1811
Znd. Eng. Chem. Res. 1991,30, 1811-1813
Kinetics of Carbon Dioxide Reaction with Sterically Hindered 2-Piperidineethanol Aqueous Solutions 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 COzand 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 thermodynamic 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 (AMP), amino alcohol is 2-amino-2-methyl-l-propanol 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 kinetics of the COz-AMP reaction have recently been investigated 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). P E 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 capacity 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 P E 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 constant 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 COZ + RNHz + RNHz+COORNHz+COO- + B RNHCOO-
RNHCOO-
+ BH+
+ HzO + RNH2 + HCOB-
(1) (2)
(3)
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 3 < Ha
