Mechanism of Fouling in Slurry Polymerization Reactors of Olefins

Article pubs.acs.org/IECR

Mechanism of Fouling in Slurry Polymerization Reactors of Olefins Maila N. Cardoso† and Adriano G. Fisch*,‡ †

Innovation and Technology Center, Braskem SA III Pólo Petroquímico, Via Oeste lote 5, Passo Raso, 95853-000, Triunfo, Rio Grande do Sul, Brazil ‡ Chemical Engineering Department, Universidade Luterana do Brasil, Avenida Farroupilha 8001, 92425-900, Canoas, Rio Grande do Sul, Brazil ABSTRACT: Slurry polymerization processes are used to produce high-density polyethylene of tailor-made properties. The formation of fouling in slurry reactors as well as in its peripheral equipment, such as external heat exchangers, is attributed either to the deposition of oligomers (wax) or to the buildup of electrical charge in small polymer particles. In order to understand the causes of fouling formation, samples were gathered from polymerization processes and they were characterized by microscopy in this research. The results show the existence of an aluminum-based layer as a common element in the samples analyzed. It is supposed that such a layer is formed by the reaction of trialkyl aluminum and water residues during the start-up procedure of the plant.

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proper fit between theory and experiments. Additionally, the quality of the surface in terms of smoothness is significant for adhesion; i.e., the larger the surface roughness the higher the attractive force.7,8 Those surface imperfections ranging in the nanometer scale are mainly important because they influence the van der Waals attractive potential.8 The particle adhesion phenomenon is also dependent on the hydrodynamic transport of the particles, which describes the transport mechanism of the particle from the bulk fluid to the solid surface as well as the opposite.3 The phenomena taking part in the particle fouling formation have already been covered in detail in the work of Henry et al.3 For olefin polymerization reactors, two types of fouling are highlighted. The concentration of particles in the reaction medium is high, ranging from 20 to 40 vol %, in which particle−particle interaction occurs at high frequency. Indeed, as the particles in the reaction medium present a distribution of size and the solvent is a material of low conductivity, particles of small size (diameter d50 < 125 μm) could agglomerate easily due to the triboelectrification phenomenon, i.e., the buildup of electrostatic charges, and the particulate cluster generated could deposit on equipment surfaces depending on hydrodynamic transport characteristics.9 In addition, individual positively charged polymer particles adhere to the reactor wall and their removal is rather difficult because of the strong adhesion force.7 Antistatic agents are commonly employed to overcome these problems.5 They act to increase the solvent conductivity, reducing the accumulation of electrical charges in the particles by a safe discharge to the earth9,10 and/or by electrical charge neutralization. Despite many similarities, not all of the slurry

INTRODUCTION The formation of deposits on a surface (fouling) is considered a problem in many chemical processes because, for instance, it deteriorates the performance of heat exchangers and/or it increases the surface roughness and, consequently, the friction factor.1−3 In order to recover the equipment performance, it is necessary to expend capital in terms of maintenance costs and production loss. The industrial technologies available for the production of polyolefins present fouling to some extent.2,4,5 In the slurry polymerization processes, the reaction medium is a suspension of particles in a liquid phase. The particulate phase is formed by fresh catalyst and by polymer produced by it. The liquid phase is composed of a hydrocarbon solvent and, in minor concentrations, of monomer, comonomer, cocatalyst, and oligomers (wax) that are dissolved in the solvent. Deposits of particles (polymer and fresh catalyst) and soluble wax might form the foulant material in a reactor operating with this reaction medium.4 Indeed, peripheral equipment and transfer lines are also affected by fouling. When fouling occurs in the reactor, the control of the reaction temperature is difficult, affecting the quality control of the product, and the environment becomes unsafe for work.2,6 These features justify the importance of understanding the mechanism of fouling formation and ways to control it in slurry polymerization reactors. The mechanism of fouling in polymerization reactors involves short to long range interactions among fluids, particles, and surfaces. The principal factors affecting the deposition of particles is described by DLVO (Derjaguin−Landau and Verney−Overbeek) theory that considers van der Waals (attractive) and electrostatic double-layer forces (repulsive).7 When DLVO theory fails in describing the experimental evidence, additional forces, such as the electrical charge buildup on particle surface (tribology), steric interactions, and ion bridging, for instance,7 should be considered to provide a © XXXX American Chemical Society

