XYLENE ISOMERIZATION UNIT CATALYST DEACTIVATION: A

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Xylene Isomerization Unit Catalyst Deactivation: Quantitative Case Study of an Industrial Unit Wanessa B. Costa*,† and Carlos A. Pires*,‡ †

Process Engineering Q1 BA, Braskem S.A., Rua Eteno, 1561, Copec, Polo de Camaçari, Camaçari, Bahia, Brasil CEP 42810-000 Chemical Reaction Engineering Laboratory, PPEQ, Escola Politécnica, UFBA, Rua Aristides Novis, n° 2, 2° andar, Federaçaõ , Salvador, Bahia, Brasil CEP 40.210-910

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/06/18. For personal use only.

‡

ABSTRACT: The isomerization of xylenes is a catalytic transformation process of metaxylene and ethylbenzene, which aims to restore the balance between both isomers and convert the ethylbenzene to xylenes. The knowledge of the catalytic deactivation behavior between stages of operation or campaigns (catalyst that has undergone successive regenerations and activations) in xylenes isomerization is very important to determine the useful life of the catalyst and, consequently, to improve the estimation of the economic viability to operate the unit, whether with a new or an old kind of catalyst. This work is a preliminary case study of catalyst deactivation, focused on pre-existing industrial data production of xylenes from a unit found in a famous industry known as Braskem, which is located in Brazil. This article involves monitoring a catalyst load throughout its useful life, which has gone through seven consecutive campaigns. The first stage of the work involved the definition of the variables that most influenced the ethylbenzene conversion and the proposition of two mathematical models, one linear and another quadratic, whose parameters were found from real plant data. The quadratic model best represented the actual behavior of the system, and the methodology used led to the conclusion that the analysis of the real ethylbenzene conversion data between the campaigns would not be sufficient to elucidate the catalyst deactivation profile due to the variability of operating conditions between the different campaigns. The catalyst irreversible activity losses (deactivation rates) between the campaigns were calculated from the quadratic models, to determine the variation of the conversion between the campaigns. Finally, an economic evaluation was done with the aim of evaluating the optimal catalyst lifetime.

1. INTRODUCTION Xylenes are very important aromatic products for the world market. Therefore, the growing demand for paraxylene (PX) has driven expansions of production capacities and the creation of new petrochemical plants. The PX is used as a raw material for PTAs (purified terephthalic acids) production. It is from the PTA that it is possible to produce the textile polyester used in the manufacture of fabrics, PET resins (polyethylene terephthalate) employed in the manufacture of many types of packaging and also industrial fibers, for tire production, materials and equipment for the electrical sector and automotive and oil industry.1 The orthoxylene (OX) market is also important for the manufacture of phthalic anhydride, which is used for the production of PVC stabilizers, plasticizers, polyester resins, alkyd resins, and synthetic dyes (flavorings and dryers for paints and pharmaceuticals).2 The PX and OX, as well as benzene, toluene, and C9 solvents, are species that have high aggregate value in the industrial environment, being produced from the cracking and catalytic reform of naphtha, after a sequence of separation and reaction processes, involving distillation, extraction, isomerization, and adsorption.3 © XXXX American Chemical Society

The isomers of xylenes and ethylbenzene are compounds that have very close boiling points,4 what makes it difficult to separate by common distillation. The PX is separated by an adsorption process, and the OX, isomer which has the highest boiling point, is separated by superfractionation, involving a distillation column with a high quantity of trays (between 130 and 170) and high reflux ratios. The metaxylene (MX) is isomerized to PX and OX, and the ethylbenzene is converted to xylenes. The unit group responsible for these processes is denominated in Braskem as LOOP C8. The Xylene Isomerization Unit, locally called Isomar, is part of the LOOP C8 and performs a primordial function for maximizing the production of orthoxylene and paraxylene due to the ethylbenzene’s transformation capacity to xylenes and isomerization of metaxylene. If this unit did not exist, there would be a buildup of ethylbenzene and metaxylene in the LOOP C8, making it impossible to produce PX and OX on a large scale. Received: Revised: Accepted: Published: A

September 20, 2018 November 20, 2018 November 27, 2018 November 27, 2018 DOI: 10.1021/acs.iecr.8b04601 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

expected to be predominant because the concentration of PX and OX is low in the unit feed. KazemeinI et al.8 studied and modeled the liquid phase deactivation of the alkylation reaction between isobutene with butene 2. They used experimental data to get the kinetics parameters to develop the model for the alkylation reaction. The deactivation model was determined empirically using an expression of the power law. However, due the diffusion limitations, it was not able to properly describe the deactivation mechanism over the entire stage of deactivation and they did not compare the results with an industrial unit. Murzin et al.9 developed an integrated model of catalytic reaction and deactivation from sitosterol hydrogenation reaction to sitostanol on a catalyst supported with platinum. The phenomenological model was based on the concept of adsorption, reaction, and contamination. They obtained simple semianalytic mathematical solutions that reveal how the operation mode of the reactor influences the reaction kinetics, including poisoning. They considered that the mechanismbased approach was successfully applied to a real catalytic process. However, they only compared the results with the planned experiments, and the real industrial data was not used to validate the model. Yeong et al.11 developed a phenomenological model representing the metaxylene isomerization, combined with the separation of xylene products using a reactor with silicate acid membranes-1. The simulated MX conversion, PX selectivity, PX yield, PX flux, and PX/OX separation factor showed good agreement with the experimental data. The mathematical model considered the parameter of catalytic deactivation. However, the presence of ethylbenzene, a component that is always present with metaxylene in an industrial environment, was not considered, and the results were not compared with a real industrial unit. Frantsina et al.12 developed a phenomenological mathematical model to control an alkane catalytic dehydrogenation industrial plant to produce the linear alkyl benzene (LAB). They used experimental bench data to regress model parameters and considered catalytic deactivation in process modeling. The results from this work showed that the combination of full-scale industrial and computational experiments, considering the physicochemical analysis and kinetic modeling, helped optimize the process of C9−C14 n-alkane dehydrogenation on a Pt catalyst. Fayon et al.10 studied the recovery of xylenes in the LOOP C8 units, considering a steady-state simulation that aims optimizing cycle length for isomerization catalyst. The costs of separating ethylbenzene as final product were evaluated, and the best configuration to the produce ethylbenzene was defined. The simulation results showed that the Isomar optimal campaign is approximately six months. Longer campaigns affect operating costs elevations. In the other hand, shorter campaigns increase the amount of unit shutdowns for catalyst regeneration and replacement. The study was conducted assuming that the life of the catalyst was 3.5 years and the catalytic deactivation rate was 2.6 °F/week. On the basis of the information contained in the literature, it was possible to verify that there are several works that present good mathematical modeling of the catalytic deactivation, considering data extracted from planned experiments. Works that use industrial data or that try to model industrial units appear in less quantity. No studies involving the evaluation of a catalyst deactivation rate throughout the campaign and

