| In recent years, soda industry in China is of overcapacity, and the contradiction between supply and demand is intensified increasingly. To carry out innovation of technology and reduce production cost are becoming the developing direction of enterprises. In the process of ammonia soda, the condensate produced by ammonia cooling tower is usually distilled into the ammonia distillation tower again in order to recover ammonia as much as possible, which increases the energy consumption. Ammonia with high water content was reused in ammoniating process, which decreases the concentration of refined brine, thus preventing sodium utilization rate from enhancing. Therefore, how to reduce the energy consumption of the process of ammonia distillation and the water content of ammonia recovered, which can reduce production cost, is becoming a major technical requirements of the current enterprises.In the paper, the condensate produced by ammonia cooling tower was taken as feed and refined brine was used as absorbent. Membrane absorption technology to recover and reuse ammonia based on the principle of reverse direct contact membrane distillation (R-DCMD-MAAR) was studied. Ammonia in simulated condensed ammonia solution was transferred to refined brine, at the same time, mass transfer of water vapor whose direction of mass transfer was the same as that of ammonia was inhibited. So the rate of water content of ammonia recovered and reused was reduced and heat of phase transition produced by vaporization of water was decreased, which thereby could reduce the energy consumption of the process and made a substantial increase in the concentration of refined brine. This process of R-DCMD-MAAR effectively combined with the ammoniation process of refined brine, thus avoiding the complexity of membrane desorption for absorption liquid after conventional membrane absorption process.To save the refined brine, the membrane aeration process of the absorption liquid (ammonia salt water) in the seaweed membrane aeration module was studied to determine the amount of aeration and pure water supplement, thus the refined brine could be reused. On basis of this, structure parameters of membrane absorption module (internal diameter (ri) of PVDF hollow fiber hydrophobic membrane used in the membrane absorption module, packing density (δ) of module), parameters of membrane absorption process (the temperature (T) of feed, the temperature difference (ΔT) between shell inlet of absorbent and tube inlet of feed in the absorption module, the concentration (CA) of feed and the velocity (vA) of feed) on the performance(JA, JW, α, RW) of R-DCMD-MAAR was studied experimentally. A semi theoretical and semi empirical formula to describe the mass transfer coefficient KA of ammonia was established, and the parameters of the process optimization were determined. The study of orthogonal experimental on the operation process showed that the order of effects of various factors on JA was:CA> T> ΔT>vA; the order of effects of various factors on Jw was:T>ΔT>CA>vA and the order of effects of various factors on a was:CA>ΔT>T>vA. The calculated value of KA in terms of the formula was close to its experimental value, and relative deviation between calculated and experimental value was within ±8.00%. After the performance optimization of R-DCMD-MAAR, JA could reach 0.86 kg/(m2h) and JW was-0.11 kg/(m2·h), and RW was 0 when T was 40 ℃,ΔT was 15℃, CA was 5.5 wt% and vA was 0.15 m/s. The mass transfer direction of ammonia and water vapor was opposite.For R-DCMD-MAAR, in the case of no obvious reduction in ammonia absorption efficiency, the water content of the ammonia recovered is low, and so as the process energy consumption. Therefore, the study can provide theoretical and technical support for the technical improvement of ammonia-soda process. |
