Some realities behind cooler/condensers

Nitric acid plants involve energy-intensive processes, beginning with the exothermic oxidation of ammonia over a catalyst with air, at reaction temperatures ranging from 810 - 940°C (1500 - 1725°F). Various methods and equipment configurations are utilised to recover a large portion of this released heat energy (boilers and boiler feedwater heaters, gas heaters, and so forth). In many monopressure, high pressure plants (550 - 960 kPa [80 - 140 psig] at the compressor discharge), once the process gases are cooled to around 175 - 230°C (350 - 450°F), continued recovery of the remaining heat energy is generally considered uneconomical.

The remaining heat energy is removed to cooling towers via cooler/condensers (which in some plants are known by other names, such as ‘weak acid condenser’). Novatech believes cooler/condensers experience the most complex thermodynamic processes in a typical nitric acid plant. A host of chemical and thermodynamic processes occur within, coupled with simultaneous fluid dynamics and heat transfer processes. This complexity is believed to be even greater than that experienced in absorbers (which do not experience the significant condensation of water vapour occurring in cooler/condensers).

This article focuses on cooler/condensers, presenting some of Novatech’s field and troubleshooting experience, and the resolution issues seen within cooler/condensers. The phenomenon of reboiling will be discussed, with some operational examples, followed by additional experiences pertinent to cooler/condenser operational concerns.

Reboiling 101

One of the most common thermodynamic processes experienced in (and sometimes upstream of) cooler/condensers is a phenomenon known colloquially as ‘reboiling’. This article is not intended to present an in-depth, scholarly exploration of the thermodynamic and chemical aspects of the ‘reboiling’ phenomenon – the inquisitive reader desiring a greater depth of scholarly knowledge is directed to the works of other authors. Rather, a general overview of reboiling, based on actual experience and observation, is presented herein for those who may be new to this industry, and who desire an initial, basic understanding from a practical perspective.

A simple description of reboiling is as follows: on first formation of liquid water condensate on a sufficiently cold surface (and forming as a relatively thin film), nitrogen tetroxide (N₂O₄) is rapidly absorbed into the water and reacts to exothermically produce nitric and nitrous acids, raising the film temperature slightly. If one could view this small zone of ‘first condensation’, in some cases it might appear to be shifting back and forth by just a few millimeters. In actuality, this film is repeatedly evaporating and reforming, thus giving the illusion of movement. Although not present in every case, the concept of ‘motion’ provides an easily visualised illustration for the reader in this discussion.

Some of the factors involved in reboiling are an abundance of very hot gas right at the surface of the film, coupled with the slight exotherm mentioned, and limited heat transfer in the immediate region where the film is forming. A similar phenomenon can sometimes be observed visually in an uninsulated glass pipe flowing slightly superheated steam. Although the exotherm mentioned previously is missing in the case of steam, many of the other thermodynamic factors are present, or if absent, can be easily created by gently blowing on the glass.

To read the full article subscribe here to receive a free copy of World Fertilizer.

Read the article online at: https://www.worldfertilizer.com/special-reports/10102025/some-realities-behind-coolercondensers/