Effect of Humidity on NPK Fertilizer Drying

ABSTRACT

The drying process within rotary coolers during the manufacture of granular NPK fertilizer plays an important role in the production of fertilizer granules possessing both a low moisture content and low caking propensity. A theory for fertilizer drying has been developed which takes into account the low critical relative humidity of complex fertilizer found at high temperature. The theory proposes that the gradient between the partial vapour pressure of moisture in the air and the vapour pressure moisture adjacent to the surface of the fertilizer granule, is the rate controlling stage in the drying of hot granular fertilizer rather than the internal diffusion of moisture within the particle.     

GRANULAR FERTILIZER DRYING AND AGGLOMERATION IN STORAGE

ABSTRACT

In the manufacture of granular NPK fertilizer the product is cooled before packaging and storage in moisture-proof bags. It has been shown that the temperature of the fertilizer prior to packing is significant in that at high temperatures, drying of the granules takes place in the bag which causes an increase in the humidity of the air surrounding the granules and thus an increase in moisture content at the granule - granule interface. This surface moisture was shown to increase the likelihood of agglomeration in the fertilizer by a capillary adhesion / unconfined yield stress model. An iterative model was set up to establish conditions that would prevent drying occurring, which takes into account fertilizer drying rate, fertilizer cooling rate cooling rate and the effect of coaling oils on the drying mechanism.     

粒状肥料专用回转式多筒烘干机

分析粒状钙镁磷肥采用传统单筒烘干机不能达到出口产品标准的原因,介绍将传统回转式单筒烘干机改造成回转式多筒烘干机的设备结构和使用效果。改造后的多筒烘干机可使颗粒肥产品粒度均匀,强度高,含水率低,成品率提高20~30百分点,特别适用于粒度1~5 mm产品的烘干。     

Drying of porous granular materials

Drying characteristics have been measured for two types of granular compound fertilizer and for granular mono calcium phosphate. Air was forced through a fixed bed, and the humidity of the air leaving the bed was analysed by means of an infrared photometer.All results confirm the assumption of vapour diffusion as the rate determining mechanism. Experiments with varying temperature and air humidity show thA shrinking core model was successfully used for predicting the drying behaviour of two of the materials. For the third material the drying rate fell r     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

ABSTRACT

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

粒状复合肥薄层干燥过程的试验研究

通过实验 ,对复合肥的干燥过程进行研究 ,分析干燥风温和复合肥颗粒大小对干燥速率的影响 ,并对以上 2因素分别进行拟和 ,得出多因素的薄层干燥的半经验公式。     

CALCULATION OF VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS

Calculation methods for different types of drying operations (vacuum-coductive drying, drying by pressure drop, self-freezing before sublimation) from multiliquid systems were presented. Kinetic relations of vacuum drying were obtained which allow calculation of drying equipment and choise of optimal operating conditions. The existence of balance invariants removing multicomponent liquid systems - single-valued relations between pressure, moisture content, liquid and exhausted vapour phase composition and product temperature were found. These relations are used to reduce the number of experiments needed to analyze the dryer.     

高塔熔体造粒法复混肥生产工艺中增设烘干工序的探讨

在高塔熔体造粒法复混肥生产工艺装置中增设烘干工序,其目的是肥料通过烘干机时,在热风的作用下,使肥料中的水分得以蒸发,从而提高肥料的颗粒强度。实践证明,肥料在烘干前含水质量分数为5%～6%,烘干后含水质量分数<1%。增设烘干工序后,使高塔熔体造粒法复混肥生产工艺生产低氮高磷配方复混肥成为可能。     

VACCUM DRYING CHARACTERISTICS OF COCONUT

The drying characteristics of shredded coconut under vacuum and at atmospheric pressure have been studied. It was found that a decrease in the absolute pressure within the drying compartment retarded the drying rate of coconut. A model for drying of coconut is proposed on the assumption that the surface of coconut is partially covered with layers of moisture and oil. It was found that oil seepage to surface was responsible for lower drying rates at low absolute vacuum pressures within the dryer.     

