Thermodynamic Behavior of Electrolytes in Mixed Solvents

9 The Extractive Distillation Process for Nitric Acid Concentration Using Magnesium Nitrate

Downloaded by PURDUE UNIVERSITY on June 9, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch009

J. G. SLOAN Imperial Chemical Industries Ltd., Organics Division, Stevenston, Ayrshire, Scotland, KA20, 3 LN

Enhancement of relative volatility in the nitric acid-water sys tem by the presence of magnesium nitrate as dissolved salt component makes possible an economic and reliable process for making high strength nitric acid. The process has a con tinuous extractive distillation stage producing 90% HNO vapor which is further rectified to nearly 100% concentration. Diluted magnesium nitrate solution from the still base is read ily reconcentrated to the preferred feed strength of 72% Mg(NO ) by vacuum evaporation. Steam provides process heat supply for the distillation and evaporation sections, some 2.5 parts being needed to concentrate one part HNO from 60% to 99.5% concentration. Extended commercial opera tion has confirmed that this is a robust and satisfactory pro cess. 3

3

2

3

T

hough there are c o m m e r c i a l processes for the p r o d u c t i o n of h i g h strength n i t r i c a c i d (98-100 w t % HNO3) directly f r o m a m m o n i a using oxygen, the b u l k of such a c i d is m a d e f r o m the weaker aqueous solutions, ca. 60 w t % H N O 3 , w h i c h are p r o d u c e d b y conventional air oxidation plants. Since the nitric acid-water system has an azeotrope of 68.2 w t % H N O 3 at 760 m m H g , concentration of weaker acids b y direct distillation is not possible. F o r m a n y years, w h e n a m m o n i a oxidation plants operated at atmospheric pressure a n d produced acid containing 45-55 w t % H N O 3 , the h i g h strength product required by the explosives and dyestuffs industries was made b y extractive distillation with sulfuric a c i d as the t h i r d component. T h e a m m o n i a oxidation plant was thus associated w i t h a sulfuric acid concentration plant i n w h i c h essentially the water present i n the original aqueous nitric acid was removed, using such commercial 128

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

9.

SLOAN

Extractive

Distillation

129

Process

a c i d concentrating units as the P a u l i n g Pot, G a i l l a r d tower, d r u m concentrator, or M a n t i u s concentrator.

Such processes still f i n d c o m m e r c i a l acceptance.

As a m m o n i a oxidation technology developed, it became possible to produce aqueous weak nitric a c i d of super-azeotropic composition, t y p i c a l l y 6 9 - 7 3 w t % HNO3.

Such acid is theoretically distillable to higher concentration i n one step.

At the same time, acid of azeotropic composition is returned to the plant absorbers; the higher the weak a c i d concentration, the lower is the recycle of azeotrope. Extractive distillation processes are still w i d e l y used for n i t r i c a c i d concentration.

Because the operational and maintenance problems associated w i t h


sulfuric a c i d concentration plants are considerable, and their capital cost substantial, attention has been directed p e r i o d i c a l l y to the use of extractive agents other than sulfuric a c i d .

Phosphoric a c i d (I) acts like sulfuric a c i d but poses

similar problems of reconcentration.

Solutions of certain m e t a l l i c salts, i n par-

ticular metallic nitrates, p e r m i t similar enhancement of relative volatility a n d are readily reconcentrated i n straightforward evaporation e q u i p m e n t , o f f e r i n g the possibility of a compact integrated concentration process. Extractive D i s t i l l a t i o n of N i t r i c A c i d i n the Presence of M e t a l Salts.

The

nitrates of magnesium (2,3, 4), l i t h i u m (4,5), potassium, c a l c i u m (3, 4), sodium, a l u m i n u m , iron (3), b a r i u m (4), and zinc (6) have been studied for this purpose, and i n general terms it has been f o u n d that: 1. Potassium, s o d i u m a n d b a r i u m nitrate produce very little alteration i n the relative volatility of nitric a c i d - w a t e r . 2. Magnesium, zinc and l i t h i u m nitrates have a m u c h greater effect on the equilibrium. 3. C a l c i u m nitrate has less effect, at a given concentration, than magnesium, zinc, or l i t h i u m but, b e i n g more soluble, it can have greater overall effect. Table I summarizes the effect of such nitrates on the azeotropic composition i n the nitric a c i d - w a t e r system. A d d i t i o n of potassium nitrate increases the azeotropic composition, but other nitrates decrease it, the a m o u n t b y w h i c h the azeotrope is displaced b e i n g proportional to the amount of nitrate added.

