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The influence of carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions
Chemical EngineeringScience, 1964, Vol. 19, pp. 95-103.Pmon
PrcssLtd., Oxford.Printedin Gmt Britain.
The influence of carbonation ratio and total amine concentration on carbon dioxide absorption in aqneous monoethanolamine solutions G. A~TARITA, G. MARRUCCI
and F. GIOIA
Istituto di Chimica Industriale dell’Universit8 di Napoli, Via Mezzocann one 16, Napoli, Italy (Received 4 April 1963 ; in revised fortn 25 August 1963) Abstract-The influence of total amhe concentration, CO,and of carbonation ratio, a, on the process of COS absorption in aqueous MEA solutions is considered. Analysis of the chemical reactions taking place in the liquid elucidates the pattem of reactions leading to the thermodynamically favoured products. Experiments confirm the reaction mechanism proposed, and the process considered is shown to be: when b:< 05, fast second order chemical absorption, with a stoichiometric coefficient of 2 molen lvIEA per mole COS; when Q > 0.5, slow pseudo-first-order chemical absorption with an overall kinetic constant depending on the OH- concentration.
The second reaction, formation of /3-amino ethyl carbonic acid, would take place only in a basic solution where the pH was 11 or greater; as the pH of even a slightly carbonated MEA solution is less than 10, the formation of j?-amino ethyl carbonate can be neglected. Carbon dioxide may undergo three reactions in an aqueous MEA solution:
CARBON
dioxide ’ absorption in aqueous monoethanolamine solutions has been studied by several workers [l-131: the reactions taking place in the liquid are substantially known [ld], and the reliability of classica1 chemical absorption theory for the investigation of this system has been proved [5-1 l] ; some sets of data are available concerning performance of industrial absorbers [12, 13, see also 211. Much work has been done in recent time whose results are so far unpublished [14-181. This paper reports the conclusions reached from the analysis of the above-said work, and presents a complete discussion of the chemical reactions involved. The main experimental contribution consists of data on absorption rates in highly carbonated solutions.
Monoethanolamine, HOC,H,NH2, has two functional groups, -NH2 and - OH, either of which could react with CO,: NHCOO-H+,
substituted
-0H +COz-’ carbonic acid.
- OCOO-H+,
substituted
(a)
CO2 + OH-
-) HCO;
W
CO2 + HzO
-) H&Os
(c)
The relative importante of reactions (a) to (c) depends on their rate constants, and the concentrations of RNH2 and OH-, which depend on the total amine concentration a, and on the carbonation ratio a. Reaction (c) is pseudo-fìrst-order, as the concentration of water may be wnsidered wnstant. The concentrations at equilibrium of the species present (RNH,, RNH:, RNHCOO-, HCO;,CO;, H+, OH-) have been calculated from published [ 1, 19, ZO]equilibrium constants as functions of the parameters a, and a [4], with the simplifying hypothesis that the wncentration of free COz in the solution is negligible, and that the equilibria are not altered at the ionic strength encountered. These calculations show that :
CHEMISTRYOF THEPROCESS
-NH,+CO,-,carbamic acid
CO, + RNH2 -) RNHCOOH (R = HOC,Hb-)
95
G. ASTARITA, G. MARRUCCI and F. GIOIA
6) when a < 0.5, the main carbonated product is monoethanolammonium carbamate, RNHCOO-RNH;, (ii) when a = 0.5, further COZ absorption causes reversion to the bicarbonate, RNH: HC03, (iii) carbonate is formed only when a is very small, a condition not encountered in practice. The concentration of carbonate, which never exceeds 1.6 x lom3 gmol/l, may thus be ignored. Although at high ionic strength, such as may be encountered in practicc, the equilibrium constants may have somewhat different values, the statements (i) to (iii) may stil1 be considered correct. Moreover, the ionic strength of an amine solution is rather low, because of the low dissociation constant of amine, and the low concentration of the only bivalent ion, CO;. In the pH range encountered in this system, MEA carbamic acid RNHCOOH is almost completely dissociated, and it is neutralized by MEA to form monoethanolammonium and MEA-carbamate ions. When COa is absorbed in a solution where a < 0.5, the following sequence of reactions takes place : CO2 + RNH, RNH2 + Hf
When a > 0.5, the equilibrium concentrations, to a first approximation, of RNHCOO- and HCO; are given by [4]: (RNHCOO-) (HCO;)
Hence one mole of COZ combining causes the disappearance of one mole of carbamate and the formation of two moles of bicarbonate, according to the following overall reaction: RNHCOO-
+ RNH:
+ CO2 + 2Hz0 + + RNH: + 2HC03
(e)
The question is, what mechanism gives rise to reaction (e)? It is obvious that RNHCOO- must be converted to bicarbonate: FAURHOLT [l] shows that the conversion of carbamate to bicarbonate takes place through reversion of carbamate to free amine and COZ, followed by the reaction of carbon dioxide with hydroxyl ions : RNHCOO- + H+ + RNH, + COZ, fast CO2 + OH- --PHCO;, slow HzO + H+ + OH-
+ RNHCOO- + H+ fast, although not instantaneous (a) + RNHj virtually instantaneous
COZ + 2RNHz + RNHCOO-
= a,(l-a) = a,(2a-1)
RNHCOO-
(b)
+ HzO -P RNH2 + HCO;
The overall reaction (e) is therefore composed of the following steps :
(d)
1. COz + OH- + HCO;, slow; absorbed reacts with OH- and lowers pH,’
The stoichiometric coefficient of the overall reaction (d) is thus 2 moles amine per mole CO* ; this has been confirmed experimentally by means of data from laminar jet absorbers [5,10,11]. It should be remembered that it is the overall stoichiometric coefficient, and not the coefficient of the slowest step, that is to be used in the penetration theory equations for second-order chemical absorption. In industrial practice, carbonation ratios greater than 0.5 are often encountered [21], and it is therefore important to consider what happens when CO, is absorbed in highly carbonated (c( > 0.5) solutions. As has been stated above, further CO2 absorption results in bicarbonate formation when a > 0.5, as this is the only way in which tl can increase beyond 0.5 and the stoichiometric coefficient become less than 2.
COZ (b)
2. RNHCOO- + H+ + RNH2 + COt, encouraged by pH lowering, 3. RNH2 + H+ -+ RNH:, formation.
encouraged by RNH,
4. COZ + OH- + HCO;, slow; the CO2 liberated by 2. (b) 5. 2Hz0 -+ 2H+ + 20H-, correct value.
to maintain K, at its -
add: RNHCOO+ 2HCO;
+ CO2 + 2H,O + RNH:
+ (e)
The slow step (b) has to be realised twice, because two bicarbonate ions have to be formed; the two RNH: ions required for neutralization are: (i) the 96
Carbonation ratio and total amine concentration on carbon dioxide absorption in aquecus monoethanolaminesolutions one originally neutralizing RNHCOO- and (ii) the one formed by the dissociation of the amine liberated by the carbamate reversion. An alternative mechanism for bicarbonate formation is reaction (c), COz f HzO + HCO; + + H+, and the rate of this reaction becomes of the same order of magnitude as that of reaction (b) when the pH falls to about 9, which occurs when a > 0.5. Thus, when a > 0.5, reaction (c) takes part in the overall mechanism as fellows: 1. 2. 3. 4.
