370
D. W. VAN KREVELEN AND C. M. E, BAANS
ELIMINATION OF CARBON MONOXIDE FROM SYNTHESIS GASES BY ABSORPTION I N CUPROUS SALT SOLUTIONS1 D. W. VAN KREVELEN A N D C. M. E. BAANS Central Laboratory of the Netherlands State Mines, Geleen, Holland Received January 16, 1 9 4 INTRODUCTION
Almost complete elimination of carbon monoxide from synthesis gas is of paramount importance for nitrogen fixation plants. Especially in factories operating on a basis of semi water gas, ammoniacal solutions of copper salts are used for the elimination of the residual quantities of carbon monoxide which remain in the gas after catalytic conversion with steam. Many investigations on this absorption process have been performed and many valuable experimental data obtained. However, until now the quantitative laws of equilibria and the velocity of the carbon monoxide absorption process have not been described in the literature and only suppositions have been made as to the mechanism of the reaction and the constitution of the reaction complex. In this paper we are communicating the results of a detailed investigation of the following subjects: ( 1 ) the composition of the reaction complex formed by the absorption of carbon monoxide in ammoniacal cuprous salt solutions, (2) the equilibria between carbon monoxide and cuprous salts in concentrated ammoniacal solutions, and (3) the rate of absorption of carbon monoxide in these solutions. Nomenclature: a = specific packing surface (sq.m./cu. m.) A total cuprous salt concentration B = total free ammonia concentration before carbon monoxide absorption c = dimensionless constant C = concentration of reaction product, Cu(NHs)rCO+ e , = ionic concentration C.,, C;, = apparent equilibrium constants (Cu) = concentration of Cu(NH8): d, diameter of scrubber packings D = diffusivity g = acceleration due t o gravity H = Henry’s law coefficient I ionic strength of the solution (= +Zc,z:) 1
-
-
k
= mass transfer coefficient
K , K’ = real equilibrium constants m = gram-moles of monovalent copper per gram-mole of carbon monoxide absorbed nA = differential rate of absorption (kg. moles/(sq. m.)(sec.) (atm.)) N A = experimental overall rate of absorption (same dimension) p = partial pressure p,, = partial carbon monoxide pressure in atmospheres Pco = carbon monoxide pressure in millimeters of mercury u = superficial liquid velocity (meters per second) u superficial gas velocity (meters per second)
-
ABSORPTION O F CARBON MOKOXIDE BY CUPROUS SALTS
371
I. PREPARATION AiVD AKALYSIS O F T H E REACTION PRODUCT I N AMMOKIbCAL CUPROUS SALT SOLUTIONS
Earlier investigations on the composition of the carbon monoxide-cuprous salt complex In 1908 Manchot and Kewton Friend (5) stated that in acid, neutral, and very weakly ammoniacal cuprous chloride solutions the following complex compound is formed after absorption of carbon monoxide: CuCl. 2H20. CO They supposed, however. that in concentrated ammoniacal solutions quite different reaction products are formed. The varying behavior of ammoniacal solutions has been neglected by other investigators. For example, Moser and Hanika (6) assumed the existence of the complex CuCl ' 2 H 2 0 . C 0 both in acid and in ammoniacal solutions of cuprous chloride (the complex CuCI.2HzO. CO does exist). Brueckner and Groebner (2) supposed that in ammoniacal solutions the compound Cu[(NH3),CO]C1is formed, but its existence has not been proved. 4 s cuprous chloride also dissolves in some ammonium salts and the solution thus obtained can absorb carbon monoxide, Brueckner and Groebner assumed that in those neutral solutions compounds of the type Cu[(NH4Cl)4CO]C1 are formed. I t is obvious that, for a quantitative treatment of equilibria and reaction rate of the carbon monoxide absorption, an exact knowledge of the mechanism of the absorption reaction is necessary. As the available literature did not afford sufficient data, we decided to perform supplementary experimental work. z , = ionic valency
activity coefficient of compound i overall activity coefficient viscosity = density
y, = yt = fi =
p
ko(RT)/Co iL = kL/cL
P =
General subscripts
G = gas phase L = liquid phase i = interface gas-liquid
Dimensionless groups Reo = Reynolds number of gas flow ReL = Reynolds number of liquid flow Sc = Schmidt number
(=);
(= 2)
372
D. W. VAK KREVELES AND C . M. E. B U N S