The

azeotrope is e l i m i n a t e d completely at salt concentrations of 45, 48, 54, or 64% b y weight for a d d i t i o n of magnesium, zinc, l i t h i u m , or c a l c i u m nitrates respectively. F o r a continuous extractive distillation process to be possible there must be adequate enhancement of the nitric a c i d - w a t e r relative volatility, and a system e q u i l i b r i u m w h i c h permits v i r t u a l l y complete separation of n i t r i c a c i d f r o m m a g n e s i u m nitrate, the latter t a k i n g u p the water content of the weak a c i d feedstock.

This requires a d d i t i o n to the weak n i t r i c a c i d of solutions of m a g -

nesium nitrate usually containing 60 w t % or m o r e of M g ( N O s ) 2 .

U n d e r these

conditions a nitric a c i d - w a t e r relative volatility of greater than 2.0 is obtained at the l o w end of the l i q u i d phase concentration at a n i t r i c a c i d mole fraction below 0.05 {4, 7).


THERMODYNAMIC

Table I.


Weight of metal nitrate in liquid phase

BEHAVIOR

O F ELECTROLYTES

E f f e c t o f Nitrates o n A z e o t r o p i c Wt % ofHN0

in the azeotrope

3

KN0

NaN0

0 10 20 30

68 75 78 81

68 67 63 57

40

85

45

3

Al(N0 )

Fe(N0 ) 3

3

3 3

3

68 61 52 Other azeotropes appear

68 60

50 60 70

48

I80H

frlHVPRATE

I40r-

60

Figure 1

Magnesium

nitrate-water

70

dO

solubility

—I

dO

l _

IOO

diagram


9.

SLOAN

Extractive

Distillation

131

Process

Composition in H N 0 — H 0 3

in the presence LiN0


of:

Ca(N0 )

3

2

3

ZnN0

2

Mg(N0 )

3

3 2

68 58 47 37

68 60 52 42

68 52 45 30

68 58 44 32

25

32

10

14

8 0

20 8 0

0

0

M a g n e s i u m Nitrate Solutions. nesium n i t r a t e - w a t e r system.

A n u m b e r of hydrates exist i n the m a g -

F i g u r e 1 is a solubility d i a g r a m showing, for i n -

stance, a hexahydrate melting at 8 9 . 9 ° C and a dihydrate melting at 130.9°C w i t h intervening eutectic mixtures (8).

Progressive thermal dehydration b y m e l t i n g

a n d evaporation of water forms lower hydrates but at temperatures f a r above 1 2 0 ° C , a n d if the t i m e of heating is prolonged, hydrolysis takes place w i t h loss of nitrogen oxides a n d the f o r m a t i o n of basic compounds. Aqueous solutions of m a g n e s i u m nitrate are a p p r e c i a b l y denser a n d more viscous than water.

T a b l e II illustrates data (9) o n the densities (in g/ml) of

concentrated solutions at h i g h temperatures. F i g u r e 2 illustrates the viscosity variations i n concentrated solutions (9). F r e e z i n g points of aqueous solutions m a y be obtained f r o m the solubility d i a g r a m , F i g u r e 1. T h e b o i l i n g point at 760 m m H g is shown i n F i g u r e 3. It w i l l be seen f r o m these graphs that at solution concentrations above 70 w t % Mg(NC>3)2, the freezing point rises rapidly (more rapidly than the b o i l i n g point) a n d the viscosity rises r a p i d l y also.

Table II.

F o r ease of h a n d l i n g therefore, solution

V i s c o s i t y V a r i a t i o n s i n C o n c e n t r a t e d S o l u t i o n s (9) % by weight

Temp (°C) 100 120 140 150

MgfNO,)^

60

62

64

66

68

70

72

1.564 1.553

1.588 1.575 1.564

1.612 1.601 1.588

1.636 1.624 1.612

1.660 1.648 1.637 1.631

1.684 1.672 1.662 1.657

1.708 1.696 1.686 1.680

Zhurnal Priktal K h i m i e



132

THERMODYNAMIC

SO

55

Figure 2.

GO

BEHAVIOR O F E L E C T R O L Y T E S

65

Viscosity of magnesium

7o

"75

nitrate

solutions

concentrations used i n the extractive distillation process do not n o r m a l l y exceed 72-74 w t % M g ( N 0 ) . 3

2

T h e specific heat of solid anhydrous M g ( N O s ) 2 m a y be calculated f r o m the equation: Cp (cal/g mole) = 10.68 + 71.2 X 1 0 " T + 1.79 X 3

MftT~

2

( T i n ° K ) (JO) T h e equivalent values i n cal/g are f o u n d i n T a b l e III.

