Reactivity ratio estimation for 2-acrylamido-2-methylpropane sulfonic acid (AMPS)/AAm and AMPS/AAc copolymers: A comparative experimental study

Transcription

1 Reactivity ratio estimation for 2-acrylamido-2-methylpropane sulfonic acid (AMPS)/AAm and AMPS/AAc copolymers: A comparative experimental study Alison J. Scott and Alexander Penlidis Institute for Polymer Research (IPR), Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Water-soluble polymers have been used for a wide variety of applications due to their versatility. Watersoluble polymers are found in many different industries including agriculture, food, plastics, pulp and paper, mining, petroleum, textiles, and waste water treatment. [1] Often, they are used as both processing aids and components of final products. Addition of water-soluble polymers to industrial processes tends to improve control of fluid motion, which may involve drag reduction, fluid thickening, or flocculation. [2] One of the major applications of water-soluble polymers is as processing aids in enhanced oil recovery (EOR). [3] Specifically, water-soluble polymers have the ability to increase the viscosity of the aqueous phase during oil recovery, which increases the efficiency of the overall process. Some of the most common copolymer systems used in EOR are acrylamide (AAm) and acrylic acid (AAc) copolymers. However, these AAm/AAc copolymers, like many other water-soluble polymers with high molecular weights, are shear sensitive. That is, when the copolymer is subjected to high temperatures and stresses, there is potential for the polymer backbone to break. [4] This directly affects the polymer's efficiency in enhanced oil recovery, as the polymer in this case will not be able to increase the aqueous phase viscosity as much as was originally desired. The shear degradation of AAm/AAc copolymers in EOR is common, as the polymer is subjected to stirring and pumping, as well as high temperatures, flow rates and pressures. Once the backbone of the polymer is broken, the damage is irreversible. Thus, it is essential to minimize degradation in EOR applications. One solution that has been proposed to limit shear degradation of AAm/AAc copolymers is the addition to the polymerization recipe of a third (co)monomer, such as 2-acrylamido-2-methylpropane sulfonic acid (AMPS). AMPS has the potential to improve main chain stability in harsh environments; the steric hindrance provided by the sulfonic group in AMPS is expected to control potential degradation of the polymer backbone. [5] The end goal for this investigation is to synthesize a water-soluble terpolymer based on 2-acrylamido-2- methylpropane sulfonic acid, acrylamide, and acrylic acid. First, though, obtaining accurate reactivity ratios for AMPS/AAm and AMPS/AAc will provide additional insight into the ternary system. This presentation will focus on the largely unstudied copolymer (binary) systems (AMPS/AAm and AMPS/AAc), and extensions to the ternary system will be covered in a complementary poster. Ultimately, the experimental data and insights based on them will make it possible to synthesize tailormade copolymers and terpolymers with desirable properties for specific applications including, but not limited to, enhanced oil recovery. Reactivity Ratio Estimation For comparison purposes, reactivity ratio estimation experiments for both AMPS/AAm and AMPS/AAc were designed using two techniques: Tidwell-Mortimer [6],[7] and the Error-in-Variables-Model. [8] Reactivity ratios were estimated by applying the cumulative copolymer composition model (using the direct numerical integration, DNI, approach) to the collected experimental data through EVM. [9]