Preparation of the carbon monoxide complex An ammoniacal solution of ammonium chloride containing about 220 g. of ammonia per liter was saturated with cuprous chloride a t 70°C. The solution was heated at 80°C. with finely divided copper in order to be sure of complete reduction to the cuprous salt. Then it was cooled to room temperature with continuous stirring. A crystalline precipitate was formed, consisting of colorless needles of Cu(NH&Cl. The mother liquid contained (in gram-moles per liter): CuC1,2.77; NHdCl, 0.78; SH3,11.44. In the dry state the compound Cu(NH&CI is fairly stable. Concentrated ammonia water was saturated with this salt at 60°C., and 50 ml. of the resulting solution was transferred to a Pyrex absorption vessel (see figure 1); immediately the absorption was started by bubbling pure carbon monoxide through the solution. This gas was saturated beforehand with ammonia by passing it through a vessel containing concentrated ammonia water. b
FIG.1. Pyrex absorption vessel
As the solution cooled the compound Cu(NH3)&I was precipitated, but the quantity of precipitate diminished after cooling to room temperature, owing to its reaction with carbon monoxide. Thereupon the vessel was cooled by ice water and for some hours carbon monoxide was introduced a t such a rate that solution and crystals were just kept in motion. Meanwhile the solution attained a state of supersaturation and suddenly a voluminous mass of colorless, more or less cubic crystals appeared. The introduction of carbon monoxide was continued for an hour after the last needles of Cu(NH3)&1 had disappeared, in order to complete the reaction. Analysis of the carbon monoxide complex The analysis of the reaction product obtained caused great difficulties, as the compound proved to be extremely unstable. I t was impossible to dry the crystals without causing considerable decomposition, and it was necessary to analyze the compound in situ. The following method proved to be satisfactory.
373
ABSORPTIOS OF C A R B O S MOSOXIDE BY CUPROUS SALTS
Vessel B (figure 21, filled with crystals and mother liquor, was connected t o a weighed receiver (C) containing dilute sulfuric acid. A calibrated cylinder (H) was connected to C. By means of nitrogen (eia a) a large part of the mother liquor was displaced from B into C and the carbon monoxide which was formed was collected in the cylinder H. The stopcock c was closed, the gas volume and pressure in H were measured, and C was weighed again. The copper and ammonia in the acid solution were determined. An example of an analysis of the mother liquor was as follows: gram-moles
gram-moles
C u . .. . . . . . . . . . . . . . . . . . . . . . . . C1. . . . . . . . . . . . . . . . . . . . . . . . .
0.0624 0.0622
NHo. . . . . . . . . . . . . . . . . . . . . . . . . .
co. ..........................
0.2380 0.0502
6
FIG.2. Apparatus
for the analysis of the copper-ammonia-carbon
monoxide complex
Receiver C was emptied, connected with B, and evacuated. The crystals in B were mashed three times with about 30 ml. of concentrated ethyl alcohol (from A) which had been cooled to - 10°C. Thereupon vessel B lvas weighed rapidly and connected with the absorption vessels D and El and El. Sitrogen was introduced into B (via a ) , and B was heated slowly. The gases formed were swept out by the nitrogen; r a t e r was absorbed in D (filled with calcium oxide and heated by boiling water); alcohol and ammonia were absorbed in vessels E1 and El (filled xith concentrated sulfuric acid and cooled by ice), and the residual gases were collected in the holder F and analyzed. The residue in B m s weighed and analyzed.
374
D. W. VAN KREVELEN AND C. M. E. BAANS
The results of three experiments were as follows:
I cu.. . . . . . . . . . . . . . . . . . .
c1. . . . . . . . . . . . . . . . . . . . ~
NHa . . . . . . . . . . . . . . . . . .
co . . . . . . . . . . . . . . . . . . .
DRAM-MOLES PEP DRAM-AIOM OF COPPEP
1.00 0.95 2.83 0.71
1.00 0.93 2.95 0.87 1.27
HsO . . . . . . . . . . . . . . . . . .
1.00 1.01 2.98 1.04 1.05
In fact the ratio Cu:Cl must be 1.0. From quantitative absorption experiments the maximum absorption capacity of ammoniacal cuprous chloride has been proved to be 1 mole of carbon monoxide per gram-atom of copper. Considering these facts, together with the above analysis of the prepared complex, it can be stated that the formula of the copper-ammonia-carbon monoxide complex must be CU[(NH~)~CO]CI.H~O.