Over the years, various catalyst technologies for aromatic C8 (C8A) isomerization have been developed with the aim of maximizing the yields of industrial processes. In addition to high activity and selectivity, a catalyst must exhibit high stability with the maintenance of maximum yields for a long time. The combination of high activity and adequate selectivity and stability over the life of the product is a challenge for the producer. The successive regenerations and activations of these catalysts between campaigns make them less active. Proper campaign management and optimization of reaction parameters allow the determination of the appropriate period for the replacement of the asset (catalyst) and allows raising the business profit margins. Articles published on the deactivation and useful life of the catalyst related to the isomerization of xylenes and ethylbenzene transformation do not concentrate on industrial units. The works are done in the laboratory, whose environment is well controlled, and do not consider the common variations of an industrial process. Researchers, such as Froment, Bischoff, Butt, and Levenspiel, have widely described the catalysts deactivation mechanisms and mathematical modeling in the presence and absence of mass transfer limitations. Other authors have also reported works in the literature to elucidate the deactivation mechanism in catalysts. Henriques et al.5 used an orthoxylene isomerization system on zeolite to characterize the coke formation. They identified that the temperature directly influences the composition and coke formation. At lower temperatures, it is strongly adsorbed in the zeolite. At high temperatures, the coke tends to be more aromatic and is retained in the catalyst’s pores. Guisnet et al.6 studied coke formation from the propene (13 kPa and 450 °C) in two zeolites types, HEMT and HFAU, through bench scale experiments. They verified that its formation in both zeolites belonged to the same family, but the coke formation was faster in the zeolite of higher acidity (HEMT). The removal of coke through regeneration was also studied, and the zeolite with larger pore size (HFAU) showed easier coke removal due to the easy oxygen access to the pores of the support. In relation to the work involving mathematical models of reactions, including catalytic deactivation, it can be observed that the deactivation parameter in the models is normally empirical, being represented by a linear, exponential, or hyperbolic function. Al-Khattaf et al.7 studied the isomerization reactions of xylenes in zeolite ZSM-5 using a commercial simulator, which reproduced the operational conditions of a fluidized bed reactor. Experimental runs were carried out in the riser simulator using a reactor bench-scale equipment with an internal recycle to get kinetic parameters. They achieved the main objective that was to resolve the controversy as to whether xylene isomerization reaction takes place with a good comparison between experimental data and model predictions, considering an exponential function of deactivation as a function of time. They proved that mutual interconversion between PX and OX (a 1,3-methyl shift) occurs with the same rate as the conversion of MX to OX (a 1,2-methyl shift) over ZSM-5 zeolite catalyst. However, they did not consider the presence of ethylbenzene, a component that is always present with metaxylene in an industrial environment. Another important factor is that, in some industries, since Isomar feed is richer in MX and EB, the 1,2-methyl shift route is B

DOI: 10.1021/acs.iecr.8b04601 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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The furnace is then the main equipment responsible for the heating (vaporization) of the feed (stream 01) and the recycle hydrogen (stream 02). The PAREX effluent constitutes the isomar feed, which is a rich mixture in metaxylene/ethylbenzene and poor in orthoxylene/paraxylene. Hydrogen has the function of minimizing the coke formation on the catalyst surface and participates in the conversion intermediate reactions of ethylbenzene to xylenes at equilibrium. The hydrogen make up replenishes the small amount of H2 consumed in the isomerization reactor by saturation, dealkylation, and cracking reactions. Consequently, the required amount of hydrogen make up is a function of the catalyst selectivity. Xylenes and ethylbenzene isomerization reactions occur in a fixed bed reactor with radial and centripetal flow. Isomar reactor is formed by two coaxial cylinders and the catalyst fills the space between these cylinders. The vaporized feed enters at the top of the reactor and is directed to the side wall. The vapor phase travels radially through the fixed bed and is directed to the central tube. The effluent from the reactor then flows through the central pipe to the bottom of the reactor. The reactor effluent (stream 05) is sent to a separator vessel, after passing through a set of heat exchangers. The separation of the gaseous phase (stream 06) and the liquid phase (stream 07) occurs on the separator. The gaseous phase mixes with the make up hydrogen (stream 03) and is sent to the furnace with the aid of a centrifugal compressor. The liquid phase is sent to the deheptanizer distillation column, responsible for separating fuel gas (stream 08) and light hydrocarbons (stream 09). These streams are generated due to the secondary and undesired reactions occurring in the reactional system. The deheptanizer bottom product (stream 10) is sent to the clay tower for removing olefinic compounds. The clay tower effluent (stream 11) is sent to the xylene fractionation unit. 2.3. Xylene Isomerization Unit Description: Isomar Reactions. Isomar catalytic process is composed of several reactions. The main ones comprise the xylenes isomerization to equilibrium16,19,20 and ethylbenzene conversion to xylenes in equilibrium as well.17 Secondary reactions are responsible for reducing unit yields and are related to the reaction severity: dealkylation (ethylbenzene or xylene are converted into benzene and fuel gas),21 transalkylation (xylenes react to form toluene and trimethylbenzene),21 hydrogenation (xylene reacts with hydrogen to form naphthenic),21 hydrogenolysis (xylene reacts with hydrogen to form toluene and methane),18 ethylbenzene disporportionation forming diethylbenzene and benzene,26 and coke formation. 2.4. Process Variables: Variables Description. The main variables of the C8A isomerization process are campaign time, processed feed mass or accumulated feed, temperature, hydrogen partial pressure, spatial velocity (WHSV), and hydrogen to hydrocarbon molar ratio. The campaign time and the processed feed mass are variables defined based on the catalytic process yield profile, in the planning of stopping the units involved with isomar (catalytic reform and toluene disproportionation units), to coincide with the regeneration of other catalysts, and in the commercial and logistical issues of the main products. C8A isomerization reactions are governed by equilibrium and reversibility conditions. As the temperature increases, the isomerization approaches the equilibrium, while the cracking reactions continue to occur. For this reason, it is desirable to

between the regenerations using the xylenes isomerization industrial data was found in the literature. Knowledge of the catalytic deactivation behavior between campaigns is of extreme importance to determine the life of a catalyst and thus improve the estimation of economic viability to operate the unit, whether with a new catalyst or an aging catalyst. The aim of this work is to evaluate the catalyst deactivation behavior during and between the campaigns of a Braskem Unit. The first stage of the work involved the definition of the variables that most influenced the conversion of ethylbenzene and the proposition of the linear and quadratic models, whose parameters were found from actual plant data. In the second stage, a simple analysis of the actual ethylbenzene conversion data between the campaigns and an analysis involving the calculation of the catalyst irreversible activity loss between the campaigns was carried out, with the quadratic models developed. Finally, an economic evaluation was carried out with the objective of evaluating the optimal catalyst lifetime. These results will serve as a model for the realization of economic forecasts related to the catalysts operating times of various catalytic processes.

2. METHODOLOGY 2.1. Xylene Isomerization Unit Description: Overview. Paraxylene and orthoxylene production process configuration, known as LOOP C8, is formed by the xylenes fractionation, PAREX, and isomar units (Figure 1). The

Figure 1. LOOP C8 Units configuration (PAREX Process, 1979, UOP-Honeywell).

xylenes fractionation unit is fed by mixed xylenes from naphtha cracking13 and heavy reformate from the naphtha reform.14 In addition to these currents, this unit is fed by a mixture of xylenes and ethylbenzene, in chemical equilibrium, from the isomar unit. The xylenes fractionation unit produces OX, C9, C10, and a stream with the mixture of EB, MX, and PX, which is sent to the PAREX unit. The PAREX unit is fed by the ethylbenzene (EB), MX, and PX mixture from the xylenes fractionation unit and a PX stream from the toluene disproportionation unit.15 The PAREX unit aims to produce PX and, in addition, sends to the Isomar Unit a mixture of EB and MX, poor in OX and PX. In the isomar unit, after the balance between the PX, MX, OX, and EB is re-established the unit effluent is sent to the xylenes fractionation unit. 2.2. Xylene Isomerization Unit Description: Unit Configuration. Xylenes isomerization unit is composed of several equipment with different unit functions, as illustrated in Figure 2. First, the stream from the PAREX unit is combined with the recycled hydrogen stream and then is preheated or vaporized in the effluent/feed exchanger and sent to the furnace to achieve the desired reaction temperature. C


