Thermodynamic Behavior of Electrolytes in Mixed Solvents

10 Use of Magnesium Nitrate in the Extractive Distillation of Nitric Acid JOSEPH A. VAILLANCOURT Downloaded by GEORGE MASON UNIV on March 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch010

Hercules Inc., 910 Market St., Wilmington, Del. 19899

The maximum boiling azeotrope of nitric acid and water (68 % nitric acid) requires extractive distillation to produce concen trated nitric acid when starting with acid that is weaker than the azeotrope. Sulfuric acid has been used for this extractive distillation, but its use requires high investment and mainte nance costs. Magnesium nitrate is being used in several com mercial plants in the extractive distillation of nitric acid. Magnesium nitrate was selected rather than other nitrate salts because it has the most favorable combination of physical properties. Although magnesium nitrate requires slightly more steam than sulfuric acid in the extractive distillation of nitric acid, this disadvantage is offset by the lower capital and maintenance costs associated with the magnesium nitrate pro cess.

T

he

demand

for concentrated nitric a c i d

9 8 + % strength,

primarily

for use i n nitration, led to the use of sulfuric acid i n the extractive distillation

of nitric acid.

T h e m a x i m u m b o i l i n g azeotrope of 68 wt % n i t r i c a c i d prevents

distilling the 5 5 - 6 0 wt % nitric acid p r o d u c e d i n an a m m o n i a oxidation plant ( A O P ) to a strength greater than 68 wt % unless extractive distillation is used.

The Sulfuric Acid Process In the process using sulfuric a c i d {see F i g u r e 1) this a c i d was, a n d i n m a n y instances still is, added to the weak nitric a c i d produced b y a n A O P before the m i x e d a c i d was f e d to the top of a distillation c o l u m n . T h e feed has been preheated i n some processes to m i n i m i z e the vapor load i n the distillation c o l u m n . E n o u g h sulfuric a c i d was added to the feed so that the vapor leaving the top of the c o l u m n was at least 98% nitric acid. L i v e steam was added to the base of the c o l u m n to provide the heat for the c o l u m n and the stripping vapor r e q u i r e d to 143

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

144

THERMODYNAMIC

BEHAVIOR

O F ELECTROLYTES

Cooling H 0 2

98% HN0

3

2% H 0

Feed Acid 55-60% HN0 _ + 93% H S0

2

Trace H S0

Condenser

3

2

2

4

4

HNO3 STILL

Downloaded by GEORGE MASON UNIV on March 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch010

Cooling HoO Live Steam Sparge

65-68% H S0 2

H S0

4

2

Cooling H 0 2

2

2

2

Concentration 93% H S0

H S0

Condenser \ Weak H S0 & HNO3

4

4

in H 0 2

4

4

Cooler

Figure

1.

HNO3 concentration

using sulfuric

acid

m i n i m i z e the nitric acid content of the sulfuric acid leaving the base of the distillation c o l u m n . A n y nitric a c i d leaving the base of the c o l u m n represented a loss of desirable product. Most of the sulfuric acid and water, i n c l u d i n g the live steam, that was fed to the distillation c o l u m n left the base of the c o l u m n as 65-68 wt % sulfuric acid. This sulfuric acid was then concentrated via v a c u u m or submerged combustion evaporation to a strength of about 93% sulfuric acid. T h e sulfuric acid was then cooled and recycled to make up the feed for the distillation column. A n y nitric acid present i n the bottoms f r o m the distillation c o l u m n was lost w i t h the water removed i n the sulfuric a c i d concentrator. A small amount of the sulfuric acid fed to the distillation c o l u m n was entrained w i t h the product 98 wt % nitric acid. T h e entrained sulfuric a c i d was not a p r o b l e m for most of the captive uses of the concentrated nitric a c i d i n nitration. H o w e v e r , the entrained sulfuric acid was a problem i n meeting some specifications of concentrated nitric acid for sale on the open market/

The Magnesium Nitrate Process In 1957 Hercules Inc. started the first unit that produced concentrated nitric a c i d for c o m m e r c i a l sales using magnesium nitrate as the extractive agent. I n this process (see F i g u r e 2) the weak nitric a c i d product f r o m an A O P is fed to the appropriate tray of a distillation c o l u m n . A concentrated solution of m a g nesium nitrate and water is fed to the proper tray i n sufficient quantity to enrich the vapors to a concentration greater than 68 wt % nitric a c i d . T h e overhead product f r o m the c o l u m n is concentrated (98-99.5 wt %) nitric acid. A portion of the concentrated nitric acid is returned as reflux to a i d i n rectification. T h e


10.

VAILLANCOURT

Extractive Distillation

of Nitric

Acid

145


bulk of the water entering w i t h the A O P feed a c i d is r e m o v e d f r o m the base of the column i n the 65-68 wt % magnesium nitrate solution. This quantity of water is then r e m o v e d i n the magnesium nitrate evaporator w h i c h operates under v a c u u m . T h e concentrated (72 wt %) m a g n e s i u m nitrate solution p r o d u c e d i n the magnesium nitrate evaporator is returned to the distillation c o l u m n . N o stripping steam is used i n the c o l u m n since water evaporated i n the c o l u m n reboiler serves as stripping vapor. T h e overhead product f r o m the c o l u m n is a high purity distillate free of sulfates and very low i n metal content, chlorides, and other impurities. Cooling H 0 2

98 % HNO3 2"% Feed Acid 55-60% HN0

HoO

3

Cooling H 0 2

Steam

H0 2

Column Reboiler

1% HNO3

Mg|N0 ) 3

Figure 2.

