2024 Fumagalli HYPER-U – a novel high-efficiency self-stripping urea process process

Since its introduction in the ‘70s, the “ammonia-“ or “self-“ stripping process has been a valid alternative to the CO2-stripping process for the industrial synthesis of urea; together they quickly replaced Total-Recycle and conventional processes due to their intrinsic competitive advantage in terms of energy consumption and consequent operational costs. In the CO2-stripping process the core operation is the carbamate decomposition at the same synthesis pressure with the aid of CO2 as stripping agent, wherefrom comes the definition of the technology. The same terminology is improperly used for the self-stripping process as the high-pressure carbamate decomposition is carried out solely by heating. The direct consequence of thermal-stripping is the considerable excess of ammonia contained in the urea solution leaving the high-pressure section which requires a purification stage at Medium-Pressure for its recovery. This plant section, which initially might have been seen as a complication with respect to the smooth simplicity of the CO2-stripping process, lays the way for an efficient overall steam consumption.
Carbamate condensation at Medium-Pressure follows a temperature profile which allows a convenient heat integration with the urea concentration stage thus leading to a significant reduction of LP steam and cooling water demand. This same concept can nowadays be found also in modern CO2-stripping processes to achieve very low steam consumption and steam network balance. Heat recovery from Medium-Pressure carbamate condensation was indeed soon recognized as mandatory for improving the efficiency of the self-stripping process such that, since its introduction in the ‘90s, it has become a standard feature applied to all new plants as well as for revamping of vintage plants. Later optimizations of the heat recovery strategy and steam network contributed to further advance the efficiency of the process, though at a lesser extent. Still, no relevant improvement has been carried out for years to the self-stripping process, so that it appears to have reached the plateau of energy optimization.
Following the demand for ever more efficient urea production plants, particularly from markets characterized by high cost of utilities, Casale has developed an improved self-stripping process to further lower the demand of medium-pressure steam and cooling water. The main innovation consists in an additional carbamate decomposition step upstream the conventional Medium-Pressure section. Carbamate condensation in the new Medium-High Pressure section, operated at 30-35 bar, features a higher temperature profile which allows to expand the heat recovery strategy; namely, the heat released by carbamate formation at MHP can be used to evaporate water in the vacuum evaporation unit. Also part of the novelty of this process, carbamate decomposition at MHP is carried out with a direct process-process heat exchange in a combined HP reactor – condenser.
MHP heat recovery joins the conventional heat recovery stages, i.e. the concentration of urea solution using MP carbamate vapors and ammonia pre-heating using LP carbamate vapors. With such arrangement the specific quantity of saved LP steam is drastically increased with respect to the state of the art of self-stripping process. Also, the consumption of cooling water for carbamate condensation is positively impacted. A lower demand of LP steam allows the reduction of MP steam supplied to the HP stripper without compromising the steam balance. With the MHP section and the combined reactor the thermal efficiency of the self-stripping process is in fact significantly improved leading to better opex and smaller carbon footprint of new plants as well as revamped self-stripping plants.

This paper is presented at the Nitrogen + Syngas 2024 International Conference & Exhibition (Gothenburg 4-6 March 2024).

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