Reactor Fouling Research Articles

Overview and some issues related to co‐firing biomass and coal

AbstractLow heating values, variable chemical compositions, peculiar physical properties, high investment cost and insecurity of biomass feedstocks supply limit the applications of biomass for energy and other processes. Co‐firing biomass and coal has potential for the development of biomass‐to‐energy capacity with significant economic, environmental, and social benefits. However, co‐firing is not straightforward, and some questions need to be addressed due to the differences in chemical compositions and physical properties of biomass and coal. This paper highlights key issues related to co‐firing, including reactor types, feeding, hydrodynamics, ash sintering, fouling, and corrosion, based on previous studies, as well as calculations and analysis. Direct co‐firing is the most common option for biomass and coal co‐firing currently, mostly due to relatively low investment needed to turn existing coal power plants into co‐firing plants. For direct co‐firing, the physical characteristics and chemical compositions of the fuel entering the combustors or gasifiers are critical to an optimum operation. Any biomass mixed with coal needs to have acceptable physical properties. More research is needed on co‐firing biomass and coal, including work on: preparation, handling, storage, and feeding of biomass feedstocks (e.g. drying, torrefaction, pelletization); co‐firing mechanisms; hydrodynamic analysis of co‐firing combustors and gasifiers; boiler/gasifier capacity, slagging, fouling, corrosion, efficiency, reliability, fuel flexibility; lower emissions and gas cleaning; catalyst poisoning; investment and operating costs.

The Advantages of Metal Complexes as Catalysts toward Polyolefins ‐ The Vinyl Polymerization of Norbornene

AbstractVarious nickel complexes bearing [N,O], [P,N] ligands as well as the supported nickel complexes were prepared and showed moderate to high catalytic activities for vinyl polymerization of norbornene. The formed polynorbornenes (PNBs) are soluble for measuring both the molecular weights and the molecular weight distributions. Comparing the results in homogeneous catalysis, the PNBs produced by supported catalysts possessed higher molecular weights and slightly broader molecular weight distributions. The reactor fouling generally happens in homogeneous catalytic systems, while spherical polymer particle morphology could be obtained without reactor fouling by using MgCl2‐supported nickel catalysts.

Microfluidic generation of metal nanoparticles by borohydride reduction

In this work a microfluidic system for the production of very small gold nanoparticles (GNP) by reduction with borohydride will be presented, as well as their direct surface modification with thiol ligands. Furthermore, the microsystem was tested for the generation of other metal nanoparticles, as for example, silver, copper and complex particles. The size distributions were determined using scanning electron microscopy and an analytical centrifugation method. Additionally, in situ UV–vis spectroscopy was applied for online monitoring and kinetic measurements. Reactor fouling in the course of the syntheses was monitored with a microscope camera, and methods like silanization and pH-adjustment were explored to minimize the fouling. The direct reduction of chloroauric acid with borohydride yielded reproducibly gold nanoparticles in the size range between 4 and 7 nm without recognizable fouling of the reactor. Between 200 and 4000 μl/min, the flow rate does not affect the size distribution of the obtained gold nanoparticle dispersions. An increasing dilution of the reactants leads to a decrease in the mean diameter of the particles. The influence of the concentration ratio (BH 4 − )/Au and the effect of the pH were also investigated, but no significant trend was observed. Surface modification of these particles was achieved in the microsystem and monitored via UV–vis measurement and size distribution determination.

Metallocene‐Catalyzed Polymerization in Nonaqueous Fluorous Emulsion

AbstractA nonaqueous organic‐in‐fluorous emulsion applicable for polymerization using water‐sensitive catalysts is presented. With statistical biphasic copolymers based on poly(4‐hydroxystyrene) containing fluorinated and nonfluorinated alkyl side chains, emulsions of hydrocarbons dispersed in perfluorocarbons were stabilized. The addition of metallocenes, activated by methylalumoxane (MAO), enabled the polymerization of olefins inside this highly hydrophobic system. High molecular weight polyolefin nanoparticles ($\overline M _{\rm n}$ up to 450 000) of polyethylene and poly(propylene) were synthesized without supporting the catalyst and without reactor fouling. In this diffusion‐controlled process, the sizes of the particles were controlled by the reaction time and the ratio of the emulsifier to solvents.magnified image

