Kinetic Modeling of Arab Light Vacuum Residue Upgrading by

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Kinetic Modeling of Arab Light Vacuum Residue Upgrading by Aquaprocessing at High Space Velocities Mazin M Fathi, and Pedro Rafael Pereira-Almao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie301380g • Publication Date (Web): 13 Dec 2012 Downloaded from http://pubs.acs.org on December 24, 2012

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Industrial & Engineering Chemistry Research

Reactor

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Gas Separation Light Products

Feed Tanks

Residua (95%) Water (5%) Preheater

Heavy

Figure 1. Experimental pilot plant representation.

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Figure 2. Representation of the kinetic model lumped products distribution.


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540oC+ ALVR k1

VGO k2

k6

k5 Distillates

k7 k3 Naphtha k4 Gas

Figure 3. Proposed lumped kinetic model.



1.2

lnkAQP = -20257(1/T )+ 29.278 R² = 0.9989

0.8

ln(k(hr-1))

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lnkTC = -20962(1/T) + 29.965 R² = 1

0.4

TC

AQP

0.0

1.39E-03

1.40E-03

1.40E-03

1.41E-03

1/T, K-1

Figure 4. ALVR TC and AQP Arrhenius follow 1st order global kinetics.


1.41E-03

1.42E-03

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0.8 Shu et al. Current Study

Global Rate Constant (hr-1)

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0.60

0.6 Yang et al. Del Bianco et al. 0.4 0.30

0.35

0.31

0.2

0 VR

ALVR

Belayim VR

Cold Lake bitumen

Residua Figure 5. Global Rate Constants for Thermal Cracking of Different Residua.



0.150 Current Model CO2, H2S, C1-C5, C2=, C3=, C4= Singh et al. C1-C5 Kataria et al. C1-C5

Gas Rate Constant (hr-1)

Mateen et al. C1-C4

0.100

Humaidan et al. C1-C5

0.050

Residue

Figure 6. Gas Rate Constants for Thermal Cracking of Different Residua.


EVR

LFVR

RBVR

MVBF

AMVR

BHVR

NGVR

0.000 ALVR

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0.24 0.20 0.17 0.16

0.20

0.20

0.21

0.21

0.17

Current Model

0.14

Singh et al. Kataria et al. Yang et al.

0.08

MVBF

AMVR

LFVR

RBVR

EVR

VR

ALVR

0

Humaidan et al.

NGVR

VGO Rate Constant (hr-1)

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Residue

Figure 7. VGO Rate Constants for Thermal Cracking of Different Residua.



0.8

y = -1.397x + 0.8128 Composition, Weight Fraction

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0.6

ALVR AQP

540C ALVR

VGO

0.4

0.2

0.0 0.00

y = -0.291x + 0.1806

0.04

0.08

0.12

Time (hr)

Figure 8. Feedstock composition profiles during AQP at 440


0.16

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540C ALVR

0.8

ALVR TC

VGO

Model Composition, Weight Fraction

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Distillate

0.6 Naphtha Gas

0.5 y = 0.998x + 0.0004 R² = 0.9995

0.3

0.2

0.0 0.0

0.1

0.2

0.3

0.4

0.5

Experimental Composition, weight fraction

Figure 9. TC model and experimental products compositions.


0.6

0.7

0.8


0.8 540C ALVR

ALVR AQP

VGO

Model Composition, Weight Fraction

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Distillate

0.6

Naphtha Gas y = 0.9983x + 0.0003 R² = 0.9998

0.4

0.2

0.0 0.0

0.2

0.4

0.6

Experimental Composition, Weight Fraction

Figure 10. AQP model and experimental products compositions.


0.8

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Kinetic Modeling of Arab Light Vacuum Residue Upgrading by Aquaprocessing at High Space Velocities Mazin M. Fathi*† and Pedro Pereira-Almao Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada. Keywords: Aquaprocessing, Arab Light Vacuum Residue, P-value, High Temperature Simulated Distillation, Ultradispersed Catalyst, Lumped Kinetic Model.

ABSTRACT

Aquaprocessing (AQP) is a novel method that offers higher conversion level under asphaltenes stability limit. It is a process of steam catalytic cracking using unsupported ultra-dispersed catalyst. Following a published work by Fathi and Pereira (2011) on upgrading a paraffinic residuum from Arab Light (AL) vacuum residue (VR) by AQP, this work investigates a proposed lumped kinetic model for the upgrading of ALVR via AQP for the first time. The model is evaluated based on experimental results conducted in a continuous up flow open tubular pilot plant reactor under short-space times and at conditions distant from coke formation. The proposed model is based on five cascaded lumps generated at and below the asphaltenes stability limit, which are 540 oC+ ALVR, VGO (455-540 oC), distillates (204-455 oC), naphtha (IBP-204 oC), and gases. The model compositions are found close to the experimental values with mean absolute percentage errors of less than 5.5%.


