Znd. Eng. Chem. Res. 1991,30, 1811-1813

Kinetics of Carbon Dioxide Reaction with Sterically Hindered 2-Piperidineethanol Aqueous Solutions

1811

Keh-Perng Shen and Meng-Hui Li* Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, Taiwan 32023, ROC

Siu-Ming Yih Tech Center, Brown and Root Braun Znc., Alhambra, California 91801

The kinetics of the reaction between carbon dioxide and an aqueous solution of a sterically hindered secondary amino alcohol, 2-piperidineethanol (PE), was studied at 40 "C by using a laboratory wetted-wall column. The reaction was found to be first order with respect to both COz and the amine. The second-order forward rate constant has a value of 195 m3/(kmobs) within the amine concentration range 0.218-1.0 kmol/m3. The product of the diffusivity and solubility for COz in PE solution, DA'/'/HA, was found by absorbing NzO into the same PE solution within the above concentration range.

Introduction Sterically hindered amines have recently been proposed

as commercially attractive solvents for carbon dioxide removal over conventional amines, such as MEA, DGA, DEA, DIPA, MDEA, and TEA, and as rate promoters for the hot carbonate process (SartOri and Savage, 197&, 1983; Savage et al., 1984; Say et al., 1984; Goldstein et al., 1986). The merits of these sterically hindered amines are their low tendency to form carbamates with COz due to the bulkiness of the substituent attached to the amino group. This results in an increased cyclic capacity and thermo- dynamic loading of COz into the solution of up to 1.0 mol of COZ/(mol of amine), which is comparable to that of tertiary amines. At the same time, the sterically hindered amines still have an appreciable reaction rate with COz, while the tertiary amines are quite unreactive and have low rate constants. An example of a hindered primary amino alcohol is 2-amino-2-methyl-l-propanol (AMP), which is the hindered form of MEA. Sartori and Savage (1983) and Roberts and Mather (1988) have presented VLE data for the system CO?-AMP solutions. The ki- netics of the COz-AMP reaction have recently been in- vestigated by Yih and Shen (1988) and shown to be first order with respect to both COz and the amine with a second-order rate constant of 1270 m8/(kmoles) at 40 OC. Besides AMP, a secondary hindered amino alcohol of commercial significance is 2-piperidineethanol (PE) (Sartori and Leder, 1978b). PE has a naphthenic ring group attached to the amino group. It also exhibits a thermodynamic loading of up to 1.0 mol of COz/(mol of amine), which means that it will have a higher cyclic ca- pacity than conventional secondary amines such as DEA or DIPA. DEA or DIPA, like primary amines, forms a quite stable carbamate with COz, which limits their COz loading to about 0.5 mol of COz/(mol of amine). The kinetics of the reaction between COz and PE has not yet been reported in the literature. The objectives of this research are to investigate the kinetic order with respect to both CO2 and PE and to obtain the forward rate con- stant at 40 "C by using a laboratory wetted-wall column.

Theory As shown by Sartori and Leder (1978a), 13C NMR

spectra for a COz-saturated AMP solution indicate that 10-20% carbamate is formed compared to 78% formed for DEA and 98% for MEA. This means that PE, a sec-

* To whom all correspondence should be addressed.

ondary hindered amine, will form even less carbamate than AMP and will have a lower carbamate stability than AMP. The reactions involved are

(1)

RNHz+COO- + B RNHCOO- + BH+ (2)

(3)

COZ + RNHz + RNHz+COO-

RNHCOO- + HzO + RNH2 + HCOB-

B is a base that could be an amine or OH-. The carbamate is almost completely hydrolyzed for the case of PE to regenerate free amine for further absorption of COP For MEA, DEA, and DIPA, the carbamates are not readily hydrolyzed and this limits their loading of COz to about 0.5 mol of COz/(mol of amine).

The formation of the zwitterion is the slow and hence rate-determining step with the rate given by

r = kmn[co~l[RNH~I (4)

In a region of fast pseudo-mnth-order reaction in which (5 )

I' = kmnCAm Cgn (6)

E = Ha (7) where E is the enhancement factor. The Hatta number, Ha, and the instantaneous reaction enhancement factor, Ei, are defined respectively as

3 < Ha << Ei

and

the following relation is obtained

With the assumption that the gas-phase resistance is negligible and with CA* = pA/HA, the specific absorption rate can be expressed as (Yih and Shen, 1988)

The slope of the straight line fitting the data of In NA versus In pa will give the order m with respect to CA. Another plot of (NA/pA(m+')/2)/(DA1/2/H~(m+1)/z) versus

0888-5885/91/2630- 181 1$02.50/0 0 1991 American Chemical Society

1812 Ind. Eng. Chem. Res., Vol. 30, No. 8,1991

Table I. Values of DA1/*/HA (kmol/(kPa m* sl/z)) for NzO and COS in PE Solutions at 40 O c a

0 0.720 992.3 0.788 1.077 0.107 0.742 992.6 0.770 1.052 0.218 0.780 993.1 0.710 0.970 0.320 0.816 995.1 0.613 0.838 1.00 1.133 996.9 0.529 0.723

a Units: amine concentration, kmol/m3; viscosity, P a q density, kg/m3.

