Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات LECTURE (11) NITRIC ACID PRODUCTION 6-6Contact Time:  An important variable in ammonia conversion is the rate of gas crossing the catalyst.  Increased flow rates lead to quicker oxygen transport rates on the catalyst surface and The removal from the catalyst surface of intermediate products.  High gas velocities and a short contact time, which is usually approximately seconds, minimize undesirable side reactions.

 The optimal velocity increases with reaction temperature.  The ammonia loading rate, which expresses the flow rate, is considered optimal at the range of kg of ammonia per gram of catalyst per day.  Gas velocities are much greater in high-pressure burners, and efficiencies are usually lower. However, the high flow rates increase catalyst losses.

6.7 Energy Recovery: Nitric acid production is a net exporter of energy. Therefore, energy recovery is a valuable resource for these facilities. External sources of fuel can be minimized through recovery of energy from the production process. There are two exothermic reactions, oxidation and absorption, that produce more energy than the total production process consumes. Steam production is the most common form of energy recovery for process lines in nitric acid production. Heat from the process is typically recovered and used to generate a large portion of the steam needed in a steam turbine generator.

 Tail gas turbines usually supply 35%-85% of the compression energy. 6.8 Platinum Recovery:  During operation the surface of the catalyst is damaged by abrasion and vaporization.  Catalyst losses increases with increasing pressure.  Platinum from the catalyst passes into the gas stream in the form of very fine particles, and its loss can substantially increase the production cost.

Two types of recovery systems catchment gauzes and mechanical filters - are usually offered. A: catchment gauzes :  The principle of catchment gauzes is to collect platinum at a temperature as high as possible while the main portion of the platinum loss is still in vapor form.  At these temperatures, platinum atoms strike the metal surface and form an alloy with the catchment metal for subsequent recovery. The system can recover up to 80% of the primary platinum losses.

The catchment gauzes, which are installed at the bottom of the burner, are composed of a mesh screen and two or more metal gauzes Catchment gauzes are returned together with the catalyst gauzes to the precious metal refining plant. B: mechanical filters The mechanical filters, which are composed of glass wool or silica fibers, are commonly installed downstream of the catalyst where the gas temperature is below 300°C. Recovery rates of 50% have been reported

:6.9: Industrial Processes Industrial plants are classified according to the pressure used:  Monopressure plants. These plants operate at the same pressure in the ammonia oxidation and absorption sections of the plant. Monopressure plants are classified as low pressure (3-5 bar) and high pressure (8-13 bar).  Dual-pressure plants. These plants operate under lower pressure in the ammonia oxidation section than in the absorption section.

High pressures also favor NO2 absorption; however, high-pressure ammonia oxidation induces greater catalyst losses and also increases power requirements unless additional equipment is installed for power recovery. High pressure processes are able to attain acceptably low NOx levels in the tail-gas which favored their adoption. Therefore, most new plants use either mono-high-pressure or dual-pressure (medium- pressure combustion, high-pressure absorption).