Conclusion Since Bjerrum (1) has proved that cuprous salts in an ammoniacal medium are always present in the form Cu(NH3); as long as the free ammonia concentration is greater than 0.001, we may conclude that the mechanism of the carbon monoxide absorption in ammoniacal solutions can be represented by the equation: Cu(NH8):
+ CO + NH3 = Cu(NH3)&Ot
In the next section we shall show that the experimental data of the absorption equilibrium can be calculated quantitatively from the proposed mechanism. 11. EQUILIBRIA BETWEEN CARBON MONOXIDE AND CUPROUS SALTS I N
CONCENTRATED AMMONIACAL SOLUTIONS
Earlier investigations Measurement of the absorption equilibrium of carbon monoxide in ammoniacal cuprous salt solutions has been the subject of several investigations. Hainsworth and Titus (3) and Larson and Teitsworth (4)measured carbon monoxide pressures of solutions of copper carbonate and copper formate. Zhavoronkow and Reshikow (9) also investigated solutions of chlorides, acetates, and lactates, whereas Zhavoronkow and Chagunawa (10) worked with solutions containing formates as well as carbonates. Notwithstanding the large number of valuable experimental data thus obtained, there is still a lack of fundamental knowledge correlating the influence of the operating variables (such as copper and ammonia concentration, nature of the negative ions, degree of saturation of the solution with carbon monoxide, etc.) on the partial carbon monoxide pressure of the solution. Moreover, we suspect inaccuracies in some of the data from the literature. This is the reason why the authors have performed a large number of new experimental measurements, on which their conclusions will be based. Afterwards the derived rela-
ABSORPTIOX OF CARBON MONOXIDE BY CUPROUS SALTS
375
tionship will be applied to the data from the literature as an additional confirmation of its correctness. The apparatus used for our measurements was reproduced from that of Hainsworth and Larson, so that for a description of it we may refer to the literature.
Derivat;on of the equilibrium equation for the carbon monoxide pressure We have demonstrated that the mechanism of the carbon monoxide absorption is the following: Cu(KH3):
+ NH3 + CO = Cu[(NH3)3CO]+
When equilibrium is attained the law of mass action can be applied:
where the y's are the activity coefficients of the individual reaction components. Writing for the factor ?'Cu(NHalaCO* ?'Cu(NHa):
= Yf
?"HI Y C O
and equating according to Henry's law [CO] = Hpco, the following equation is obtained :
Kow defining -4 = total cuprous salt concentration B = total free ammonia concentration before carbon monoxide absorption l / m = moles carbon monoxide absorbed per mole cuprous salt, we get: = A/m [CU(NH~)~CO+]
NH3 = B
- A/m
=
mB - A m ~
Substitution in equation 2 gives:
This equation gives the partial carbon monoxide pressure as a function of the composition of the solution. K and H are functions of temperature, y t will principally be a function of the ionic strength of the solution. The negative ion will only have an influence upon yt. From the experiments the values of C, will be determined as a function of the operating variables.
376
D . W. VAN KREVELEN AKD C. M. E. BAAKS
Application of equation 3 to the authors’ experiments The compositions of the solutions for which equilibrium measurements were performed are given in table 1. A survey of the experimental data can be found in table 2, where also the calculated C,,-values are given. I t is obvious that the C,,-values a t constant temperature are approximately constant ; this proves the correctness of the postulated mechanism. I n figure 3 the logarithmic values of C., (for the series BI, thus maintaining constant ionic strength) are plotted as a function of the reciprocal absolute temperature. The points lie on a straight line, from which an apparent activation energy of 11,900 cal./mole may be calculated. In figure 4 the C,,-values, reduced to 293”K., are plotted on a logarithmic scale against the ionic strength ( I = + & z : , where c, and z i are the concentration and the valency of the ion i ) for the whole series of experimental data. In the investigated concentration range TABLE 1 Composition of the solutions in gram-moles per liter SOLUIIONS USED
COYPONKNIS
B9
Bl
c u + . ........................
cu*. . . . . . . . . . . . . . . . . . . . . . . .
c1- . . . . . . . . . . . . . . . . . . . . . . . . .