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1. INTRODUCTION Petroleum residues are typically collected at the bottom of distillation units as crude oil heavy end effluents. These residues are eventually subjected to further conversion via multiple routes, blended in the fuel oil pool or utilized in pavement. They are mainly composed of heavy aromatics and large complex hydrocarbons that encapsulate multiple polar and non-polar hetero-atoms1. Residual hydrocarbon upgrading has been mainly carried out by conventional thermal processes and/or hydroprocessing and deasphalting technologies. However, these processes are characterized by low conversion, large percentage of coke2 and/or high operating cost3. Aquaprocessing (AQP) is considered an alternative economical path that increases the conversion of residua to lighter products compared to thermal cracking. This process was investigated in an open tubular pilot plant reactor using Arab Light Vacuum Residue (ALVR) for which details have been reported elsewhere4. AQP is a catalytic process that uses ultra-dispersed (UD) catalytic metals active phases for steam cracking chemistry to maintain or improve the product stability and quality5. The UD catalytic metals generate hydrogen and oxygen radicals through the catalytic dissociation of the steam and promote hydrogen and oxygen addition to the produced free hydrocarbon radicals. In a recent publication, evidences of the oxygen transfer from dissociated water to the hydrocarbon products were presented4. Following the recently published experimental results, this study aims to provide and investigate detailed kinetic information on the upgrading reactivity of ALVR by AQP. Kinetic data on the mild thermal cracking of heavy residual hydrocarbons were published by many authors. The reported global thermal cracking activation energy has a wide range of 150-350 KJ/mol depending on the oil type3, 6,

7, 8, 9, 10

. Several authors have reported kinetic data on the pyrolysis11 and

coking12 kinetics of ALVR. The characteristics of a given residual oil differ to some extent from one batch to another batch depending on stock oil cut points, different oil sources and history, and analysis methodology. Therefore, in this work ALVR was subjected to thermal cracking using the same experimental setup and methodologies used in AQP in order to establish a baseline for comparison. Heavy oil is a mixture of many components. Having a complete kinetic description of all components ACS Paragon Plus Environment

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participating in a reaction complex is involved and impractical. Combining the oil components into predefined lumps depending on type or boiling range seeks to ease the prediction of the kinetic information13. Therefore, the Lumping approach is applied in this work to represent complex reaction mechanisms with simplified reactions system. However, there exist limitations and drawbacks associated with using lumping approach for kinetic modeling. One of the issues is related to the dynamic structural changes of each reacting species present in the different lumps as the reactions progress, which are accompanied by changes in reaction behavior of each lump. Another drawback is the loss of information of each species combined in the lump and therefor loss of product distribution information. Furthermore, Weekman14 indicated that lumping kinetics is not a reliable methodology to estimate kinetics data for catalytic processes whose catalysts are prone to deactivation or compositional changes. This is because changes in the catalyst properties would be accompanied by changes in the rate constants accordingly. This last troublesome issue is avoided in the current work since the utilized UD catalytic particles leaves with the heavy effluent and new catalyst is prepared within the feedstock from fresh batches of catalytic precursors. Furthermore, thermal cracking process evades this issue entirely. Numerous kinetic models for residual oil upgrading based on lumping technique have been reported in open literature for thermal cracking and hydroprocessing; however none for a catalytic process using steam as hydrogen donor. A number of these models are summarized by Kataria et al. 7 In this study, a lumped kinetic model of five simulated distillation cascaded lumps from the upgrading of ALVR via AQP under high space velocities is proposed and investigated for the first time. The proposed lumped model can be applied for catalyst matrix optimizing research and operating conditions adjustment. A detailed kinetic study on AQP via more complex structured kinetic model considering coke formation will be investigated in future work. The investigated model in the current work assumes conditions distant from coke formation and heavy aromatics condensation reactions. The model predicts the kinetic parameters using real time upgrading experimental results. For comparison and validation purposes, the model is also tested using non-catalytic thermal cracking (TC) upgrading experimental results. Thus far,