Influence of silica calcination temperature on the performance of supported catalyst SiO2–nBuSnCl3/MAO/(nBuCp)2ZrCl2 polymerizing ethylene without separately feeding the MAO cocatalyst

Abstract The effects of support calcination temperature, an important catalyst synthesis parameter, on the overall performance of the supported catalyst [silica ES70– n BuSnCl 3 /MAO/( n BuCp) 2 ZrCl 2 ], polymerizing ethylene without separately feeding the MAO cocatalyst, were studied. The silica was calcined at 250, 450, 600, and 800 °C for 4 h. n BuSnCl 3 was used to functionalize the silica. Ethylene was polymerized using the synthesized catalysts at 8.5 bar(g) in hexane for 1 h. No reactor fouling was observed. Free-flowing polymer particles with bulk density (0.23–0.27 g/ml) and a fairly spherical morphology similar to that of the catalyst particles were obtained. Also, the particle size distribution of the polymer resembled that of the catalyst. Therefore, the replication phenomenon from catalyst to polymer took place. The narrow PSD span (1.41) indicates that the resulting polyethylenes are suitable for various mixing-intensive polymer applications. The MAO cocatalyst-free ethylene polymerization instantaneously formed a polymer film around the catalyst particle, which coated/immobilized the catalyst constituents; this is how leaching was in situ prevented which favored heterogeneous catalysis to occur. The catalysts showed fairly stable polymerization kinetics. The catalyst activity, as a function of the silica calcination temperature, varied as follows: 250 °C > 600 °C > 800 °C > 450 °C. This finding has been explained considering the relevant surface chemistry phenomena. The calcination temperature did not significantly affect the bulk density and the PDI (3.4 ≤ PDI ≤ 3.8) of the resulting polyethylenes. The low PDI substantiates the retention of single-site catalytic behavior of the experimental supported catalysts.

Preparation of spherical MgCl 2-supported bis(phenoxy-imine) zirconium complex for ethylene polymerization

The spherical MgCl 2 supports treated by methylaluminoxane (named MAO-supports) were prepared using MgCl 2·2.56C 2H 5OH adducts as support material subjected to elimination of EtOH by thermal dealcoholization treatments. Then bis(phenoxy-imine) zirconium complex, bis-[ N-(3- tert-butylsalicylidence)cyclohexylaminato] zirconium(IV) dichloride ( 1), was directly impregnated onto these MAO-supports. ICP, EDX, GC, N 2-BET, IR and XRD analyses indicated that the texture of the supports and MAO-supports, the elements on solid catalysts, and so on, were greatly relied on dealcoholization temperature. Meanwhile, it was also found that the activities of the supported catalysts for ethylene polymerization and the properties of resultant polymers were strongly dependent on dealcoholization temperature. Finally, PEs with spherical morphology and high bulk density (over 0.35 g/ml) were obtained, without reactor fouling.

Efficient Slurry-Phase Homopolymerization of Ethylene to Branched Polyethylenes Using α-Diimine Nickel(II) Catalysts Covalently Linked to Silica Supports

α-Diimine nickel(II) complexes covalently linked to a silica support were obtained by the reaction of an amino- or hydroxy-functionalized α-diimine nickel complex with silica particles treated with trimethyl aluminum. These silica-supported aryl α-diimine nickel(II) precatalysts linked via an amino (1a) or hydroxy functionality (1b−c) at the p-aryl position, and precatalysts linked via one (2a) or two (2b) hydroxy functionalities in the alkyl backbone were used in slurry polymerizations of ethylene to produce branched polyethylenes. Activation of precatalysts with ethylaluminum sesquichloride was effective even at very low Al/Ni ratios of 48. Initial polymerizations, performed at 60 °C and 150 psig ethylene, showed excellent productivities of up to 10.8 kg PE per gram of 1b (3.0 wt % nickel loading). Low productivities at temperatures above 60 °C can in part be overcome by increase in ethylene pressure. For example, a productivity of 6.0 kg PE per gram of 1b (3.0 wt % nickel loading) was obtained at 80 °C and 700 psig ethylene. The best polymerization productivities were obtained with trimethylaluminum as a traceless silica-linker, however, tetrachlorosilane and trichloroborane are attractive alternatives. Branching densities can be controlled by choice of ortho-aryl substituents. No reactor fouling occurs with these supported catalysts.