Vapor-Liquid

2

HNO3 concentration

Evaporator Reboiler

using magnesium

nitrate

Equilibrium

T h e magnesium nitrate, as well as other nitrate salts, enhances the volatility of the nitric acid while suppressing the volatility of water. This effect is similar to that achieved w i t h sulfuric acid. C o n t r a r y to most c o m m e r c i a l extractive distillations i n w h i c h the extractive agent enhances the volatility of the more volatile component i n the feed, extractive agents for nitric acid distillation must enhance the volatility of the less volatile component i n the feed (nitric acid). M a g n e s i u m nitrate was selected f r o m a m o n g several nitrate salts since it h a d a large, favorable effect on the relative volatility of nitric a c i d as w e l l as for its thermal stability a n d good physical properties i n water solution. T h e effect of the magnesium nitrate on the v a p o r - l i q u i d e q u i l i b r i u m of nitric acid and water can be seen i n Figure 3. As the concentration of magnesium nitrate i n the l i q u i d increases, the volatility of n i t r i c a c i d also increases.



146

THERMODYNAMIC

0

10 20

30 40

BEHAVIOR

O F ELECTROLYTES

50 60 70 80 90 100

WEIGHT % HN0 IN LIQUID (TOTAL BASIS) 3

Figure 3. Vapor-liquid equilibria. shown are at constant wt % MgiNO^ liquid—on an acid-free basis.

Figure 4.

Weight fraction

Values in the

of H N O 3 in vapor vs. liquid



10.

VAILLANCOURT

Extractive

Distillation

of Nitric

147

Acid

Therefore, the vapors emitted f r o m the l i q u i d of a given composition are richer in n i t r i c a c i d as the concentration of the magnesium nitrate increases. T h o u g h this might suggest that the highest possible concentration of magnesium nitrate should be used, consideration of the distillation c o l u m n conditions shows that this is not the case. In a distillation c o l u m n , as the magnesium nitrate concentration is increased, the l i q u i d rate increases more rapidly than the vapor rate; therefore, the slope of the operating line ( l i q u i d rate/vapor rate) increases. Since the n u m b e r of theoretical stages i n a distillation c o l u m n is a f u n c t i o n of the degree of separation between the v a p o r - l i q u i d equilibrium curve and the operating line, as well as the steepness of the v a p o r - l i q u i d equilibrium curve, there is an o p t i m u m range for the magnesium nitrate concentration i n the l i q u i d . If the magnesium nitrate concentration is too low, the v a p o r - l i q u i d e q u i l i b r i u m curve is not steep enough, a n d too m a n y stages are r e q u i r e d to effect the distillation. If the m a g nesium nitrate concentration is too high, the separation between the v a p o r - l i q u i d e q u i l i b r i u m curve and the operating line is too small; therefore, too m a n y stages are required. T h e effect of a 1% increase i n the magnesium nitrate content of the l i q u i d i n the distillation c o l u m n is shown i n F i g u r e 4. Since the magnesium nitrate is not a volatile component, the weight fraction H N O 3 is plotted on a magnesium nitrate-free basis. T h e concentration of m a g n e s i u m nitrate i n the l i q u i d does not vary appreciably between the feed tray a n d the base of the stripping section. T h e design of a distillation c o l u m n for the concentration of nitric acid using magnesium nitrate as the extractive agent is rather specific to the strength of the feed a c i d produced i n the A O P . As the feed a c i d concentration changes, a different n u m b e r of theoretical stages is r e q u i r e d to produce the desired overhead and bottoms purity. In the i n i t i a l design the reflux ratio a n d the concentration of magnesium nitrate are adjusted to m i n i m i z e the number of theoretical stages in the c o l u m n . Since the cost of the e q u i p m e n t a n d the cost of energy both i n crease as the vapor rate i n the c o l u m n is increased, the o p t i m u m design is a compromise that must consider economics as w e l l as v a p o r - l i q u i d equilibria. Also some allowance must be m a d e for changes i n the feed a c i d concentration, as these changes do occur to some extent i n the operation of the A O P . T h e final design is one that tolerates changes i n feed composition, reflux rate, a n d m a g nesium nitrate concentration i n a c o l u m n containing a fixed number of theoretical stages.

Economics Magnesium nitrate has been used instead of sulfuric acid i n new commercial plants since it requires lower capital investment and has lower overall operating costs.

T h e lower capital investment results f r o m the use of metal equipment for

all the components as w e l l as f r o m a more compact plant layout.