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Figure 2. Xylene isomerization unit simplified flowchart.

secondary and unwanted reactions rate elevation. Therefore, in order to maintain constant the reaction severity, the adjustment of the “pressure x temperature” pair will be required for each desired spatial velocity and will depend on the campaign in which the catalyst is subjected. Due to the deactivation by the coke formation throughout a campaign, it is necessary to raise the reaction severity, to keep the same main products reaction rate. Another important parameter to control the catalyst deactivation rate is the H2/HC molar ratio, defined as the amount of moles of hydrogen divided by the hydrocarbons moles present in the reactor. The higher the ratio, the longer is the catalyst campaign, since in general, increasing hydrogen, the rate of deactivation reduces due to saturation of coke precursors. However, the tendency for unwanted secondary reactions to occur, such as the dealkylation and hydrogenation, increase with the increasing in the H2/HC molar ratio. Typical H2/HC ratio operating values range from 3 to 5. The main response variables (dependent variables) of the C8A isomerization unit are the C8A aromatic ring loss, the ethylbenzene conversion capacity to xylenes (EBate), the isomerization capacity to paraxylene (PXate), and the isomerization capacity to orthoxylene (OXate). C8A aromatic ring loss is the result of the undesired secondary reactions. The calculation of this loss was carried out by monitoring the amount of C8A rings in the reactor inlet and outlet streams. Since the formation of C8A rings in isomerization is not expected, the C8A concentration in effluents will always be less than in the feed due to the undesired reactions. The lower the concentration of C8A in the effluent, the greater the C8A aromatic ring loss according to eq 1.

keep the operating temperature as low as possible to obtain the desired activity of the catalyst. Excessively high temperatures (>410 °C) can also lead to metal sintering in the catalyst, although it occurs more frequently during the catalyst regeneration. The normal operating temperature range in the C8A isomerization system comprises the range from 345 to 410 °C. The pressure affects the concentration of the reagents and therefore the reaction rates. Higher pressure favors the ethylbenzene saturation reactions and makes the dealkylation less favorable. Specifically, the saturation increases the amount of naphthenic C8 (NARO) available, which are key intermediates for the ethylbenzene isomerization reactions. However, a large amount of NARO increases the LOOP C8 units energy consumption, due to circulation flow elevation between the xylene fractionation, PAREX, and isomar units. Therefore, the NARO generation optimal balance should be considered to maintain the high EB conversion, without raising much circulation flow by LOOP C8. So, in ethylbenzene isomerization systems, it is common to adjust the pressure as the temperature increases, to maintain a constant level of NARO in the system. Typically, the hydrogen partial pressure range varies from 5 to 10 bar. Isomerization and transalkylation do not alter the reagents and product moles total number, so that any effect of pressure on these two reactions is considered negligible. The hydrogen purity of the recycle gas affects the H2 partial pressure inside the reactor. The H2 partial pressure affects the NARO concentration and may also affect stability, since, in general, the increase in hydrogen reduces the rate of deactivation due to the saturation of coke precursors. The spatial velocity in the catalytic bed (WHSV) is only affected by the unit feed rate, because the bed mass remains constant throughout the catalyst lifetime. The spatial velocity reduction, keeping constant the temperature and pressure, will increase the reaction severity, approaching the isomers C8 closer to the equilibrium. However, it will reduce the main reactions yield, due to the

C8Aloss =

C8A feed − C8A effluent C8A feed

(1)

The distance to equilibrium is a variable used to calculate how far from the thermodynamic equilibrium the isomers are D


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the oxychlorination and reduction process in order to carry out platinum dispersion to restore the ethylbenzene conversion capacity. However, even after performing these procedures, the catalyst does not recover all the catalytic activity of the previous campaign due to the permanent platinum agglutination, inherent to the process of deactivation by successive regenerations. The ethylbenzene equilibrium distance (EBate) is the main variable used to evaluate the behavior of isomar catalytic deactivation. EBate represents the ethylbenzene conversion capacity to xylenes by indirectly correcting the effect of the EB composition in the feed rate, since the denominator of this equation has the EB concentration variable at the equilibrium, according to the eq 2. During unit normal operation, EBate is the variable most affected by the catalytic deactivation, requiring higher flow rates in the LOOP C8 to perform the same PX and OX production. At the campaign end, the low EB conversion may even affect the paraxylene and orthoxylene production. In this case, the EB concentration in the isomar feed tends to increase because the EB conversion in the reactor decreases. To keep the same PX/OX production, it is necessary to raise the isomar feed rate to compensate the effect of EB conversion decrease. Therefore, the LOOP C8 feed rates increase until reaching the limit of some equipment, consequently requiring a unit stop to perform the catalytic regeneration procedure. Figure 3a shows the EBate normalized behavior for the seven catalyst campaigns, in chronological order. The EBate normalization was made from the division of each EBate result by the highest EBate value of its respective campaign. For example, EBate of the first campaign was divided by the highest value of first campaign Ebate. The EBate results of the second campaign were divided by the highest EBate value of the second campaign and so on. A decay can be observed of the EB conversion throughout the campaigns and the activity recovery after the regenerations. The coke formation on the surface of the catalyst and the platinum agglutination throughout the campaign will make difficult the access of the ethylbenzene to the catalyst metallic surface, reducing the EB conversion capacity to the xylenes due to the difficulty of realizing the EB hydrogenation reactions and NARO dehydrogenation. The C8A ring loss behavior, Figure 3b, the paraxylene isomerization capacity (PXate), Figure 3c, and the orthoxylene isomerization capacity (OXate), Figure 3d, were also inserted in this figure to elucidate that these key variables do not suffer the significant impact of catalytic deactivation. C8A loss calculation was directly impacted by the unit mass balance intrinsic error. For example, the flow rate of C8A rings loss for light hydrocarbons, deheptanizer top products (Figure 2 streams 08 and 09), represent about 2% of the unit effluent total flow. The mass balance of the unit is carry out by adding the Figure 2 streams 01 plus 03 and subtracting the streams 08, 09, and 11. The result of this balance should be zero; however, an error of up to 2% is allowed due to the flow instruments calibration. Therefore, since the flow of losses is much lower than the flows of the main products, monitoring C8A losses by this methodology can be influenced directly by the unit instruments calibration. As can be seen in the graph, there is no loss profile trend throughout the campaigns. Theoretically, the C8A ring losses elevation throughout the campaign is expected due to the increase in reaction severity, which is responsible for

for a given temperature and pressure and can be represented by eqs 2, 3, 4, and 5. EBate =

EBfeed − EBeffluent EBfeed − EBequilibrium

EBequilibrium = 6.1998 + 0.2458 × Tout PXate =

PX effluent − PX feed PX equilibrium − PX feed

(2) (3)

(4)

PX equilibrium = 24.495 − 0.0243 × Tout − 0.0009 × Tout 2 (5)