Vinyl polymerization of norbornene over supported nickel catalyst

AbstractA series of nickel complexes, bis(salicylideneiminato)nickel(II), were supported on spherical MgCl2 and SiO2. Scanning electron microscopy, energy‐dispersed X‐ray spectroscopy, and the BET method for surface areas measurements were utilized to examine the supporting process of the catalysts. The particle morphology of the original support is retained and replicated throughout the supported catalyst preparation and norbornene polymerization. Spherical polymer particle morphology was achieved, without reactor fouling. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2233–2240, 2006

Transition metal complexes of polymeric Schiff bases as catalyst precursors for the polymerization of ethylene

Coordination compounds derived from polymeric Schiff bases can be used as catalyst precursors for the polymerization of ethylene. After the activation with MAO, these systems are of heterogeneous nature and can prevent reactor fouling. The structure of the ligand backbone has strong influence on the polymerization activities and the properties of the resulting polyethylenes. Force field calculations using Allingers MM3 point out minimum energy structures, which help to explain the experimental results.

SCWO of salt containing artificial wastewater using a transpiring-wall reactor: Experimental results

Experimental results using a transpiring-wall reactor (TWR) for supercritical water oxidation (SCWO) containing a hydrothermal flame as an internal heat source are presented. Our reactor set-up is aimed at overcoming the problem of reactor fouling and plugging due to precipitation of salts. Two types of transpiring-wall elements with different porosity were investigated. Experiments with artificial wastewater containing methanol and sodium sulfate in concentrations up to 6 and 3 wt.%, respectively, were used for a maximum operation time of 3 h. The salt deposition in the reactor was determined by electric conductivity and ionic chromatography measurements of the reactor effluent. About 65 % of the salt introduced to the reactor was detected in the effluent. No plugging of the reactor and equipment was observed during the experiments and methanol conversion ratios higher than 99.99 % were measured.

Selective ethylene trimerization: A study into the mechanism and the reduction of PE formation

In the present study, the titanium-catalyzed ethylene trimerization in general, and more specifically, the concomitant PE formation have been studied. The polymer formation is undesirable as it not only will lead to lower 1-hexene yields, but it can cause reactor fouling under the applied conditions (30–80 °C, toluene solvent). It is, therefore, important to know which factors are involved in the formation of polymeric products and how their formation can be reduced or even prevented. The PE formation turns out to be catalyzed by at least two different species. A significant amount of PE is formed in the early stages of the reaction, caused by the presence of partly alkylated titanium species. The PE formation during later stages of the reaction is due to degraded catalyst species, which means that polymer formation is an inherent property of this catalyst system. The polymer output can be reduced largely by premixing Cp′TiCl 3 and methyl aluminoxane (MAO) prior to injection into the reactor. It was also demonstrated that the type of MAO activator/impurity scavenger is of great importance. For a low yield of PE it is essential to use an MAO that does not contain and/or is not able to generate aluminum hydride species. In the end the best results with respect to both productivity and selectivity were obtained by starting from trimethyl titanium compounds.