2 2 Experimental Procedures The comonomer ratios in the feed of each system (AMPS/AAm and AMPS/AAc) were chosen according to a recent design of experiments technique based on mechanistic models. [10] Maintaining constant total monomer concentration, ph and ionic strength is extremely important in copolymerization kinetics, as has been demonstrated previously, so the experimental procedures of Riahinezhad et al. were adopted throughout this work. [11] Polymerization occurred at 40 C and 100 rpm, and vials were removed at selected time intervals to ensure a well-defined conversion versus time plot. Polymer samples were then isolated via precipitation, filtered and vacuum dried for 1 week at 50 C. Preliminary Results The preliminary design for choosing optimal feed mole fractions for the copolymer of 2-acrylamido-2- methyl-1-propanesulfonic acid (AMPS) and acrylamide (AAm) was based on the classical work by McCormick and Chen. [12] These preliminary reactivity ratios were incorporated into both Tidwell- Mortimer and EVM design techniques, and the following results were obtained. Table 1: Preliminary Design for AMPS/AAm Reactivity Ratios r 1 (r AMPS) r 2 (r AAm) From Literature [12] Tidwell-Mortimer (T-M) EVM Conditions chosen for preliminary experiments Ultimately, the selected design (presented in the final row of Table 1) was chosen based on a combination of Tidwell-Mortimer and EVM designs, and considering process understanding. It was anticipated that choosing pre-polymerization recipes with low (f 11) and high (f 12) concentrations of AMPS would provide a substantial amount of reliable information. For the preliminary experiments, copolymerizations were independently replicated at least once at each feed composition. Preliminary reactivity ratio estimates (r AMPS and r AAm) were estimated by applying the cumulative composition model (using direct numerical integration, DNI) to the data through EVM. The joint confidence region (JCR) for preliminary reactivity ratios is presented in Figure 1. The point estimate from McCormick and Chen [12] is included in the figure for reference, and the current estimates are r AMPS = and r AAm = In Figure 1, it is clear that the reactivity ratio estimates from the literature [12] are different from our newly determined reactivity ratios; the point estimate from the literature is not contained within the JCR from the current study. However, this rather drastic difference might be due to differing reaction conditions. The work by McCormick and Chen [12] was at ph = 9 and did not consider ionic strength. The current study was at ph = 7 and ensured consistent ionic strength. Additional (but less influential) factors include different reaction temperatures (30 C vs. 40 C) and different initiators (potassium persulfate, KPS vs. 4,4'-azo-bis-(4-cyanovaleric acid), ACVA).

3 3 1.1 Preliminary JCR 1.05 (0.1320, ) Point Estimate (from Literature) r AAm r AMPS Figure 1: Joint Confidence Region and Preliminary Reactivity Ratio Estimates for AMPS/AAm Copolymer The above (experimentally determined) estimates of the reactivity ratios were used in a sequential scheme for finding new optimal feed compositions for the AMPS/AAm copolymerization, which are presented in Table 2. The reactivity ratios (and corresponding JCRs) determined using these optimally designed experiments will be presented at the conference. Table 2: Optimal Design for AMPS/AAm Reactivity Ratios r 1 (r AMPS) r 2 (r AAm) Experimentally Determined (from preliminary experiments) Tidwell-Mortimer (T-M) EVM Similarly, the preliminary design for the AMPS and acrylic acid (AAc) copolymer was based on both background literature [13] and process understanding/constraints. Ultimately, the same initial feed compositions were chosen for both the AMPS/AAc and the AMPS/AAm systems. Details regarding the preliminary design for AMPS/AAc are presented in Table 3. Table 3: Preliminary Design for AMPS/AAc Reactivity Ratios r 1 (r AMPS) r 2 (r AAc) From Literature [13] Tidwell-Mortimer (T-M) EVM Conditions chosen for preliminary experiments

4 4 As for the AMPS/AAm system, experiments for the AMPS/AAc were independently replicated at least once at each feed composition. Again, preliminary reactivity ratio estimates (r AMPS and r AAc) were calculated by applying the cumulative composition model and direct numerical integration to the data using EVM. The preliminary JCR for the AMPS/AAc reactivity ratios is shown in Figure 2. The point estimate from literature is included in the figure for reference, and the current reactivity ratio estimates are r AMPS = and r AAc = Preliminary Results (0.4778, ) Point Estimate (from Literature) r AAc r AMPS Figure 2: Joint Confidence Region and Preliminary Reactivity Ratio Estimates for AMPS/AAc Copolymer Although the point estimate from Abdel-Azim et al. [13] is still very close to the edge of the preliminary JCR, the picture in Figure 2 is better than that in Figure 1. Optimal experiments for reactivity ratio determination designed according to both Tidwell-Mortimer and EVM techniques are shown in Table 4. As before, updated reactivity ratio estimates (based on the preliminary experiments of Table 3) are used to locate optimal feed conditions for the AMPS/AAc copolymerization. However, a constraint (0.2 < f 1i < 1.0) was included when designing optimal experiments through EVM to eliminate the possibility of low conversions and experimental difficulties with precipitation. The flexibility to introduce limits on feed compositions is yet another advantage of the adopted mechanistic model- and EVM-based design of optimal experiments. Table 4: Optimal Design for AMPS/AAc Reactivity Ratios r 1 (r AMPS) r 2 (r AAc) Experimentally Determined (from preliminary experiments) Tidwell-Mortimer (T-M) Error-in-Variables-Model (constraint: 0.2 < f 1i < 1.0)

