Pore Destruction Resulting from Mechanical Thermal Expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.     

Dewatering of Biomaterials by Mechanical Thermal Expression

Dewatering by mechanical thermal expression (MTE) for a range of materials is explored using a laboratory-scale MTE compression-permeability cell. It is shown that MTE can be used to effectively dewater a range of biomaterials including lignite, biosolids, and bagasse. The underlying dewatering mechanisms relevant to MTE, namely (1) filtration of water expelled due to thermal dewatering, (2) consolidation, and (3) flash evaporation, are discussed. At lower temperatures, the dominating dewatering mechanism is consolidation, but with increasing temperature, thermal dewatering becomes more important. A major focus is an investigation of the effects of processing parameters, including temperature (20 to 200°C) and pressure (1.5 to 24 MPa), on material permeability, a fundamental dewatering parameter. It is illustrated that permeability is particularly dependent on the processing temperature, owing to changes in both the material structure and the water properties. In addition, a comparison of permeability in the direction of applied force (axial) and perpendicular to the direction of applied force (radial) is presented. It is shown that, due to alignment of particles under the applied force, the permeability and, hence, rate of water removal in the radial direction is greater than in the axial direction. SEM micrographs are presented to illustrate the particle alignment.     

Dewatering of Biomaterials by Mechanical Thermal Expression

Dewatering by mechanical thermal expression (MTE) for a range of materials is explored using a laboratory-scale MTE compression-permeability cell. It is shown that MTE can be used to effectively dewater a range of biomaterials including lignite, biosolids, and bagasse. The underlying dewatering mechanisms relevant to MTE, namely (1) filtration of water expelled due to thermal dewatering, (2) consolidation, and (3) flash evaporation, are discussed. At lower temperatures, the dominating dewatering mechanism is consolidation, but with increasing temperature, thermal dewatering becomes more important. A major focus is an investigation of the effects of processing parameters, including temperature (20 to 200°C) and pressure (1.5 to 24 MPa), on material permeability, a fundamental dewatering parameter. It is illustrated that permeability is particularly dependent on the processing temperature, owing to changes in both the material structure and the water properties. In addition, a comparison of permeability in the direction of applied force (axial) and perpendicular to the direction of applied force (radial) is presented. It is shown that, due to alignment of particles under the applied force, the permeability and, hence, rate of water removal in the radial direction is greater than in the axial direction. SEM micrographs are presented to illustrate the particle alignment.     

Effect of mechanical thermal expression drying technology on lignite structure

Mechanical thermal expression (MTE) is a developing nonevaporative lignite dewatering technology. It has been proved to be effective to dewater high moisture content in low-rank coals via the application of mechanical force and thermal energy at elevated temperatures. In this paper, the dewatering behavior of the Xiaolongtang lignite in Yunnan province, China during the MTE process was studied with three process parameters: time, temperature, and pressure. Meanwhile, the mechanism was also explored of how variations in temperature and pressure during the MTE process affect the surface oxygen functional groups and pore structure, which was mainly conducted by means of Fourier transform-infrared spectrometer (FTIR) and mercury intrusion porosimetry (MIP). Increases in MTE temperature and pressure resulted in significant reductions in residual moisture content and moisture-holding capacity, along with the increase in fixed carbon content and content reductions of other elements, especially oxygen content, this could be largely attributed to the destruction of the surface oxygen functional groups and porosity in the lignite. Technologically, the optimal conditions for temperature and pressure are 150–220°C and 6–10?MPa. The residual moisture content of the lignite treated by MTE at 200°C, 10?MPa is lower than 8%; the dewatering rate reaches over 76% with the calorific value being approximately 22?MJ/kg. Carboxyl and hydroxyl groups break down at drying temperatures above 120°C and constant applied pressure 10?MPa; with the pore volume significantly reduced, only few pores (diameter?相似文献   

Comparison of some physico–chemical properties of Victorian lignite dewatered under non-evaporative conditions

A Victorian lignite, designated Loy Yang low ash, run of mine (LYLA (R)) has been dewatered using mechanical thermal expression (MTE) at 150–200 °C and 6–25 MPa and by hydrothermal dewatering (HTD) at 200–300 °C and the products compared. Total acidity values for all samples as measured by a pyrolysis thermogravimetric Fourier transform infrared (TG–FTIR) method were similar to those measured by barium ion exchange. Stronger (carboxylic) acid values determined by pyrolysis TG–FTIR tended to be lower than ion exchange values, except for the 300 °C HTD sample, for which both methods gave similar values although these were much lower than at all of the other treatment temperatures. Equilibrium moisture contents (EMC) for the MTE products and the 200 °C HTD product were similar to those of the original lignite at relative vapour pressure (RVP) ⩽ 52%, but lower at RVP 92%. EMC values for 300 °C HTD products were all lower than for the original lignite, indicating that processing temperature was the most important factor in determining these properties. CO₂ adsorption surface area was also mainly a function of processing temperature, decreasing with increasing temperature. However, the pore volume as determined by mercury porosimetry was influenced by whether dewatering was effected by MTE or HTD, the mechanical pressure applied in the MTE process resulting in a lower porosity.     