CO2 + HzO -+ HCO; + H+, slow, RNHCOO- + H+ + RNH, + CO, RNH2 + H+ -+ RNH: COz + HzO + HCO; + H+, slow,
add: RNHCOO2HCO;
+ 2Hz0 + CO2 + RNH:
(c)
(i)
Stirred tank data
Runs lasting 3 hr were performed, with the absorption rate V measured at 5 min intervals; at the end of each run, the equipment was shut off until the next day. With c1> 0.5, the absorption rate was almost constant over a 3-hr run. A typical V VSa curve is given in Fig. 1. The data for a < 0.5 wil1 first be considered. As long as a < 0.5, a second-order reaction mechanism is involved, and three possible regimes are to be considered : (a) If(RNH,)2c& B 1, có being the physical solubility of COz, the absorption rate V is given by :
where ki is the physical absorption coefficient, and S is the interface area.
+ (e)
(b) If 1 4 (RNH,)/&
Recent work [22] on the catalysis of reaction (c) by arsenite ions suggests the possibility of increasing carbonation rates in highly carbonated amine solutions.
+ ,/[k,(RNH,)t’]
t’ being the average life of surface elements, and k,, the kinetic constant of reaction (a), V = kgS(RNHJ2 (c) If
,/[kU(RNH,)t’]
EXPERIMENTAL
(2)
Q (RNH,)/2có
V = @ka(RNH,)]Scó
Experiments of carbon dioxide absorption in MEA solutions have been carried out on five different absorbers, namely : (i) a batch absorber, consisting of a magnetically stirred tank containing 500 cm3 of liquid. (ii) a 3.0 cm o.d. stainless steel wetted wal1 column. (iii) a 36 spheres ‘(3.7 cm o.d.) string-of-balls absorber. (iv) a 60 cm packed height column, 8-0 cm i.d., packed with + in. Rachig rings. (v) a 390 cm packed height column, same as (iv). Details of the experimental technique are available [ 14-181, as well as tabulated data. Only typical sets of data and fìnal cross-plots wil1 be given in the following. The gas phase was in every case pure carbon dioxide saturated with water vapour ; the temperature, 18°C; the pressure, 1 atm; absorption rates were determined volumetrically. The packed columns were fed concurrently. Liquid solutions were prepared at least 24 hr before starting the runs, in order to ensure that the solutions were in equilibrium.
(3)
where D is the diffusivity of CO2 in the liquid. As is known from classica1 chemical absorption theory, ki and t’ are related to each other by the equation : ki2 = Dit’
I
0.1
0
Slirrcd
tank
dolo
1 0.5 e
FIG. 1
97
(4)
I I
G. ASTARITA,G. MARRUCCI and F. GIOIA
where W is the liquid volume, c’ is the CO2 concentration which makes equilibrium to the actual HCO; and OH- concentrations, and k is a pseudofirst-order kinetic constant, related to the kinetic constants kb and k, of reactions (b) and (c): k = k, + k&OH-)
as suggested by the model of statistically random surface renewal [23]. Equations (1,2 and 3) can be calculated explicitly if the values of kE and S are known, in as much as the value of k, is known [6]. For the absorber used, the value of S was estimated at 50 cm’, by assuming that the interface was a tone having the base equal to the cross section of the tank, and the height equal to the depth of the vortex formed at the normal stirring rate. This allows to plot equation (3), as done in Fig. 2. In the same figure the absorption rate data are plotted, the abscissa having been calculated from the tabulated data of Ref. [4]. The line of equation (2) has been plotted as the best line of slope 1.0 through the data in the region of interest: its position gives the value k;S = 0.80 cm31 sec. This value allows to plot equation (3), and has been used in subsequent calculations. The agreement between data and theory is quite satisfactory, thus confirming the reaction mechanism discussed. As literature data [5, 6, 10, 111 on absorption rate at a = 0 indicate the same agreement, it can be concluded that the chemical absorption process when a < 0.5 is by now quite well understood. When a > 0.5, the mechanism of reaction discussed would indicate a pseudo-ílrst-order slow chemical absorption process-where slow means that the actual absorption coefficient equals k;. If the bulk concentration, c, of unreacted CO1 stays constant, the absorption rate is given by:
Equation (6) shows that k > k,; literature data on k, values [24-261 are rather uncertain, but an order of magnitude of 0.02 sec-’ may be considered reliable. Hence, the value of k Wfor the stirred tank used is surely larger than 10.0 cmy/sec, as compared with 0.80 cm3/sec for k:S. Hence it may be assumed that equation (5) can be written as: V = k;S(có - c’)
k;S
(7)
Values of V at constant a are plotted in Fig. 3 VSa,, the horizontal line being the value of k&!G$,. The behaviour of the data suggests the validity of equation (7), and hints to the possibility that the values of (k&%, - V) should be proportional to the equilibrium concentration c’. The latter can be calculated either from back-pressure data [21], or from interpolation through the data of Ref. [4], being : c’ oc (HCO;)/(OH-)
(8)
Figure 4 is a plot of (k$C$, - V) VSboth the ratio (HCO;)/(OH-) and the equilibrium back-pressure. It can be seen that, though this correlation brings all the data on one correlating line, the slope of the same is about 0.25, instead of the expected value 4c
Stined
có - c’
l’ 7 k;S(có - c) = k W(c - c’) = 2.’
(6)
10-c
(5)
0 m=0.5625 D =0.6250
tank
I
I
0.4
I ooI gmol/C
kW
FIG. 3
98
\
1 5
q A
a ~0.6675
A 0
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions back-
io_s_
Y
prersure,
The upper limit is compatible with equation (6), if the values of k* (27-29) and of k, (24-26) taken from the literature are considered:
mm. HIJ
p’:
_,o_,
k = 0.02 + 8500 (OH-)
(12)
.
i
.“-s-/
and the concentration of hydroxyl ions is evaluated from the data of Ref. [4], which indicate for (OH-) values of the order of magnitude of 10e6 gion/l in the range of a, and a values of interest.
-lo-s
(iii) 10'
10' [HCO;l FIG.
