| In this paper, a new technique for treatment of high concentration formaldehyde with inorganic compounds, straight-chain saturated monohydric alcohols and straight-chain halogenated hydrocarbons by hollow fiber membrane contactors was developed. Moreover, mass transfer process of the membrane absorption and resource utilization of formaldehyde was studied. The main conclusions are as follows:1.Treatment of high concentration formaldehyde with inorganic absorbentsThree inorganic compounds, water, sodium bisulphite and ammonia, were chosen as absorbents to compare their efficiency to remove formaldehyde under the same conditions. It was found that sodium bisulphite might be a proper absorbent when considering various kinds of influencing factors. Then, some important factors, such as the inlet concentration and flow rate of the feed gas, the pH, flow rate and temperature of the absorbent were thoroughly studied. Experimental results showed that under proper operation conditions, the inlet concentration was 2.9×10-2m·s-1, the flow rate of formaldehyde was 616.1 mg·m-3, and the pH, flow rate and temperature of the absorbent were 6.5, 8.1×10-3m·s-1 and 30℃, respectively, the outlet concentration of formaldehyde was as low as 0.3mg·m-3, the removal efficiency of formaldehyde was 99.96% and the overall mass transfer coefficient was 6.52×10-5m·s-1. Moreover, the absorption product, CH2(OH)SO3Na, could easily be separated in saturated solution when NaHSO3 was added. The solution of NaHSO3 could be recycled as absorbent only with proper dilution.2.Treatment of high concentration formaldehyde with straight-chain saturated monohydric alcoholsSome straight-chain saturated monohydric alcohols (C=2～5) were used as absorbents to compare their efficiency to remove formaldehyde under the same conditions. Although their efficiency to remove formaldehyde did not show significant difference, it is feasible to use n-pentanol as absorbent when considering various kinds of influencing factors. Then, some important factors, such as the inlet concentration and flow rate of the feed gas, flow rate and temperature of the absorbent were thoroughly studied. Experimental results showed that under proper operation conditions, the inlet concentration of formaldehyde was 1188.6mg·m-3, the inlet flow rate of formaldehyde was 5.3×10-2m·s-1, the absorbent flow rate was 8.1×10-4m·s-1 and the absorbent temperature was 25℃, the outlet concentration of formaldehyde was 1.7mg·m-3, the removal efficiency of formaldehyde was 99.85% and the overall mass transfer coefficient was 1.16×10-4m·s-1. Moreover, it was found that the absorption product can be separated through distillation, and the recovery efficiency of formaldehyde can be reached 95%. The absorbent, n-pentanol, could be recycled.3.Treatment of high concentration formaldehyde with straight-chain halogenated hydrocarbonConsidering the viscosity, saturated vapor pressure and other properties of straight-chain halogenated hydrocarbon, it was found that 1-bromooctane and 1-chlorooctane might be proper absorbents. Then, The effect of some key parameters, such as the inlet concentration and flow rate of the feed gas, flow rate and temperature of the absorbent on the removal efficiency of formaldehyde and the overall mass transfer coefficient were thoroughly studied. When 1-bromooctane was used as absorbent, the inlet concentration of formaldehyde was 99.7mg·m-3, the inlet flow rate of formaldehyde was 5.5×10-3m·s-1, the absorbent flow rate was 2.4×10-3m·s-1 and the absorbent temperature was 5.8℃, the outlet concentration of formaldehyde was 1.7mg·m-3, the removal efficiency of formaldehyde was 98.3% and the overall mass transfer coefficient was 7.55×10-6m·s-1. When 1-chlorooctane was used as absorbent, the inlet concentration of formaldehyde was 150.6mg·m-3, the inlet flow rate of formaldehyde was 5.5×10-3m·s-1, the absorbent flow rate was 3.2×10-3m·s-1 and the absorbent temperature was 5.7℃, the outlet concentration of formaldehyde was 1.3mg·m-3, the removal efficiency of formaldehyde was 99.1% and the overall mass transfer coefficient was 8.76×10-6m·s-1. Further investigations indicated that formaldehyde gas could be easily emitted and the absorbents could be recycled from the absorption products under proper temperature. The recovery efficiency of formaldehyde was reached more than 96% and 90% for 1-bromooctane and 1-chlorooctane, respectively.4.Effect of the parameters of hollow fiber membrane contactor on the removal efficiency of formaldehydeWhen 1-bromooctane was used as absorbent, the effect of some important membrane parameters, such as length-diameter ratio, fiber packing density and hollow fiber membrane materials on the removal efficiency of formaldehyde were thoroughly studied. Experimental results showed that under the operation conditions, the inlet concentration of formaldehyde was 198mg·m-3, the inlet flow rate of formaldehyde was 5.5×10-3m·s-1, the absorbent flow rate was 3.2×10-3m·s-1, and the absorbent temperature was 30℃, the removal efficiency of formaldehyde were increased with the increase of the length-diameter ratio and the removal efficiency reached 100% when length-diameter ratio exceed 13.64. The removal efficiency of formaldehyde were increased and then declined with the increase of the packing density. And the removal efficiency reached maximum value when the packing density was about 36.8%. In addition, the removal efficiency of polyethersulfone hollow fiber membrane contactor was higher than that of polypropylene hollow fiber membrane contactor under the same operation conditions.5.Effect of the coexisted acid gases on the treatment and resource utilization of formaldehydeEffect of some coexisted acid gases, such as SO2, H2S and CO2, on the treatment and resource utilization of formaldehyde were studied when 1-bromooctane was used as absorbent. The removal and recovery efficiency of SO2 and H2S were studied when inlet concentration from 50 to 600mg·m-3, and the removal and recovery efficiency of CO2 were studied when inlet concentration from 500 to 2500mg·m-3. Experimental results show that, when the inlet flow rate of formaldehyde was 5.5×10-3m·s-1, the absorbent flow rate was 2.4×10-3m·s-1, and the absorbent temperature was 30℃, the removal efficiency of SO2 was lower than 29% and the recovery efficiency of SO2 was higher than 97%. The removal efficiency of H2S was higher than 94% and the recovery efficiency of H2S was lower than 11%. The removal efficiency of CO2 was lower than 49% and the recovery efficiency of CO2 was higher than 74%. Under the consideration of the real concentrations of SO2 (0.86μg·m-3), H2S(0.009μg·m-3) and CO2(648mg·m-3)in the air, SO2 and H2S may have tiny effect on the removal and recovery efficiency of formaldehyde. However, CO2 may influence the removal and recovery efficiency of formaldehyde. Therefore, CO2 should be removed at the pretreatment stage to ensure the resource utilization efficiency of formaldehyde.
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