rate phase, the transition phase, and the falling rate phase. Moreover, several validation experiments were carried out with different operating conditions to identify the values of the unknown model parameter. The optimal stirring conditions were investigated by calculating total drying times for different sequences of stirring and no stirring periods.  相似文献   

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