10'
/[OH-]
4
of 1.0. The authors have at the moment no explanation to offer for this result, except the uncertainty on the value of k:S. (ii)
Wetted-wal1 column data
Data on carbon dioxide absorption in MEA solutions with a > 0.5 have been collected on a wetted wal1 column with liquid flowing on its outer surface. Colunm length could be adjusted, and 3 different lengths have been used, namely 20,40 and 80 cm. For all the runs performed, the measured absorption rate was almost equal to the rate of physical absorption. The latter had previously been determined experimentally, and compared wel1 with theoretical predictions for wetted-wal1 column physical absorption [ 161. This indicates that the pseudofirst-order kinetic constant k was in any case smaller than l/t’, t’ being given by: t’ = z/us
String-of-bah data
The string-of-balls absorber can be considered the ideal equipment for studying slow-Grist-order chemical absorption processes involving k values of the order of magnitude of 0.1 sec-‘. In fact, t’ values (diffusion time, equal to the time required by a surface element to travel along one sphere) for the typical table-tennis bal1 absorber are of the order of 0.1 sec, and therefore kt’ 4 1. At the same time, the total residence time tp over a large number of spheres (thirty-six for the absorber used in this work) is of the order of 10 sec, hence yiélding measurable values of ktp (order of magnitude of 1). The theoretical equations are well known [30], and can be used to evaluate ktp values from absorption rate data. Reference [30] indicates that, when the following two conditions are fulíìlled, N” being the N.T.U. for physical absorption : Nog1
1
String-ofPhysiml
10“
Z being the column height, and IC~the surface velocity of the liquid. In the conditions of our experiments, the highest value of t’ was: f >
00)
0.02 sec- l
-’
(11) 99
(14) 0
bolls dato obsorption /
3
t
The wetted wall data hence indicate that k was in any case less than 0.13 sec- ‘. As previously discussed, k is larger than 0.02 sec- 1; hence, the true value of k has been proved to lie, within two rather close limits:
(13)
k 4 N”/t,
(9)
t&, = 1.5 sec;
_
Lim
of physicol
saturotion,\l=
Lc;
>”
10-3
I 101
10
L,CC/min Fis.
5
I 10'
G. ASTARITA,G. MARRUCCI and F. GIOIA
the slow fnst order (kt’ á 1) chemical absorption equation is : v = L(1 + k$)(có - c’)
(15)
where L is the volumetric liquid flowrate. For the conditions of our experiments, both conditions (13) and (14) were always fulfilled. In fact, (13) implies that physical absorption should result in practically saturated exit liquid; this is coníìrmed by a set of data on physical absorption reported in Fig. 5. The group N”/tp can be calculated theoretically [30-321, is independent of liquid flowrate (both N” and tp are proportional to L-‘13) and has for the absorber used. the value 0517 sec- ‘. As it has been proved that k is less than 0.13 sec-l, condition (14) can be considered always fulfilled. Experiments have been carried out only with not too highly carbonated solutions, for which c’ is negligible, in order to avoid the difficulty discussed concerning the data of Fig. 4. Equation (15) can hence be written as: I’ = L(1 + kt&@
(16)
G being the solubility of COz in water, and Qibeing a correction factor which takes into account the solubility lowering due to the ionic strength of the solution. Following the indications of Ref. [33 and 41, the value of CPhas been calculated as: log@ = - 094(2aa,)
(17)
2aa,, being the ionic strength of the solution if the carbonate ion concentration is neglected. A typical set of chemical absorption data is given in Fig. 6. The same data are cross-plotted in Fig. 7 as suggested by equation (16). The straight line 103-
String-of-bolk data &,= 3.40, 0 =0.58 Series 4 of toble
I
FIG. 6
0.1 10
I
I
102
101
L, cc/min
FIG. 7
through the data is the best line of slape -213, as required by the fact that tp is proportional to Lm213. From the plot of Fig. 7, the value of k can easily be calculated, through the theoretical value of tp. Final results are reported in Table 1, (OH-) values having been taken from Ref. [4]. Table 1 Series
ao(gmol/l)
0
k(sec-1)
1 2 3 4
1.92 1.10 0.686 340
0.62 0.705 0.57 0.58
0.065 oaI4 0.115 0.084
(OH-) cBion/l) 8.0 5.5 31.0 7.8
x x x x
10-7 10-7 10-7 10-7
It can be seen that k values are of the expected order of magnitude, though somewhat higher than equation (12) would predict. (iv) and (v)
Packed column data
The data obtained with the two packed columns wil1 now be considered on the basis of the ASTARITA and BEEK model [30]. First, an order-of-magnitude analysis on the parameters of interest wil1 be made; as the value of the same depend on the liquid flowrate, the value L = 1 l/min will arbitrarily be chosen as‘ a characteristic value, for which al! the parameters are calculated. Original data on physical absorption are tabulated in Ref. [34]; they show that the H.T.U. for liquid-side mass transfer was 37 cm. Actual liquid phase residence times have not been measured; their value can be estimated [34] by means of empirical correlations [35] at 0.15 sec per cm of packed height. Equivalent diffusion times can be
100
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions 190 cm packed tower 60
cm,pocked
iowcr
00’1.72, Series
o ‘0.625 L of toble
dolo 2
physicol
lim
10
I
0.1
obsorption
01
L, L/min FIQ.
through equation (4) and empirical correlations (36) of effective interfacial areas at the order of magnitude of 0.03 sec. As only experiments with a > 05 have been performed, it may be stated that: (a), kt’ values are in every case largely smaller than 1, hence the process considered is a slow-Erst-order process ; (b), ktJN” values are in any case smaller than 1 -notice that both tp and N” decrease with increasing flowrate, their ratio being approximately constant at the characteristic value of 5.5 sec; therefore, case “B” of ASTARITA and BEEK’S model applies. On this basis, it can be seen that the 60 cm column (ktP < 1) is expected to give absorption rates ahnost equal to the corresponding rates of physical absorption, while the 390 cm column is expected to give absorption rates correlated by equation (16). Typical sets of directly measured V’ VS L data are given in Figs 8 and 9; Fig. 10 is a plot of the best correlating lines for al1 the series of runs perestimated
Thir set of dato is lim Of
I
10
I
1, limh FIG. 10
8
formed on the 390 cm col-. The values of V’ as chacorresponding to L = 1.0 l/min-assumed racteristic of the absorption rate at any given liquid composition-are reported in Table 2. Table 2. Series
Volumetric absorption rates atL = 1.Ol/min
Packed hei& (cm)
A B :
390 390 390 390
F G H
z 390 390
1 L
60 60
N
z
0 (gnf$l) 0900
o6oo
1.20 2.36 1.61 0.475 2.21 0.475 0.0 (water)
0.635 0.533 0.635 0.710 0.618 0.810 -
2.71 2.43 2.06 1.90 1.67 0.98
1.67 1.72 3.40 0.0 (water)
0.670 0.625 0.565 -
0.81 0.81 0+30 0.80
For the 60 cm column the rate of chemical absorption practically coincides with the rate of physical absorption. The data for the 390 cm column show the expected trend, the absorption rate decreasing with increasing a, and with increasing a, as does the OH- concentration (4). Values of k can again be estimated; of course, no value of k can be evaluated from the 60 cm column data. CONCLUSIONS
L, LI min
FIG.9
Figure 11 is a fìnal plot of k values calculated from the string-of-balls and the packed column data. In the same figure equation (12) is plotted. 101
G. ASTARITA, G. MAIWJCCIand F. GIO~
. String-of -bah o Packed tower
data data
10-1
k =0.13
11. When a > 0.5, pseudo-first-order slow chemical absorption takes place, the overall reaction being substantially reaction (e). The kinetic constant can be evaluated from equation (12), (OH-) values being calculated from the tabulated data of Ref. [4] or by direct pH measurement. Absorption rates decrease with increasing a, and with increasing a.
sec-1
.