Received: June 29, 2016 Revised: August 17, 2016 Accepted: August 19, 2016

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DOI: 10.1021/acs.iecr.6b02490 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Samples’ Features H2/C2 ratio plant

catalyst system

industrial pilot pilot pilot

a a b c

sample from heat exchanger pipeline reactor wall reactor wall transfer line between reactors

Al/Ti ratio

R1

R2

C4/C2 flow rate (kgC4·tC2−1)

[η] (dL·g−1)

density (kg·m−3)

9−12 9−12 1−4 1−4

1−5 0−0.2 0−0.2 0−0.2

0.1−0.5 0 0 0

10−50 0.05−0.1 0.05−0.1 0.05−0.1

1.0−2.5 >3 >3 >3

940−960 n.a.d n.a. n.a.

a Ziegler−Natta (MgCl2-based support) TEA as cocatalyst; polymerization run without antistatic. bZiegler−Natta (spherical MgCl2-based support) TEA as cocatalyst; polymerization run using antistatic. cPrepolymerized Ziegler−Natta DEAC as cocatalyst; polymerization run using antistatic. dn.a., not analyzed.

the operation of both industrial and pilot plants, it is a common procedure to recirculate solvent throughout the equipment. A small quantity (100−600 mmol L−1) of triethyl aluminum (TEA) was added to the solvent as a scavenger for impurities, such as water and other oxygenated compounds. All catalysts used in the polymerization reactions presented an average particle size (d50) ranging from 9 to 12 μm. Additional information on the samples and the respective polymerization conditions are given in Table 1. The samples were extracted from the respective surface by using either a sharp knife or a spatula. The collected samples presented a flat thin shape, normally rectangular. Samples were analyzed by scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM−EDS; benchtop TM-1000 from Hitachi). Samples were not covered with any metal due to the low-potential electron beam. Analyses were conducted on both surfaces of each sample, i.e., on the metal side and on the slurry side. The EDS was conducted on a selected area of the sample. Intrinsic viscosity ([η]) was determined using an Ubbelohde type viscometer (Lauda D20 KP) with decalin as solvent at 135 °C. The sample solutions were prepared using ca. 15 mg of polymer dissolved in 15 mL of decalin at 145 °C. Nine measurements were performed for each sample, but only four, which presented ts values differing by less than 0.1 s, were used in the calculations of intrinsic viscosity using eq 1:

polymerization technologies require the addition of antistatic agents to control these types of fouling, probably due to differences either in the equipment designs or in components (types and concentration) in the reaction medium. Oligomers (wax) that are dissolved in the solvent tend to deposit on the surface. The presence of wax is attributed to the dissolution of polymer of low molecular weight as well as to the fusion of polymer chains, mainly those of high comonomer incorporation presenting a melting temperature lower than the respective polymerization temperature.2 The formation of wax deposits on surfaces is caused by phase separation, which is thermodynamically driven, of the soluble polymer from solvent to a cold surface.4,6 The rate of deposition on equipment surfaces is particularly dependent on the surface temperature, suspension temperature, and average melting temperature of the wax.4 The deposition of such a foulant cannot be described by the particle mechanism, but it depends on the coupling of thermal and hydrodynamic transports. From this short introduction, it is suggested that the fouling generation and its control are complex issues, which depend on several variables of the polymerization technology. In this sense, it is necessary to consider an associative cause for fouling in slurry reactors of ethylene considering particularities as solvent (e.g., nature and quality), catalyst system (e.g., MgCl2- or SiO2based catalyst, size distribution of catalyst particle, prepolymerization), polymerization conditions (e.g., temperature of reactor medium), product characteristics (e.g., homopolymer, copolymer), and equipment designs (e.g., heat exchanger project). Despite many years of developments, the interplay of these variables on fouling formation in the polymerization reactors and in peripheral equipment is still unclear; thereby the predominant mechanism is not determined. With the aim of bringing out new aspects of fouling formation in slurry reactors designed for olefin polymerization, this work presents analytical data of fouling and inferences on the mechanism of deposition are made from these data.

