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The principle of evaporation of ammonium nitrate

 

Evaporation is produced by the heating action of nitric acid in the solution contained in the solvent is vaporized , to improve the concentration of solute in the solution . In the production of ammonium nitrate , the ammonium nitrate solution is evaporated , leaving the vaporization of water in the solution , the concentration of nitrate increased. Evaporation operation of the device is called an evaporator.
 
Evaporation may be carried out below the boiling point or boiling point , which is far more efficient than the former . The industrial evaporation are carried out under boiling conditions . How much latent heat and evaporation of liquid vaporization when needed , depending on the temperature of the heat supply and the evaporation process , the pressure . Therefore, the thermal evaporation process is part of the evaporator is also a heat exchanger.
 
Heating the solution evaporated heat needed , the method can be used by the partition wall or direct thermal contact with the heat source material is supplied . Such as: water, steam , steam and high- boiling liquid , infrared radiation, hot flue gas , current, etc. can be used as a heat source for evaporation . The most widely used industrially is heated by the wall pipe, steam-heated water evaporator . On the one hand the operation of this water vapor in the evaporator as a heat source heat supply also other aqueous steam generation itself , for ease of distinction, the former is called the heating steam , which is known evaporative steam ( secondary steam ) .
 
Evaporation operation may be at atmospheric pressure or under reduced pressure . Evaporated under reduced pressure called evaporated in vacuo.
 
Used in the production of ammonium nitrate film single- effect vacuum evaporator, the heat source saturated steam .
 
Saturated steam as a heat source which has the following advantages:
 
( 1 ) large enthalpy ;
 
( 2 ) high heat transfer efficiency ;
 
( 3 ) condensing temperature stable ;
 
( 4 ) control of the heating process is simple ;
 
( 5 ) is commonly used modern factory for easy sharing and utilization of heat .
 
The case of a large degree of superheat superheated steam , should be warming to become saturated steam supply heating . But slightly superheated water vapor , in the actual production is more advantageous because it does not condense into water in the steam delivery pipe is longer .
 
2 vacuum evaporation process
 
Since the ammonium nitrate solution at higher concentrations , its high boiling point , such as 92% solution of ammonium nitrate
 
Its boiling point is 162 ℃. This solution requires a high boiling point higher heat carrier heating . However, high concentrations of ammonium nitrate solution at high temperature (180 ℃ above ) decompose and cause an explosion . Therefore, appropriate for the ammonium nitrate solution was evaporated under a reduced pressure , thus heating the heating medium at a lower steam pressure only . Therefore Sec predetermined steam pressure evaporator heating is not higher than 8 kg / cm 2 operation, in order to make the process of evaporation of good thermal conductivity , the heating steam to be between the boiling point of the ammonium nitrate solution is evaporated , a sufficient temperature difference. For this purpose, the heating steam pressure sufficient to enable the ammonium nitrate solution was evaporated under a reduced pressure , the boiling point of the solution can thus reduce the effective temperature difference is increased . 82% ammonium nitrate solution as a boiling point at atmospheric pressure is 131 ℃, but the boiling point at 500 mm Hg negative pressure was 94 ℃ Figure 71.
 
Ammonium nitrate solution at 71 under vacuum and atmospheric boiling point and solubility of ammonium nitrate .
 
Solvent in the solution due to the heat required for gasification , the gasification temperature is decreased with the increase . Therefore, evaporated in vacuo , the evaporation heat of water vapor consumed one kg than at atmospheric pressure or under pressure consumed as much. E.g., at 90 atm ( absolute pressure ) 1 kg of water evaporated consumed 485.6 kcal , and in the degree of vacuum of 700 mm Hg , the same evaporation of water for 1 kg 574.7 kcal of heat is required .
 
The amount of steam by the heat than the atmospheric pressure or under heating steam consumption seen in the amount of ammonium nitrate solution was evaporated under vacuum consumes as much, and the diameter of the pipe due to equipment limitations should be appropriate to increase the flow rate , but the vacuum evaporation has the following advantages :
 
( A ) the ammonium nitrate was evaporated in vacuo and the solution of the low operating pressure (negative pressure ) to reduce the risk .
 
( 2 ) may be evaporated by low pressure steam or steam . Such as the use of some of the evaporator and is produced by evaporation of steam. Or the use of low temperature waste heat to complete.
 
( 3 ) apply to deterioration due to high temperature of the solution was evaporated . Such as ammonium nitrate solution that is easy to produce at a high temperature decomposition and decomposition can promote faster and cause an explosion. So the solution was evaporated in vacuo evaporation selected preferably ammonium nitrate .
 