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no kinetic data have been published on the application of AQP to paraffinic residuum such as vacuum residue from Arabian Light. The original set of the new conditions examined in this work to acquire kinetic information were found sufficiently different with respect to any published result so far on this novel process and in the patented work, which extends the knowledge windows of operability of AQP. The paraffinic nature of ALVR and the relative instability of its asphaltenes in the virgin residue limit considerably the yield of lighter products these residuals produce with respect to naphthenic oils when submitted to low severity thermal cracking. The few papers existing in the literature on AQP have all been produced with residuals from naphthenic oils, which are much more reactive than the ones from paraffinic oils. The set of conditions described in the few papers and the patent of Aquaprocessing when applied to ALVR do not allow noticeable improvements in yields; whereas for naphthenic residuals the literature shows these can be brought to 430 oC or more at 2-3 hr-1 of space velocity without reaching the asphaltenes stability limit and under AQP produce significantly higher yields than thermal cracking. The new set of shorter space times (other than the ones mentioned in the US Patent No. 5885441) and related kinetic information are required to subject the oil to higher temperatures (≥ 430 oC) to activate the catalyst and avoid asphaltenes deposition. Deep understanding of the reaction kinetics is mandatory for the efficient design and operation of AQP to upgrade paraffinic origin residua. The generated kinetic information are vital in understanding the upgrading mechanism to fine tune the catalyst matrix and operating conditions as to maximize the production of the desired products and therefore minimize oil availability to condensation reactions. Therefore, the investigation on the kinetics of ALVR AQP to obtain added yields over thermal cracking is an original work. The kinetic information published in this article is the first on AQP as well as the first on upgrading a residuum of paraffinic origin via AQP. Hydrocarbon oils with high amount of paraffins are not suitable for upgrading by AQP. In an unpublished work by the authors of this study on upgrading Arab Light Atmospheric Residue (ALAR) using AQP it was shown that the asphaltenes in the ALAR are more prone to precipitation than those in the ALVR. The high instability of the ALAR asphaltenes is caused by the presence of high amounts of


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paraffins and shortage in the resins fraction. Resins fraction improve the solvation and stability of asphaltenes in the oil medium1, 3 while paraffins destabilize the equilibrium between the oil medium and the asphaltenes particles causing them to condense. The negative effect of paraffins on the asphaltenes stability becomes more pronounced at high temperatures. This was verified after subjecting ALAR to 428 oC, which was the maximum temperature before the condensation reactions pick up the pace at 5hr1

. As indicated earlier, 430 oC is the least temperature required to activate the UD catalyst, which is

higher than the onset temperature of asphaltenes condensation reactions. 2. EXPERIMENTAL PROCEDURE 2.1. Feed Preparation Procedure Arab Light vacuum residue provided by Saudi Aramco Oil Company is used to prepare the feedstock. The received Arab light vacuum residue consists of 18.3 VGO and 81.7 wt% 540 oC+ hydrocarbons. The feedstock was prepared by efficiently dispersing and suspending 600 ppm of catalytic metals particulates within the residual oil medium4. Metals weight ratio of 3:1 (K:Ni) was used following the procedure reported by Pereira et al.5 The feedstock was prepared at 40 oC in the form of water in oil (W/O) catalytic emulsion that is composed of 94 wt% ALVR, 5 wt% water, 1 wt% surfactant, 140 ppm nickel and 460 ppm potassium precursors salts. In order to reduce the viscosity, the ALVR it was blended with 25 wt% naphtha before starting the emulsification process, which resulted in viscosity reduction from 36,100 cP to 300 cP at 40 oC. The prepared catalytic emulsion is then decomposed in a continuous flow decomposition reactor at 350 oC and atmospheric pressure with a residence time of 2 minutes to form the catalytic suspension. Most of the blended naphtha and water were vaporized during the decomposition step. The remaining entrapped naphtha and water were separated from the suspension downstream of the reactor in a hot separator leaving behind the particulates of the catalytic metals active phase suspended within the residual oil medium. 2.2. Reactivity Test and Analysis Procedure AQP reactivity tests were carried out at 260 psi in a 100 cm3 up flow isothermal tubular reactor pilot


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plant, as shown in Figure 1, for which details were published elsewhere4. Thermal cracking experiments were conducted at liquid hourly space velocities (LHSV) of 2 and 2.5 hr-1 at temperatures of 400, 405, and 408 oC. Taking into consideration the minimum catalyst activation temperature of 430 oC, the experimental conditions at which AQP was evaluated were 435 oC and LHSV of 5-7.5 hr-1, 440 oC and LHSV of 6-8.5 hr-1, and 445 oC and LHSV of 8-10.5 hr-1. Thermal cracking experiments are conducted at near Aquaprocessing conditions in order to compare their performances. The feedstock at 100 oC was introduced into the system with a Zenith Precision Metering Gear Pump model H-9000. The oil was then passed through a preheating zone to raise its temperature up to 260 oC. For the AQP experiments, superheated steam at 330 oC was injected upstream of the reactor at a mass ratio of 5% with an Alltech HPLC pump model 426. After that, the total feed temperature was gradually raised to 355 oC before entering the reactor from the bottom. It is worth mentioning that the same preheating procedure was followed for the thermal cracking and AQP experiments. After reaching the reaction temperature and pressure and the desired flow rate in the reactor the system is allowed to stabilize. The upgraded oil exiting the reactor, from the top, enters a hot separator maintained at 320 oC, where heavy product was collected at the bottom and lighter fraction was collected from the top. Distillates from the lighter fraction were collected in a low pressure separator maintained at room temperature and the non-condensed gas was sent to an online gas analyzer and a wet test meter for gas composition and volume measurement, respectively. The collected liquid products and recorded gas volumes and compositions were used to close at least two mass balances in predetermined periods for each experimental condition. Aliquots from the heavy products are collected for asphaltenes stability measurement, which was repeated at each severity level. The converted and unconverted oil compositions were determined with High Temperature Simulated Distillation (HTSD) method. The HTSD analysis was carried out with a high temperature gas chromatography instrument from Agilent modified by Separation Systems to calculate the amount of 540 oC+ hydrocarbons using the ASTM D7169-05 method described by Carbognani et al.15 Asphaltenes