.
cl
.
I
I
! 10-7
10-6 [OH-]
t . 10-5
, gmol /C
FIG. 11
The order-of-magnitude agreement is satisfactory ; scattering of the data and some discrepancy is probably justified by the complexity of the elaboration procedure, and by the unaccounted for influence of ionic strength on k. The authors fee1 that the agreement is good enough not to reject the reaction mechanism proposed, and advance the following two conclusions : 1. As long as CC< 0.5, second order fast chemical absorption takes place, the overall reaction being substantially reaction (4. A stoichiometric coefficient of 2 is recommended, the absorption equation being:
Acknowle&ements-The authors wish to acknowledge the contribution of several chemical engineering students, some of whom are by now working in industry, who have collaborated in the three yean during which the experiments referred to in tbis paper were carried out. Their names fellow: Messrs. A~vIoo1, BoNADIEs,FEDERICI, ErZO,ORIIM, PATIERNO,PIIRRO’ITI,SAP~NARA, SPAGNUOLO,TONCELLI. The thorough revision of the manuscript by Professor P. V. DANCKWERTS is acknowledged.
NOTATION (M: mole. L: length. T: time.) ao c c’ c’o zJ D k ki kE
Values of (RNHJ can be calculated from the tabulated data of Ref. [4], an approximate equation being :
I+ s t’ tP US
V V’ W Z 01 a,
(RNH,) = a,(l - 2a) Absorption rates increase with increasing ao and decreasing a, as long as viscosity effects do not influence the value of k;.
ML-3 Total amine concentration M L-3 Concentration of unreacted CO8 ML-3 Equilibrium value of c M L-8 CO3 physical solubility M L-3 Value of C’O in water L2 T-1 Carbon dioxide diífusivity T-’ Pseudo-first-order overall kinetic constant Kmetic constant of reaction i), units of concentration are gmol/l, of time are sec L T-1 Physical absorption ccefficient LST-1 Liquid flowrate N.T.U. in liquidside controlled mass transfer L3 Interface area T Diffusion time, as defined in diierent cases T Total liquid phase residence time L T-1 Surface velocity of liquid M T-1 Absorption rate L3 T-1 Volumetric absorption rate L3 Volume of liquid L Height of wetted-wall cohunn Carbonation ratio, mole COa/mole amine Correction factor = C’O/ CT
REFERENCES
JENSENM. B., JORGENSEN E. and FAUR~IOLT C., Acts Chem. Stand. 19.54 8 1137. JORGENSEN E. and FAURHOLT C., Acts Ckem. Stand. 1954 8 1141. JORGENSEN E., Acts Chem. Stand. 1956 10 147. AsïARlTA G., MARRUCCIG. and GIOIA F., Bollettino Classe Scienxe Fisiche Matematiche e Natura& Accademia Lincei, February 1963. ASTARITA G., Id. Ch. E. Thesis, Univ. Delaware, Newark 1960. G., Ckem. E&pg. Sci., 1961 16 202. &TARITA ASTARITA G., Chim. e Zn&str. 1960 42 849. ASTUUTA G., Rit. Sci. 1960 30 658. ASTARITA G., unpublished report 1962.
102
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions EMMER;R. E., PhD Dissertation, Univ, Delaware, Newark 1958. EMMERT R. E. and PI~~RD R. L., Amer. Inst. Chem. Engw. J. may 1962 8. CRYDERD. S. and MALO~Y J. O., Trans. Amer. Inst. Chem. Engrs. 194131827. !%xNEmwN A. L. and LEIBUSCH A. G., J. Appl. Chem. USSR 1946 19 869; ibid. 1950 23 1253. GIOLAF., Chem. Eng. Thesis, Univ. Naples, 1962. SPAGNUOLO A., Lab. Notebook, Istituto Chimica Industriale, Unlv. Naples, 1962. Izzo M., Lab. Notebook, Istituto Chimica Industriale, Univ. Naples, 1962. BONADIes V., Lab. Notebook, Istituto Chimica Industriale, Univ. Naples, 1962. FEDERICIG., Lab. Notebook, Istituto Chimica Indu@ale, Univ. Naples, 1962. BA= R. G. and PDTCHING G. D., J. Res. Nat. Bur. Sbnd. 195146 349. HOUGEN0. A. and WATSONK. M., ChemicalProcess %nciples, Vol. II. Wiley, New York, 1947. KOHLA. L. and RIESENFELD F. C., Gus Purifcation. McGraw-Hill, New York, 1960. ROBERTS D. and DANCKWFXTS P. V., Chem. Engng. Sci. 1962 17 961. DANCKWEXTS P. V., in HARTNI&S Recent Adwnces in Heat and Mass Tram+. McGraw-Hill, New York, 1961. PINSENTB. R. W., PMRSON L. and ROUGHTON F. J. W., Trans. Faraday. Sec. 1956 52 1512. MEDAE., Trans. Faroday Sec. 1956 52 1519. HIMMELBLAU P. M. and BABB A. L., Amer. Inst. Chem. Engrs. 1. 1958 4 143. FAURHOLT C. Lob. Lanabkor. Forstk. Aarsskr. 1 (1924). JENSENA. and FAURHOLT C., Acta Chem. Stand. 1952 6 385. NLJSINOR.A.T.O., PhD Dissertation, Delft 1958. A~TARITAG. and BEEKW. J., Chem. Engw. Sci. 1962 17 665. LYNNs., STaAAlXMEXER P. and KRAMERS H., Chem. Ehgng. Sci. 1953 4 63. A~TARITAG., Rit. Sci. 1961 31 ((II-A) 177. A~TARITAG. and MARRUCCIG., unpublished report, 1962. TONCELLI R., Chem. Eng. Thesis, Univ. Naples, 1961. DAVIDSON J. F., Trans. Inst. Chem. Engrs. 1959 37 131. SHUWAN H. L., ULWCH C. F., PROIJLXA. Z. and ZIMMERM ANJ. O., Amer. Inst. Chem. Engrs. J., 1955 1 253.
R&um&L*article traite de l’intluence de la concentration totale d’amine, ao, et du rapport de carbonation, a, sur l’absorption du COS dans des solutions aqueuses de MEA. L’analyse des rhctions Chiliques qui se déroulent dans le liquide permet d’hcider le mhnisme qui conduit auxproduitsfavorisés par la thermodynamique. Les exphiencea wntìrment ce mhnisme, selon lequel l’absorption est rapide, obeit &une loi du 2h ordre, avec un coe&ient stoichiometrique de 2 moles de MEA par mole de COs lorsquea c 0,5, tandis que la rhction est lente, du pseudo le’ ordre avecuneconstantecinhique globale d6pendant de la wncentration en OH- lorsque a > 0,s.