B10
1
Ell
0.88 0.93 0.94 0.01 0 . 0 2 0 . 0 2 1.12 1.71
0.59 0.01 1.71
HCOO-. .................... NHa ( t o t a l ) . . . . . . . . . . . . . . . . . 8.25 NHa (free complex) . . . . . . . 7.15 NHa (free). . . . . . . . . . . . . . . . . . 5.93
5.78 5.57 3.77
+
a linear relationship appears to exist. Only the C,-value for the formate solution is higher. All the data for the chlorides can be represented by the equation: log,,
c,
=
11’900 - 0.0401 - 8.790 2.3RT ~
(4)
Table 3 gives the calculated and experimental values of C,.
Application of equation 3 to Larson’s data for copper formate 80htiOnS Our equation will now be checked with data from the literature, viz., for absorption equilibria in copper formate solutions. Table 4 shows the composition of Larson’s solutions and the C,-values calculated from his equilibrium measurements. This series of C,-values can be satisfactorily correlated by the equation : log10
c,, = 2.3RT ~13’500 - 0.0401 - 9.830
In figures 5-8 Larson’s data are plotted and are compared with the values predicted according to equation 5 (the drawn curves). The agreement proves to
TABLE 2 Experimental data
0.164 0.044 0.017
B1
51. . . . . . . . .
I
I
10
2 3 4 5
0.768 0.344 0.272 0.147 0.082
1 2 3 4 5
0.351 0.203 0.096 0.090 0.051
1
0.738 0.543 0.292 0.227 0.085
1
1 I
I
91.. . . . . . .. I
I
20
I I
1
'I 1
1
~
1
1
0 815 0 561 0 297 0 913 0 826 0 780 0 663 0 533 0 0 0 0 0
666 563 403 392 250
I
30
2 3 4 5 1
22.8
I
B6.
: I
1 2
0.085 0.179 0.110
1 2 3
0.060
1
0.113 0.099 0.108
22.3
B5
. . . . . . ..'
B7 . . . . . . . . . ~
23.2
~
I
1 2
0.158 0.183 0.116
I
1
0.209
1
2 3
0.199 0.168
1 2 3
0.838 0.841 0.836
1 2 3
0.853 0.833 0.826
1 22.7
I
I B10
21.7 1 i i
i B11
..
2 3
0.056 0.062
i
22.7
B9
1
0 175 0 152 0 154
1
0 200 0.309 0.232
I
20.4
I B8
0.133 0.101 0.111
2 3
1
0 712 0 595 0 537 0 427
I
22.0
377
1 1
, 1
'
I 1
11 '
1
0.306 0.282 0.314 0.286 0.265 0.268 0 323 0 351 0 247 0 386 0 376 0 354 0 757 0 757 0 754 0 784
0 765 0 777
!'
2 51 2 37 2 37
1.
2 44
378
D. W. VAN KREVELEN AND C. M. E . BAANl3
--.--.-&-01
30 31
12 13 3 4 3 5 36 37
-1
01
FIG.3 FIG. 4 FIG.3. Plot of the logarithmic values of C., as a function of the reciprocal absolute temperature. FIG.4. Plot of C.,-values against the ionic strength. 0 , chloride solutions; 0 , formate solutions. TABLE 3 Calculated and experimental values of C,, I
T
1.
1.76
4.. . . . . . . . . , , . ,
2.59 2.47 1.52 2.41 1.76 1.16 1.73
273 283 293 303 296 295 293 296
SERIES
(CALCUUTED) ca,
'K.
6..............
6.. . . , . . . . . . . . , 7,, . . , . . . . . . . . . 8,, . . . . . . . . , . . . 9., , , . . . . . . . . . . 10.. , . , . , . . . . , , .
4.7 2.2 1.05 0.53 0.79 0.85 1.06 0.78
4.8 2.4 1.15 0.58 0.81 0.81 1.04 0.75 0.86 0.89 0.98
TABLE 4 Composition of the solutions used bg Larson and Teitsworth i n gram-moles per liter
I
C., (293'K.) . . . . . . . . . . . , , . . . . . . . . . . . . C,. C,.
(313'K.). . . . . . . . . , . , . . , . . . . . . . . . . (333'K.) . . . . . . . . , . . . . . . . . . , . . . . . .