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stability measurement was determined by measuring its tendency to precipitate using the Peptizing Value (P-value) technique. This technique was first developed by DiCarlo et al.16 and it measures the intrinsic stability between asphaltenes fraction and maltene fraction of the residual oil. Residual oil with a P-value of 1.20 is considered at the minimum asphaltenes stability limit. 3. KINETIC MODELING OF ALVR AQP 3.1. Proposed Kinetic Modeling and Presumption One of the objectives of conducting the AQP experiments at different conditions is to develop a kinetic model that can be used to best estimate the upgrading kinetic parameters. For this purpose, enough experimental data points were generated and collected to facilitate developing a kinetic model that closely matches the experimental results. The proposed kinetic model assumes conditions distant form coke formation and asphaltenes precipitation. Since heavy oil upgrading mostly follows 1st order kinetics9, the proposed model in this work is an arrangement of seven first order kinetic reactions of five HTSD cascaded lumps, which are unconverted 540 oC+ ALVR, vacuum gas oil (VGO), distillates, naphtha, and gases distributed as shown in Figure 2. The kinetic model configuration resembles the orientation of an inverted pyramid, where the 540 oC+ ALVR and the gaseous product are located at the inverted pyramid base and tip, respectively. The selected experimental conditions do not allow for coke formation or asphaltenes deposition inside the pilot plant. Modeling of coke formation in pilot plant experiments is troublesome and the collected data are inaccurate and are characterized by very low repeatability and reproducibility based on the following reasons: 1. Once formed, the generated coke would stick inside the pilot plant lines, which would make mass balances impossible to close. 2. Coke is not inert and can escalate the cracking reaction and therefore can eventually accelerate further coke formation17. 3. Coke deposition inside the reactor internal wall forms a dynamic coke layer that grows in


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thickness with time. Once coke is formed on the internal walls of the reactor, it would reduce the reactor volume, hindering the oil flow and reducing its targeted space-time. Furthermore, the coke layer on the internal reactor wall possesses higher resistance to heat transfer (higher heat transfer coefficient), which would reduce the reaction temperature below the targeted temperature and raise difficulties in temperature control. Therefore, in order to avoid the above troublesome issues the experimental conditions are carefully selected as to avoid coke deposition. As a result, the proposed kinetic model does not take into consideration coke formation. Mild thermal cracking reactions aim toward thermally breaking the heavy oil molecules into smaller hydrocarbon products. From literature, the mild thermal cracking reactions (visbreaking) are considered irreversible for modeling purposes18,19. Under the selected experimental conditions in this work, the proposed kinetic model was assumed irreversible, which obliges the specific reaction rates to be greater than zero, for the following reasons: 1. The upgrading was taking place in an open reaction system; under the relatively low pressure and high temperatures most of the light components generated by the upgrading reactions would be in the gas phase, experiencing reduced space times than anticipated at the reaction conditions. This obliges that the reacting species are not in equilibrium with the cracked products, which indicates that the upgrading reactions under the selected operating conditions are irreversible. 2. The cleavage of C-C bonds requires lower energy than the cleavage of C-H bonds20, which indicates that the cracking of the heavier hydrocarbons’ C-C bonds are more favored. In Addition, the produced lighter hydrocarbons are relatively kinetically stable under the considered reaction conditions3,6,8,21; the activation energies of the cracking reactions of the lighter hydrocarbons are higher than those of the heavier molecules. This signifies that the formation reactions of lighter hydrocarbons are irreversible under the considered reaction conditions.