103
PrcssLtd., Oxford.Printedin Gmt Britain.
The influence of carbonation ratio and total amine concentration on carbon dioxide absorption in aqneous monoethanolamine solutions G. A~TARITA, G. MARRUCCI
and F. GIOIA
Istituto di Chimica Industriale dell’Universit8 di Napoli, Via Mezzocann one 16, Napoli, Italy (Received 4 April 1963 ; in revised fortn 25 August 1963) Abstract-The influence of total amhe concentration, CO,and of carbonation ratio, a, on the process of COS absorption in aqueous MEA solutions is considered. Analysis of the chemical reactions taking place in the liquid elucidates the pattem of reactions leading to the thermodynamically favoured products. Experiments confirm the reaction mechanism proposed, and the process considered is shown to be: when b:< 05, fast second order chemical absorption, with a stoichiometric coefficient of 2 molen lvIEA per mole COS; when Q > 0.5, slow pseudo-first-order chemical absorption with an overall kinetic constant depending on the OH- concentration.
The second reaction, formation of /3-amino ethyl carbonic acid, would take place only in a basic solution where the pH was 11 or greater; as the pH of even a slightly carbonated MEA solution is less than 10, the formation of j?-amino ethyl carbonate can be neglected. Carbon dioxide may undergo three reactions in an aqueous MEA solution:
CARBON
dioxide ’ absorption in aqueous monoethanolamine solutions has been studied by several workers [l-131: the reactions taking place in the liquid are substantially known [ld], and the reliability of classica1 chemical absorption theory for the investigation of this system has been proved [5-1 l] ; some sets of data are available concerning performance of industrial absorbers [12, 13, see also 211. Much work has been done in recent time whose results are so far unpublished [14-181. This paper reports the conclusions reached from the analysis of the above-said work, and presents a complete discussion of the chemical reactions involved. The main experimental contribution consists of data on absorption rates in highly carbonated solutions.
Monoethanolamine, HOC,H,NH2, has two functional groups, -NH2 and - OH, either of which could react with CO,: NHCOO-H+,
substituted
-0H +COz-’ carbonic acid.
- OCOO-H+,
substituted
(a)
CO2 + OH-
-) HCO;
W
CO2 + HzO
-) H&Os
(c)
The relative importante of reactions (a) to (c) depends on their rate constants, and the concentrations of RNH2 and OH-, which depend on the total amine concentration a, and on the carbonation ratio a. Reaction (c) is pseudo-fìrst-order, as the concentration of water may be wnsidered wnstant. The concentrations at equilibrium of the species present (RNH,, RNH:, RNHCOO-, HCO;,CO;, H+, OH-) have been calculated from published [ 1, 19, ZO]equilibrium constants as functions of the parameters a, and a [4], with the simplifying hypothesis that the wncentration of free COz in the solution is negligible, and that the equilibria are not altered at the ionic strength encountered. These calculations show that :
CHEMISTRYOF THEPROCESS
-NH,+CO,-,carbamic acid
CO, + RNH2 -) RNHCOOH (R = HOC,Hb-)
95
G. ASTARITA, G. MARRUCCI and F. GIOIA
6) when a < 0.5, the main carbonated product is monoethanolammonium carbamate, RNHCOO-RNH;, (ii) when a = 0.5, further COZ absorption causes reversion to the bicarbonate, RNH: HC03, (iii) carbonate is formed only when a is very small, a condition not encountered in practice. The concentration of carbonate, which never exceeds 1.6 x lom3 gmol/l, may thus be ignored. Although at high ionic strength, such as may be encountered in practicc, the equilibrium constants may have somewhat different values, the statements (i) to (iii) may stil1 be considered correct. Moreover, the ionic strength of an amine solution is rather low, because of the low dissociation constant of amine, and the low concentration of the only bivalent ion, CO;. In the pH range encountered in this system, MEA carbamic acid RNHCOOH is almost completely dissociated, and it is neutralized by MEA to form monoethanolammonium and MEA-carbamate ions. When COa is absorbed in a solution where a < 0.5, the following sequence of reactions takes place : CO2 + RNH, RNH2 + Hf
When a > 0.5, the equilibrium concentrations, to a first approximation, of RNHCOO- and HCO; are given by [4]: (RNHCOO-) (HCO;)
Hence one mole of COZ combining causes the disappearance of one mole of carbamate and the formation of two moles of bicarbonate, according to the following overall reaction: RNHCOO-
+ RNH:
+ CO2 + 2Hz0 + + RNH: + 2HC03
(e)
The question is, what mechanism gives rise to reaction (e)? It is obvious that RNHCOO- must be converted to bicarbonate: FAURHOLT [l] shows that the conversion of carbamate to bicarbonate takes place through reversion of carbamate to free amine and COZ, followed by the reaction of carbon dioxide with hydroxyl ions : RNHCOO- + H+ + RNH, + COZ, fast CO2 + OH- --PHCO;, slow HzO + H+ + OH-
+ RNHCOO- + H+ fast, although not instantaneous (a) + RNHj virtually instantaneous
COZ + 2RNHz + RNHCOO-
= a,(l-a) = a,(2a-1)
RNHCOO-
(b)
+ HzO -P RNH2 + HCO;
The overall reaction (e) is therefore composed of the following steps :
(d)
1. COz + OH- + HCO;, slow; absorbed reacts with OH- and lowers pH,’
The stoichiometric coefficient of the overall reaction (d) is thus 2 moles amine per mole CO* ; this has been confirmed experimentally by means of data from laminar jet absorbers [5,10,11]. It should be remembered that it is the overall stoichiometric coefficient, and not the coefficient of the slowest step, that is to be used in the penetration theory equations for second-order chemical absorption. In industrial practice, carbonation ratios greater than 0.5 are often encountered [21], and it is therefore important to consider what happens when CO, is absorbed in highly carbonated (c( > 0.5) solutions. As has been stated above, further CO2 absorption results in bicarbonate formation when a > 0.5, as this is the only way in which tl can increase beyond 0.5 and the stoichiometric coefficient become less than 2.
COZ (b)
2. RNHCOO- + H+ + RNH2 + COt, encouraged by pH lowering, 3. RNH2 + H+ -+ RNH:, formation.
encouraged by RNH,
4. COZ + OH- + HCO;, slow; the CO2 liberated by 2. (b) 5. 2Hz0 -+ 2H+ + 20H-, correct value.
to maintain K, at its -
add: RNHCOO+ 2HCO;
+ CO2 + 2H,O + RNH:
+ (e)
The slow step (b) has to be realised twice, because two bicarbonate ions have to be formed; the two RNH: ions required for neutralization are: (i) the 96
Carbonation ratio and total amine concentration on carbon dioxide absorption in aquecus monoethanolaminesolutions one originally neutralizing RNHCOO- and (ii) the one formed by the dissociation of the amine liberated by the carbamate reversion. An alternative mechanism for bicarbonate formation is reaction (c), COz f HzO + HCO; + + H+, and the rate of this reaction becomes of the same order of magnitude as that of reaction (b) when the pH falls to about 9, which occurs when a > 0.5. Thus, when a > 0.5, reaction (c) takes part in the overall mechanism as fellows: 1. 2. 3. 4.
