PROCESS AND INSTALLATION FOR THE PRODUCTION OF AMMONIUM NITRATE

Abstract: The invention is carried out in accordance with the following steps: introduction under pressure of a 50-60% nitric acid solution (a) and anhydrous ammonia (b) into a tubular T reactor (3), leading to a solution of ammonium nitrate and to a gas phase; separation of the ammonium nitrate solution from the vapour phase in a discharge chamber (4) of the reactor; subjecting the ammo­ nium nitrate solution to a sudden pressure drop in a further vacuum discharge chamber (7), and immediate passa­ ge of the ammonium nitrate solution through a combined evaporator-concentrator (9) adjacent the discharge chamber (7), resulting in separate streams of concentrated ammonium nitrate solution (e) and residual vapour (g). According to the invention, ammonium nitrate solu­ tions of more than 95% concentration are yielded and are suitable for granulation and prilling treatment without a heat supply from outside the system.

Description: This invention relates to a process and installation for the production of concentrated aqueous solutions of ammonium nitrate. Introduction Ammonium nitrate, which is used as a fertilizer and in the manufacture of explosives, is produced industrially by the neutralization reaction between dilute nitric acid of a concentration of 50 to 60 weight % and anhydrous ammonia. The resulting ammonium nitrate solution is then concentrated sufficiently to be used as feed for granulation or prilling units (95 weight % or more). The reaction between nitric acid and ammonia is highly exothermic with in theory the evolution of sufficient heat to evaporate all the water accompanying the 50 - 60 weight % nitric acid. However, for practical purposes the vapour evolved in this neutralization is not hot enough to be useful in the subsequent step of concentration by evaporation of the resulting ammonium nitrate solution, and so the system must be supplied with external heat. The point is that the processes described in many patents relating to the preparation of ammonium nitrate, which can be broken down into vacuum or atmospheric pressure processes and positive pressure processes, do not solve the problem satisfactorily: A.- Vacuum or atmospheric pressure processes. The ammonium nitrate yielded has a concentration of 70 to 85 weight % and the vapour produced in the reaction is not hot enough to be used in the step of evaporating the ammonium nitrate solution; its only use is for preheating the reactor inlet and outlet streams. An evaporation stage using external heating is therefore necessary. Other disadvantages include the considerable size of the necessary equipment and the substantial energy costs of breaking down the usually contaminated ted residual vapour of the reaction system. B.- Pressure processes In some cases where working is carried out at a pressure of 4-8 bars absolute and with the use of nitric acid in a concentration of approximately 60 weight % or more, ammonium nitrate concentrations of 95 weight % or more are yielded without a supply of external heat; however, for this to be possible the nitric acid feed temperature must be 140°C and the reactor temperature must be 200-240°C. These high temperatures lead to corrosion problems (special materials have to be used) and risks of explosion in the reactor which can be avoided only by means of sophisticated control systems. The process for producing concentrated solutions of ammonium nitrate according to WO-A-87,000,150 uses only one depressurization step of the NH₄NO₃ solution which is produced in a tubular reactor. This depressurization step allows the separation between the ammonium nitrate solution from the vapour phase, this latter being used to preheat the reactants before entering into the tubular reactor. This process produces only 88.3% of ammonium nitrate. Thus, the purpose of the invention is to obtain a greater percentage of ammonium nitrate. Description of the invention This invention relates to a process for preparing concentrated ammonium nitrate solutions and to the installation for performing the process. According to the process, ammonium nitrate solutions of a concentration greater than 95% and suitable for subsequent granulation or prilling treatment are yielded without a supply of external heat to the system and starting from nitric acid having a concentration of 50 to 60% weight % and from anhydrous ammonia in moderated conditions. This invention also makes it possible to combine in a single step and apparatus the neutralization reaction between nitric acid and ammonia and the concentration of the ammonium nitrate solution yielded by using the heat generated in the reaction, a low contaminants content residual vapour also being yielded which is useful for preheating the reactor feed streams to optimal operating conditions. The installation in which ammonium nitrate is prepared by the process according to the invention is of reduced size and easy scalation and designed to ensure efficient utilization of the energy produced in the reaction by means of the high speeds and turbulence occurring in the reaction so that the system is easily operable and safe since the quantities of concentrated ammonium nitrate solution present in the installation at any time are very small. A detailed description will be given hereinafter of the process and installation according to this invention, reference being made to the accompanying drawing in which small letters denote streams and numbers denote elements of the installation. The feedstocks of the reaction, namely 50 - 60 % weight nitric acid (a) and anhydrous ammonia (b), are heated with residual vapour from the reaction (c) in heat exchangers (1) and (2) to temperatures of 50 - 75 °C and 70 - 90°C respectively and are introduced into a tubular T reactor (3) at a pressure of 4 - 6 bars absolute. Contact takes place between the two reactants at high speed and with high turbulence in the reactor (3). The two-phase neutralization reaction occurs at high speed along the narrow cross-section chamber of the reactor (3), generating enough heat to evaporate a major part of the water of the nitric acid solution. The reaction products are delivered to a discharge chamber (4) maintained at a pressure of 3 - 4 bars absolute and at a temperature of 150 - 170°C; the vapour phase rises from the chamber (4) to a concentration chamber (5) to be collected at the top as the residual vapour stream (c)which goes to the heat exchangers (1) and (2) to preheat the feedstocks. The liquid phase, in the form of a 75 - 85 weight % ammonium nitrate solution, is collected at the bottom of the chamber (6) and rises therefrom because of a pressure difference in a stream (d) to the chamber (7) above the concentrator. The chamber (7) is maintained at a pressure of 0,3 bar absolute by means of a vacuum unit (8), so that a flash distillation occurs from the 3 - 4 bars absolute of the bottom discharge chamber up to this pressure. From the flash chamber (7) the ammonium nitrate solution descends through a number of pipes (9) of the concentration chamber (5) in the form of a film. During this descent and at temperatures of 125 - 145°C the ammonium nitrate solution exchanges heat with the rising vapour from the discharge chamber so that water is evaporated and concentrations of 95% or more are obtained. The concentrated solution is finally collected at the bottom (10) of the concentration chamber to provide the stream (e) of concentrated ammonium nitrate product. A condensate stream (f) is also collected in the zone where the pipes of the concentration chamber (11) end. The vacuum unit (8) removes residual vapour from the flash chamber (7) to form the stream (g) which is used to evaporate the ammonia by heat exchange in the evaporator (12) before the ammonia passes through the heat exchanger (1). EXAMPLE Three different cases I, II and III will be given hereinafter as an example of the operation of the installation with various concentrations of the nitric acid feeding the reactor and/or of its temperature. FEED STREAMS Initial temperature of nitric acid (°C) 25 25 25 Concentration of nitric acid (wt.%) 52 52 50 Nitric acid feedstock temperature (°C) 55 70 70 Ammonia feedstock temperature (°C) 80 80 80 DISCHARGE CHAMBER Pressure in discharge chamber (bars abs.) 3 3 3 Temperature of nitrate solution (°C) 165 167 162 Concentration of nitrate solution in wt.% 75.4 77.1 73.1 Rate of flow of vapour (kg/t of nitrate) 400 429 419 FLASH CHAMBER Pressure at the top (bar abs.) 0.3 0.3 0.3 Temperature in flash chamber (°C) 93 95 91 Concentration of nitrate solution in wt.% 81 83 79 Rate of vapour flow (kg/t of nitrate) 99 95 103 CONCENTRATION CHAMBER Temperature of nitrate solution (°C) 129 129 129 Condensation temperature (°C) 134 134 134 Concentration of outgoing solution in wt.% 95 96 95 Condensate (kg/t of nitrate) 214 184 256 Useful vapour (kg/t of nitrate) 324 319 303 Surplus vapour (kg/t of nitrate) 116 134 47