Physico-chemical properties of Loy Yang lignite dewatered by mechanical thermal expression

Mechanical thermal expression (MTE) is a pressure dewatering process, which is carried out at elevated temperatures. The process is being investigated as a means of lowering greenhouse gas emissions from existing lignite fired power stations by reducing the moisture content of lignites. The investigation was carried out to identify how variations in temperature and pressure during the MTE process affect the physico-chemical properties of MTE treated Loy Yang lignites. Increases in MTE temperature (<250 °C) and pressure (<12.7 MPa) resulted in significant reductions in residual moisture content, moisture holding capacity and sodium levels, which have been largely attributed to the destruction of porosity in the lignite.     

Modelling the mechanical thermal expression behaviour of lignite

A process known as mechanical thermal expression (MTE) is used to dewater lignite to improve its heating value. A numerical model is developed to investigate the compression dewatering stage of the MTE process at ambient (20 °C) and elevated temperature (150 °C). This model includes details not previously investigated such as the wall friction, the compressive yield stress of the lignite network as a function of void ratio, and the relationship between the yield stress and the absolute permeability. This allows gradients of these properties to develop as dewatering progresses. The results from the model are compared with the void ratio, hydraulic pressure drop and permeability, which are independently determined experimentally. A major simplification in this work is that only primary consolidation is incorporated in the model. The results clearly show that secondary consolidation does occur in lignite, but it is not significant except at low applied pressures, or for compression times which are much longer than would be of industrial interest.     

Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals

The mechanical thermal dewatering (MTE) process has been shown to effectively dewater high moisture content low rank coals via the application of mechanical force at elevated temperatures.Using mercury intrusion porosimetry (MIP) as an investigative tool, this study examines how MTE processing conditions, such as temperature and pressure, affect the compressibility, pore size distribution, apparent (skeletal) density and shrinkage behaviour of three low rank coals sourced from Australia, Greece and Germany. As both pore filling and sample compression occurred at high mercury intrusion pressures, all MIP data were corrected for compression effects by using compressibility values derived from mercury extrusion data.The MTE process is shown to produce a low porosity coal, which, depending upon the processing conditions used, undergoes further shrinkage upon oven drying at 105 °C. An increase in MTE temperature (above about 85 °C) led to an increase in mesopore volume, which is caused by a hardening of the coal structure, leading to pore volume retention and a consequent reduction in percent shrinkage on oven drying. The increase in measured mesopore volume is also associated with an increase in measured surface area.The reverse trend is seen with increasing MTE pressure, where both the macro and mesopore volume decrease with pressure, causing the percent shrinkage to increase accordingly. This effect may be due to an increase in capillary forces caused by a decrease in the average pore diameter. The percent shrinkage increased up to a pore volume of about 0.1 cm³/g, beyond which no further reduction in pore volume was achieved. The decrease in mesopore volume is also associated with a decrease in measured surface area.Compressibility values derived from mercury extrusion data show that the MTE process has little impact on the network strength of the skeletal network structure of all three coals investigated. Likewise, the skeletal density remained relatively unchanged.The reduction in water content, pore volume and the changes in shrinkage behaviour under increasingly severe MTE conditions are suggestive of the physical changes that accompany increased coalification (rank) within the lignitic range.     

Assessment of the water quality produced from mechanical thermal expression processing of three Latrobe Valley lignites

Mechanical Thermal Expression (MTE) is a developing non-evaporative brown coal (lignite) dewatering technology. This study assesses the physical and chemical properties of MTE product water expressed from three Latrobe Valley lignites. MTE product water was found to be acidic, contain concentrations of dissolved organic and inorganic components of up to 5.3 g/L and was found to contain suspended colloidal coal particles of up to 1.7 g/L. The concentration of contaminants in the product water was found to be related to different coal types. The selection of coals for future MTE processing will have significant implications for the quality of water produced. Many investigated water parameters are in breach of selected water use guidelines and remediation of the water from this process is required prior to reuse or disposal.     