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Figure 3 shows the proposed kinetic model, which resembles the heavy oil hydrocracking kinetic model first proposed by Ancheyta et al.22 However, a different kinetic configuration is used that employs only seven first order kinetic rate constants assuming that the gaseous product is originating from the 540 o

C+ ALVR, exclusively, at the short space times considered for this study. Consequently, the VGO,

distillates and naphtha formed at the short space times do not undergo further cracking to produce gases due to their kinetic stability as indicated earlier. The experimental compositions of the 540 oC+ ALVR fraction prior and subsequent its conversion to the other four lumps were used to estimate the overall global kinetic rate constants. These experimentally estimated rate constants were used to optimize each individual reaction rate constant for converting the 540 oC+ ALVR to the other four lumps

, and

, as shown in Figure 3.

3.2. Kinetic Model Evaluation Steps First, the experimental feedstock and products compositions were determined using the HTSD method and the total processed and collected oil weights. Plug Flow Reactor (PFR) model is considered in this study to evaluate the kinetic data taking into consideration constant flow rates and assuming negligible radial gradients and invariable oil densities. The general PFR mass balance relationship for each lump can be expressed as

The mass flow rate of lump j written as , where

, where

in eq 1 is a constituent of the total oil mass flow rate

and can be

is the weight fraction of lump j. The total oil flow rate can be written as

is the oil density estimated experimentally to be ~1 g/cm3. Since the oil density was

assumed invariable, therefore

; and upon substitution into eq 2 gives the general mass balance

equation for constant volumetric flow rate


9

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where


is the reaction rate for lump in wt%/hr, V is the reactor volume in cm3, and t is the space time

in hours (hr). Equation 2 is the PFR mass balance design equation for constant volumetric flow rate, which was combined with the kinetic model rate expression to develop the mass balance on each lump in terms of specific reaction rates and lumps’ concentrations

Equations 3-7 are a system of Ordinary Differential Equations (ODE) of first degree. The solution of this system involves direct integration of the left hand sides and numerical integration of the right hand sides by midpoint approximation method utilizing Riemann summation technique, as shown in eq 8

where is the sequence number of space time t (1-6), as demonstrated in Table 1, refers to each lump and

is the reactions rate constant or combination of rate constants (

) according to each

ODE. To establish the solution, each ODE (eqs 3-7) was programmed in the form of eq 8 into an Excel spreadsheet, as shown by eqs 9-13


10


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Distillates (d):

Naphtha (n):

Then, given the space times and the experimental products composition data the specific reaction rates (

were estimated assuming initial values via the Excel optimizer solver23. For the model to

converge on the experimental result, its solution has to meet three criteria. First criterion: the summation of squared differences between both sides of eqs 9-13 has to be minimized as much as possible. To meet this criterion, least-square regression method was utilized to predict the specific reaction rates that would result in the minimum total squared difference between the predicted and experimental net composition change, determined from eq 14 at each space-time


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Second criterion: the estimated specific reaction rates have to be proportional to the reaction temperature profile by satisfying the following Arrhenius equation

where j is the number of the reaction rate constant (1-7). Third criterion: the global experimental specific reaction rates and the model predicted specific reaction rates of directly converting the 540 oC+ ALVR to the other four lumps have to be as close as possible, or

. In order to

determine the global reaction rate constant, the mass balance equation of 540 ,

was

combined

with

the

first

order

kinetic

rate

o

C+ ALVR, expression,

, and integrated as shown by eq 16

since

then,

was then determined from the slope of the plot of

vs.

using eq 17. In order to

close the gap between the global and model predicted kinetic rate constants, symmetrical constraints were placed in the aforementioned Excel optimizer. The solution resulting in specific reaction rates that satisfy the above three criteria was considered converging. These accepted specific reaction rates were then programmed collectively with the feedstock composition and the proposed kinetic model into POLYMATH® ODE solver to estimate the products composition numerically using Runge-KuttaFehlberg (RKF45) method through 100 subintervals at each space-time. The Mean Average Percentage Error (MAPE) between the product’s model predicted and experimental compositions are calculated from eq 18


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where,

and

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are as defined above and T is total number of experimental points evaluated, which is

determined as

. Absolute Percentage Errors (APE) between the

experimental global and model predicted kinetic parameters were calculated using eq 19

For the comparison purpose, the thermal cracking kinetic rate constants at the higher conditions of AQP (435-445 oC and 5-10.5 hr-1, respectively) were estimated from the predetermined rates constants using Arrhenius equation (eq 21) derived from eq 15, as shown below

In order to estimate products compositions of thermal cracking at the higher conditions, the kinetic model was utilized. 4. RESULTS 4.1. Kinetic Parameters In order to have a better understanding on effect of water splitting on the heavy oil upgrading kinetics, following is an explanation on the water splitting role. From the kinetics point of view water is believed to reduce the specific reaction rate of asphaltenes formation through reacting with its precursors24. This interaction is favored by high pressure and temperature and involves hydrogen transfer from water to the hydrocarbon radicals and asphaltenes precursors. This transfer reduces the rate of hydrogen abstraction reactions from the different species in the oil4. At high temperature, hydrogen transfers from water to oil by either water self-ionization, under high pressures, or through thermo-catalytic splitting to mixtures of hydrogen, hydroxyl and oxygen radicals or combination thereof25. Heavy hydrocarbon oils ACS Paragon Plus Environment