Lz
1.31 0.29 0.08
w
L4
110
(0.84)
0.99
0.80
0
0
0
3.20 7.17 (4.81) (3.13)
3.20 5.87 3.66 1.68
1.37 4.47 3.90 2.30
7.5 1.17 0.29 0.09
6.0 1.17 0.27
7.9 0.10
ABSORPTION OF CARBON MONOXIDE BY CUPROUS SALTS
379
be very good, Another illustration of the correlation of calculated and experimental Cep-valuesis given in table 5 . (This table also contains the C,-values of our own experimental series B11 when the formate solutions were used.)
D
0.81
t
FIG.5 FIG.6 FIG.5. Cu+, 0.84 gram-mole per liter; free NHs, 2.64 gram-moles per liter; solution L2 Fro. 6. Cu+, 0.84 gram-mole per liter; free NHs, 3.13 gram-moles per liter; solution L3
0.4 0,2
0
0.2 0.4
0.6
QB
FIG.7
FIG.8 FIG.7. Cu+, 0.99 gram-mole per liter; free KHa, 1.68 gram-moles per liter; solution L4 FIG.8 . Cu+, 0.80 gram-mole per liter; free S H s , 2.30 gram-moles per liter; solution L10
Significance of the formula of Zhavoronkow and Chagunawa Zhavoronkow and Chagunawa derived a very simple relation between the carbon monoxide pressure and the quantity of carbon monoxide which is absorbed. Their formula is: - 1=
vco
1 -+v m
1 V,aPco
where Vco = milliliters of carbon monoxide absorbed per milliliter of solution V , = maximum absorption, milliliters of carbon monoxide per milliliter of solution
380
D. W. VAh? KREVELEN AKD C. M. E . BAANS
P c o = carbon monoxide pressure in millimeters of mercury 01 = constant depending upon temperature. As in our nomenclature Vco = R T A / m and 8, = R T A , substitution gives 1 1 -1 m= 1 or 01 = (7) CUPCO (m 1)Pco 760(m 1)pCo
+
-
~
-
According to our formula 3 we have
c,
=
m (m - l)(mB - A)pco
so that equation 7 can only be valid if m/(mB - A ) is constant. The latter is only true as long as B (free ammonia concentration) is large and A (copper concenTABLE 5 Illustration of correlation of calculated and experimental C.,-values SERIES
i L2.. . . . . . . . . . . . .
L3
L4,
L 1 0 . .. . . . . . . . . . .
B l l . ,. . . . . . . . . . .
9K
3.04
3.20
3.20
7.3 1.32 0.30 0.08
273 293 313 333
7.1 0.29 0 08
273 293
1.37 1.74
273 293 313 333
,
313 273 333
Ii
295
1
7.1 1.29 0.29
~
'
1 I
8.3 0.09
'I
1.25
i
1.31 0.29 0.08 7.5 1.17 0.29 0.09 6.0 1.17 0.27
7.9 0.10
1.16
tration) is small. For these solutions the curves l / p c o = f ( m ) become straight lines. As the solution becomes saturated with carbon monoxide (small m-values) the lines will bend off to the point l / p c o = 0; m = 1. Figures 9 to 11 prove that the experimental data are in complete agreement with theory . 111. RATE OF ABSORPTION OF CARBOX MONOXIDE I N CONCENTRATED AMMOXIACAL CUPROUS SALT SOLUTIOSS
Description of the apparatus The absorption experiments were performed with mixtures of nitrogen and carbon monoxide. The latter was stored in a small gas holder from which it was
ABSORPTION OF C.4RBOPi MOKOXIDE BY CUPROUS SALTS
381
m
t 6.
5.
4
0
2
4
6
E
10 12
14
16
10
FIG. 9
FIG. 10 FIG.9. Cu+, 0.59 gram-mole per liter; free NHa, 5.93 gram-moles per liter; solution B1 FIG. 10. Cu+,0.84 gram-mole per liter; free XHs, 2.64 gram-moles per liter; solution L2
FIG.11. Cu',
0.99 gram-mole per liter; free XH3, 1 6 8 gram-moles per liter; solution L4
transferred t o the apparatus by means of water pressure. It \vas thoroughly mixed x i t h pure nitrogen After sampling, the mixture flowed through a scrubber n-here it was washed by means of copper solution. The exit gases were sampled,
382
D. W. VAN KREVELEN AND C . M. E. BAANS
freed from stripped ammonia, and passed through a gas meter. Gas and liquid velocities mere measured in calibrated flow meters. Temperatures in the absorber were measured a t three points, viz., a t the entrance and the exit of the liquid and somewhere in the middle of the column. Entering and exit gas and liquid were carefully analyzed.