Table 11. Values of Partial Pressure and Absorption Rate for Absorption into PE Solutions at 40 O c a

amine concn PA $I X 108 h L X los t, 0.107 0.938 3.910 0.105 3.50 0.344

0.696 2.961 0.105 4.01 0.314 0.447 1.937 0.105 3.64 0.335 0.093 0.483 0.110 4.04 0.328

0.218 0.938 4.810 0.105 3.60 0.343 0.662 3.320 0.105 3.25 0.368 0.347 2.131 0.110 3.46 0.369 0.091 0.475 0.105 2.50 0.438

0.320 0.938 5.010 0.105 3.50 0.355 0.658 3.581 0.105 3.20 0.377 0.324 1.769 0.105 3.32 0.368 0.056 0.308 0.105 2.91 0.402 0.021 0.117 0.105 3.20 0.377

1.00 0.938 7.170 0.105 4.00 0.362 0.603 4.150 0.105 2.52 0.493 0.168 1.497 0.105 3.40 0.404 0.037 0.324 0.105 3.10 0.429

"Units: amine concentration, kmol/m3; PA, kPa; 4, kmol/s; h, m; L, m3/s; t,, a.

In CB will give the order with respect to CB. Lastly, k,, can be obtained from a log-log plot of eq 10.

Experimental Section The apparatus and experimental and analytical proce-

dures were the same as those described by Yih and Shen (1988). The absorption rates were determined in a short wetted-wall column. For chemical absorption, C02 was mixed with nitrogen to give a range of partial pressures, and the absorption rate was measured from the product of the gas flow rate and the difference in the inlet and outlet COz composition in the gas as determined by gas chromatography. When pure COz was absorbed, the ab- sorption rate was measured by a soap-film meter. The value of DA'/2/HA for C02-PE is calculated from the re- lation (DA'/~/HA)c~,-~E =

( D A ' / 2 / H A ) N ~ o - P E ( D ~ ' / 2 / H ~ ) ~ ~ ~ ~ * ~ / (DA'/~/HA)N,O-H,O (11)

Results and Discussion Table I gives the measured data of DA'/'/HA at 40 OC

for N20-PE and the calculated values for C02-PE ac- cording to eq 11. With use of the wetted-wall column, the COP absorption rates for various C02 partial pressures for COP absorption into PE solutions at 40 "C are presented in Table 11. The specific absorption rates of COz and Hatta numbers as a function of partial pressures of COz are given in Table 111. A plot of In NA versus In pA in Figure 1 gives a slope of 1, which means that m = 1. In Figure 2, a plot of In ((NA/pA)/(DA'/'/HA)) versus In CB gives a slope of l/z, which indicates that n is also 1. As can be seen in Table 111, the three amine solutions with amine concentrations of 0.218,0.32, and 1.0 kmol/m3 ex-

Table 111. Specific Absorption Rates for Various COS Partial Pressures for COz Absorption into PE Solutions at 40 "C"

amine (NA/PA)/ concn PA x lo-* NA x lo8 (DA1/2/HA) Ha 0.107 0.938 4.59 4.65 2.374

0.696 3.47 4.74 2.268 0.447 2.27 4.83 2.343 0.093 0.54 5.52 2.320

0.218 0.938 5.64 6.20 3.384 0.662 3.90 6.07 3.505 0.347 2.39 7.10 3.510 0.091 0.56 6.34 3.824

0.320 0.938 5.88 7.48 4.171 0.658 4.20 7.62 4.298 0.324 2.07 7.62 4.247 0.056 0.36 7.67 4.439 0.021 0.14 7.96 4.298

1.00 0.938 8.39 12.37 7.446 0.603 4.87 11.17 8.689 0.168 1.75 14.41 7.886 0.037 0.38 14.21 8.106

"Units: amine concentration, kmol/m3; pa, kPa; NA, kmol/ ( m 2 4 OJA/PA)/(DA"'/HA),

101

n

5 f

10-1 1

4 8

B

4 8

0

100 a a 4 10-1 a 8 4 ,0-2

P xlo-2(kPa) CQ2

Figure 1. Specific absorption rate as a function of C02 partial pressure to determine reaction order with respect to COP.

a

a a 100 a 5 1 6 6 7

10-1 ' 6 6 7

cg (-31 Figure 2. Determination of reaction order with respect to PE.