OXate =

OX effluent − OX feed OX equilibrium − OX feed

OX equilibrium = 22.297 + 0.1799*Tout − 0.0007*Tout 2

(6) (7)

where C8Afeed and C8Aeffluent refer to the mass percentage of aromatics C8 (EB + PX + OX + MX) in the reactor inlet and outlet, respectively; EBfeed and EBeffluent refer to the mass percentage of the ethylbenzene related to C8A in the reactor inlet and outlet, respectively; PXfeed and PXeffluent refer to the mass percentage of the paraxylene related to C8A in the reactor inlet and outlet, respectively; OXfeed and OXeffluent refer to the mass percentage of the orthoxylene related to C8A in the reactor inlet and outlet, respectively; EBequilibrium is the mass percentage of ethylbenzene related to C8A at equilibrium;24 PXequilibrium is the mass percentage of paraxylene related to xylenes in equilibrium;24 OXequilibrium is the mass percentage of orthoxylene related to xylenes in equilibrium;24 and Tout is the reactor outlet temperature (°C). 2.5. Process Variables: Isomar Unit Data Collection and Treatment. The information needed to carry out this work was imported from the laboratory analysis management system (LIMS) and the isomar industrial DCS data collection system. The acquired data was treated in an EXCEL worksheet to purge the inconsistent information (erroneous laboratory analysis results, unit stop days, loss of instrument indication, sudden change feed composition, etc.) and perform all yield calculations and mass balance. The virgin catalyst was loaded into the isomar reactor in 2005 and in 2018 completed seven campaigns, having gone through six regenerations. The first catalyst campaign was the longest, with 854 days of operation. The seventh campaign was the one of smaller duration, with 358 days of operation, followed by the second campaign with 392 days. The third and fifth campaigns had similar durations (mean of 630 days). The fourth campaign has the highest amount of reliable data (690 data) and the sixth campaign had 523 days of operation, being the third shorter campaign of the catalyst. 2.6. Key Process Variables for Determining Catalytic Deactivation. The isomar catalyst mainly goes through two types of catalytic deactivation. The first happens during the campaign and occurs mainly due to the coke formation on the catalyst surface. The second occurs during the catalyst regeneration and activation process due to the permanent platinum agglutination.22 After the end of the campaign, the coke deposited on the catalyst surface is removed by combustion with oxygen to restore its activity. Thereafter, the coke-free catalyst go through E


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happened because the isomerization reactions occur due to the catalyst acidic function. The surface area involved with the catalyst acidic function is much larger than the surface area involved with the metal sites, so the isomerization capacity does not go through significant changes even with the coke formation. The PXate and OXate profiles throughout the campaigns remained virtually constant. As seen previously, the variable that most suffers the influence of catalytic deactivation throughout the campaign is the ethylbenzene conversion capacity to xylenes (EBate). For this reason, this variable was defined as a reference for the evaluations, in order to obtain the understanding of the catalyst deactivation behavior in the campaigns. 2.7. Catalyst Deactivation throughout the Campaign. On the basis of the literature and plant experiences, the variables that may affect EBate were chosen: feed rate, reactor outlet temperature, H2 partial pressure, NARO content at the unit feed, processed or accumulated feed, day of campaign, recycle H2 purity, H2/HC molar ratio, EB content of the unit feed, unit mass balance, and PXate and OXate. Linear multiple regression was used to identify the most relevant factors influencing the EBate response variable, with the help of MINITAB commercial program, for the seven catalyst campaigns. First, multiple linear regression was performed, with all factors influencing the EBate response variable for the seven campaigns. Then, the factor that had the highest p-value was expunged from the regression, and a new regression was performed for each campaign. This procedure was performed successively until there were no more variables with the p-value above 0.05. Only the feed rate, H2 partial pressure and reactor temperature were kept in all regressions, regardless of the pvalue result, since these factors are very important for the catalyst performance, as recognized in the literature and in the unit daily operation experience. After determining those statistically significant variables (factors) that affected EBate, multiple linear regressions and multiple quadratic regressions were performed for the seven campaigns in order to establish the model that best represents the plant data and the catalytic deactivation throughout the campaign. 2.8. Catalyst Deactivation between Campaigns. The big question to determine the deactivation rate between campaigns is to make them comparable. Ideally, all campaigns should have been carried out under the same operational conditions (feed, temperature, and pressure). However, the fact is that those industrial campaigns are typically carried out under different operating conditions, increasing the challenge of phenomenon modeling. The solution developed to make the campaigns comparable consisted in using the equations defined in item 2.7, where the operational data of the seven campaigns were applied to each of these equations. In this scenario, the effect can be compared of several operating conditions on EBate and then its variation between adjacent campaigns studied in order to determine the catalyst irreversible activity loss (IAL). For example, the seven campaigns operational conditions were individually inserted into the equation found for the first campaign, resulting in the adjusted EBate behavior for all the seven campaigns. The IAL was then calculated throughout the campaigns by subtracting the adjusted EBate from the adjacent campaigns: first−second, second−third, third−fourth, fourth− fifth, fifth−sixth, and sixth−seventh. This same procedure was

Figure 3. (a) Normalized EBate, (b) C8A Losses, (c) PXate, and (d) OXate. Campaigns 1st (blue ◇), 2nd (red □), 3rd (green △), 4th (purple ×), 5th (light blue × with vertical line through center), 6th (dark blue +), and 7th (orange ○).

keeping or stopping the reduction of ethylbenzene conversion, but is also responsible for raising the C8A cracking rate. The paraxylene and orthoxylene isomerization capacity, PXate and OXate, respectively, was not significantly influenced by the catalytic deactivation (Figure 3, panels c and d). This F


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than the hydrogen recycle purity. Then the H2pp was more significant than the H2 recycle purity. The H2/HC molar ratio is always controlled at its lower limit to ensure lower energy consumption throughout the campaign. Therefore, there is no variation of the H2/HC ratio to control the EBate, making this variable not significant. The unit mass balance error also did not influence in the EBate calculation. However, as previously seen, the mass balance error affect the calculation of C8A losses. PXate and OXate, variables representing how far the equilibrium paraxylene and orthoxylene isomers are, respectively, did not influence the correlation because they are relatively stable throughout the campaign, as can be seen in Figure 3 (panels c and d). 3.2. Catalyst Deactivation throughout the Campaign: Linear Modeling. The multiple linear regression was performed with the six significant factors obtained through the study of the variables, as explained in the previous item, resulting in eq 8.

performed for the other equations, using the operational conditions of the other six campaigns, in order to verify if the IAL result had the same behavior. Since it is not possible to change the operational conditions performed in each campaign, this work sought to compare the effect of these various operating conditions on the EBate in order to study and determine the catalyst irreversible activity loss (IAL) between the campaigns.

3. RESULTS AND DISCUSSION 3.1. Catalyst Deactivation Throughout the Campaign. NARO content at the unit feed, processed or accumulated feed, and EB content at the unit feed, were the factors considered as statistically significant, as their respective p-values remained below 0.05 for all interactions in the seven campaigns. The independent variable feed rate, reactor outlet temperature, and H2 partial pressure (H2pp) were also considered statistically significant, although they were not unanimous at their respective p-values in all interactions. The other variables were not considered statistically significant because they did not have p-value results below 0.05 in all campaigns. Table 1 shows the final result of the p-values for the linear regression of the EBate response variable with the six factors.

EBate = C + a × F + b × T + c × H 2pp + d × NARO + e × AF + f × EB

where C is a constant; a, b, c, d, e, and f are the regressed parameters for each campaign, as summarized in Table 2; F is the feed rate, t/h; T is reactor outlet temperature, °C; H2pp is H2 partial pressure, bar; NARO is hydrocarbon naphthenic C8 content at the unit feed, % wt; AF is processed or accumulated feed, kt; and EB is ethylbenzene content at the unit feed, % wt. 3.3. Catalyst Deactivation throughout the Campaign: Quadratic Modeling. The quadratic approach, represented by eq 9, was also performed to verify if the quadratic model may be even better than the linear model.

