Orthogonal test results and analysis of vegetable dehydration and drying

Orthogonal test results and analysis of vegetable dehydration and drying

In this paper, the pulse fluidized drying of high moisture crop vegetable was studied. In the experiment, the peeled scallion was divided into four parts along the axial direction, however, it was cut into sections along the transverse direction, each section was 2-3cm, washed with water, and dried. After the free moisture on the surface of the material was removed, the drying test could be started. The initial moisture content of the scallion was 85% (wet basis), and the porosity of the gas distributor was 25%. The experimental indexes were moisture content and unit heat consumption.

（1） Influence of four factors on drying process

The results show that the wind temperature is the main factor affecting the precipitation amplitude and unit heat consumption. With the increase of wind temperature, the precipitation amplitude increases, and the unit heat consumption also increases; with the increase of wind speed, the precipitation range increases, and the unit heat consumption increases, but the influence of wind speed is not as significant as that of wind temperature; 20 minutes before drying, the material layer thickness (bed static height) has an effect on precipitation amplitude However, with the drying process going on, the moisture content of the material is getting lower and smaller, and the influence of the thickness of the material layer on the drying process can be ignored; the frequency of air flow fluctuation also has an impact on the drying process. The test shows that when the air flow fluctuation frequency is 4Hz, the precipitation amplitude is large and the energy consumption is low.

The results of orthogonal test show that: the order of influencing precipitation amplitude is: wind temperature (a), wind speed (b), air flow fluctuation frequency (d), material layer thickness (c); the primary and secondary order of influencing unit heat consumption is wind temperature (a), wind speed (b), material layer thickness (c) and airflow fluctuation frequency (d); the better level of water content is A3, B3, C2, D2; test index is: A3, B3, C2, D2 The better level of unit heat consumption is A1, B1, C2 and D2

（2） Feasibility of VAV drying process

A large amount of water needs to be removed from vegetables. The weight of dried products after dehydration is only 1 / 10 of the weight of fresh vegetables, and the volume of dried products is only 1 / 10 of that of fresh vegetables. It can be observed from the test that if the wind speed remains constant during the whole drying process, in the later stage of drying, the materials will boil violently and be broken seriously, and even small pieces of materials will be blown out of the machine. Therefore, it is required to divide the drying process into several periods, and use different air volume in different periods, so as to maintain a good pulsating fluidization state in the whole drying process, without causing any damage The above phenomenon occurs due to the reduction of material weight.

Through several groups of experiments on the dehydration and drying of Welsh onion, we think that the following process is more reasonable: if the bed thickness of wet material is 150 mm and the hot air temperature is 75 ℃, the moisture content can be reduced to less than 10% after 50 min of drying. The pulsating air velocity in the first 20 min is 0.98-1 M / s, and the pulsating frequency is 4 Hz; the pulsating air velocity in the middle 20 min is 0.5-0.6 M / s, and the pulsating frequency is 4 Hz; and the pulsation is 10 min after drying The flow velocity is 0.4-0.5, and the pulsating frequency is still 4Hz. The dry product obtained in this way can not only obtain the required moisture content, but also be neat without breakage, and the appearance color is also good.