CO2 + HzO -+ HCO; + H+, slow, RNHCOO- + H+ + RNH, + CO, RNH2 + H+ -+ RNH: COz + HzO + HCO; + H+, slow,
add: RNHCOO2HCO;
+ 2Hz0 + CO2 + RNH:
(c)
(i)
Stirred tank data
Runs lasting 3 hr were performed, with the absorption rate V measured at 5 min intervals; at the end of each run, the equipment was shut off until the next day. With c1> 0.5, the absorption rate was almost constant over a 3-hr run. A typical V VSa curve is given in Fig. 1. The data for a < 0.5 wil1 first be considered. As long as a < 0.5, a second-order reaction mechanism is involved, and three possible regimes are to be considered : (a) If(RNH,)2c& B 1, có being the physical solubility of COz, the absorption rate V is given by :
where ki is the physical absorption coefficient, and S is the interface area.
+ (e)
(b) If 1 4 (RNH,)/&
Recent work [22] on the catalysis of reaction (c) by arsenite ions suggests the possibility of increasing carbonation rates in highly carbonated amine solutions.
+ ,/[k,(RNH,)t’]
t’ being the average life of surface elements, and k,, the kinetic constant of reaction (a), V = kgS(RNHJ2 (c) If
,/[kU(RNH,)t’]
EXPERIMENTAL
(2)
Q (RNH,)/2có
V = @ka(RNH,)]Scó
Experiments of carbon dioxide absorption in MEA solutions have been carried out on five different absorbers, namely : (i) a batch absorber, consisting of a magnetically stirred tank containing 500 cm3 of liquid. (ii) a 3.0 cm o.d. stainless steel wetted wal1 column. (iii) a 36 spheres ‘(3.7 cm o.d.) string-of-balls absorber. (iv) a 60 cm packed height column, 8-0 cm i.d., packed with + in. Rachig rings. (v) a 390 cm packed height column, same as (iv). Details of the experimental technique are available [ 14-181, as well as tabulated data. Only typical sets of data and fìnal cross-plots wil1 be given in the following. The gas phase was in every case pure carbon dioxide saturated with water vapour ; the temperature, 18°C; the pressure, 1 atm; absorption rates were determined volumetrically. The packed columns were fed concurrently. Liquid solutions were prepared at least 24 hr before starting the runs, in order to ensure that the solutions were in equilibrium.
(3)
where D is the diffusivity of CO2 in the liquid. As is known from classica1 chemical absorption theory, ki and t’ are related to each other by the equation : ki2 = Dit’
I
0.1
0
Slirrcd
tank
dolo
1 0.5 e
FIG. 1
97
(4)
I I
G. ASTARITA,G. MARRUCCI and F. GIOIA
where W is the liquid volume, c’ is the CO2 concentration which makes equilibrium to the actual HCO; and OH- concentrations, and k is a pseudofirst-order kinetic constant, related to the kinetic constants kb and k, of reactions (b) and (c): k = k, + k&OH-)
as suggested by the model of statistically random surface renewal [23]. Equations (1,2 and 3) can be calculated explicitly if the values of kE and S are known, in as much as the value of k, is known [6]. For the absorber used, the value of S was estimated at 50 cm’, by assuming that the interface was a tone having the base equal to the cross section of the tank, and the height equal to the depth of the vortex formed at the normal stirring rate. This allows to plot equation (3), as done in Fig. 2. In the same figure the absorption rate data are plotted, the abscissa having been calculated from the tabulated data of Ref. [4]. The line of equation (2) has been plotted as the best line of slope 1.0 through the data in the region of interest: its position gives the value k;S = 0.80 cm31 sec. This value allows to plot equation (3), and has been used in subsequent calculations. The agreement between data and theory is quite satisfactory, thus confirming the reaction mechanism discussed. As literature data [5, 6, 10, 111 on absorption rate at a = 0 indicate the same agreement, it can be concluded that the chemical absorption process when a < 0.5 is by now quite well understood. When a > 0.5, the mechanism of reaction discussed would indicate a pseudo-ílrst-order slow chemical absorption process-where slow means that the actual absorption coefficient equals k;. If the bulk concentration, c, of unreacted CO1 stays constant, the absorption rate is given by:
Equation (6) shows that k > k,; literature data on k, values [24-261 are rather uncertain, but an order of magnitude of 0.02 sec-’ may be considered reliable. Hence, the value of k Wfor the stirred tank used is surely larger than 10.0 cmy/sec, as compared with 0.80 cm3/sec for k:S. Hence it may be assumed that equation (5) can be written as: V = k;S(có - c’)
k;S
(7)
Values of V at constant a are plotted in Fig. 3 VSa,, the horizontal line being the value of k&!G$,. The behaviour of the data suggests the validity of equation (7), and hints to the possibility that the values of (k&%, - V) should be proportional to the equilibrium concentration c’. The latter can be calculated either from back-pressure data [21], or from interpolation through the data of Ref. [4], being : c’ oc (HCO;)/(OH-)
(8)
Figure 4 is a plot of (k$C$, - V) VSboth the ratio (HCO;)/(OH-) and the equilibrium back-pressure. It can be seen that, though this correlation brings all the data on one correlating line, the slope of the same is about 0.25, instead of the expected value 4c
Stined
có - c’
l’ 7 k;S(có - c) = k W(c - c’) = 2.’
(6)
10-c
(5)
0 m=0.5625 D =0.6250
tank
I
I
0.4
I ooI gmol/C
kW
FIG. 3
98
\
1 5
q A
a ~0.6675
A 0
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions back-
io_s_
Y
prersure,
The upper limit is compatible with equation (6), if the values of k* (27-29) and of k, (24-26) taken from the literature are considered:
mm. HIJ
p’:
_,o_,
k = 0.02 + 8500 (OH-)
(12)
.
i
.“-s-/
and the concentration of hydroxyl ions is evaluated from the data of Ref. [4], which indicate for (OH-) values of the order of magnitude of 10e6 gion/l in the range of a, and a values of interest.
-lo-s
(iii) 10'
10' [HCO;l FIG.