Supported zirconocene catalyst for preventing reactor fouling in ethylene polymerization

AbstractThe commercial interest of metallocene complexes for olefin polymerization has led to additional efforts to prepare suitable metallocene complexes efficiently and economically. Ethylene polymerization was carried out with a series of heterogeneous catalysts which were prepared in various Zr/silica ratios by immobilization of Ind2ZrCl2 preactivated with methylaluminoxane (MAO) on silica. This method to form the catalyst system resulted in a polymerization catalyst with reduced fouling tendencies and improved reactor operability. Polymerization of ethylene was conducted in Buchi reactors in a slurry phase under mild pressure. Some of the physical properties of the obtained polymers were also determined. Copyright © 2005 Society of Chemical Industry

Continuous Synthesis of Gold Nanoparticles in a Microreactor

A continuous flow microreactor was used for the synthesis of gold nanoparticles (5 to 50 nm) directly from a gold salt (HAuCl(4)) and a reducing agent (ascorbic acid). Experimental parameters were optimized to obtain narrow size distributions, which were at average two times narrower than those obtained in a conventional synthesis. Additionally, two approaches, i.e., elevation of pH during reaction and hydrophobization of internal reactor surfaces were tested to suppress reactor fouling.

Immobilization and activation of vanadium(III) and titanium(III) single‐site catalysts for ethylene polymerization using MgCl2‐based supports

AbstractSignificant increases in the activity of vanadium(III) amidinate catalysts for ethylene polymerization have been obtained by immobilization on a MgCl2‐based support prepared by reaction of AlEt3 with a MgCl2/ethanol adduct. Catalyst immobilization and activation on this type of support prevents the rapid decay in activity observed under homogeneous polymerization conditions with unsupported catalysts. Stable polymerization activity is also observed with analogous titanium(III) complexes. Polyethylene with narrow molecular weight distribution and spherical particle morphology is obtained without reactor fouling. Copyright © 2005 Society of Chemical Industry

Dynamic On‐Line Reoptimization Control of a Batch MMA Polymerization Reactor Using Hybrid Neural Network Models

AbstractA hybrid neural network model based on‐line reoptimization control strategy is developed for a batch polymerization reactor. To address the difficulties in batch polymerization reactor modeling, the hybrid neural network model contains a simplified mechanistic model covering material balance assuming perfect temperature control, and recurrent neural networks modeling the residuals of the simplified mechanistic model due to imperfect temperature control. This hybrid neural network model is used to calculate the optimal control policy. A difficulty in the optimal control of batch polymerization reactors is that the optimization effort can be seriously hampered by unknown disturbances such as reactive impurities and reactor fouling. With the presence of an unknown amount of reactive impurities, the off‐line calculated optimal control profile will be no longer optimal. To address this issue, a strategy combining on‐line reactive impurity estimation and on‐line reoptimization is proposed in this paper. The amount of reactive impurities is estimated on‐line during the early stage of a batch by using a neural network based inverse model. Based on the estimated amount of reactive impurities, on‐line reoptimization is then applied to calculate the optimal reactor temperature profile for the remaining time period of the batch reactor operation. This approach is illustrated on the optimization control of a simulated batch methyl methacrylate polymerization process.

Potentials and Limitations of Automated Parallel Emulsion Polymerization

AbstractThe application of automated parallel synthesizer robots for the investigation of polymerization processes is of major interest at present. In this contribution we describe the application of the emulsion polymerization of styrene and vinyl acetate. The preparations of emulsions and latexes were investigated in detail and compared to “conventional” stirred tank reactors. In particular the influence of the vortex mixing as well as the limitations regarding solid content and reactor fouling are addressed. magnified image

Application of exponentially weighted principal component analysis for the monitoring of a polymer film manufacturing process

Multivariate statistical representations have been widely used in the process manufacturing industries for process performance monitoring, in particular for the detection of changes in current operation and the onset of process disturbances or faults. Applications of the technology have focused to a lesser extent on manufacturing processes where drift occurs over time as part of normal process operation, e.g., due to reactor fouling, machine wear, ramping of temperatures during process operation, and changes due to set-point adjustments. In this paper, an extension to the methodology based on the statistical projection technique of principal component analysis (PCA) is proposed for the monitoring of processes where drift and set-points changes are common place, i.e., exponentially weighted PCA. The technique is illustrated through its application to a polymer film manufacturing process where the representation is required to adapt quickly to changes in the process that are part of normal operating procedures, but remain sensitive to the detection of deviations from normal operation.

