Design and experiment on system for concentrating biogas slurry with vacuum evaporation

Abstract: Large quantity of residues is produced during the process of biogas fermentation. The biogas slurry can be used as resource after concentration treatment. To better use the slurry in the process of biogas slurry concentration, the method of a vacuum evaporation was used. A vacuum evaporation system of 20 L for concentrating biogas slurry was designed based on the physical and chemical properties of biogas slurry. The parameter design and equipment selection for every part of the system included an evaporator (the main equipment), condenser, circulating cistern and vacuum pump (auxiliary equipment). Also, we used the biogas slurry from Henan Agricultural University as the experimental material to test the efficiency of the vacuum evaporation device. The results showed that, the organic matter content of biogas slurry had no significant change when the evaporation temperature was less than 80℃ while it reduced dramatically when the evaporation temperature was more than 80℃. The change rate of the organic matter content reached 9.4% when the evaporation temperature changed from 80℃ to 100℃. The contents of nitrogen, phosphorous and potassium in the biogas slurry decreased with the increased of evaporation temperature. Especially, the content of nitrogen had the most dramatic change and it decreased 13%. The contents of the organic matter, nitrogen, phosphorous and potassium increased with the rise of vacuum degree for the system. When the vacuum degree of the system was high, the evaporation temperature was low and the evaporation capacity of biogas slurry increased. Under the condition explored in this paper, the absolute vacuum degree (intensity of pressure in evaporation tank) reached at 0.025 MPa and 67℃, and the largest evaporation capacity was 8 324 mL/(m2?h) with the concentrating rate of 28.75%. The evaporation capacity of 7 700 mL/(m2?h) and the concentrating rate of 26% could be reached when the vacuum degree was kept from 0.04 to 0.05 MPa and from temperature at 75 to 80℃. Also, the device can prevent the loss of active ingredients in the biogas slurry. In this paper, we explored the feasibility and provided reference in the concentration technology of biogas slurry. The energy consumed by this designed system was from the electricity. In fact, this system can be combined with solar energy, which can be used to heat the biogas slurry and be regarded as the power source of the vacuum evaporation system. If so, the device will be energy saving because of the addition of solar energy. This will be the focus of the future research.