Table III.

Specific Heat of A n h y d r o u s M g ( N 0 )

Temperature 25 100 120 140 160 180 200

3

(°C)

C

p

2

(Cal/g) 0.229 0.260 0.269 0.278 0.287 0.296 0.304


Extractive Distillation

Process


SLOAN

Figure

4.

Integral heat of solution

of magnesium

nitrate


134

THERMODYNAMIC

BEHAVIOR

OF ELECTROLYTES

T h e h y d r a t i o n of anhydrous m a g n e s i u m nitrate evolves heat, 25,730 cal/g mole M g ( N 0 ) 3

2

Mg(N0 ) • 6H 0 3

2

2

(II).

L i k e w i s e , the dissolution of

M g ( N 0 ) or the hydrates i n water or the a d d i t i o n of further water to these so3

2

lutions also evolves heat ( 1 2 , 1 3 , 1 4 , 1 5 ) .

F i g u r e 4 illustrates the molar integral

heat of solution of M g ( N 0 ) , the value for i n f i n i t e d i l u t i o n b e i n g 21,575 cal/g 3

mole.

2

F r o m these figures, the enthalpies of magnesium nitrate solutions m a y

be computed.


The Ternary System Nitric Acid-Water-Magnesium

Nitrate

T h e displacement of the azeotropic composition b y progressive a d d i t i o n of magnesium nitrate has been shown i n Table I above. have been determined (3, 4).

V a p o r - l i q u i d equilibria

F i g u r e 5 depicts e q u i l i b r i u m vapor compositions

i n the ternary system at the b o i l i n g point, w h i l e F i g u r e 6 shows b o i l i n g points i n the system at 760 m m H g .


9.

Extractive

SLOAN

Distillation

135

Process


HNOj

m

20 A *

0 5 1

/ \ / \ \ /\ V/\ J / /\ >

< ae

Figure

6.

Magnesium

nitrate-nitric

-ro

acid-water

boiling points (760

mm)

Since i n an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains i n the l i q u i d phase, a n d since because of the s i m i l a r i t y of the molar latent heats of n i t r i c a c i d a n d water there is substantially constant molar l i q u i d overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process.

It is appropriate to represent

the e q u i l i b r i u m situation as a pseudo-binary system for each magnesium nitrate concentration, and F i g u r e 7 shows v a p o r - l i q u i d e q u i l i b r i a on a nitric a c i d - w a t e r basis at a series of m a g n e s i u m nitrate concentrations f r o m zero to 0.25 mole fraction i n the l i q u i d phase. F i g u r e 5 shows that w h e n nitric acid solutions containing 5 0 - 6 0 w t % H N O 3 are m i x e d w i t h m a g n e s i u m nitrate solutions c o n t a i n i n g 6 0 - 7 0 w t % Mg(NOa)2, the e q u i l i b r i u m vapor composition at the b o i l i n g point does not exceed 8 5 - 9 0 wt% H N O 3 .

Thus, to achieve concentrations higher than this the process must

p r o v i d e for rectification of the top vapor. T h e r m a l data for the ternary system have not been widely reported, but may be evaluated as required for process calculations f r o m available data for the nitric a c i d - w a t e r a n d m a g n e s i u m n i t r a t e - w a t e r b i n a r y systems.


136

THERMODYNAMIC

BEHAVIOR

O F ELECTROLYTES


lo -

.|

O

MOLE

Figure

7.

-a FRACTION

-3 H N O ^ tN

System magnesium equilibrium,

Process Conditions for Extractive

-5

-4l-lQPtfc>

(

~ Ha.O

-C

-7

BINARY)

nitrate-nitric acid-water, pseudo binary basis

liquid-vapor

Distillation

Consider the concentration of 60 w t % n i t r i c a c i d b y extractive distillation w i t h 7 0 - 7 5 w t % solutions of magnesium nitrate. It m a y be seen f r o m F i g u r e 5 that the vapor composition above b o i l i n g solutions is at the level 8 5 - 9 0 w t % H N O 3 over a wide range of mixtures, f r o m 3.5 parts of magnesium nitrate solution


9.

Extractive Distillation

SLOAN

137

Process

per part of weak nitric a c i d u p to about 8 parts.