Enhanced liquid spreading due to porosity

A conceptual pore level model (Chem. Eng. Sci. 57 (2002) 3401) of spreading of liquid over internally wet porous particles is applied to explain reported rector level enhancement in wetting of trickle bed reactor (Ind. Eng. Chem. Res. 36 (1997) 5133). It is confirmed that a symbiotic relationship exists between internal and external wetting of porous particles whereby each enhances the other. Further, it is illustrated that liquid spreading in porous solids is driven more by porosity than by contact angle. A major implication of this phenomena is that liquid will spread more on less wettable (but porous) surfaces in comparison to more wettable (but nonporus) surfaces and a reinterpretation of experiments involving spreading of liquid over porous solids is required. On a large scenario, it is hoped that present exercise will be in important step towards explaining the complex reactor level macro phenomena by simple and conceptual pore level micro models.     

孔隙随机结构对非均质多孔泡沫导热性能的影响

多孔泡沫是一类低密度、高比表面积、具有独特性能的新型功能材料。实际多孔泡沫材料通常是非均质的,即其孔隙结构分布是随机的。为研究非均质多孔泡沫材料的导热性能,提出用孔隙均匀度作为表征孔隙结构分布随机性的参数,以多孔石墨泡沫为例,分析孔隙均匀度对多孔泡沫有效热导率的影响。数值计算结果表明：孔隙结构分布越不均匀,多孔泡沫材料的导热性能越差。根据计算结果提出了非均质石墨泡沫相对有效热导率的预测式,并与现有文献报道的结果进行了比较,发现当前结果呈现了孔隙结构随机性对材料有效热导率的影响,与ORNL实验结果更吻合。     

Experimental study on the effects of drying methods on the stabilities of lignite

The drying processes are always applied prior to the transportation or utilization of lignite, and result in notable changes in the stabilities of lignite. In this paper, the study on the effects of nitrogen and MTE drying process on the physico-chemical properties and stabilities of Zhaotong lignite was carried out. The briquettes produced by MTE drying in this study were 150 mm in dimension, and so had a much larger particle size than nitrogendried samples. Nitrogen adsorption, mercury intrusion porosimetry and scanning electron microscopy all suggested that drying was accompanied by the transformation of larger pores into smaller ones. Compared to nitrogen drying, the pore structures could be stabilized by the MTE process. The soluble salts were removed during MTE drying which resulted in the decrease in ash and the concentrations of some of the major metals.The removal of water enhanced the hydrophilicity of nitrogen dried samples, but did not affect the hydrophilicity of MTE dried samples. The moisture holding capacity of MTE dried samples reduced faster than nitrogen dried samples with the decrease of residual moisture content. Themoisture readsorption processes of MTE dried sampleswere strongly inhibited due to themuch larger particle size of sample produced byMTE drying than nitrogen drying. The susceptibility to spontaneous combustion, indicated by cross point temperature and self-heating tests, of nitrogen and MTE dried samples increased with the decrease of residual moisture content. The MTE dried samples are more liable to spontaneous combustion than nitrogen dried samples with the same residual moisture and particle size. However, the larger particle size of the MTE product made it more stable with respect to spontaneous combustion and also moisture readsorption.     

Estimation and determination of the permeability of porous ceramics

The possibility of estimating the permeability of porous ceramic materials both with mercury and with gas liquid pore diagrams and based on model concepts concerning the porosity and average size of the constituent particles in the material was demonstrated. It was shown that the permeability of a porous material and the average pore radius can be directly determined with the linear segment of the reverse GLP curve and the capillary sinuosity can be calculated with it.     

Fabrication and characterisation of cellular alumina articles produced via radiation curable dispersions

A general and versatile method for the production of cellular materials from radiation curable solvent-free colloidal ceramic dispersions containing pore formers has been developed. By this technique cellular ceramic articles with a precisely controlled porosity, cell size and shape are obtained for compositions containing solid pore formers. Monolithic bulk samples are obtained by thermal curing, whereas thin films and multi-layered articles are advantageously produced by UV curing. In this work the influence of three different spherical pore former types, PE, PS and PMMA, on the processing and final properties of the porous materials using alumina as model material is studied. The effect of pore former type and concentration on rheology, curing behaviour, debinding and sintering steps as well as thermal conductivity and mechanical strength of the sintered cellular materials is presented. It is also shown that the choice of pore former type modifies the sintering behaviour and resulting properties.     

