Zhanyong Li

Uniformity Issue in Microwave Drying

Microwave drying is an advanced drying technique in which heat is generated inside the material due to transformation of high-frequency electromagnetic energy so the liquid moisture is intensively evaporated and transported toward the material surface. However, an inherent problem in microwave drying is nonuniform heat generation, which leads to uneven drying. In this article, the factors causing nonuniform microwave heating were identified, and feasible approaches to solving the nonuniformity problem were analyzed in terms of improving the uniformity of the electromagnetic field distribution and random or organized dislocation of the processed material in virtually nonuniform microwave field. The R&D trends in microwave drying toward reducing uneven heating were pointed out and discussed.

Determination of Moisture Diffusivity by Thermo-Gravimetric Analysis under Non-Isothermal Condition

Abstract: In drying of solids, the diffusion model based on Ficks second law is usually applied to interpret the moisture migration within the solid. Then the temperature dependence of the moisture diffusivity, generally described by an Arrhenius-type equation, is obtained through the drying kinetics. In this article, a nonisothermal (linearly increasing temperature) procedure was used to determine the moisture diffusivity as a function of temperature with the complex optimization method, and the result was accessed by comparison with a classical isothermal procedure. All the experiments were conducted in a thermogravimetric analyzer (TGA) for accurately recording the mass loss from the sample and easily programming the heating profile.

Dewatering and Drying in Mineral Processing Industry: Potential for Innovation

Wet beneficiation of minerals necessarily requires removal of large amounts of water—typically contaminated—before further processing of the concentrated ores can be carried out. Often such concentrates are transported over large instances by ground or sea transport. For economic reasons dewatering/drying are essential unit operations. Often the capacity requirements of the mining operations are very large, the product processed has low unit value, and the material is abrasive. The selection of dryers is often arbitrary. No clear guidelines can be found by noting current practices in different countries. This article provides a global view of the types of dewatering equipment and dryers currently used in the mineral processing industries, provides guidelines for selection of drying systems, and recommends emerging innovative technologies, such as superheated steam and pulse combustion drying for future applications in this industry. A few case studies are examined.

Investigation of Flow Behaviors and Bubble Characteristics of a Pulse Fluidized Bed via CFD Modeling

The bubbling flow in a 2D pulsed fluidized bed is simulated using a developed Eulerian-Eulerian two-fluid modeling approach. Parametric studies are carried on for pulsed inflow of the gas phase with rectangular and sawtooth patterns where three pulsating frequencies of 0.4, 4, and 40 Hz are used. The flow instabilities are observed to develop in the pulsed fluidized bed and the mechanisms leading to the instabilities are discussed. The formation, coalescence, split-up of air bubbles, and their trailing wakes inside the bed are described and discussed in detail. Bubble rising velocity and size are obtained and analyzed. Bed expansion and fluctuation ratio are calculated to evaluate the pulsed fluidization quality. Numerical results improve the understanding of the pulsed fluidization.

Production of a solid fuel using sewage sludge and spent cooking oil by immersion frying.

Sewage sludge and spent cooking oil are two main waste sources of modern Chinese cities. In this paper, the immersion frying method using spent cooking oil as the heating medium was applied to dry and convert wet sewage sludge into a solid fuel. The drying and oil uptake curves were plotted to demonstrate the fry-drying characteristics of the sewage sludge. Parametric studies were carried out to identify the governing parameters in the frying drying operation. It was found that at frying oil temperatures of 140-160°C, the wet sewage sludge could be dried completely in 6-9 min and converted into a solid fuel with a high calorific value of 21.55-24.08 MJ/kg. The fuel structure, chemical components, pyrolysis and combustion characteristics were investigated and the experimental results showed the solid fuel had a porous internal structure and a low ignition temperature of 250°C due to presence of oil. The frying drying mechanism was also discussed.

EFFECT OF A TEMPERING PERIOD ON DRYING OF CARROT IN A VffiRO-FLUlDIZED BED

ABSTRACT Drying of diced carrot in a vibrofluidized bed was studied experimentally for various air temperatures, bed heights and size of the cubes. Effect of a tempering period that mav be implemented into ¦ drying cycle was examined with respect to drying kinetics and energy consumption. Although two stage drying with a tempering period increases the overall drying time, it shortens the drying time in a VFB dryer thus reduces energy consumption

Characteristics of Single Droplet Impact on Cold Plate Surfaces

Droplet collision with cold solid surfaces is a fundamental phenomenon observed in low-temperature processes. In this work, a single droplet impacting on a cold surface was investigated by analyzing changes in droplet shape and dimensions recorded with a high-speed camera. At low surface temperatures (−5°C and −10°C), the droplet spreads in a very short period upon impact and then retracts, but to a diameter larger than that at room temperature, dependent on surface material property and texture. Higher impact velocity expands the spreading diameter and promotes the retraction to an extent, but at lower surface temperature (i.e., −20°C) the droplet is quickly frozen with smaller change in diameter on the stainless-steel surface rather than on the Teflon plate. The change of droplet shape upon impacting on a 30° inclined surface was also studied. The surface texture of the stainless-steel plate produced by the polishing process has an obvious effect on the droplet dimension retraction. A proper selection of wall material and process conditions can alleviate or prevent wall deposition.

Drying of a Root Crop in Vibro-Fluidized Beds

ABSTRACT Drying of sliced corm of Moyu which represents a group of hard-to-dry herbaceous plants was carried out in a batch laboratory vibrated fluid bed to determine drying kinetics and identify operating parameters for required product quality. Results from laboratory studies were verified in a pilot-scale continuous VFB dryer. Field trials performed over the Moyu harvest season confirmed the resulls extrapolated from laboratory data.

Drying of Photosynthetic Bacteria in a Vibrated Fluid Bed of Solid Carriers

ABSTRACT A highly heat-sensitive and xerolabile suspension of photosynthetic bacteria was dried in batch and continuous vibrofluidized beds of porous and nonporous solid carriers. Experimental drying rate and particle temperature curves as well as growth tests for fresh and dried bacteria show that this is a technically feasible process, and product obtained is of required quality.

Control of Sludge-to-Wall Adhesion by Applying a Polarized Electric Field

An application of a polarized electric field to reduce adhesion of a biological sludge to the dryer wall was studied experimentally by contact drying of a sludge cake deposited on a heated metal plate serving as the cathode. It was found that the adhesion intensity is greatly reduced due to a water layer formed at the heated plate, when a fraction of water in the sludge migrates to the cathode by electro-osmosis, as well as by gas bubbles, which are generated by the electrochemical reaction. The voltage gradient from 4 to 5 V/cm was found optimal for drying 10-mm-thick sludge samples. Under these conditions, the electric energy consumption amounted to 3.87 kWh/t. An increase in cathode temperature reduced the sludge-adhering intensity and shortened drying time. The beneficial effect of an electric field on reducing sludge adherence decreases with increasing wall temperature.