10'
/[OH-]
4
of 1.0. The authors have at the moment no explanation to offer for this result, except the uncertainty on the value of k:S. (ii)
Wetted-wal1 column data
Data on carbon dioxide absorption in MEA solutions with a > 0.5 have been collected on a wetted wal1 column with liquid flowing on its outer surface. Colunm length could be adjusted, and 3 different lengths have been used, namely 20,40 and 80 cm. For all the runs performed, the measured absorption rate was almost equal to the rate of physical absorption. The latter had previously been determined experimentally, and compared wel1 with theoretical predictions for wetted-wal1 column physical absorption [ 161. This indicates that the pseudofirst-order kinetic constant k was in any case smaller than l/t’, t’ being given by: t’ = z/us
String-of-bah data
The string-of-balls absorber can be considered the ideal equipment for studying slow-Grist-order chemical absorption processes involving k values of the order of magnitude of 0.1 sec-‘. In fact, t’ values (diffusion time, equal to the time required by a surface element to travel along one sphere) for the typical table-tennis bal1 absorber are of the order of 0.1 sec, and therefore kt’ 4 1. At the same time, the total residence time tp over a large number of spheres (thirty-six for the absorber used in this work) is of the order of 10 sec, hence yiélding measurable values of ktp (order of magnitude of 1). The theoretical equations are well known [30], and can be used to evaluate ktp values from absorption rate data. Reference [30] indicates that, when the following two conditions are fulíìlled, N” being the N.T.U. for physical absorption : Nog1
1
String-ofPhysiml
10“
Z being the column height, and IC~the surface velocity of the liquid. In the conditions of our experiments, the highest value of t’ was: f >
00)
0.02 sec- l
-’
(11) 99
(14) 0
bolls dato obsorption /
3
t
The wetted wall data hence indicate that k was in any case less than 0.13 sec- ‘. As previously discussed, k is larger than 0.02 sec- 1; hence, the true value of k has been proved to lie, within two rather close limits:
(13)
k 4 N”/t,
(9)
t&, = 1.5 sec;
_
Lim
of physicol
saturotion,\l=
Lc;
>”
10-3
I 101
10
L,CC/min Fis.
5
I 10'
G. ASTARITA,G. MARRUCCI and F. GIOIA
the slow fnst order (kt’ á 1) chemical absorption equation is : v = L(1 + k$)(có - c’)
(15)
where L is the volumetric liquid flowrate. For the conditions of our experiments, both conditions (13) and (14) were always fulfilled. In fact, (13) implies that physical absorption should result in practically saturated exit liquid; this is coníìrmed by a set of data on physical absorption reported in Fig. 5. The group N”/tp can be calculated theoretically [30-321, is independent of liquid flowrate (both N” and tp are proportional to L-‘13) and has for the absorber used. the value 0517 sec- ‘. As it has been proved that k is less than 0.13 sec-l, condition (14) can be considered always fulfilled. Experiments have been carried out only with not too highly carbonated solutions, for which c’ is negligible, in order to avoid the difficulty discussed concerning the data of Fig. 4. Equation (15) can hence be written as: I’ = L(1 + kt&@
(16)
G being the solubility of COz in water, and Qibeing a correction factor which takes into account the solubility lowering due to the ionic strength of the solution. Following the indications of Ref. [33 and 41, the value of CPhas been calculated as: log@ = - 094(2aa,)
(17)
2aa,, being the ionic strength of the solution if the carbonate ion concentration is neglected. A typical set of chemical absorption data is given in Fig. 6. The same data are cross-plotted in Fig. 7 as suggested by equation (16). The straight line 103-
String-of-bolk data &,= 3.40, 0 =0.58 Series 4 of toble
I
FIG. 6
0.1 10
I
I
102
101
L, cc/min
FIG. 7
through the data is the best line of slape -213, as required by the fact that tp is proportional to Lm213. From the plot of Fig. 7, the value of k can easily be calculated, through the theoretical value of tp. Final results are reported in Table 1, (OH-) values having been taken from Ref. [4]. Table 1 Series
ao(gmol/l)
0
k(sec-1)
1 2 3 4
1.92 1.10 0.686 340
0.62 0.705 0.57 0.58
0.065 oaI4 0.115 0.084
(OH-) cBion/l) 8.0 5.5 31.0 7.8
x x x x
10-7 10-7 10-7 10-7
It can be seen that k values are of the expected order of magnitude, though somewhat higher than equation (12) would predict. (iv) and (v)
Packed column data
The data obtained with the two packed columns wil1 now be considered on the basis of the ASTARITA and BEEK model [30]. First, an order-of-magnitude analysis on the parameters of interest wil1 be made; as the value of the same depend on the liquid flowrate, the value L = 1 l/min will arbitrarily be chosen as‘ a characteristic value, for which al! the parameters are calculated. Original data on physical absorption are tabulated in Ref. [34]; they show that the H.T.U. for liquid-side mass transfer was 37 cm. Actual liquid phase residence times have not been measured; their value can be estimated [34] by means of empirical correlations [35] at 0.15 sec per cm of packed height. Equivalent diffusion times can be
100
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions 190 cm packed tower 60
cm,pocked
iowcr
00’1.72, Series
o ‘0.625 L of toble
dolo 2
physicol
lim
10
I
0.1
obsorption
01
L, L/min FIQ.
through equation (4) and empirical correlations (36) of effective interfacial areas at the order of magnitude of 0.03 sec. As only experiments with a > 05 have been performed, it may be stated that: (a), kt’ values are in every case largely smaller than 1, hence the process considered is a slow-Erst-order process ; (b), ktJN” values are in any case smaller than 1 -notice that both tp and N” decrease with increasing flowrate, their ratio being approximately constant at the characteristic value of 5.5 sec; therefore, case “B” of ASTARITA and BEEK’S model applies. On this basis, it can be seen that the 60 cm column (ktP < 1) is expected to give absorption rates ahnost equal to the corresponding rates of physical absorption, while the 390 cm column is expected to give absorption rates correlated by equation (16). Typical sets of directly measured V’ VS L data are given in Figs 8 and 9; Fig. 10 is a plot of the best correlating lines for al1 the series of runs perestimated
Thir set of dato is lim Of
I
10
I
1, limh FIG. 10
8
formed on the 390 cm col-. The values of V’ as chacorresponding to L = 1.0 l/min-assumed racteristic of the absorption rate at any given liquid composition-are reported in Table 2. Table 2. Series
Volumetric absorption rates atL = 1.Ol/min
Packed hei& (cm)
A B :
390 390 390 390
F G H
z 390 390
1 L
60 60
N
z
0 (gnf$l) 0900
o6oo
1.20 2.36 1.61 0.475 2.21 0.475 0.0 (water)
0.635 0.533 0.635 0.710 0.618 0.810 -
2.71 2.43 2.06 1.90 1.67 0.98
1.67 1.72 3.40 0.0 (water)
0.670 0.625 0.565 -
0.81 0.81 0+30 0.80
For the 60 cm column the rate of chemical absorption practically coincides with the rate of physical absorption. The data for the 390 cm column show the expected trend, the absorption rate decreasing with increasing a, and with increasing a, as does the OH- concentration (4). Values of k can again be estimated; of course, no value of k can be evaluated from the 60 cm column data. CONCLUSIONS
L, LI min
FIG.9
Figure 11 is a fìnal plot of k values calculated from the string-of-balls and the packed column data. In the same figure equation (12) is plotted. 101
G. ASTARITA, G. MAIWJCCIand F. GIO~
. String-of -bah o Packed tower
data data
10-1
k =0.13
11. When a > 0.5, pseudo-first-order slow chemical absorption takes place, the overall reaction being substantially reaction (e). The kinetic constant can be evaluated from equation (12), (OH-) values being calculated from the tabulated data of Ref. [4] or by direct pH measurement. Absorption rates decrease with increasing a, and with increasing a.
sec-1
.