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such as short residua crack at conditions near or above water critical condition. Water self-ionization, for a given pressure, is disfavored near the critical temperature26; therefore, water self-splitting at heavy oil upgrading temperatures is improbable unless very high pressures are employed. In order to overcome this issue a different approach must be employed to induce water splitting rate at lower pressures. The UD catalyst is utilized in this case to promote water splitting at lower pressures. The UD catalyst improves the kinetics of water dissociation via free radicals mechanism4. Furthermore, the catalytic particles also improve hydrogen addition reactions kinetics to the thermally generated hydrocarbon free radicals. This in turn reduces the recombination reactions rates of heavy hydrocarbon radicals, thereby improving the reacting oil stability and reducing asphaltenes formation. This rational explanation is demonstrated by the enhanced oil conversion through extending the windows of operability of the upgrading process through oil stability enhancement achieved by Aquaprocessing4. Figure 5 shows the Arrhenius plot of the global kinetics of thermal cracking and AQP. It is noted that ALVR upgrading reactions fits the presumed first order kinetics for both thermal and AQP with activation energy of 174 and 168 KJ/mol, respectively. These estimated values are within the range reported for the thermal cracking of heavy hydrocarbon oils3, 6, 7, 8, 9, 10. Comparison between the AQP and the thermal cracking global reactions rate constants on the ordinate of Figure 4 indicates that the AQP demonstrates faster kinetics at relatively similar activation energies. The small difference between the activation energies indicates that the UD Ni/K catalyst was mainly involved in improving the oil stability, thereby increasing its availability for conversion rather than lowering the activation energy. As a result of this improved stability the reaction mechanism shifts from generating polycyclic compounds to producing more valuable products on the expense of asphaltenes for a given residence time when compared to thermal cracking. In order to verify the kinetic finding of this work, it is necessary to compare the generated thermal cracking kinetic data against available data from literature.


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Page 26 of 37

Global Kinetics Shu et al.27 conducted a study on the visbreaking of Cold Lake Bitumen (CLB) in a batch reactor. Their reported kinetic information indicated a global rate constant of 0.60 hr-1. Comparison between this specific reaction rate and that of ALVR (0.31 hr-1) at 400 oC indicates that ALVR thermal cracking proceeds by slower kinetics when compared to CLB. This could be explained by comparing their API gravities and SARA fractions shown in Table 24, 28. The breaking of single bonds between long chain saturate species and between them and other species in the oil is undemanding. Bonds become more stable and therefore hard to break as the degree of their association increases 20. The cracking rate of bonds between different species decreases in the following order29; Saturates > Aromatics > Resins > Asphaltenes. The presence of long chain saturates in the form of cross linkages between heavier species induces faster global cracking kinetics when compared with oils with less saturates contents3. Recently Yang30 and his coworkers proposed 8 parallel lumps kinetic model for upgrading of undisclosed type vacuum residue. They estimated a global rate constant of 0.30 hr-1 for the thermal cracking of the heavy oil, which is in close agreement global rate constant estimated for ALVR in this study. Other global rate constants estimated by different authors30-31 for the thermal cracking of different residua are also found in reasonable agreement with the estimated value in this study. Figure 5 shows the global rate constants estimated by a number of authors in addition to the one estimated in the current study for the thermal cracking of different residua.

GAS Singh et al.9,

32

proposed a five lumps kinetic model with seven first order rate constants for the

thermal cracking of four vacuum residua, namely North Gujarat VR (NGVR), Bombay High VR (BHVR), Arab Mix VR (AMVR), and Mathura VB Feed (MVBF). They conducted their study in a semi batch reactor setup and estimated the (C1-C5) gas generation rate constants to be 0.07 and 0.03 hr-1 for NGVR and BHVR, respectively. Using similar experimental setup and kinetic model Kataria et al.9