Table 1. p-Value Significance Test campaign significant factors feed rate reactor outlet temperature H2 partial pressure feed NARO content accumulated feed feed EB content

1st

2nd

3rd

4th

5th

6th

7th

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.04 0.00

0.58

0.64

0.00

0.00

0.00

0.41

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

(8)

EBate = C + a × F + b × T + c × H 2pp + d × NARO + e × AF + f × EB + g × F 2 + h × T 2 + i × H 2pp2 + j × NARO2 + k × AF2 + l × EB2

(9)

where g, h, i, j, k, and l are the regressed parameters for each campaign. In the first interaction, we considered all quadratic terms resulting in six linear factors with their respective six quadratic factors. However, not all quadratic factors presented significance; it means, the p-value of the quadratic factor was above 0.05. Then, in each interaction of the respective campaign, the quadratic factor with the highest p-value was excluded from the interaction to perform a new regression. This procedure was repeated until all p-values of the factors resulted in below 0.05. The regressed parameters for each campaign are summarized in Table 3.

The other variables that were not classified as significant go through a new analysis in order to identify the reasons why they did not affect EBate. The campaign days represented by the catalyst operating time was not as significant due to longer campaigns with the lower feed rate and vice versa. Then the accumulated feed, which also takes into account the operating time, was more significant than simply operating time. The recycle hydrogen purity was used to calculate the hydrogen partial pressure. As the reaction is carried out in the gaseous phase, the H2pp represents a more complete variable

Table 2. Linear Regression Constants for the Respective Campaign with Uncertainty of Each Regressed Parameter campaign 1st 2nd 3rd 4th 5th 6th 7th

C −166.7 −94.9 −42.3 −133.8 −1.4 −159.6 118.1

± ± ± ± ± ± ±

a 67.3 59.1 42.2 37.2 37.8 32.2 41.0

−0.2237 −0.2121 −0.3542 −0.3957 −0.3533 −0.4442 −0.0623

± ± ± ± ± ± ±

b 0.0553 0.0729 0.0329 0.0462 0.0458 0.0503 0.0583

0.5574 0.4125 0.2096 0.4954 0.1707 0.5584 −0.2647

± ± ± ± ± ± ±

c 0.1822 0.1682 0.1167 0.1033 0.1073 0.0891 0.1141

0.168 −0.237 4.117 1.679 1.193 0.185 3.596 G

± ± ± ± ± ± ±

d 0.255 1.005 0.547 0.553 0.553 0.439 0.571

1.798 0.454 1.791 1.702 1.136 2.191 1.663

± ± ± ± ± ± ±

e 0.203 0.358 0.219 0.215 0.271 0.1643 0.208

−0.019054 −0.033560 −0.032389 −0.020897 −0.024751 −0.023254 −0.02223

± ± ± ± ± ± ±

f 0.001639 0.00428 0.001724 0.00116 0.001903 0.00153 0.00228

−0.1416 −0.4012 −0.1539 −0.4005 −0.3600 −0.2446 −0.4165

± ± ± ± ± ± ±

0.0604 0.0542 0.0722 0.0758 0.0555 0.0464 0.0996


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0.000056 ± 0.00001

linear

0.000014 ± 0.000004

0.01776 ± 0.00645 −0.4099 ± 0.1862

± ± ± ± ± ± ± 2521 −0.4632 −13.32 8.75 18.973 −0.07098 −0.2814 ± ± ± ± ± ± ±

Table 4. Models Validation

± ± ± ± ± ± ± ± ± ± ± ± 0.0305 ± 0.0242 0.000022 ± 0.000002 0.000031 ± 0.000004

5th

957 0.585 −5.29 6.37 5.12 −0.05714 −0.2793 −0.00779 0.00729 −0.333 −0.1869 0.000025 520 0.423 2.71 0.359 0.618 0.00309 0.0488 0.00338 0.00348 ± ± ± ± ± ± ± ± ±

4th

−762 −1.665 4.03 1.545 0.697 −0.04809 −0.323 0.01041 −0.00461 35.3 0.469 0.0936 0.457 0.1765 0.0036 0.0679 0.00396 ± ± ± ± ± ± ± ±

3rd

−47.8 −1.217 0.3484 2.929 15.509 −0.05978 −0.3809 0.00684

1038 0.497 5.5 5.29 1.819 0.00998 0.0459 0.00423 0.0072 0.348 0.0786 0.000017 ± ± ± ± ± ± ± ± ± ± ± ± 0.0000054 ± 0.000002 0.01158 ± 0.00614

2nd

−1292 −2.304 7.29 5.85 −2.751 −0.10425 −0.233 0.01833 −0.00943 −0.426 0.1706 0.000128 2993 1.109 15.54 0.579 0.1932 0.00316 0.318 0.00855 0.0202 ± ± ± ± ± ± ± ± ±

1st

−8131 −4.752 42.99 0.205 1.577 −0.02239 −0.86 0.03491 −0.0554

quadratic 2

campaign

Fcal

Ftab

R (adj)

Fcal

Ftab

R2 (adj)

1st 2nd 3rd 4th 5th 6th 7th

563 303 804.2 868.2 867.3 759.5 881.1

2.13 2.13 2.12 2.12 2.12 2.13 2.13

90.79% 86.12% 91.55% 91.89% 93.64% 93.39% 94.68%

524.4 400.2 1001 1361.9 596.7 745 1279.1

2 1.8 2 1.9 1.8 2 1.9

91.46% 93.76% 94.73% 96.74% 94.89% 94.18% 97.48%

With the exception of the second campaign, all the campaigns obtained adjusted R2 greater than 90%; nevertheless the quadratic models presented adjusted R2 larger than the linear models, since the insertion of more parameters in the quadratic model contributes to R2 elevation. All models are considered significant as Fcal was higher than Ftab. The graphical analysis of the linear and quadratic model response for the EBate was performed in comparison with the real EBate, both normalized according to Figure 4. Both models presented good representability in relation to the plant data for the seven campaigns analyzed. However, the quadratic model was chosen to represent the ethylbenzene conversion capacity behavior during the campaign due to the higher R2 presented. 3.4. Catalyst Deactivation between the Campaigns. If the regenerations and activations of the catalyst between the campaigns had ideal results, the catalyst would always have the same conditions as a new catalyst. This would imply overlapping EBate profiles, and theoretically, the catalyst would have infinite use. Due to activity losses caused during the material recovery process, EBate curves of actual campaigns should move down, always with a greater slope than the previous campaign. In this case, the EBate variation between two adjacent curves at a given time should increase with the increase in the number of campaigns. The variation of the EBate between adjacent campaigns would be the index of catalyst irreversible activity loss (IAL), which should increase with the increase in the number of campaigns. In the case of this study, the IAL was calculated between the campaigns course third−fourth, fourth−fifth, fifth−sixth, and sixth−seventh. The results are presented in Figure 5. IAL curves are shown with great amplitude variation and did not follow the theoretical tendency of rising, with larger slopes as the number of campaigns increased. Similar behaviors were observed between the pairs of campaigns, which do not show the influence catalyst aging under real working conditions. This effect was caused by different operating conditions between the campaigns, for the same accumulated feed, as an attempt to maintain the catalytic activity always high. This operational procedure makes the real IAL curves with no practical use for the development of this work, and it is necessary to adopt another strategy. The new approach proposed is theoretical and considers the quadratic models developed to determine the variation of the EBate between the campaigns correcting the effect of the divergent operating conditions.