4
5
2
1
14
FIQ.12. Flow sheet of the apparatus for determination of the rate of abaorption 1 = vessel containing carbon monoxide 2 = vessel containing alkaline water (under pressure) 3 = air or nitrogen (30 cm. of mercury overpressure) 4 = cylinder containing pure nitrogen 5 = safety 6 = flow meters for gases 7 = absorption liquid 8 = flow meter for liquid 9 = mixing column 10 = sampling flask for entering gas mixture 11 = absorption column 12 = sampling flask for exit gas 13 = bubbler containing dilute sulfuric acid (for ammonia removal) 14 = measuring cylinder 15 gas meter
-
The absorber was a glass tube of height 40 cm. and 30 mm. internal diameter, filled with 5.5-mm. glass Raschig rings. Figure 12 shows a flow sheet of the apparatus. The composition of the solutions used is given in table 6 , while table 7 shows the data of the absorption experiments.
383
ABSORPTIOS O F CARUON AlOh'OXIDE BY CUPROUS SALTS
Discusszon of results .Is we have demonstrated, the mechanism of the carbon monoxide absorption in these concentrated ammoniacal solutions can be formulated as follows:
Cu(SH3):
+ SHs + CO
=
Cu(NH3)3CO*
It is well known that thia reaction proceeds extremely fast. As the physical sohbility of carbon monoxide is very small, it seems probable that the reaction zone TABLE 6 Composztion of washing liquids
65 68 71 74 75 77 79 80 87 89 95 103 106 111 112
1
0 69 0 715 o 45 1 0 69 o 43 0 68 o -14 1 39 I 1 435 0 4351 0 715' 0 67 1 16 0 70 1 005 1 77 1 1 445
1
'
0 01 1 0 01 o 00 0 01 o 01 0 00 o 01 1 0 01 1 0 01 0 00 0 02 0 00 0 02 0 00 0 00 0 02 0 02
0.69 0.50 0.42 0.51 0.40 0.49 0.61 2.16 2.13 0.52 1.10 0.78 1 225 0 69 1.02 2.27 1.54 1.30 1.035
~
0.245 0.276 0.078 0.326 0.113 0.319 0.145 0,062 0.120 0.115 0.000 0.077 0.008 0 180 0.108 0.065 0.153 0.153 0.228
, ~
9.28 7 38' 0 47 4 09 0 31 5.87 5.02 400 013 8.09 6 221 0 45 7.80 6 72 0 18 8.10 629 045 11.23 9 861 0 44 8.64 4 95 0 87 2 40 0 91 6.22 2.13 0 95' 0 31 15.84 13.97' 0 34 4.25 I 2.661 0:26 12 4 i 9 671 0 04 8 15 6 40 0 35 11.03 8 78 0 24 11.31 7 09 0 59 9.33 5 6 9 ~0 67 10.72 7 6ZI 0 13 10.25 7 63 0 36
1
1007 1027 1003 1016 986 1015 973 1079 1100 1028 977 1036 1016 1013 1014 1087 1075 1050 1034
1
1,365 1.27 1.22 1.34 1.27 1.33 1.30 1.26 1.16 1.04 1.51 1.20 1.44 1.37 1.49 1,486 1.39 1.50 1.46
x
10-3 1.44 1.34 0.66 1.50 0.75 1.45 1.06 2.36 2.50 0.86 1.12 1.01 1.45 1.23 1.35 2.46 2.32 1.79 1.73
(formate) 1
198
1
1
0.935 0.01
1.72
o
O.Oo0
7.00
4 30
1.71
0.000
7.12
4.42 0.75
1.17
1.73
1 1029 1 1.17
1.73
74
(chloride)
1041
n.ill coincide with the phase boundary and that the rate of the absorption process will be determined by the mass transfer of reactants and reaction products. Taking this assumption as a working hypothesis the schedule of the absorption mechanism given in figure 13 can be made. Then for a thin cross-section of the scrubber, the rate of absorption, expressed as kilogram-moles per square meter of contact surface per second, can be written as follows: nA 1
=
kO(P0
For nomenclature see footnote 1
- PJ
=
k d C , - C')
(8)'
384
D. W. VAN KREVELEK AND C. M. E. BAANS
. 2
9 1
.. . .. .. ..