5 5 It is anticipated that the EVM design will provide more precise parameter estimates (that is, smaller joint confidence regions) for both AMPS/AAm and AMPS/AAc. An investigation of both techniques (T-M and EVM) and more details will be presented at the conference; this will confirm the hypothesis and provide a clear comparison between the Tidwell-Mortimer and EVM design approaches for the first time in the literature. References [1] "Water-Soluble Polymers," in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, 2000, pp [2] DOW Chemical Company, "POLYOX Water-Soluble Resins," [3] K. C. Taylor and H. A. Nasr-El-Din, "Water-Soluble Hydrophobically Associating Polymers for Improved Oil Recovery: A Literature Review," Journal of Petroleum Science and Engineering, vol. 19, pp , [4] A. Zaitoun, P. Makakou, N. Blin, R. Al-Maamari, A. Al-Hashmi, M. Abdel-Goad and H. Al-Sharji, "Shear Stability of EOR Polymers," in Society of Petroleum Engineers International Symposium, The Woodlands, Texas, [5] Q. Li, W. Pu, Y. Wang and T. Zhao, "Synthesis and Assessment of a Novel AM-co-AMPS Polymer for Enhanced Oil Recovery (EOR)," in International Conference on Computational and Information Sciences, [6] P. W. Tidwell and G. A. Mortimer, "An Improved Method of Calculating Copolymerization Reactivity Ratios," Journal of Polymer Science: Part A, vol. 3, pp , [7] P. W. Tidwell and G. A. Mortimer, "Science of Determining Copolymerization Reactivity Ratios," Journal of Macromolecular Science, Part C, vol. 4, pp , [8] N. Kazemi, T. A. Duever and A. Penlidis, "A Powerful Estimation Scheme with the Error-in- Variables Model for Nonlinear Cases: Reactivity Ratio Estimation Examples," Computers and Chemical Engineering, vol. 48, pp , [9] N. Kazemi, T. A. Duever and A. Penlidis, "Reactivity Ratio Estimation from Cumulative Copolymer Composition Data," Macromolecular Reaction Engineering, vol. 5, pp , [10] N. Kazemi, T. A. Duever and A. Penlidis, "Design of Experiments for Reactivity Ratio Estimation in Multicomponent Polymerizations Using the Error-In-Variables Approach," Macromolecular Theory and Simulations, vol. 22, pp , [11] M. Riahinezhad, N. Kazemi, N. McManus and A. Penlidis, "Effect of Ionic Strength on the Reactivity Ratios of Acrylamide/Acrylic Acid (sodium acrylate) Copolymerization," Journal of Applied Polymer Science, vol. 131, p , [12] C. L. McCormick and G. S. Chen, "Water-Soluble Copolymers. IV. Random Copolymers of Acrylamide with Sulfonated Comonomers," Journal of Polymer Science: Polymer Chemistry Edition, vol. 20, pp , [13] A.-A. A. Abdel-Azim, M. S. Farahat, A. M. Atta and A. A. Abdel-Fattah, "Preparation and Properties of Two-Component Hydrogels Based on 2-Acrylamido-2-methylpropane Sulphonic Acid," Polymers for Advanced Technologies, vol. 9, pp , 1998.