Dual pressure system for producing nitric acid and method of operating thereof

A production plant for producing nitric acid at reduced power consumption and reduced emissions, comprising:

• an air compressor providing compressed air;
• a supply for a first oxygen-rich gas, a mixing of the first oxygen-rich gas and of compressed air providing part of a first oxygen-containing gas;
• a mixing apparatus, for mixing the first oxygen-containing gas with an ammonia gas stream, to produce an ammonia/oxygen-containing gas mixture;
• an ammonia converter operable at a pressure equal to or higher than P1 and lower than P2, for oxidising ammonia in the ammonia/oxygen-containing gas mixture, to produce a NOx gas/steam mixture comprising water and nitric oxide;
• a means for regulating a concentration of ammonia and/or of oxygen in the ammonia converter, including a means for controlling a flow of the first oxygen-rich gas in the oxygen-containing gas and/or a means for controlling a flow of the ammonia gas stream, for maintaining the oxygen to ammonia molar ratio inside the ammonia converter at a ratio of at least 1.2;
• a first gas cooler/condenser downstream the ammonia converter, to produce an aqueous diluted nitric acid mixture and a gaseous NOx stream;
• a NOx gas compressor for compressing the gaseous NOx stream, to produce a compressed NOx gas stream at a pressure P2;
• an absorption tower for absorbing NOx gases from the compressed NOx gas stream in water, to produce a stream of raw nitric acid-containing residual NOx gas and a tail gas comprising NOx gases, comprising an absorption tower tail gas outlet for evacuating the tail gas;
• a heat exchange system located upstream the first gas cooler/condenser for heating a tail gas stream with the heat from the NOx gas/steam mixture coming from the ammonia converter;
• a second gas cooler/condenser for separating and condensing steam from the compressed NOx gas stream before the stream is provided to the absorption tower;
• a second oxygen-containing gas, having either:
• a) a pressure equal to or higher than P1 and up to P2, for supplying oxygen downstream the ammonia converter and upstream the NOx gas compressor;
• or
• b) a pressure higher than P2, for supplying oxygen to the compressed NOx gas stream;
• a means for controlling the flow of the second oxygen-containing gas such that a tail gas stream contains at least 0.5% by volume oxygen;
• and
• a first pressure release means located downstream the heat exchange system, for expanding a tail gas stream, to produce a first expanded tail gas at a pressure equal to or higher than P1 and lower than P2, wherein the first pressure release means can at least partly power the NOx gas compressor and/or the air compressor;
• characterized in that the production plant further comprises:
• a first and/or a second means for splitting a gas stream, wherein
• (i) the first means for splitting is a means for splitting a tail gas stream into a first tail gas stream and a second tail gas stream, wherein the first tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with the first oxygen-rich gas and compressed air, and wherein the mixing of compressed air, the first oxygen-rich gas and the first tail gas stream provides the first oxygen-containing gas, and
• (ii) the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth tail gas stream, wherein the third tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas provides the second oxygen-containing gas, and wherein the second oxygen-containing gas is supplied downstream the ammonia converter and upstream the NOx gas compressor;
• or
• the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth gas stream, and wherein the third tail gas stream is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas;
• and the pressurization of the mixed third tail gas, compressed air and the first oxygen-rich gas in a means for pressurizing provide the second oxygen-containing gas at a pressure higher than P2, and wherein the second oxygen-containing gas is supplied downstream the NOx gas compressor and upstream the absorption tower.