ts − to −

K ts K to

− 1 = c[η]e 0.32c[η] (1)

where [η] is the intrinsic viscosity (dL·g−1), ts and to are the elution times (s) for the solution and for decalin, respectively, K is the viscometer constant, and c is the solution concentration (g·dL−1) at 135 °C (eq 2).

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c(135 °C) =

EXPERIMENTAL SECTION Samples of fouling were collected from an industrial process of slurry polymerization of ethylene. Such a process uses a cascade of polymerization reactors (stirred tanks) to produce highdensity polyethylenes with tailor-made properties. Comonomer (1-butene) was exclusively added to the second reactor aiming for the maximization of polymer properties. For comparison reasons, samples of fouling from a pilot plant of the same cascade polymerization technology were also obtained and characterized. It is worth mentioning that the heat transfer occurs using an external heat exchanger in the industrial plant, while in the pilot plant it occurs using a reactor jacket. Prior to

c(25 °C) 1.107

(2)

Polymer density was obtained in accordance with standard method ASTM D-1505.

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RESULTS AND DISCUSSION Analytical Characterization of the Fouling. Fouling from the Industrial Slurry Plant. The sample of fouling was taken from the pipeline between the reactor and the external heat exchanger (double pipe). An overview of the metal-side surface of the fouling sample is depicted in Figure 1. It is possible to notice that this fouling is formed by aggregation of well-defined polymer particles of average diameter of ca. 100 B


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Industrial & Engineering Chemistry Research

Figure 1. Overview of the metal-side surface of the fouling samples from the industrial plant.

Figure 3. Micrographs and microanalysis of the slurry-side surface of fouling samples from the industrial plant: (a) overview and (b) detail.

A more detailed micrograph of the metal-side surface of the fouling is found in Figure 2a, in which a domain that does not exhibit a defined morphology (cloudy region) is clearly visible among the individual particles. Figure 2b shows this cloudy region in detail. Microanalysis was conducted on this domain, and Fe, Si, Al, and Mg besides C and O were found as the typical composition of this material. Some bright domains (small bright points) in Figure 2 are related to materials of high Fe content (22.2 wt %), which, probably, is a residual content of the pipe construction material. Figure 3 depicts the slurry-side surface of the industrial fouling sample. It is noticeable from microanalysis that the composition of this side of the fouling is preponderantly C and O. This result is substantially different from the respective metal-side surface (Figure 2b), mainly in terms of the metals Al and Mg. Additionally, the presence of the cloudy region is not evident and, in terms of morphology, the particles are not flat, but are roughly spherical, of average diameter of ca. 100 μm. With this result showing differences of composition between the fouling sides, it is possible to affirm that the fouling mechanism is somewhat different from that occurring on the surface at the beginning. Fouling from the Reactor Wall of the Pilot Plant. Similar analysis was accomplished for the fouling samples collected from the reactor wall of the pilot plant operating with spherical

Figure 2. Micrographs and microanalysis of the metal-side surface of fouling samples from the industrial plant: (a) overview and (b) cloudy domain.