T h e upper

portion of the distillation c o l u m n and the condenser and p i p i n g for handling the concentrated nitric acid are fabricated f r o m the same materials whether sulfuric


148

THERMODYNAMIC

BEHAVIOR

OF

ELECTROLYTES


a c i d or magnesium nitrate is used as the extractive agent. T h e savings i n e q u i p m e n t costs are realized i n the base of the c o l u m n , the c o l u m n reboiler, the extractive agent concentrator, and the piping that handles the hot extractive agent. W h e n magnesium nitrate is the extractive agent, a l l of this e q u i p m e n t can be fabricated f r o m conventional stainless steel. W h e n sulfuric a c i d is the extractive agent, a l l of this e q u i p m e n t must be fabricated f r o m l i n e d steel e q u i p m e n t that utilizes glass, teflon, or brick as the corrosion resistant barrier. T h e difference i n the cost of this portion of the plant results i n a sizeable capital saving w h e n magnesium nitrate is used as the extractive agent. Some savings i n structural steel and foundation costs are also realized f r o m reductions i n the e q u i p m e n t weights. A single stage vacuum evaporator can be used to remove the water f r o m the magnesium nitrate c o m p a r e d w i t h the two or more v a c u u m or submerged combustion stages r e q u i r e d for sulfuric a c i d . T h e more compact layout results i n reduction of costs for b u i l d i n g steel, foundations, a n d shorter runs of process a n d utility p i p i n g . T h e lower operating costs of the m a g n e s i u m n i t r a t e - n i t r i c a c i d - w a t e r extractive distillation process are caused p r i m a r i l y b y lower maintenance costs c o m p a r e d w i t h the sulfuric a c i d - n i t r i c a c i d - w a t e r extractive distillation. T h e hot magnesium nitrate solutions are less corrosive to the equipment than the hot sulfuric a c i d solutions despite the more exotic materials of construction used to handle the sulfuric acid. In a d d i t i o n , w h e n repairs or changes are r e q u i r e d i n the equipment and piping, they are easier and cheaper to make i n the magnesium nitrate process than i n the sulfuric acid process. W h e n magnesium nitrate is used, the stainless steel equipment requires no special skills and less advanced planning compared w i t h the lined equipment required w h e n sulfuric acid is used. F e w e r a n d less c o m p l i c a t e d e q u i p m e n t a n d p i p i n g spare parts are r e q u i r e d for m a g nesium nitrate c o m p a r e d to sulfuric acid. A n additional operating saving can be realized b y reusing a portion of or all of the water evaporated i n the m a g n e s i u m nitrate concentrator. T h i s water can generally be reused as absorption water i n the A O P or as wash water i n a nitration process. T h e sulfuric a c i d content of the water evaporated f r o m the sulfuric a c i d concentrators or the corrosivity of this stream generally negates its reuse i n other portions of the plant. This ability to reuse water when magnesium nitrate is the extractive agent generally results i n savings i n the waste treatment costs c o m p a r e d w i t h the use of sulfuric a c i d as the extractive agent. T h e use of magnesium nitrate as the extractive agent does, however, have some disadvantages i n comparison w i t h sulfuric acid. Since magnesium nitrate is a less efficient extractive agent than sulfuric acid, the distillation c o l u m n generally requires more theoretical stages and a slightly higher reflux ratio w h e n magnesium nitrate is used as the extractive agent. T h e higher reflux ratio for magnesium nitrate use means a higher steam consumption i n the c o l u m n reboiler. However, this is offset partially by a lower steam consumption i n the equipment used to evaporate water f r o m the extractive agent. T h e slightly higher steam consumption w h e n magnesium nitrate is used requires a correspondingly higher


10.

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Extractive

Distillation

of Nitric

Acid

149

electrical consumption i n operating the cooling tower if used, or higher consumption of electricity to p u m p cooling water if it is used o n a once-through basis.


Future Outlook Another disadvantage of using m a g n e s i u m nitrate instead of sulfuric a c i d would be the disposal of the evaporated water if all of it cannot be reused i n other processes. W h e n the evaporated water is neutralized prior to disposal, the magnesium nitrate process results i n nitrate salts, whereas the sulfuric acid process results i n sulfate salts being discharged. In the past, sulfate salts have been more acceptable i n plant effluents than nitrate salts. F u t u r e e n v i r o n m e n t a l considerations m a y prohibit the disposal of either of these salts, thereby negating this advantage of the use of sulfuric acid. In the recent past new c o m m e r c i a l facilities have been b u i l t using magnes i u m nitrate i n extractive distillation to produce concentrated n i t r i c a c i d f r o m A O P acid. T o the author's knowledge no new c o m m e r c i a l facilities have been built using sulfuric acid as the extractive agent for concentrating A O P acid. N e w facilities generally use sulfuric a c i d to concentrate spent nitration acids only if they already contain sulfuric acid.

Conclusions T h e preference for m a g n e s i u m nitrate i n c o m m e r c i a l operations indicates that the advantages of using m a g n e s i u m nitrate o u t w e i g h the disadvantages. That is, the lower investment a n d maintenance costs inherent i n the use of magnesium nitrate more than offset the increased steam consumption, even i n areas of the w o r l d where energy costs have been h i g h historically. R E C E I V E D June 23, 1975.


Thermodynamic Behavior of Electrolytes in Mixed Solvents

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