510 0.552 2.64 4.25 2.04 0.01048 0.0545 0.00444 0.00332 0.244 0.1007 0.000008

−188.7 −0.4914 0.6457 0.284 21.014 −0.03719 −0.2295

The regressions were validated from the premise that the significant model has to have Fcal > Ftab (Fisher-Snedecor distribution)23 and R2 as close to unity as possible (Table 4).

C a b c d e f g h i j k l

Table 3. Quadratic Regression Constants for the Respective Campaign with Uncertainty of Each Regressed Parameter

6th

31.5 0.0493 0.0876 0.414 0.1565 0.00439 0.0437

7th

982 0.0676 5.04 3.15 0.1517 0.00855 0.0706


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Figure 4. Real and calculated EBate behavior.

EBate calculated profiles as a function of the accumulated feed were determined for all the campaigns operational conditions. The curves referring to the operating conditions of all the campaigns were both calculated and analyzed, and the results showed similar behaviors. By way of illustration, Figure 6 shows EBate profiles for all campaigns, using the operational conditions of the seventh campaign. In other words, the operational conditions of the seventh campaign (feed rate, temperature, pressure, NARO content, EB content, and accumulated feed) were inserted in the seven quadratic equations regressed for the respective seven campaigns, in order to evaluate the impact of seventh campaign operational conditions in the other campaigns. Figure 6 also allows interpreting how the results of the other campaigns would be if the operational conditions of the seventh campaign were applied. For example, when the unit was processing 100 kt accumulated feed, EBate of the seventh campaign presented the result of 16.5%. However, if the

Figure 5. Difference between real EBates from successive campaigns.

The starting point to make this comparison adequate, for the interests of this work, was to define the response profile of the comparable EBate in the various operating conditions carried out in the campaigns, using the same accumulated feed. The I


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expected. In the second campaign, the EBate decay was one of the most pronounced because the platinum scattering procedure was not yet operationally adopted. The catalyst used in the third campaign had an EBate profile as if it were new, since the EBate values were the highest and the slope was one of the lowest found. This same behavior was followed in the fifth campaign. In the case of the third campaign, the company began to adopt the platinum dispersion procedure. The result was better than with the catalyst used in the first campaign since platinum was probably not dispersed properly. Due to the operational problems that occurred in the first and second campaigns, they will not be considered in the next stages of this work. At the beginning of each campaign, the fifth campaign showed a conversion greater than the fourth campaign and the seventh campaign showed a behavior very similar to the sixth campaign. Between the fourth and fifth campaigns, it was necessary to remove all the catalyst from the reactor to perform a mandatory inspection (NR-13) inside the equipment. After the inspection, the catalyst was sifted to remove the particles with diameter out of specification. The loss found was 10%, and the same amount of new catalyst was added to the catalyst used and inserted back into the reactor. In this scenario, the catalyst used in the fifth campaign was more active than the fourth campaign due to the portion of new catalyst inserted in the reactor. The catalytic activity of the seventh campaign was approximately equal to the sixth campaign since there was a change of operating procedure in the stability and continuous injection of the oxidizing agent during oxychlorination, improving platinum dispersion on the catalyst surface. Another important aspect can be seen in applying the operational conditions of the seventh campaign in other

Figure 6. Calculated EBate behavior for the seven campaign with the operational data from 7th campaign. * EBate = EBate × confidentiality factor (arbitrary number peak so as to not reveal the actual value of EBate due to manufacturer technology catalyst confidentiality issues).

catalyst were in the third campaign, the EBate result would be 22.5%. This shows a IAL (catalyst irreversible activity loss) of 6%. The calculated EBate profiles were declining for all campaigns, and their slopes were higher according to the following campaign sequence: third, fifth, fourth, first, sixth, seventh, and second. This result does not appear to be consistent since the theoretical slope growth sequence should take into account the actual sequence of the campaigns, since the catalyst deactivation also increases in this sequence. This same graph was performed for the other six operating conditions, and the behavior of the results presented a very similar profile. The explanation of these results was found in the operational history of the unit. In the first campaign, the new catalyst had a more stable EBate decay than other campaigns, as might be

Figure 7. Catalyst irreversible activity loss (IAL). J


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calculated EBate results presented higher values compared to the real values. The advantage of working with higher EBate may result in reduced LOOP C8 energy consumption and/or increase PX and OX production. On the other hand, these operational conditions closer to the maximum limits increase the loss of C8 aromatics because they favor secondary reactions, which also reduce the campaign time due to the increase of the coke formation, requiring the catalyst regeneration more frequently. In fact, in order to obtain the optimum operational point, it is necessary to carry out economic evaluations taking into account the advantages and disadvantages of the operation with different severities in the unit. The optimum operating point of the unit should fluctuate according to raw material prices, energy cost, catalyst activity, cost of regeneration, etc. Thus, over the life of the catalyst, these economic evaluations must be updated, taking into account that the scenarios are always undergoing changes. In fact, in the first catalyst campaigns, the operational limit variables (temperature and pressure) were not reached at the catalyst campaign end. For this reason, the unit could have been operated closer to the maximum temperature and pressure limits to maximize ethylbenzene conversion. In relation to the campaign time of this catalyst, some facts corroborated for the accomplishment of the seven campaigns instead of the fours indicated by the manufacturer. Between the first and second campaigns, platinum dispersion was not performed. Therefore, the catalyst did not realize the oxychlorination and reduction procedure, which, although recovering its activity by platinum dispersion, lose some activity by other mechanisms such as sintering. The addition of 10% of catalyst in the fifth campaign also helped in the activity reestablishment. The fact that the unit was not operated in its maximum operating conditions in the first campaigns also helped to extend the catalyst lifetime. 3.7. Economic Evaluation. A simulation of the useful life of this catalyst was carried out with the objective of evaluating the financial impact of the catalyst irreversible activity loss (IAL), considering the following assumptions. (a) Accumulated Feed, it was considered the seventh campaign (693kt), since this campaign had the operating conditions closest to the maximum limits. (b) EBate in the first campaign, the calculated EBate from the third campaign was considered, since it had the highest value between the campaigns. However, considering the seventh campaign, the operational conditions were the highest conversion the catalyst could provide. (c) IAL, the general average of the catalyst irreversible activity losses calculated for the campaigns pairs third−fourth and fifth−sixth was used, considering the accumulated feed of 693 kt. The overall average resulted in 2.4%, so if the other campaigns had been carried out without the impacts previously addressed, an IAL around 2.4% was to be expected between campaigns, when processing a feed of 693 kt. (d) Other campaigns estimation− second campaign EBate was calculated by subtracting EBate from the first campaign minus IAL (2.4%). Taking into account that IAL is the catalyst irreversible activity loss, the second campaign can not have the same yield as the first campaign, since the catalyst suffered irreversible activity loss. Then, to estimate the conversion capacity of the other campaigns, IAL was used, as previously calculated in this work. The EBate of the other campaigns followed the same rational. The economic evaluation was made based on the financial margin calculation of the PX production block (Catalytic