μm. Indeed, the particles are flat as they adhere to the surface of the metal. C


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Figure 4. Micrographs and microanalysis of the slurry-side (a, a′) and metal-side (b, b′) surfaces of fouling samples from the reactor wall of the slurry pilot plant. Catalyst system: nonspherical Ziegler−Natta (MgCl2-based support) + TEA.

powder are visible in the slurry-side micrographs of Figure 4, parts a and a′, respectively. The fouling material was analyzed in terms of intrinsic viscosity in an attempt to get information about its composition. An intrinsic viscosity around 1.0 dL·g−1 was found. Considering that the polymer made in the pilot plant at the respective moment presented an average intrinsic viscosity higher than 3.0 dL·g−1, it suggests that the fouling was formed by deposition of wax. It is important to notice that the heat transfer in the pilot plant exclusively occurs using a reactor jacket rather than using an external heat exchanger (double pipe) as in the industrial plant. In this sense, the hydrodynamics of flow and the lower heat transfer coefficient of the pilot reactor could impinge on conditions for wax deposition. Unfortunately, there is not enough experimental data to deeply investigate this result. Figure 5 depicts the fouling sample found when the pilot plant was operating with the spherical MgCl2-based catalyst. The qualitative compositions of both sides (see Figure 5a,c) are similar, formed predominantly by carbon and oxygen. In addition, the compositions of both surfaces have minor quantities of magnesium, chlorine, and titanium, which are

and nonspherical MgCl2-based Ziegler−Natta catalyst. Figure 4a,a′ depicts the slurry-side surface of the fouling sample collected when the plant was operating with nonspherical MgCl2-based catalyst, which is the same used in the industrial plant. As evidenced by microanalysis, the composition is mainly C and O but there is some content of Al. The microanalysis of the respective metal-side surface (Figure 4b,b′) shows the same component, but the Al content is higher than the respective slurry-side surface. Additionally, Cl has been also detected in the metal-side surface of the sample. In terms of morphology, both surfaces of the fouling sample from reactor wall exhibit cloudy regions; however, the cloudy domain is more intense in the metal-side surface as well as the Al content. According to this analysis, the sample of fouling from the pilot plant bears similarities in terms of composition to the industrial plant. Therefore, the initial mechanism of fouling might also be of similar characteristics. Opposite to the industrial fouling sample from the exchanger pipeline (Figure 3), the fouling material found in the reactor wall of the pilot plant presents a poor morphology, despite the catalyst being the same. Some particles of different sizes and a D


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morphology. Indeed, the average diameter of the particles adhered to the reactor wall is ca. 2 μm, suggesting that the fouling is formed by deposition of small particles of the catalyst. Also evident in Figure 5b,c is the presence of the cloudy domain as a thin layer between the deposit, which is partially formed by particles, and the reactor wall surface. The aluminum content is somewhat higher in the metal-side surface (Figure 5c). The analysis of the fouling sample from the pilot plant that works with a spherical catalyst also evidences the same composition features in the metal-side surface of the fouling in comparison with the nonspherical catalyst operating in both industrial and pilot plants. It validates the suggestion about a unique mechanism as a starting point for fouling in the slurry plants analyzed. In the case of slurry polymerization, the use of a low dielectric constant (e.g., n-heptane) increases the accumulation of electrostatic charges over the particles, mainly those of sizes ca. 1 μm.7 To overcome this problem, some commercial technologies of polymerization employ antistatic compounds in the reaction medium to increase its electrical conductivity. On the other hand, alkyl aluminum compounds, which are commonly used as cocatalyst, also present a similar effect when added in proper concentrations.11 The particulate material of the fouling depicted in Figure 5, which is clearly formed by the agglomeration of small catalyst particles, could be explained by the increase of electrical charges over the particles due to the low concentration of TEA in the reaction medium (Al/Ti = 1−4). Sample from the Transfer Line of the Pilot Plant. Figure 6a,a′ shows the micrographs for the slurry-side surface of the fouling sample. Microanalysis indicated the absence of Al or any other metal in the composition. The same results were found for the metal-side surface of this sample. Both samples do not show any well-defined morphology. This suggests that wax and/or polymer fines produce the fouling material. As a rationale from this result, the aluminum-based compound, when present on the metal-side surface, favors further adherence of particles of polymer or of fresh catalyst. Because of the hydrodynamic flow pattern and of the restriction of heat transfer coefficient, the deposition of wax occurs preferentially. Mechanism of Fouling. A rationale for the fouling mechanism could be built considering the microscopic examination of the samples and the respective microanalysis. The analytical results from the metal-side surface revealed that the fouling mechanism starts with the formation of a thin layer of aluminum compounds on the metal surface. This layer seems to play the role of an initial seed for further deposition of polymer particles and/or fresh catalyst, as the formation of such an aluminum-based layer on the metal surface leads to the increase of roughness.7,8 The polymer particles adhered on the former aluminumbased layer are classified as fines, as evidenced by the average particle diameter that is smaller than 125 μm. Additionally, very small particles of catalyst are also deposited on this previous layer. The adherence of small particles of polymer and catalyst could also occur either by the buildup of static charges or by settling in those regions of low velocities. The increase of roughness due to the aluminum-based layer makes easier the adherence of particulate materials on the surface.7 The presence of materials of poor morphology in the fouling samples is attributed to the concomitant adherence of soluble polymer (e.g., oligomers) to the layer of particles. In addition,

