宽区压榨毛毯-纸幅体系微观结构的研究

【目的】了解毛毯-纸幅体系在压榨过程中的结构变化以及压缩速度、不均匀压力作用对于湿纸幅水分脱除动力学的影响。【方法】将毛毯和纸幅视作一个综合作用体系加以研究,首先研发了一套专门实验装置,通过压榨模拟实验研究压缩过程毛毯-纸幅体系微观结构上的变化,进而研究不同压缩速度、不同毛毯表面结构以及不同浆种对压缩过程所需压力的影响。【结果】①宽区压榨毛毯-纸幅体系微观结构为非均匀性结构; 靠近渗透毛毯表面最为致密,且最上层纸幅会嵌入毛毯纸幅空隙中,使得毛毯-纸幅体系整体渗透性降低,所需压榨压力增加。②采用铁丝面和中网面模型压缩过程中,当加压试验速度从51 mm/min变为99 mm/min时,后者压力峰值约为前者的4倍; 加压试验速度从99 mm/min变为124 mm/min时,后者压力峰值约为前者的2.3倍。但当压缩速度从124 mm/min变为99 mm/min(减小20%)时所需作用力减小了50%左右,并不符合达西定律。③在同一加压速度下,采用铁丝网面模拟压榨过程所需的压力是采用3种毛毯表面模型模拟压榨过程所需的压力的5倍左右。而采用3种毛毯表面模型模拟压榨过程所需的压力值几乎相同。④采用中网面毛毯表面模型时,当没有加入中间刚性层,试验速度从51 mm/min变为99 mm/min时,压缩过程所需的压力并没有明显的变化。当试验速度从99 mm/min变为124 mm/min(即增加25%)时,所需压力增加150%左右。在试验速度为51 mm/min时,压缩过程所需压力基本一致; 当试验速度为99 mm/min时,采用中间刚性层实验组所需的压力峰值比没有采用的组大; 当试验速度变成124 mm/min时,现象却相反。【结论】在压榨过程中纸幅模型在厚度方向上呈现不均匀性。纸幅模型在压缩过程中,压缩速度相同时压榨毛毯表面模型选用的不同,所需的载荷也不相同,当采用铁丝网面(即理想化平整的压榨表面)时,所需的压力最大; 而压缩速度越大,所需的压力值也就越大,且压力的大小与速度的变化关系并不符合达西定律。在纸幅模型层之间加入中间刚性层之后,相同的加压试验速度下,采用理想化细密平坦的压榨表面所需压力的峰值会减小。

Research on microstructure of felt-sheet system (FSS) in press section with the extended pressing nip

Objective] The micro structure of felt-sheet systems (MSFSS) and its characteristics influence dewatering efficiency during pressing and is the basis upon which to develop new pressing technologies and new press felts for modem high-speed paper machines.Methods] The press felt and wet sheet were considered as one system,the felt-sheet system (FSS),in this study.First,a set of special experimental devices for the study was developed.Then,the change of the MSFSS in the press process was studied through a model experiment for pressing (MEP) using the device.Finally,the effects of different compressing velocities,surface structures of the press felt,and pulp types on the compressing force needed were determined.Results] The traditional hypothesis that the MSFSS is uniform,was incorrect.In fact,MSFSS was uneven and it appeared that,along the Z direction of the wet web and the pressure profile under the press roller's force,the contacting area between web and felt can be divided into two parts-the ideal key pressing layer and the contact layer,in which there is a non-compressed layer with high permeability.Conclusion] The FSS in the direction of thickness appears uneven with stratification,and during the press process as the wet web is compressed unevenly.The non-compressed layer of FSS had advantages for the dewatering and compression process.The surface near the permeable felt was the most compact;the upper web layer would be implanted into the interspace of the FSS,which would reduce the permeability of FSS and lead to an increase in the necessary press pressure.The relationship between the pressure and compressing velocity did not conform to Darcy's law.When the velocity exceeds a critical value,stratification will play an important part in press compression.It was also discovered that when the paper model was compressed using the wire press surface (an ideal flat permeable press surface),a peak force approximately six times higher than that for different felt surface models was required.