转筒干燥器工艺计算顺序及参数优选

针对磷肥和磷复肥以及其他物料干燥工序常用的转筒干燥器暴露的设计问题,阐述了工艺计算之前应掌握的原始数据.介绍了工艺计算顺序及参数优选,认为许多物料的干燥都应考虑其预热段的容积;转筒干燥器的水分蒸发强度须在流体力学相似和热工条件相似的工况下选用;由于进料口处有漏风,因此热风炉至转筒干燥器的进气口处应设置低压热风机,才能达到预期的水分蒸发强度,这对燃煤热风炉尤为重要.     

Heat and mass transfer in granular potash fertilizer with a surface dissolution reaction

Potash is a widely used granular fertilizer and when exposed to high humidities it readily adsorbs water vapour forming a liquid electrolyte solution on each particle. Heat and mass transfer due to air flow through granular potash beds is studied experimentally and numerically. A one dimensional experimental setup is used to measure the temperature and air humidity response and mass gain of a potash bed subjected to a change in air flow. A porous media mathematical model is developed to predict the transient temperature and moisture content distributions. The processes are modelled as nonequilibrium heat and mass transfers between the porous solid and air flow gaseous phases. The state of the surface electrolyte solution is modelled by the thermodynamics of electrolyte solutions. Experimental and numerical results show non‐equilibrium internal moisture and heat transfer processes exist with significant differences in the pore air and particle temperature and surface relative humidity.     

CALCULATION OF VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS

ABSTRACT

Calculation methods for different types of drying operations (vacuum-coductive drying, drying by pressure drop, self-freezing before sublimation) from multiliquid systems were presented. Kinetic relations of vacuum drying were obtained which allow calculation of drying equipment and choise of optimal operating conditions. The existence of balance invariants removing multicomponent liquid systems - single-valued relations between pressure, moisture content, liquid and exhausted vapour phase composition and product temperature were found. These relations are used to reduce the number of experiments needed to analyze the dryer.     

Drying of Porcine Dermal Tissue Via Pressure Swing Adsorption

Due to its close concordance with human tissue, porcine dermal tissues are often used as an acellular implant for skin regeneration in the field of tissue engineering. Generally, moisture within the dermal tissue has to be removed to preserve them until time for clinical applications. To investigate the drying kinetics of porcine dermal tissue under different drying conditions, a novel pressure swing adsorption dehydration (PSAD) system has been proposed for preliminary moisture removal. In this study, several process parameters were varied, namely, amplitude of pressure swing, depressurizing-pressurizing phase duration, and chamber temperature. In addition, a desiccant bed was incorporated to the experimental rig to maintain drying chamber humidity. Employing this pressure-swing drying technique, up to 50% of moisture from porcine dermal tissue could be easily removed within a relatively short period of time. Experimental results indicated that lower depressurized levels and higher frequency of pressure drops and chamber temperature improved dehydration rate. Color change for the dermal tissue was observed to be generally low, indicating low oxidation. Scanning electron microscopic study of the pore structure from pressure-swing adsorption dried dermal tissue showed samples having numerous internal channels. PSAD, therefore, has proven to be a highly effective drying technology to rapidly obtain partially dried dermal tissue while minimizing product quality deterioration.     