Figure 5. Detailed micrographs of the slurry-side (a) and metal-side (b, c) surfaces of fouling samples from the reactor wall of the pilot plant. Catalyst system: spherical Ziegler−Natta (MgCl2-based support) + TEA.

identified as bright spots in Figure 5b. This suggests that the catalyst particles are part of the material deposited. As corroborative evidence, Figure 5c shows clearly that the fouling is formed as an agglomeration of particles of spherical E


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Figure 6. Micrographs and microanalysis of the slurry-side (a, a′) and metal-side (b, b′) surfaces of fouling samples from the reactor transfer line of the slurry pilot plant.

polymerization technology could employ a significant level of scavenger during the start-up procedure to eliminate residual impurities, predominantly water, from the solvent. The product of the reaction of triethyl aluminum and water is a partially hydrolyzed adduct, which adheres to the metal surface due its low solubility in aliphatic solvents.12,13 Equation 3 illustrates the reaction of trialkyl aluminum (AlR3) and traces of water, forming an oligomeric oxygen-containing compound.14 For high quantities of water, the product of the complete hydrolysis of AlR3 is aluminum hydroxide.15

Figure 7. Scheme of fouling mechanism in slurry polymerization process.

small particles of polymer could be formed due to the poor fragmentation of the catalyst during the first moments of the polymerization. The mechanism of fouling for the slurry polymerization process is sketched in Figure 7. The formation of the aluminum-based layer on the metal surface, which is a central element of the suggested mechanism, might be related to the use of high content of alkyl aluminum as a scavenger, i.e., alkyl aluminum used to eliminate impurities in the reaction medium. It is important to notice that the slurry

It is worth mentioning that the results disclosed in this study, which support the proposed mechanism, were gathered analyzing samples from different parts of the process. Besides, as the reaction medium recirculates through the process equipment (reactors, heat exchanger, and pipelines, for instance), the proposed mechanism of fouling can occur in any part of the process. F


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(12) Malpass, D. B.; Palmaka, S. W.; Smith, G. M.; Rogers, J. S. Hydrocarbon Soluble Alkylaluminoxane Compositions Formed by Use of Non-Hydrolitic Means. U.S. Patent 5,777,143, July 7, 1998. (13) Sangokoya, S. A.; Wiegand, K. E. Production of HydrocarbonSoluble Hydrocarbylaluminoxanes. U.S. Patent 5,847,177, Dec 8, 1998. (14) Giannetti, E.; Nicoletti, G. M.; Mazzocchi, R. Homogeneous Ziegler−Natta Catalysis. II. Ethylene Polymerization by IVB Transition Metal Complexes/methyl Aluminoxane Catalyst Systems. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2117. (15) Schoenthal, G. W.; Slaugh, L. H. Process for Preparing Aluminoxanes. U.S. Patent 4,730,071, March 8, 1988.