A,,
. . . . . . . .. .. ... .. ... .. . .. .. . . .. .. . . .. . .. . .. . .. . . . . . . . . .. . . . . . . . .. . i d cd t-m 6 2 A, m d a m h 3 - 3 M - 3 2
.
.
2
l B d O R P T I O S OF' CARBOS MOSOXIDE BY CUPROUS SALTS
385
386
D. W. VAN KREVELEN AND C. M. E. BAANS
At the interface equilibrium exists, so that
Moreover, the rate of supply of free copper to the reaction zone will be equal to the rate of transfer of the carbon monoxide complex from the reaction zone, thus:
ci - c,
= (CU),
-
(CU)i
From equation 8 we derive:
(CUI< = (CU),
ir
!
- 12.4 k,
concantration
imWhC*.l rscWon zona
FIG.13. Scheme for the absorption mechanism
and by combination of equations 8 and 10:
ci =
(CUI,
+ CL - (CUIi = CL + IC,, n A
Substitution of these expressions in equation 9 gives:
or
* The diffusion coefficients (and therefore the mass transfer coefficients) of Cu(iYH3): and [Cu(NHa)aCO]+are assumed to be equal, v i e . , 1.0 X 10-8 sq. m./sec. (determined by the authors). ' As the ammonia is present in large excess, it is assumed that (XHs), equals (iYHa)m (the logarithmic mean ("a)).
ABSORPTIOX O F C A R B O S MOSOXIDE BY CUPROUS SALTS
387
In order to apply this equation to the whole scrubber we, have to use the overall yalue of n A (= N A , the experimentally determined rate of absorption) and average values for the other variables. Defining: (Ceq(XH3)mpom
+ 1)dV~
=
P
Ce,Q(XH3)mAV; = Q
-C
Ceq((CU)~("a)po
L ) ~=
R
C,,(Cu)~(SH~),Sn= S where the index m indicates the logarithmic mean value, n e get by substitution: k,
=
Pkc-Q ___ R ko - s
As kc and k , are functions of the superficial gas and liquid velocities, vie use the dimensionless equations (see 7 and 8):
So we get CL
=
Cop' COR'
Q'
- S'
where
*
p'
=
p-RT
Q'
=
Q
S' = S $
Equation 12 can be solved by trial and error. In table 7 the values of co and C L are giyen for each experiment. For the whole series of experiments they prove to be fairly constant. The average values of c0 and c L are 0.029 and 0.017, respectively. The correctness of our working hypothesis, cit., that the rate of absorption of carbon monoxide in concentrated ammon acal solutions is only determined by mass transfer processes, can be proved now by independent determinations constants cG and c L .
388
D. W. VAN KREVELEN AXD C. M . E. BAANS
Independent debrminations of cG and c L The value of cG has been determined in the same apparatus by experiments on water evaporation into a current of air (at 20°C.). In figure 14 the experimental data are plotted as a function of the above-ment oned dimensionless groups. From these data a cG-valueof 0.026 can be calculated. The value of c L has been determined from experiments on carbon dioxide absorption in water at 20°C., also in the same apparatus. From the data, which are reproduced graphically in figure 15, a c,-value of 0.014 can be calculated. Th e jact that these independently determined eo- and cL-values are about the same as those found j r o m the carbon monoxide absorption experiments i s a proof of the correctness of our assumptions.