( 4 ) Since the boiling point of the solution is reduced , the heat loss is much less than the heat loss of the evaporator at atmospheric or pressurized operation.
 
3 film evaporator operating principle
 
By operation of : There are three main types of evaporators :
 
( 1 ) natural circulation evaporator ;
 
( 2 ) forced circulation evaporator ;
 
( 3 ) film evaporator.
 
The first two evaporator residence time of the solution in the device requires several tens of minutes or even hours . This not only large-scale production can not meet the needs of the ammonium nitrate solution and evaporation is unfavorable . Increase residence time because of its high temperature , and easy to produce decomposition and explosion. Therefore, the use of film evaporator to meet their needs.
 
In order to facilitate understanding of the role 's film evaporator , you can look at the situation gradually heated vertical tube , see Figure 72. Attached
 
Only a small temperature difference between the heating steam and liquid , the device only acts as a vertical tube heater. Tube solution is a natural convection . Increase the boiling temperature difference occurs because of the impact hydrostatic head of steam bubbles are generated near the top position of the tube .
 
Temperature difference continues to increase, the bubble due to the aggregation , formation of vapor and liquid are alternately ejected . And increases with temperature , alternating spray more frequently , the ratio of the vapor bubble is also growing . Temperature increases again , close to the wall surface solution was first heated gasification . Then as a result of the steam expansion account , together form the central pillar of steam pipes, high-speed rise. I.e. liquid extraction columns in this high-speed steam vent into a film known as " crawling ." Rapid rise along the wall , and evaporation was continued . Finally, from the top of the pipe is running out of steam fast suspension - emulsion .
 
When the temperature difference is too large, destruction of the film , the wall is covered with a lot of steam and into the dry state, and finally decrease the heat transfer coefficient K and evaporated strength.
 
Film evaporator , if under 710 mm Hg vacuum , then steam nozzle outflow speeds of up to 100-160 m / sec , driven by steam speed control column was moving along the wall to climb up to 20-80 meters / sec . If the next intrinsics 735 mm Hg vacuum operation, the steam column runs up to its speed of sound , then the steam is visible column drive faster solution speeds along the wall crawling . Therefore, the material in the evaporator tube residence time just a few seconds only. Based on this , the special design of the device so that the speed of evaporation of the solution up to the speed of sound , which is supersonic evaporator. Such a short time , heat-sensitive solution , even slightly higher evaporation temperature , and will not suffer the risk of thermal decomposition .
 
Film evaporator heating tube is relatively long , the general length of the tube diameter 100-150 times . When boiling solution of foam generated heat during ascent easily broken , it is suitable for medium viscosity and easy generation of foam concentrate liquid evaporation . As the liquid flows fast, so easy to generate boiler scale . Film evaporator, due to the film speeds greatly improve the heat transfer coefficient , but because it is the boiling point of the feed , a heating tube into the evaporator , which boils evaporation , so the impact is very small hydrostatic head . Feeding tube was just one path to reach its desired concentration requirements, it is no longer circulating evaporator , while also reducing the boiling point elevation loss resulting from the temperature difference . So this bell evaporator production intensity is greater . Compared with other types of equipment has the following advantages :
 
( A ) in the heating tube velocity, the residence time is short , thermal decomposition of ammonium nitrate is not easy .
 
( 2 ) heat transfer coefficient , high heat transfer efficiency, large-scale production strength.
 
( 3 ) heat-sensitive ammonium nitrate , prime urine can be evaporated, and to a high concentration without decomposition in fear.
 
( 4 ) the impact of factors of production intensity evaporator ;
 
Evaporator production strength, i.e. the amount of water evaporated per unit of heat transfer area per unit time can be calculated with the equation Heat legend :
 
q = KFΔt, evaporated W kg / hr of water needed W. (Nt) ( kW / h ) of heat.
 
Therefore, the intensity of production of the evaporator is:
 
V = ( kg / m 2 · hr ) ;
 
q- heat load , kcal / h ;
 
K - heat transfer coefficient , thousands Camille 2 · h · ℃;
 
F - heat transfer area , m 2 ;
 
Δt - there is a temperature difference between the school , ℃;
 
N - secondary steam enthalpy , kcal / kg ;
 
T - the solution boiling point , ℃.
 