CONCLUSION The analyses of fouling samples revealed the formation of an aluminum-based layer on the equipment surfaces that acts as a precursor layer for further depositions of materials, mainly particles. The formation of such a precursor layer is attributed to the elimination of impurities (e.g., water) by trialkyl aluminum compounds occurring at the start-up procedure after a plant breakdown. The product that is formed by the reaction of water and trialkyl aluminum presents low solubility in the hydrocarbon solvent; thereby it deposits on available surfaces forming fouling. As the deposition of this former aluminum-based layer on the metal surface leads to the increase of roughness, after this initial step, further deposition of small particles occurs easily. Concomitant anchoring of soluble components present in the reaction medium, such as wax, can also happen on this rough surface. The results from the current research will serve as guidelines for the development of solutions and/or industrial procedures aiming to reduce or even eliminate the formation of the aluminum-based layer on the equipment surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: adriano.fi[email protected]. Tel.: +55 51 3477 4000. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.N.C. acknowledges Braskem S.A. for financial support (Project ZPIB 201360).

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REFERENCES

(1) Bott, T. R. Fouling of Heat Exchangers; Elsevier Science & Technology Books: Amsterdam, The Netherlands, 1995; Vol. 26. (2) Saeda, S.; Suzaka, Y. Method for Preventing Fouling in the Polymerization of Olefins. U.S. Patent 3,956,252, May 11, 1976. (3) Henry, C.; Minier, J.-P.; Lefèvre, G. Towards a Description of Particulate Fouling: From Single Particle Deposition to Clogging. Adv. Colloid Interface Sci. 2012, 185−186, 34. (4) Towles, T. W.; Skinner, J. E.; DePierri, R. G.; Kendrick, J. A. Method for Controlling Fouling in Slurry-Type Polymerization Reactors. U.S. Patent 7,381,777 B1, June 3, 2008. (5) Soares, J. B. P.; Mckenna, T. F. L. Polyolefin Reaction Engineering; Wiley-VCH: Weinheim, Germany, 2012. (6) Buchelli, A.; Call, M. L.; Brown, A. L.; Bird, A.; Hearn, S.; Hannon, J. Modeling Fouling Effects in LDPE Tubular Polymerization Reactors. 3. Computational Fluid Dynamics Analysis of a Reacting Zone. Ind. Eng. Chem. Res. 2005, 44, 1493. (7) Oliveira, R. Understanding Adhesion: A Means for Preventing Fouling. Exp. Therm. Fluid Sci. 1997, 14, 316. (8) Katainen, J.; Paajanen, M.; Ahtola, E.; Pore, V.; Lahtinen, J. Adhesion as an Interplay between Particle Size and Surface Roughness. J. Colloid Interface Sci. 2006, 304, 524. (9) Bayat, M. H.; Abdouss, M. Antielectrostatic Agent Addition in Low-Dielectric-Constant Polymerization Media. J. Appl. Polym. Sci. 2012, 125, 1979. (10) Mousavi, M.; Hakim, S.; Nekoomanesh, M. Effect of Fatty Amine and Perfluorocarbon as Anti-Fouling Agent on the Catalyst Activity and Titanium Oxidation State in Slurry Polymerization of Ethylene. J. Appl. Polym. Sci. 2006, 102, 257. (11) Bushick, R. D.; Stearns, R. S. Relationship between the Ionic Nature of Some Organoaluminumtransition Metal Catalysts and the Rate of Polymerization. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 215. G


Mechanism of Fouling in Slurry Polymerization Reactors of Olefins

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