15

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estimated similar specific reaction rates for the (C1-C5) gas generation from the thermal cracking of NGVR and BHVR. In a recent study using similar experimental setup and kinetic model Humaidan et al.10 conducted a thermal cracking study on vacuum residues from three different crude types, namely, Lower Fars (LF), Ratawi Burgan (RB), and Eocene. They reported relatively higher specific reaction rates for the (C1-C5) gas generation. Figure 6 shows the gas generation rate constants for the thermal cracking of different vacuum residua reported by the above authors. These rate constants are lower than the estimated rate constants for the gas generation from the thermal cracking of ALVR (0.11 hr -1). This variation is expectable by comparing the boiling ranges of the collected gases and the types of oils being studied. Singh, Kataria, Humaidan and their coworkers collected their gas samples sampling vessels8, 33 for offline compositional analysis after passing them through cold traps maintained at 0 to 3 oC8, 34. The current study was conducted in a flow system and the total gas product was sent to an online gas analyzer4 after passing through a low pressure separator maintained at room temperature (22

o

C) and

pressure. Therefore, the rate constant for gas generation from the thermal cracking of ALVR refers to the gas that is composed of CO2, H2S, C2 =, C3=, and C4= in addition to C1-C525.

Naphtha The direct naphtha generation from the thermal cracking of ALVR is negligible as indicated by

,

which approaches zero in the case of thermal cracking, when compared to the contribution of VGO and distillates. This is related to the cascade nature of the upgrading arrangement, where each lump is mainly involved in producing the immediate subsequent lump, in terms of boiling point range, rather than the furthest ones. This result is in reasonable agreement with the result reported from the kinetic study conducted by Mohaddecy35 and Singh32 and their coworkers. In contrast to the direct ALVR conversion, naphtha production from VGO and, to a lower extent, from distillates is more favored via thermal cracking as indicated by

and

, respectively. This result was also presented by Mohaddecy


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Page 28 of 37

and Singh who reported rate constants of 0.04 and 0.05 hr-1 for naphtha generation from the cracking of (320 oC+ hydrocarbon) and (350-500 oC hydrocarbon), respectively.

Distillates Comparison between the specific reaction rates for distillates production from the VGO and 540+ oC ALVR indicates that the distillates production is more favored from VGO as indicted by its faster kinetic constant

, that is higher than

by more than 12 fold. This result is supported by the findings

of Mohaddecy et al.35 and Singh et al.32 who reported rate constants of 1.68 and 1.20 hr-1 for the distillates generation from the VGO, respectively.

VGO In this study the specific reaction rate for generating VGO from the thermal cracking of ALVR was estimated as 0.17 hr-1. This result is in reasonable agreement with the figures reported in literature7-9, 30, 32-33

, as shown in Figure 7.

Table 3 shows the upgrading model and global experimental kinetic parameters of thermal cracking and AQP generated in this study. As can be depicted from Table 3 the experimental and model based results are in close agreement with a maximum APE between the experimental global and model predicted kinetic parameters of 3.4%. Comparison between the global and model kinetic parameters shows APE of less than 4% for the activation energies and of less than 1% for the specific reaction rates. Comparison between the AQP and thermal cracking model specific reaction rates shows a faster kinetics trend with the former process except for

and

improved with AQP as indicated by its faster kinetics,

, and

. The 540+ oC ALVR conversion was . Furthermore, gas generation pace

was dropped under AQP conditions as indicated by its lower rate constant,


. The increase in

,

,

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and


and reduction in

in AQP is expected due to reduction in tendency to form larger

polycondensed molecules. Data comparisons in Table 3 indicate that the VGO conversion to distillates is the fastest reaction-taking place amongst the reaction complex with the lowest activation energy. This in turn indicates that a major fraction of VGO is considered an intermediate product for converting the residual oil to distillates. Although the VGO has the highest conversion specific reaction rate to the distillates; nevertheless, it shows a slower concentration declining profile when compared to the 540 o

C+ ALVR, as shown in Figure 8. This can be explained knowing that the VGO undergoes

simultaneous conversion and generation from the unconverted 540 oC+ ALVR, which is a characteristic of intermediate products. The conversion rate of the VGO to distillates was improved largely with AQP as indicated by its specific reaction rate

, which is higher than that of thermal cracking at similar

temperatures. Naphtha production from VGO is more favored via thermal cracking as indicated by higher

. However, faster kinetics is observed for the naphtha generation from the 540+ oC ALVR and

distillates through AQP as indicated by higher

and

, respectively. Nonetheless, from Table 3 it is

seen that the direct naphtha production from the 540+ oC ALVR is subordinated by higher activation energy and lower rate constant

, which approaches zero in the case of thermal cracking, when

compared to the contribution of the distillates. This is related to the cascade nature of the upgrading arrangement, where each lump is mainly involved in producing the immediate subsequent lump, in terms of boiling point range, rather than the furthest ones. This can be visualized by comparing with

and

with

in Table 3. This implies that the naphtha production kinetics is mostly dominated

by . This signifies that the differential increase in compared to the differential increase in

with thermal cracking is of less importance when

with AQP due to its higher impact on naphtha production.