Drying of Porcine Dermal Tissue Via Pressure Swing Adsorption

Due to its close concordance with human tissue, porcine dermal tissues are often used as an acellular implant for skin regeneration in the field of tissue engineering. Generally, moisture within the dermal tissue has to be removed to preserve them until time for clinical applications. To investigate the drying kinetics of porcine dermal tissue under different drying conditions, a novel pressure swing adsorption dehydration (PSAD) system has been proposed for preliminary moisture removal. In this study, several process parameters were varied, namely, amplitude of pressure swing, depressurizing-pressurizing phase duration, and chamber temperature. In addition, a desiccant bed was incorporated to the experimental rig to maintain drying chamber humidity. Employing this pressure-swing drying technique, up to 50% of moisture from porcine dermal tissue could be easily removed within a relatively short period of time. Experimental results indicated that lower depressurized levels and higher frequency of pressure drops and chamber temperature improved dehydration rate. Color change for the dermal tissue was observed to be generally low, indicating low oxidation. Scanning electron microscopic study of the pore structure from pressure-swing adsorption dried dermal tissue showed samples having numerous internal channels. PSAD, therefore, has proven to be a highly effective drying technology to rapidly obtain partially dried dermal tissue while minimizing product quality deterioration.     

熔融尿液联产复合肥的生产工艺

介绍熔融尿液联产复合肥的生产技术:利用氯化钾溶于尿液中可形成低熔化合物,采用喷浆圆盘造粒、无干燥工艺流程,生产各种规格高浓度尿基复合肥,该工艺生产尿基复合肥不用干燥产品水分可达标,大大降低了投资和生产成本,技术与经济优势明显。     

Vacuum Drying of Oak Wood

ABSTRACT

Vacuum drying, j,e drying under absolute gas pressure of about 10? Pa. is an efficient means of reducing the process period and of producing good quality wood. We will examine here continuous vacuum drying where the plank surfaces are kept at a constant temperature, which remains above the boiling point, and moisture flowing to the surface is extracted from the kiln.

We have carried out an experimental study of oak drying under such conditions. The drying rate and moisture content profile of the sample (40 mm thick) are recorded during the whole drying period.

A model of continuous drying is established from general conservation equations with the main approximation that the air is rapidly extracted. The two constitutive equations of the model which describe temperature and water content fields are of a diffusive type and coupled through coefficients. The adequate boundary equation is not a convective one, but expresses a hygroscopic equilibrium between the vapour in the chamber and the wood surface. The mass diffusive coefficient can be adjusted to the drying rates through capillary pressure and bound water diffusion functions. The wood heterogeneity (seasonal growth) is the main factor of discrepancy in these functions. The simulated drying rates correspond with the experimental ones.     

Vacuum Drying of Oak Wood

Vacuum drying, j,e drying under absolute gas pressure of about 10⊃ Pa. is an efficient means of reducing the process period and of producing good quality wood. We will examine here continuous vacuum drying where the plank surfaces are kept at a constant temperature, which remains above the boiling point, and moisture flowing to the surface is extracted from the kiln.

We have carried out an experimental study of oak drying under such conditions. The drying rate and moisture content profile of the sample (40 mm thick) are recorded during the whole drying period.

A model of continuous drying is established from general conservation equations with the main approximation that the air is rapidly extracted. The two constitutive equations of the model which describe temperature and water content fields are of a diffusive type and coupled through coefficients. The adequate boundary equation is not a convective one, but expresses a hygroscopic equilibrium between the vapour in the chamber and the wood surface. The mass diffusive coefficient can be adjusted to the drying rates through capillary pressure and bound water diffusion functions. The wood heterogeneity (seasonal growth) is the main factor of discrepancy in these functions. The simulated drying rates correspond with the experimental ones.     

EXPERIMENTAL MEASUREMENT OF TEMPERATURE AND MOISTURE PROFILES DURING APPLE DRYING

An experimental system has been designed and constructed to investigate the drying rate, temperature and moisture profiles developed within food materials during forced convective air drying. The moisture profile was determined by employing the technique of gamma ray densitometry, and embedded thermocouples were used to sense temperature at different locations. The overall drying rate was measured using an electronic balance. A microcomputer and a computer front end were implemented for data acquisition and experimental control. Sample temperature and moisture profiles measured during drying are presented as a function of both drying time and the distance to the surface of the specimen. Both profiles described the drying phenomena inside the sample. The evaporation front was observed in a narrow region of the apple specimen, this front moved into the sample as drying progressed. The amount of apple shrinkage was determined to be linearly related to the distance which the evaporation front had recededfrom the initial surface.