As can be seen the analytical : a material for an evaporator evaporation , its strength depends on the production of △ t, i.e. the heating temperature of the steam and the temperature difference between the steam evaporated. Improve the heating steam pressure and vacuum evaporator will help to improve the degree of difference between the effective temperature , thereby increasing the evaporator production strength. However, by increasing the heating steam to the steam pressure often original limitations, because the vacuum evaporator to increase the boiling point of the solution to reduce the concentration of the solution is increased , the occurrence of adverse effects on the heat transfer , particularly at the boiling point of the ammonium nitrate solution below its freezing point , it is easy to produce crystallization, so the production is difficult to maintain . Of course, the improved structure of the apparatus from reducing temperature loss , also help to increase the temperature difference . Derived from the above analysis ; improve the evaporator to increase the intensity of production should primarily focus on transfer coefficient k .
 
K can be calculated using the following general formula :
 
K =
 
Where a1-- tube heating steam condensing heat transfer coefficient , kcal / m 2 · when · ℃
 
a2-- boiling heat transfer coefficient , kcal / m 2 · when · ℃;
 
 --- Wall thermal resistance , kcal / m · when · ℃;
 
 --- Wall thickness , m ;
 
 --- Wall thermal conductivity, kcal / m 2 · when · ℃
 
 --- Fouling thermal resistance , kcal / m · when · ℃;
 
 --- Dirt thickness , m ;
 
 Dirt thermal conductivity, kcal / h m 2 · · ℃;
 
Now let's discuss how to reduce the thermal resistance were to increase the overall heat transfer coefficient K.
 
In general, the condensation heat transfer coefficient than the boiling heat transfer coefficient a1 a2 much larger , and therefore not transfer the principal contradiction. It should be noted if they contain small amounts of condensable gas evaporation condensation , the heat transfer coefficient will be greatly reduced , so that the heat transfer effect is reduced . Such as: steam heating section 4440 a11 , a11 and the evaporator 's Sec 50000 as an evaporator section and the evaporation of steam , wherein the partially condensed gas containing NH3, N2, H2, CH4 caused. Therefore, in the design of the evaporator , you must pay attention to the discharge of the inert gas , to have non-condensable gas vent pipe to Yasumasa heating chamber heat transfer efficiency.
 
Secondly fouling resistance under many occasions , often the major factor affecting the heat transfer effect . Water is used in the production of nitric untreated hard water , which contains a large number of compounds of calcium , magnesium, silicon , etc. , after the evaporator using a very long time, it is easy to generate boiler scale . Dimensionless evaporator must be cleaned regularly , otherwise significantly reduce the heat transfer coefficient , the production ability. To this end, using a variety of effective decontamination method with a base such as dilute hydrochloric acid solution and washed .
 
The main factors affecting the heat is boiling heat transfer coefficient a2, a2 film evaporator heat affected by the following factors:
 
( 1 ) an effective temperature difference: steam heating temperature difference between the ammonium nitrate solution has a significant impact on the flow rate of the solution in the tube , the flow rate was increased with the increase △ t , a2 value also increases.
 
( 2 ) increase the degree of vacuum : the flow rate of the steam heating the evaporator tubes is increased, the solution flow rate is increased , a2 value also increases.
 
( 3 ) ammonium nitrate solution was evaporated to a small viscosity , a dilute concentration , the flow rate is increased to increase the value of a2 .
 
( 4 ) the liquid level on the evaporator tube or tube also affects the heat transfer coefficient . According to the experiment , or that the evaporator , the heat transfer coefficient max.
 
Nature ( 5 ) of the solution : the solution weight , concentration , viscosity related. And severe and affect the heat transfer coefficient of viscosity of the solution , wherein the viscosity of the Effect of the large and heavy . When the device is the same as the dilute solution was evaporated , a2 is large compared to the concentrated solution was evaporated .
 
(6) Effect of heating steam amounts of: heating the evaporator chamber , a large quantity of steam is heated , the air flow between the accelerating tube , the tube wall of the film away, the effective increase of the temperature difference △ t , a2 are increased. But △ t can not increase indefinitely , otherwise △ t over a large area of saturated steam heating pipe increases ; inside saturated steam district heating pipe increases ; film away the outer wall of the tube , so that the effective temperature difference △ t increases, also increases. △ t but can not increase indefinitely , otherwise △ t is too large, the saturated steam heating pipe increased area ; heat pipe evaporator steam velocity is too large, the heat entrained ammonium nitrate solution drops on small , difficult in the separator is detached , resulting in the evaporation operation is interrupted . △ t is generally better to take between 20 ~ 35 ℃.

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