Borane-functionalized silica supports: In situ activated heterogeneous zirconocene catalysts for MAO-free ethylene polymerization

We treated silica with tris(pentafluorophenyl)borane, B(C 6F 5) 3, to create borane-functionalized support, SiO 2–B(C 6F 5) 3 which was then used as a support and co-catalyst for the in situ activated dichloro-zirconocene (Cp 2ZrCl 2/TIBA) and dimethyl-zirconocene catalyst systems (Cp 2Zr(CH 3) 2) for ethylene polymerization. The surface modifications of SiO 2–B(C 6F 5) 3 was investigated by SEM–EDX, FTIR and XPS to determine the distribution of fluorine and boron atoms on the silica surface as a measure of the presence of B(C 6F 5) 3 and interaction with the siloxane groups of the silica support. Catalytic performances are presented in the form of ethylene consumption rate profiles. The results show that the independent of the time dedicated to the in situ activation of the metallocene, all pre-activated systems showed activity decay. In contrast, the in situ activated systems either showed little or no activity decay. The presence of TIBA was adequate to make an active MAO-free catalyst system. The pre-activated and in situ activated metallocene systems produced polyethylene with slightly different molecular weights (MWs) and molecular weight distribution (MWD). MWDs were quite narrow ∼3. The bulk density of polyethylene product is highest for the in situ activated metallocenes, but there is no difference between the products of dichloro- and dimethyl-zirconocenes. Pre-activated and in situ activated catalysts show no reactor fouling.

Polymerisation of ethylene or propylene with heterogeneous metallocene catalysts on clay minerals

Heterogeneous metallocene catalysts avoid reactor fouling in olefin polymerisations. For technical use the metallocene catalysts must be fixed on carriers. Silica is one of the favoured inorganic carriers. But the activities of heterogeneous metallocene catalysts on silica in ethylene and propylene polymerisation are often lower than those of comparable homogeneous catalyst systems. We tested the clay minerals kaolin and montmorillonite as inorganic carriers for the polymerisation of ethylene and propylene with commercially available titanocenes and zirconocenes and also with new types of titanocenes. The heterogeneous catalysts on kaolin were less active in ethylene polymerisation as comparable homogeneous catalysts and they were not active in propylene polymerisation. In contrast the heterogeneous catalysts on montmorillonite are often more active in ethylene or propylene polymerisation than comparable homogeneous systems. As cocatalysts trimethylaluminium (AlMe 3, TMA) and triisobutylaluminium (Al( i Bu 3), TIBA) were used for heterogeneous polymerisations. As catalysts commercially available titanocenes and zirconocenes and also four titanocenes with unusual structures were tested which Lang et al. had synthesised. The high activities in ethylene or propylene polymerisations with montmorillonite may be due to the special structure of the montmorillonite carrier.

Metallocene derivatives as active supports of zirconium‐based catalysts for ethene polymerization

AbstractCyclopentadienyl magnesium chloride (MgClCp) and its functionalized derivatives represent original and interesting supporting materials to heterogenize metallocene catalysts for olefin polymerizations. The synthesis of MgClCp, its functionalization, and the preparation of a catalytic system in which the ZrCl2(Flu)+ moiety is joined on the support through a cyclopentadienyl ligand are reported. This catalyst was tested in ethene polymerization, and both the catalytic activity and properties of the produced polymer were measured. Its performance was compared with that shown by the catalyst ZrCl2CpFlu employed under the same conditions for both unsupported and conventional supports, such as MgCl2. The results showed a remarkable improvement in terms of the activity and polymer properties with these heterogenized catalysts. Moreover, this system showed stability toward leaching processes and was characterized by good morphological control of the growing polymer. Finally, catalysts in which [HB(3,5‐Me2pyrazolyl)3]ZrCl and [HB(3,5‐Me2pyrazolyl)3]ZrClOtBu+ moieties were bonded to a functionalized MgClCp− support were also synthesized and tested. The results showed that the proposed supports could be usefully used to heterogenize tailored metallocene homogeneous catalysts. In fact, new catalysts were prepared that combined the peculiar advantages of both heterogeneous and homogeneous catalysts and overcame the disadvantages of the latter, such as a lack of morphology and reactor fouling. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4243–4248, 2001