Claims: A process for the production of concentrated aqueous solutions of ammonium nitrate starting from 50-60 weight % nitric acid and anhydrous ammonia introduced into a tubular reactor in which an exothermic neutralization reaction occurs between the two reactants giving a solution of ammonium nitrate and producing a gas phase of vapour generated by the heat evolved in the reaction, said gas phase being separated from the ammonium nitrate solution in a pressurized discharge chamber, characterized in that it is carried out in the following steps: (a) subjecting the ammonium nitrate solution separated from the gas phase in said pressurized discharge chamber to a sudden pressure drop in a vacuum discharge chamber with separation of a further quantity of vapour which is removed through a vacuum line; and (b) immediate passage of the ammonium nitrate solution through a combined evaporator-concentrator adjacent the discharge chamber of the above step where heat exchange occurs with a vapour stream consisting of the gas phase separated from the ammonium nitrate solution in said pressurized discharge chamber, resulting in the end-product stream of concentrated ammonium nitrate solution and in a residual vapour stream. A process according to claim 1, characterized in that the feed streams to the reactor are at a pressure of 4 to 6 bars absolute and at a temperature of 50 to 75°C in the case of the nitric acid and of 70 to 90°C in the case of the ammonia. A process according to claim 1, characterized in that the said discharge chamber is at a pressure of 3 to 4 bars absolute and at a temperature of 150 to 170°C. A process according to claim 1, characterized in that the combined avaporator-concentrator is at a presure of 0,2 to 0,4 bar absolute and at a temperature of 125 to 145°C. An installation for the operation of the process according to claims 1-4, comprising a tubular reactor and a discharge chamber of the reactor, characterized in that: (a) said tubular reactor is a tubular T-reactor; (b) said discharge chamber of the reactor operating under pressure is provided with an outlet for a liquid stream in the bottom part; (c) there is provided a combined evaporator-concentrator which forms part of the top part of the discharge chamber and through which vapour originating in the discharge chamber rises, and which has at the bottom a condensate outlet; (d)there is provided a vacuum flash chamber which is adjacent to the top part of the combined evaporator-concentrator and where the liquid stream separated in the discharge chamber arrives as a result of pressure differences, thereafter to continue descending along the combined evaporator-concentrator; (e) there is provided a collector which is disposed at the bottom of the combined evaporator-concentrator and from which the stream of concentrated product is removed; (f) there is provided a vacuum unit whose line is connected to the flash chamber; (g) there is provided an ammonia feed stream evaporator using the heat of the residual vapour removed through the vacum line; and (h) respective heat exchangers are provided to preheat the feed streams using the heat of the residual vapour leaving the top part of the combined evaporator-concentrator.

Inventors: LOPEZ NINO HERNANSAEZ, JOSE LUIS, OLIVARES DEL VALLE, JOAQUIN, SALCEDO GOMEZ, RAFAEL
Applicants: UNION EXPLOSIVOS RIO TINTO, S.A.
Application year: 1988
Main classification: C01C1
IPC codes: C01C1