.
cl
.
I
I
! 10-7
10-6 [OH-]
t . 10-5
, gmol /C
FIG. 11
The order-of-magnitude agreement is satisfactory ; scattering of the data and some discrepancy is probably justified by the complexity of the elaboration procedure, and by the unaccounted for influence of ionic strength on k. The authors fee1 that the agreement is good enough not to reject the reaction mechanism proposed, and advance the following two conclusions : 1. As long as CC< 0.5, second order fast chemical absorption takes place, the overall reaction being substantially reaction (4. A stoichiometric coefficient of 2 is recommended, the absorption equation being:
Acknowle&ements-The authors wish to acknowledge the contribution of several chemical engineering students, some of whom are by now working in industry, who have collaborated in the three yean during which the experiments referred to in tbis paper were carried out. Their names fellow: Messrs. A~vIoo1, BoNADIEs,FEDERICI, ErZO,ORIIM, PATIERNO,PIIRRO’ITI,SAP~NARA, SPAGNUOLO,TONCELLI. The thorough revision of the manuscript by Professor P. V. DANCKWERTS is acknowledged.
NOTATION (M: mole. L: length. T: time.) ao c c’ c’o zJ D k ki kE
Values of (RNHJ can be calculated from the tabulated data of Ref. [4], an approximate equation being :
I+ s t’ tP US
V V’ W Z 01 a,
(RNH,) = a,(l - 2a) Absorption rates increase with increasing ao and decreasing a, as long as viscosity effects do not influence the value of k;.
ML-3 Total amine concentration M L-3 Concentration of unreacted CO8 ML-3 Equilibrium value of c M L-8 CO3 physical solubility M L-3 Value of C’O in water L2 T-1 Carbon dioxide diífusivity T-’ Pseudo-first-order overall kinetic constant Kmetic constant of reaction i), units of concentration are gmol/l, of time are sec L T-1 Physical absorption ccefficient LST-1 Liquid flowrate N.T.U. in liquidside controlled mass transfer L3 Interface area T Diffusion time, as defined in diierent cases T Total liquid phase residence time L T-1 Surface velocity of liquid M T-1 Absorption rate L3 T-1 Volumetric absorption rate L3 Volume of liquid L Height of wetted-wall cohunn Carbonation ratio, mole COa/mole amine Correction factor = C’O/ CT
REFERENCES
JENSENM. B., JORGENSEN E. and FAUR~IOLT C., Acts Chem. Stand. 19.54 8 1137. JORGENSEN E. and FAURHOLT C., Acts Ckem. Stand. 1954 8 1141. JORGENSEN E., Acts Chem. Stand. 1956 10 147. AsïARlTA G., MARRUCCIG. and GIOIA F., Bollettino Classe Scienxe Fisiche Matematiche e Natura& Accademia Lincei, February 1963. ASTARITA G., Id. Ch. E. Thesis, Univ. Delaware, Newark 1960. G., Ckem. E&pg. Sci., 1961 16 202. &TARITA ASTARITA G., Chim. e Zn&str. 1960 42 849. ASTUUTA G., Rit. Sci. 1960 30 658. ASTARITA G., unpublished report 1962.
102
Carbonation ratio and total amine concentration on carbon dioxide absorption in aqueous monoethanolamine solutions EMMER;R. E., PhD Dissertation, Univ, Delaware, Newark 1958. EMMERT R. E. and PI~~RD R. L., Amer. Inst. Chem. Engw. J. may 1962 8. CRYDERD. S. and MALO~Y J. O., Trans. Amer. Inst. Chem. Engrs. 194131827. !%xNEmwN A. L. and LEIBUSCH A. G., J. Appl. Chem. USSR 1946 19 869; ibid. 1950 23 1253. GIOLAF., Chem. Eng. Thesis, Univ. Naples, 1962. SPAGNUOLO A., Lab. Notebook, Istituto Chimica Industriale, Unlv. Naples, 1962. Izzo M., Lab. Notebook, Istituto Chimica Industriale, Univ. Naples, 1962. BONADIes V., Lab. Notebook, Istituto Chimica Industriale, Univ. Naples, 1962. FEDERICIG., Lab. Notebook, Istituto Chimica Indu@ale, Univ. Naples, 1962. BA= R. G. and PDTCHING G. D., J. Res. Nat. Bur. Sbnd. 195146 349. HOUGEN0. A. and WATSONK. M., ChemicalProcess %nciples, Vol. II. Wiley, New York, 1947. KOHLA. L. and RIESENFELD F. C., Gus Purifcation. McGraw-Hill, New York, 1960. ROBERTS D. and DANCKWFXTS P. V., Chem. Engng. Sci. 1962 17 961. DANCKWEXTS P. V., in HARTNI&S Recent Adwnces in Heat and Mass Tram+. McGraw-Hill, New York, 1961. PINSENTB. R. W., PMRSON L. and ROUGHTON F. J. W., Trans. Faraday. Sec. 1956 52 1512. MEDAE., Trans. Faroday Sec. 1956 52 1519. HIMMELBLAU P. M. and BABB A. L., Amer. Inst. Chem. Engrs. 1. 1958 4 143. FAURHOLT C. Lob. Lanabkor. Forstk. Aarsskr. 1 (1924). JENSENA. and FAURHOLT C., Acta Chem. Stand. 1952 6 385. NLJSINOR.A.T.O., PhD Dissertation, Delft 1958. A~TARITAG. and BEEKW. J., Chem. Engw. Sci. 1962 17 665. LYNNs., STaAAlXMEXER P. and KRAMERS H., Chem. Ehgng. Sci. 1953 4 63. A~TARITAG., Rit. Sci. 1961 31 ((II-A) 177. A~TARITAG. and MARRUCCIG., unpublished report, 1962. TONCELLI R., Chem. Eng. Thesis, Univ. Naples, 1961. DAVIDSON J. F., Trans. Inst. Chem. Engrs. 1959 37 131. SHUWAN H. L., ULWCH C. F., PROIJLXA. Z. and ZIMMERM ANJ. O., Amer. Inst. Chem. Engrs. J., 1955 1 253.
R&um&L*article traite de l’intluence de la concentration totale d’amine, ao, et du rapport de carbonation, a, sur l’absorption du COS dans des solutions aqueuses de MEA. L’analyse des rhctions Chiliques qui se déroulent dans le liquide permet d’hcider le mhnisme qui conduit auxproduitsfavorisés par la thermodynamique. Les exphiencea wntìrment ce mhnisme, selon lequel l’absorption est rapide, obeit &une loi du 2h ordre, avec un coe&ient stoichiometrique de 2 moles de MEA par mole de COs lorsquea c 0,5, tandis que la rhction est lente, du pseudo le’ ordre avecuneconstantecinhique globale d6pendant de la wncentration en OH- lorsque a > 0,s.
103