Processing of porous ceramics: Piezoelectric materials

The paper reviews processing techniques used to produce porous ceramics with tri-dimensionally interconnected porosity, in a wide range of pore volumes and pore size and distribution. Attention is focused on the development of porous electroceramics and especially piezoelectric PZT materials. The porosity can be introduced through dry or wet techniques. In the dry techniques, a fugitive phase is added to the perovskitic powder by mechanical mixing. Wet techniques involve the manipulation of suspensions and a better control of the final morphology and microstructure of the samples can be achieved by the colloidal approach. The whole spectrum of techniques for generation of porosity is surveyed; it includes burnout of volatile particles or thermally unstable sponge structures, generation of porosity by foaming, slip casting, tape casting, direct consolidation, solid freeform fabrication, die pressing. Porosities of up to 70% are obtained in aerogels by sol–gel processing. The pore size distribution and microstructural differences resulting from various processing parameters influence the physical properties, particularly the acoustic/piezoelectric response of PZT materials.     

Porosity and particle size effects on the gas flow characteristics of porous metals

Several models have been proposed in the past which relate the gas flow characteristics of a porous material to the pore microstructure. Such approaches require metallographic measurements to predict the permeability or inertial flow coefficients. The present study approaches the problem through the dominant adjustable extrinsic processing variables. Specifically, porosity and mean particle size are used to predict the gas flow coefficients for porous 316L stainless steel. Although such an approach proves successful, it is established that traditional processing variables (compaction pressure, sintering temperature and sintering time) have a significant influence on pore shape and therefore on gas flow. Such factors must be included in any comprehensive future predictive models of gas flow through porous metals.     

Liquid Permeability of Packed Particles: Why Perpetuate the Carmen—Kozeny Model?

Liquid transport through the interstices of packed particles is commonly described using the Carmen–Kozeny mean hydraulic radius model, which calibrates the effective pore dimension from mean macroscopic parameters. However, the experimental aqueous permeability of sets of porous powder compacts varying widely in porosity and pore structure was shown to be much better described in terms of the linear mean pore size determined from mercury penetration porosimetry. Here it is shown that the latter model is supported by studies of the permeability of porous rock and percolation theory.     

Hierarchically porous alumina ceramic catalyst carrier prepared by powder bed fusion

Hierarchical porous ceramic catalyst carriers, which exhibit good catalytic performance, are widely used in the petrochemical industry. However, the fabrication of ceramic carriers with hierarchically porous structures is highly challenging for conventional preparation processes. Thus, a strategy for designing and manufacturing hierarchically porous alumina ceramic catalyst carriers using aluminium trihydrate as raw material and powder bed fusion (PBF) as the forming process is proposed herein. PBF process parameters were optimised to define the processing window for creating ceramics with complex structures. Controllable pore characteristics in nano- and microscales has been achieved by combining dehydroxylation, PBF, and post-sintering processes. The effects of raw material composition and process parameters on crush strength, porosity, and specific surface area were systematically investigated. The resulting porous ceramics exhibit a crush strength of 86.03 ± 18.10 N/cm, specific surface area of 1.958 ± 0.123 m²/g, and porosity of 64.85 ± 1.15% with a multipeak distribution at 95 ± 1.23 nm and 17.76 ± 0.14 μm. The possibility of complicated monolithic catalyst carrier structures with bionic leaf vein characters has been validated for potential industrial applications.     

Kinetics and mechanism during mechanical/thermal dewatering of lignite

In order to increase the efficiency of lignite fired power stations the mechanical/thermal dewatering (German abbreviation: MTE, Mechanisch/Thermische Entwässerung, also used for ‘mechanical/thermal expression’) was developed at the University of Dortmund as an energy efficient process for the reduction of the water content of lignite prior to combustion [1-3], [Patentschrift DE 44 34 447 A1, 1994; Patent EP 0 784 660 B1; WO 96/10064, 1996; VDI-Berichte 1280 (1996) 165]. While a 25 t/h demonstration plant has been constructed at the Niederaußem power station of RWE in Germany and came into operation in 2001 [4], [Proceedings of the VGB/EPRI Conference, 2001] additional detailed research has been done on the process fundamentals at the University of Dortmund. Experiments for three different lignites from Germany, Greece and Australia are presented in this paper and it is shown, that the dewatering kinetics depending on time, temperature and pressure can be described by a new model derived from soil-mechanical fundamentals and rate-process-theory [5], [Mechanismen und Kinetik der Mechanisch/Thermischen Entwässerung von Braunkohle, 2001]. Due to differences in lignite composition the experimental determination of some model parameters for each coal is necessary. From the activation energy which is determined from experiments concerning dewatering kinetics it can be deduced, that even during secondary consolidation (creep) the drainage of water is the dominating process. The experiments also provide a clear distinction between the effect of the so-called ‘thermal dewatering’ due to heating of the lignite and the subsequent mechanical expression.