Processing Characteristics Research Of The Screw Steam Explosion Machine

Plant fiber pretreatment is the first step of the plant fiber utilization. Steam Explosionpretreatment of plant fiber, which has lots of unique advantages, such as the producing offibrous plant fiber, is gradually introduced in the field of wood plastic composites. Howevermost of the steam explosion equipments are tank and could not make the continuousproduction. Our research group designed screw steam explosion machine and made the plantfiber steam explosion pretreated continuous successfully. In this paper, the structure of thescrew steam explosion machine was descripted briefly and the process of the plant fiberpretreatment with steam explosion was introduced, also the processing characteristics of thescrew steam explosion machine was studied deeply. With a series of experiments, the diestructural parameters including die gap size and die end size, the material parametersincluding material particle size, the initial moisture content and pre-immersion time, and theprocess parameters including feeding volume, spindle screw speed and the steam explosiontimes were studied to show the effect on the pressure and the temperature of the steamexplosion and the quality of the material after the steam explosion. The pressure werereflected by the backpressure and the electric current of the steam explosion machine, thetemperature were reflected by the change of the changes in moisture content before and afterthe steam explosion, and the quality of the material was reflect by the material screeningvalue.During the continuous steam explosion process, the material was transported through therotation of the screw, and were sealed, compacted and heated for the friction among thematerial and between the material and the screw. When the material is transported to the dieexit, the material is sealed and heated to a certain extent, and suddenly the material weretransported to the atmospheric environment, the pressure is reduced quickly, allowing aportion of the water in the fiber bundles to vaporize and do work. The energy released canbreak hydrogen bonds and even covalent bonds in the fiber bundles and facilitates thedegradation of amorphous hemicellulose and lignin. Thus the tissue structure of the originalfiber bundles is destroyed and fiber bundles dissociate from the intercellular layer intoindividual fibrocytes, thereby completing preparation of plant fibers with a large aspect ratio.With the decrease of the die gap size and the increase of the die terminal length, thebackpressure of the steam explosion and the motor current increased, the moisture content ofthe material after steam explosion process decreased, and the screen value increased. Becausethe smaller of the die gap size and the longer of the die terminal length, the bigger of the resistance when transported through the die of the material, the bigger the reaction of thematerial on the screw, so the bigger of the backpressure. At the same time the temperature ofthe material increased because of the increase of the friction on the material. When thematerial came out from the die, the pressure decreased suddenly, and the heated water in thematerial turned into steam, the higher of the temperature, the more of the steam, the less of themoisture content of the material after steam explosion. Also, with the increase of thetemperature and the pressure, the intensity of the steam explosion effect on the materialbecame much stronger, and the dissociation degree of the fibrous fiber cells from the materialincreased, it showed in improved screen value of the steam explosion treated material. But ifthe die gap size and the die end size are too small, the friction would be too large and thetemperature is too high to make the machine work continuously.With the increase of the initial particle size of the material, the resistance of the materialincreased when transported through the die, so the backpressure and the current increased.The friction increased too because of the increase of the resistance, so the temperatureimproved to make much more water in the material into steam when the steam explosionhappened, and the moisture content reduced after the process. But the screen value reduced,and that because the particle size of the material is too big to make the steam come out fromthe inside of the material. If the initial moisture content of the material is too low, the materialtemperature would be too high, and when the material came to the die it would be scorchedand smoke would be produced, at last the quality of the steam explosion treated material waseffected and also the continuous production was stopped. It is useful to make the initialmoisture content more and the soaking time much longer, because that would make thematerial much softer and be good for the tear of the material. But the temperature and thesteam explosion strength would be reduced.With the increase of the feeding volume, the quantity of the material pass through the dieevery minute increased which caused the improvement of the friction, so the backpressure andthe current increased. Also the temperature caused by the increased friction raised too, butthose temperature would be absorbed by all of those material, so the average temperature ofeach unit material reduced and that why much less water turned into steam and the moisturecontent changed small before and after the steam explosion process. With the improve of thespindle screw speed, the back pressure and the current reduced, but the moisture content andthe screen value changed not so much. With the increase of explosion times, the backpressureand the current increased and the moisture content lost every time reduced and the moisturecontent reduced. The fiber became much smaller and the fibrous fiber increased. Orthogonal structure showed that the effect of initial moisture content on thebackpressure, current value and the change of the moisture content was the most obvious, thedie gap size effect ranked on the second, the effect of spindle speed effected the mostunobvious. As for the screen value, the die gap size affected the most and the feeding volumeand spindle screw speed had less effect on it.