A Study On Cleaner Production/ Process Optimization & Control For De-Adsorption Column At Urea Unit

The NH3 & CO2 which was contained in the wastewater from urea unit (called: ammonium carbonate solution) was recovered by de-absorption valve column. Because the column had some shortage such as low capacity, low separation efficiency, low load fluctuation and lack of control measures, H2O concentration in the outlet gas from the top of the column was too high to be harmful to the water balance of the urea system and the NH3 in the outlet stream from the bottom of the column was higher than expected too. So, this process had to be modified. At present, the retrofit options include replacing the float valve tower with packed column and increasing the height and diameter of the tower. But it required large investment and might face the restriction of workshop layout.Regarded to the above problems, we used the principle of cleaner production (CP) (reduction the pollutants at the source), adopted two approaches of CP (reformation the process and equipment, operation improvement), by analyzing the process, studying the characteristics of unit operation, investigating the status quo and options of the same unit of other plants, combined with the conditions of the aimed plants and modification funds, we decided to use the existing column, just adjusted the structure of the components in the column and optimize the process.Using chemical process simulation software and liquid mechanics calculation software to simulate and re-design, we found the main reasons of the low performance of the column. A computer control system was designed.After modification, the capacity of the column was increased from 6-7 m3/h to 4-15 m3/h, meanwhile, H2O concentration in the outlet gas was decreased from 56% to 35% and NH3 in the waste water was reduced from 1000 mg/1 to 270mg/l. At the same time, computer control system made the process run more steadily. The implementation of the project achieved good both environmental and economical benefits by using the principle of CP, the capital payback was 2 month. Meantime, the methodology and experience of process optimization and computer control not only could be adopted in urea sector; it also could be used for reference for other sectors.