campaigns. Due to the fact that the seventh campaign showed the greatest severity between the campaigns, the initial values of the EBates calculated are higher than those presented in the other graphs performed with the other operating conditions. 3.5. Catalyst Deactivation between the Campaigns: Determination of Catalyst Irreversible Activity Loss (IAL). The catalyst irreversible activity loss (IAL) were calculated for the pairs of campaigns third−fourth, fifth− sixth, and sixth−seventh, considering all the seven campaigns operation conditions. The IAL profiles found were similar for all operating conditions presented. It can be observed that there is a very similar behavior between the irreversible deactivation profile of the third and fourth campaigns and the fifth and sixth campaigns in accordance with Figure 7 (panels a and b). The irreversible deactivation profile between the sixth and seventh campaigns shows an initial growth trend, which can be attributed to the excellent dispersion procedure of the platinum carried out between the sixth and seventh campaigns, thus not significantly deactivating. However, throughout the campaign, the catalyst irreversible activity loss increased, demonstrating the aging advanced state of the material. This result indicates that if the catalyst is regenerated again, the next campaign will be carried out in a smaller activity negatively impacting the business, since the irreversible activity loss is high. 3.6. Unit Performance Evaluation. Taking into account the catalytic deactivation results throughout the campaign and the catalyst irreversible activity loss between the campaigns, the unit performance analysis and a catalyst performance estimate can be done to the unit that has a similar catalyst in any other life cycle. In order to carry out the catalyst performance evaluation over its useful life, the operational conditions of the seventh campaign were chosen since it is the campaign in which the temperature and pressure were worked closer to the maximum limits of the unit, under more severe conditions compared to the previous campaigns. Table 5 shows the real EBate from the seven catalyst campaigns, considering the campaigns completed. The table Table 5. Real and Calculated EBate Evaluationa campaign

total real EBate (%)

partial real EBate (%)

calculated EBate (%)

1st 2nd 3rd 4th 5th 6th 7th

14.16 12.44 19.73 16.62 16.29 14.66 15.5

16.41 12.44 20.59 18.25 17.36 14.97 15.5

18.7 12.88 21.13 18.66 18.76 16.25 15.53

EBate = EBate × confidentiality factor.

a

also shows the partial real EBate, considering partial campaigns in relation to the accumulated feed of 693 kt, and the calculated EBate in relation to the accumulated feed of 693 kt, considering the operational conditions from the seventh campaign. By analyzing the calculated EBate, it can be concluded that the campaigns prior to seventh could have been carried out in operational conditions closer to the respective maximum limits, in order to reach higher EBate values, knowing that the K


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Industrial & Engineering Chemistry Research Reformer, Toluene Disporportionation and LOOP C8 Units), taking into account the costs with the raw material and energy, as well as credits from the sale of the main products, coproducts, and intermediate products. The LOOP C8 feeds were kept constant in all campaigns. The only process feed variable that was changed between campaigns was EBate. Therefore, the financial impact between the campaigns was only due to the change in EBate. The LOOP C8 raw materials prices, as well as energy and products, were considered in accordance with those practiced internally at Braskem. The result of the margin will be illustrated by an index, where the financial margin was divided by Isomar feed flow due to confidentiality of results. Figure 8 illustrates the catalyst lifetime cycle simulation result with the financial impact of irreversible activity loss. Ten

Figure 9. Catalyst exchange economic evaluation. Return and Investment units is MMUS$/10 years.

highest return and the highest investment. The “2 UL” considers that the catalyst carried out two campaigns, being substituted in the sequence to carry out two more campaigns until completing the period of 10 years. In “9 UL”, the catalyst operated until the ninth year, carrying out nine campaigns. The tenth year was realized, considering the yields of the new catalyst. The ratio results of the return divided by investment indicate that, in any one situation presented, the operational return was obtained. However, there is a point of maximum ration that would be the desired, provided financial return for the company. The investment in the catalyst could still be lower if the platinum is recovered to reduce the amount invested in the other catalyst exchanges, which could generate higher returns. The ratio simplified return divided by investment shows that the best time for catalyst replacement would be in the seventh campaign.

Figure 8. Ratio between financial margin and EBate. *EBate = EBate × confidentiality factor.

campaigns were estimated. The 9th and 10th campaigns considered an EBate with a value below the IAL. In accordance with Figure 8, the financial margin accompanies the EBate reduction until the sixth campaign. However, after the seventh campaign, the margin reduction is proportionally much higher than the EBate reduction, reaching a negative value at the 10th campaign. The analysis of the best time to perform the catalyst exchange can also be performed using Figure 8. For example, a simplified economic evaluation was performed for a hypothetical unit that processes 1314 kt/year of feed and has a typical space velocity of 3.0,24 corresponding to 50000 kg of catalyst. The cost of a typical xylene isomerization catalyst, which has 0.3% platinum, was estimated from the following information: (i) platinum quotation in July,25 (ii) platinum import cost of 1.6 times platinum value, and (iii) technology cost, which was considered twice the internalized platinum total value. The catalyst estimated total cost for the hypothetical unit is $19.7 million. Figure 9 illustrates the financial return as well as the investment to be made in 10 years of unit operation. To obtain these values, variations in the campaign number were considered during 10 years of operation, using the 12 months campaigns duration. The return of each scenario was always calculated in relation to the base scenario, which considers the useful life of the catalyst for 10 years. In other words, the catalyst has gone through 10 campaigns in the base scenario. For example, “1 UL” means that the catalyst operated only for one campaign, being replaced by a new catalyst of the same technology at the beginning of the next cycle. Specifically, in 10 years there were 10 new volumes of catalyst, and consequently, the unit always operated at maximum yields, obtaining the

4. CONCLUSION The losses associated with the deactivation of the xylene isomerization unit catalyst were determined using a set of preexisting industrial data. The work was structured as follows: process study, acquisition and treatment of the plant pre-existing real data, study and choice of the dependent variable most affected by the catalytic deactivation, determination of the factors that most affect the key variable of catalytic deactivation, multiple linear regressions and multiple quadratic regressions for the seven campaigns in order to establish the model that would best represent the plant data and the catalytic deactivation throughout the campaign, establishment and adaptation of the operational conditions for all the campaigns to allow the determination of the catalyst irreversible activity loss (IAL) between the campaigns and, last, results final analysis. The application of this methodology demonstrated how complex is the analysis and understanding of the catalytic deactivation rate between the campaigns of an industrial unit. In this work, it was also demonstrated that the determination and estimation of the catalyst irreversible activity loss between the campaigns can not be performed only by comparing the campaigns real data, since each campaign presents different operational conditions. A simple comparison can lead to misunderstanding of catalytic phenomena and taking actions that may compromise the company’s results. The analysis of the catalyst irreversible activity loss between the campaigns showed that the first campaign was the most L



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ACKNOWLEDGMENTS The authors acknowledge the support of the Coordination for the Improvement of Higher Education Personnel (CAPES) and Braskem.

stable in terms of catalytic deactivation throughout the campaign, but the most active campaign was the third campaign due to the platinum dispersion procedure. Probably platinum was not fully dispersed in the new catalyst and its performance deteriorated in the second campaign, where only the coke burn stage was performed, causing the second campaign to perform worse. The beginning of the fifth campaign also presented a more active performance than the beginning of the fourth campaign, since between these campaigns the mandatory internal inspection of the reactor was carried out, and as a consequence, the replacement of 10% of the new catalyst mass was carried out, adding more activity to the bed. The seventh catalytic campaign also started with activity very similar to the sixth campaign, which can be attributed to the operational conditions closest to the maximum limit in the seventh campaign and excellent procedure of regeneration and activation of the catalyst between the sixth and seventh campaigns. However, the seventh campaign showed an increase in the deactivation rate when compared to the sixth campaign as the campaign was developed, demonstrating the advance in catalyst aging as expected, due mainly to the advance in catalyst aging. In addition, the fact of operating under more severe conditions also increases the formation of coke, favoring the catalytic deactivation throughout the campaign. In comparison with the real plant data, it was not possible to observe these behaviors of deactivation profiles, since the isomar catalytic system presented flexibility of temperature and pressure elevation. The last campaigns and especially the seventh campaign was carried out at a higher level of hydrogen partial pressure and temperature, therefore ensuring the ethylbenzene conversion practically at the same level of the last campaigns. Probably, units that operate in operational conditions close to the maximum limits will present other profiles of IAL. However, the steps performed in this work would also be able to provide the determination of IAL catalyst for most reaction systems. The performance of this work allowed the development of structured steps to calculate the catalytic deactivation rate during and between the campaigns of the isomar catalyst. In addition to this fact, the result of the work demonstrated that innumerable internal and external influences can affect the catalytic deactivation rate; however, once the cause of the influence has been identified, it is possible to determine and know the IAL as seen throughout the work. The knowledge and elaboration of the steps proposed in this work to determine the catalytic deactivation rate by real plant data may serve as the basis for several economic evaluations of the catalyst exchange processes and the elaboration of an attractive investment matrix, contributing to maximize the financial margin of the company business.