In the particular case of 5 parts

of 72 w t % Mg(NC>3)2 solution a n d 1 part of 60 w t % n i t r i c a c i d , the m i x t u r e has the composition 60 w t % M g ( N 0 ) , 10 w t % H N 0 , a n d 30 w t % H 0 w i t h an 3

2

3

e q u i l i b r i u m vapor composition of 88 w t % H N 0

2

at the b o i l i n g point.

3

A rec-

t i f y i n g section of the c o l u m n gives a top product of nearly 100 w t % H N 0 , w h i l e 3

the nitric a c i d content at the base of the distillation c o l u m n m a y be taken as zero. A suitable reflux ratio m a y be d e t e r m i n e d b y trial a n d error.

F o r the case of a

3:1 ratio w i t h the heat requirement supplied b y an external reboiler, the reboiler outlet l i q u o r composition is 66.6 w t % M g ( N 0 ) . 3

2

A s s u m i n g constant m o l a r

overflow, the vapor rate is 1.15 parts per part of H N 0 3


3

distilled, m a k i n g the

column base composition 59 w t % M g ( N 0 ) . This molar composition (0.15 mole 2

fraction) applies throughout the s t r i p p i n g section of the c o l u m n , a n d the appropriate e q u i l i b r i u m curve is selected f r o m F i g u r e 7.

G r a p h i c a l methods (16,

17) m a y be used to calculate the n u m b e r of theoretical plates r e q u i r e d for the separation. A variety of feed compositions a n d reflux ratios m a y be thus e x a m i n e d , preferably b y c a r r y i n g out detailed plate-to-plate e q u i l i b r i u m calculations w i t h check heat balances, as the t h e r m a l effects are substantial, a i m e d at o p t i m i z i n g the reflux ratio (representing operating cost) against the n u m b e r of theoretical plates (representing capital cost).

In particular terms, I C I has f o u n d that feed

ratios between 4:1 a n d 7:1 (parts of magnesium nitrate solution per part of weak nitric a c i d feed) are possible, w i t h reflux ratios i n the range 2:1 to 4:1.

The

theoretical plate requirement for the complete c o l u m n is between 15 a n d 20. W i t h i n this range the process w i l l concentrate 60 w t % H N 0

3

to 99.5 w t % using

72 w t % M g ( N 0 ) as extractive agent a n d d e n i t r a t i n g it to less than 0.1 w t % 3

2

HNO . s

H e a t R e q u i r e m e n t of the Process.

H e a t is r e q u i r e d for v a p o r i z a t i o n i n

the extractive distillation c o l u m n , a n d for the reconcentration of m a g n e s i u m nitrate solution. Overall thermal effects caused b y the magnesium nitrate cancel out, a n d the heat d e m a n d for the complete process depends on the amount of water b e i n g removed, the reflux ratio e m p l o y e d , a n d the t e r m i n a l (condenser) conditions i n distillation a n d evaporation.

T h e composition a n d temperature

of the m i x e d feed to the still influence the relative heat demands of the evaporation a n d distillation sections. wt% H N 0

3

F o r the concentration of 60 w t % H N 0

3

to 99.5

using a still reflux ratio of 3:1, a still pressure of 760 m m H g , a n d an

evaporator pressure of 100 m m H g , the theoretical overall heat re q u i re me nt is 1,034 k c a l / k g H N 0 . 3

I d ' s C o m m e r c i a l Process.

In 1960 I C I constructed a concentration plant

using this extractive distillation process (18) w i t h a capacity of 16,000 tonnes/ a n n u m of product acid (99.5 w t % H N 0 ) w h i c h has subsequently been extended. 3

A flowsheet is g i v e n i n F i g u r e 8, a n d the process description is as follows. W e a k nitric a c i d (normally 60 w t % H N 0 ) a n d concentrated m a g n e s i u m 3

nitrate solution (72 w t % M g ( N 0 ) ) enter at the feed point of a n extractive dis3

2

tillation c o l u m n . T h e rectifying section above the feed point has a water-cooled



NITRATE EVAPORATOR

z

Figure 8.

Flowsheet

of the concentration

GO°/a MACNBtttUM

p REBOILER

CONCENTRATED MAQNCS1UM NITRATE TANK

S T

COLUMN

STRIPPING

u

FEEb

RECTIFYING COLUMN

C O O L E R

HITRIC ACID

0>f

nitrate

process

NITRIC ACID STORAQC

3 3 5 % NITRIC ACID

REFLUX

•

NITRIC ACID

.

Thermodynamic Behavior of Electrolytes in Mixed Solvents

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