Designing energy-efficient hardware and technological structure of absorption refrigeration units for ammonia production

Abstract

The object of research is the equipment and technological design of absorption refrigeration units (ARUs) for the technological system of secondary condensation of large-scale ammonia production. Improving the energy efficiency of ARU is an urgent problem in the general process of reducing operating costs for natural gas in these industries as a whole. Based on the results of analytical studies, the feasibility of combining absorption-refrigeration and vapor-ejector cycles was substantiated, which ensures a decrease in the boiling temperature of a weak water-ammonia solution in the cube of the generator-rectifier and an increase in the condensation pressure in the ARU cycle. Under such circumstances, it becomes possible to increase the concentration of the refrigerant due to the rectification of steam with a part of the liquid refrigerant without using a pump with the removal of the dephlegmator from the ARU circuit. Experimental studies and material-thermal calculations of ARU cycles were carried out to determine the basis of comparison and the proposed version of the ARU scheme. It has been proven that the new technological design of ARU provides an increase in cooling capacity from 3.22 MW to 3.6 MW (by 12 %), the thermal coefficient from 0.527 to 0.551 (by 4.6 %), a decrease in the circulation ratio from 7.77 to 7.1 (by 8 %), and a decrease in the secondary condensation temperature by 2.5 ℃. It is shown that for the proposed version of the technological design of ARU, there is a change in specific costs – an increase in electricity by 1.48 kWh/t NH3 and a decrease in natural gas by 0.41 nm3/t NH3. Taking into account existing cost indicators for natural gas and electricity, the application of the proposed technology ensures a decrease in annual operating costs by UAH 7 million (USD 185,000), and therefore an increase in the economy of ammonia production as a whole.