hibit Hatta numbers that are greater than 3. For eq 7 to be valid, the value of the Hatta number has to be greater than 3, Le., eq 5 should be satisfied. Thus, data of these three amine solutions are used to determine the rate constant for the COO and amine reaction. A plot of In NA versus In (PACB'/~DA'/'/HA), in Figure 3, gives the value

Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1813

DEA = diethanolamine DIPA = di-2-propanolamine MDEA = methyldiethanolamine MEA = monoethanolamine PE = 2-piperidineethanol TEA = triethanolamine CA* = dissolved gas concentration in equilibrium at the

CB = amine concentration, kmol/m3 DA = diffusion coefficient of gas in liquid, m2/s DB = diffusion coefficient of reactant in liquid, m2/s E = enhancement factor Ei = enhancement factor for instantaneous reaction Ha = Hatta number HA = Henry’s law constant, (kPa.m3)/kmol h = wetted-wall column height, m kL = liquid-phase mass transfer coefficient, m/s k,, = reaction rate constant, m3/(kmol.s) L = liquid flow rate, m3/s m = order of reaction with respect to C 0 2 NA = specific absorption rate, kmol/(m2.s) n = order of reaction with respect to amine pA = C02 partial pressure, kPa r = reaction rate, kmol/(m3-s) tc = contact time, s Z = stoichiometric coefficient Greek Symbol 4 = absorption rate, kmol/s

gas-liquid interface, kmol/m3

Registry No. PE, 1484-84-0; C02, 124-38-9.

Literature Cited Goldstein, A. M.; Brown, E. C.; Heinzelmann, F. J.; Say, G. R. New

FLEXSORB Gas Treating Technology for Acid Gaa Removal. Energy Prog. 1986, 6, 67.

Roberts, B. E.; Mather, A. E. Solubility of C02 and H a in a Hin- dered Amine Solution. Chem. Eng. Commun. 1988, 64, 105.

Sartori, G.; Leder, F. Process for Removing Carbon Dioxide Con- taining Acidic Gases from Gaseous Mixtures Using a Basic Salt Activated with a Hindered Amine. U.S. Patent 4112050, Sept 5, 1978a.

Sartori, G.; Leder, F. Process and Amine-Solvent Absorbent for Removing Acidic Gases from Gaseous Mixtures. US . Patent 4112051, Sept 5, 1978b.

Sartori, G.; Savage, D. W. Process for Removing Acid Gases with Hindered Amines and Amino Acids. U.S. Patent 4094957, June 13, 1978~.

Sartori, G.; Savage, D. W. Sterically Hindered Amines for COz Re- moval from Gases. Ind. Eng. Chem. Fundam. 1983,22, 239.

Savage, D. W.; Sartori, G.; Astarita, G. Amines as Rate Promotera for Carbon Dioxide Hydrolysis. Faraday Discuss. Chem. SOC. 1984, 77, 17.

Say, G. R.; Heinzelmann, F. J.; Iyengar, J. N.; Savage, D. W.; Bisio, A.; Sartori, G. A New, Hindered Amine Concept for Simultaneous Removal of COz and HzS from Gases. Chem. Eng. Prog. 1984 (October), 80, 72.

Yih, S. M.; Shen, K. P. Kinetics of Carbon Dioxide Reaction with Sterically Hindered 2-Amino-2-methyl-1-propanol Aqueous Solu- tions. Ind. Eng. Chem. Res. 1988,27, 2237.

Receiued for reuiew June 11, 1990 Accepted March 11,1991

O1 1 PE(kmol /d ) t 0 0.218

0

0

10-1 I,, , , , , , , , , , , , , , , , , ;f 100

2 3 4 2 3 4 10-2 10-1

Figure 3. Determination of rate constant kz for the COz and PE reaction.

of k2 as 195.0 m3/(kmol-s) for COZ-PE at 40 “C. As noted by Sartori and Savage (19831, steric hindrance

has an adverse effect on the C02-amine rate constants. For secondary amines, DEA has second-order rate con- stants of the order of between 10oO and 2000 m3/(kmol.s). DIPA, which has a more bulky alkyl group attached to the amino group than DEA, has a second-order rate constant of between 400 and 700 m3/(kmol.s). PE has an even more bulky group, a naphthenic group, attached to the amino group. The value of kz found in this work for CO,-PE is smaller than that for DIPA but still larger than the rate constant for a tertiary amine, such as MDEA, which has a k2 value of the order of 10 m3/(kmol.s). Therefore, the value of the rate constant found in this work seems reliable and reasonable.

Conclusions The kinetics of the reaction between C02 and PE was

investigated at 40 “C by using a laboratory wetted-wall column. The reaction was found to be first order with respect to both COz and the amine. The second-order forward rate constant has a value of 195.0 m3/(kmol.s) within the amine concentration range 0.218-1.0 kmol/m3. Values of DA112/HA were also reported within this con- centration range by N20 absorption into the same PE solutions.

Nomenclature AMP = 2-amino-2-methyl-1-propanol