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NOMENCLATURE LOOP C8 = group of units responsible for producing PX and OX PAREX = paraxylene separation unit ISOMAR = xylenes isomerization unit HEMT, HFAU, and ZSM-5 = synthetic zeolites types LIMS = laboratory, analysis management system DCS = digital control system

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COMPOUNDS PX = paraxylene OX = ortoxylene MX = metaxylene EB = ethylbenzene HC = hydrocarbon PET = polyethylene terephthalate PVC = polyvinyl chloride PTA = purified terephthalic acids LAB = linear alkyl benzene C8A = C8 aromatic hydrocarbons NARO = nonaromatic hydrocarbons C4 = hydrocarbons having 4 carbon atoms C5 = hydrocarbons having 5 carbon atoms C6 = hydrocarbons having 6 carbon atoms C7 = hydrocarbons having 7 carbon atoms C8 = hydrocarbons having 8 carbon atoms C9 = hydrocarbons having 9 carbon atoms C10 = hydrocarbons having more than 10 carbon atoms XMEB = mixture of xylenes rich in ethylbenzene

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VARIABLES

EBate = ability to convert ethylbenzene to xylenes at equilibrium PXate = ability to isomerize paraxylene to the equilibrium OXate = ability to isomerize orthoxylene to the equilibrium ppH2 = hydrogen partial pressure WHSV = catalytic bed space velocity IAL = catalyst irreversible activity loss UL = catalyst useful lifetime C8A loss = C8 aromatic ring loss C8Afeed = mass percentage of aromatics C8 (EB + PX + OX + MX) in the reactor inlet C8Aeffluent = mass percentage of aromatics C8 (EB + PX + OX + MX) in the reactor outlet EBfeed = mass percentage of the ethylbenzene related to C8A in the reactor inlet EBeffluent = mass percentage of the ethylbenzene related to C8A in the reactor outlet EBequilibrium = mass percentage of ethylbenzene related to C8A at equilibrium PXeffluent = mass percentage of the paraxylene related to C8A in the reactor outlet PXfeed = mass percentage of the paraxylene related to C8A in the reactor inlet PXequilibrium = mass percentage of paraxylene related to xylenes in equilibrium

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wanessa B. Costa: 0000-0002-5628-1783 Carlos A. Pires: 0000-0003-4231-6495 Notes

The authors declare no competing financial interest. M


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knovel.com/hotlink/toc/id:kpFPR00003/fundamentals-petroleum/ fundamentals-petroleum (accessed Apr 17, 2018). (14) Speight, J. G. Chemistry and Technology of Petroleum, 4th ed. [Online]; Taylor & Francis: Boca Raton, FL, 2007. https://app. knovel.com/hotlink/toc/id:kpCTPE0002/chemistry-technology/ chemistry-technology (accessed Apr 17, 2018). (15) Marcilly, C. Acido-Basic Catalysis, Vol. 2 - Application to Refining and Petrochemistry [Online]; Editions Technip: Paris, 2006. https:// app.knovel.com/hotlink/toc/id:kpABCVARPE/acido-basic-catalysis/ acido-basic-catalysis (accessed May 7, 2018). (16) Iliyas, A.; Al-Khattaf, S. Xylene Isomerization over USY Zeolite in a Riser Simulator: A Comprehensive Kinetic Model. Ind. Eng. Chem. Res. 2004, 43, 1349−1358. (17) Robschlager, K.-H.; Christoffel, E. G. Reaction Mechanism of Ethylbenzene Isomerization. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18 (4), 347−352. (18) Corma, A.; Cortes, A. On the Mechanism of Catalytic Isomerization of Xylenes Molecular Orbital Studies. J. Catal. 1979, 57, 444−449. (19) Corma, A.; Sastre, E. Evidence for the Presence of a Bimolecular Pathway in the Isomerization of Xylene on Some Large-Pore Zeolites. J. Catal. 1991, 129, 177−185. (20) Morin, S.; Gnep, N. S.; Guisnet, M. A Simple Method for Determining the Relative Significance of the Unimolecular and Bimolecular Pathways of Xylene Isomerization over HY Zeolites. J. Catal. 1996, 159, 296−304. (21) Toch, K.; Thybaut, J. W.; Vandegehuchte, B. D.; Narasimhan, C. S. L.; Domokos, L.; Marin, G. B. A Single-Event MicroKinetic Model for “Ethylbenzene Dealkylation/Xylene Isomerization” on Pt/ H-ZSM-5 Zeolite Catalyst. Appl. Catal., A 2012, 425−426, 130−144. (22) Joshi, S. S.; Ranade, V. V. Industrial Catalytic Processes for Fine and Specialty Chemicals [Online]; Elsevier: Oxford, UK, 2016. https://app.knovel.com/hotlink/toc/id:kpICPFSC03/industrialcatalytic/industrial-catalytic (accessed May 7, 2018). (23) McShane-Vaughn, M. Probability Handbook - 5.16 Summary of Continuous Distributions [Online]; American Society for Quality (ASQ): Milwaukee, WI, 2016. https://app.knovel.com/hotlink/toc/ id:kpPH00001M/probability-handbook/probability-handbook (accessed May 7, 2018). (24) Ernst, S. Advances in Nanoporous Materials, Vol. 1 - 2.3.2.1 Xylene Isomerization [Online]; Elsevier: Kidlington, Oxford, UK, 2009. https://app.knovel.com/hotlink/toc/id:kpANMV0001/ advances-in-nanoporous/advances-in-nanoporous (accessed May 7, 2018). (25) Kitco. http://www.kitco.com/charts/liveplatinum.html (accessed July 17, 2018). (26) Min, H.-K.; Hong, S. B. Mechanistic investigations of ethylbenzene disproportionation over medium-pore zeolites with different framework topologies. J. Phys. Chem. C 2011, 115, 16124− 16133.

OXeffluent = mass percentage of the orthoxylene related to C8A in the reactor outlet OXfeed = mass percentage of the orthoxylene related to C8A in the reactor inlet OXequilibrium = mass percentage of orthoxylene related to xylenes in equilibrium Tout = reactor outlet temperature, °C C = constant a, b, c, d, e, f, g, h, i, j, k, and l = regressed parameters for each campaign F = feed rate, t/h T = reactor outlet temperature, °C H2pp = H2 partial pressure, bar AF = processed or accumulated feed, kt EB = ethylbenzene content at the unit feed, % wt.

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REFERENCES

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