-
-
ReG
ReL
SUMRl.\RY
A method is described by which the reaction complex of cuprous chloride and carbon monoxide in ammoniacal solutions has been prepared as a crystalline product. Its composition was proved t o be CU[(?\TH~)~CO]CI~H,O. This compound is very unstable in the crystalline state and decomposes at room temperature with loss of carbon monoxide and ammonia. It could be stated that the absorption mechanism is the following: Cu(SH3):
+ CO + S H B
=
Cu(SHS)aCO'
Using this equation the authors have derived formulae from vhich the carbon monoxide equilibrium pressure can be calculated as a function of the temperature, the copper, ammonia, and carbon monoxide content, and the ionic strength of the solution. The correlation with the authors' experimental data and with data of Larson and Teitsworth proves to be very satisfactory. The reaction between carbon monoxide and cuprous salts in a concentrated ammoniacal solution is immeasurably rapid. The rate of mass transfer of the
.4BSORPTlOK O F CARBOS YOSOXIDE BY CUPROUS S.4LTS
389
reaction product (the carbon monoxide-copper complex) is controlling for the absorption velocity. The values found for the mass transfer coefficients in this complex absorption are the same as those found for simple physical absorption and desorption in the same apparatus. The rate of absorption of carbon monoxide in neutral and Jveaklj- ammoniacal solutions proves to be much smaller than in concentrated ammoniacal solutions. I n these cases the chemical reaction velocity is partly rate controlling for the absorption process. REFEREXCES (1) BJERRUM, J . : Kgl. DanPke Videnskrrb. Selskab, Mat.-fys. Medd. 11(5) (1931), ll(10) (1932), 12(15) (1934). (2) BRUECHNER, H., .4ND GROEBKER; W . : Gas-u. Wasserfach 78, 269-i3 (1935). (3) HAIXSWORTH, W.R . , A X D TITUS, E. Y . : J . .4m. Chem. Soc. 43, 1-11 (1921). (4) LARSOS,A. R . , A N D TEITSWORTH, C. S.: J. Am. Chem. SOC.44, 2878-85 (1922). ( 5 ) MASCHOT,W., ASD FRIESD,J . SEIYTOK: Ann. 369, 100-28 (1908). (6) MOSER,L . , .4SD HAKIKA.F . : Z . anal. Chem. 67,448-56 (1925/26). (7) V a s KREYELES,D . R., A N D HOFTIJZER, P.J.: Rec. trav. chim. 66, 49-65 (1947). (8) VANKREYELEN, D . W . , HOFTIJZER, P.J., . ~ K DV A N HOOREN,C. J . : Rec. trav. chim. 66, 513-32 (1947). (9) ZHAVORONKOW, K. h l . , ASD RESHIKOW, P. h I . : J . Chem. Ind. (U.S.S.R.) 10, 41-9 (1933). (10) ZH.4VORONKOW, s.hI., A S D CHAGUNAIY.4, V. T . : J. Chem. Ind. (U.S.S.R.) 17, 25-9 (1940). APPESDIX
Rate o j absorption o j carbon monoxidc in neutral and weakly animoniacal copper solufions Only preliminary experiments have been performed for the cases when the cuprous solution ( a ) is about neutral (not containing free ammonia) or ( b ) contains ammonia combined with carbon dioxide. Case a : \Vith a solution containing 1.77 gram-moles of cuprous chloride and 6.85 gram-moles of ammonium chloride per liter an overall absorption coefficient
Kc =
An
( P - PeQuIl)
= 1.4 X
kg.-moles/(atm.) (see.) (sq. m.)
has been measured. This value is five to ten times smaller than corresponding Ko-values for ammoniacal copper solutions, so that the reaction:
CO
+ Cu(SHdC1):
=
Cu[(SH,Cl)&O]+
is not of the type of immeasurably rapid reactions. From the dimensionless Lo-equation a kc-value of 2 X lo-' for the conditions in the experiment can be calculated, so that the gas film resistance only amounts t o about 10 per cent of the total absorption resistance. The major resistance must be sought in the reaction velocity in the liquid film.
390
D. W. VAN KREVELEN AND C. M. E. BAANS
Case b: In this case the ammonia is buffered by the equilibria
+ H20 = NH3 + HCO; + COT- = SHa + HCOY
NH2COO-
XH:
so that during the absorption the consumption of free ammonia by the reaction Cu(NH3):
+ SHg + CO = Cu(n"3)aCO'
is opposed by ammonia formation due to the above-mentioned shift reactions. Also in this case the rate of carbon monoxide absorption is considerably less than for concentrated ammoniacal solutions. For a solution containing (in gram-moles per liter) : CUI
1.67
i
CU"
0.16
~
~~
HCOO-
2.35
FIXED
cos
1.84
1
TOTALNHI
8.19
we found the following absorption coefficient:
Ko =
N.4
(P -
=
2.1 X IO-' kg.-moles/(atm.)(sec.)(sq. m.)
Posuid
Obviously in this case also a chemical reaction velocity is the rate-controlling factor. It is possible that the velocity of the shift reactions is rather slow, so that during the reaction the liquid becomes neutral and the mechanism changes. Both case (a) and case (b) will have to be the subjects of further study.