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Page 30 of 37

Because most of the VGO was converted to distillates, in terms of availability, and that the distillates conversion to naphtha is more favored by faster kinetics in comparison to the 540+ oC ALVR; therefore, the distillates fraction is considered the major contributor in the naphtha production. This can be verified by comparing the activation energies for producing naphtha from the VGO, 540+ oC ALVR, and distillates, which are found to be 245-250, 188-190, and 150-169 KJ/mol, respectively. From the above rationales, it is believed that naphtha production was improved with AQP. The increase in the formation reaction rates of the VGO, distillates and naphtha via AQP has decreased the affinity of condensation reactions. Tables 4 and 5 show the percent conversion of the 540+ oC ALVR and the products P-values and compositions as functions of space-time via AQP at 440 oC and TC at 400 oC, respectively. From Table 4 and 5 it is observed that the distillates fraction leaped from zero, initially, to 21 and 14 wt% in the products generated with a space velocities of 6.5 hr-1 via AQP and 2 hr-1 via TC at P-value of 1.20. This provides an essential indication that the VGO conversion to distillates was the fastest reaction-taking place as indicated above. It is also noticed that the reduction in the compositions of the 540+ oC ALVR and VGO was accompanied by simultaneous increase in the distillates, naphtha and gas compositions in the total product. The maximum percent conversions of the 540+ oC ALVR were reported to be 33.5 and 29.5% via AQP and TC, respectively4. Micro Carbon Residue (MCR) analysis on the products samples did not show any noticeable difference in the MCR% in the products of TC and AQP. However, the asphaltenes content was observed to be higher in the TC product when compared to the product of AQP at similar conditions. This result has already been shown elsewhere4. 4.2. Proximity of Model and Experimental Data. The product experimental and model predicted compositions are found to be in close agreement, as shown in Figures 9 and 10. From Table 3 the maximum MAPE between the product model predicted and experimental compositions was found 5.4%. The aforementioned insignificant magnitude of errors in predicting the kinetic parameters (APE) and product compositions (MAPE) support the validity of the


19

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proposed model. The rate constants of the thermal cracking data shown in Table 3 at 435-445 oC and 510.5 hr-1, respectively, were estimated using Arrhenius equation (eq 21) without experimental data. This data was generated for the purpose of comparison with the AQP kinetic data generated by the model. Therefore, the MAPE could not be determined. 5. CONCLUSION Five fractions lumped kinetic model that explains the reaction system of upgrading Arab Light vacuum residue via Aquaprocessing was proposed and investigated for the first time at conditions distant from coke formation. The proposed model was found capable of predicting the upgraded product composition with MAPE of less than 5.5%. Furthermore, the model predicted kinetic parameters for converting the 540 oC+ ALVR were found close to the global experimental values with APE of less than 1% for kinetic rate constants and of less than 3.5% for activation energies. It is observed that the direct naphtha production from the 540 oC+ ALVR is less favored via thermal cracking when compared to Aquaprocessing. It is found that the distillates generation from the VGO is the fastest reaction taking place in the system as defined, which is verified by the lowest activation energy requirement and the highest specific reaction rate. While the gas production was dictated by slower kinetics via Aquaprocessing, the naphtha and distillates productions were improved by faster kinetics. Insert Here “Table 1. AQP Space Times and Temperatures.” Insert Here “Table 2. SARA and API Gravities of Unconverted ALVR and Cold Lake Bitumen.” Insert Here “Table 3. ALVR Experimental and Model Predicted Upgrading Kinetic Parameters.” Insert Here “Table 4. Conversions and Products P-values & Compositions as Functions of LHSV for AQP at 440 oC.” Insert Here “Figure 1. Experimental pilot plant representation.” Insert Here “Figure 2. Representation of the kinetic model lumped products distribution.”


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Page 32 of 37

Insert Here “Figure 3. Proposed lumped kinetic model.” Insert Here “Figure 4. ALVR TC and AQP Arrhenius follow 1st order global kinetics.” Insert Here “Figure 5. Global Rate Constants for Thermal Cracking of Different Residua.” Insert Here “Figure 6. Gas Rate Constants for Thermal Cracking of Different Residua.” Insert Here “Figure 7. VGO Rate Constants for Thermal Cracking of Different Residua.” Insert Here “Figure 8. Feedstock Composition Profiles During AQP at 440 oC.” Insert Here “Figure 9. TC model and experimental products compositions.” Insert Here “Figure 10. AQP model and experimental products compositions.” AUTHOR INFORMATION Corresponding Author Present Addresses *†Mazin M. Fathi. E-mail: [email protected]. Saudi Aramco P.O. Box 2278, Dhahran31311, Eastern Province, Saudi Arabia, Tel: (+966)509545753. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work is supported by Saudi Aramco who provided oil samples and academic financial support. It is also supported by Alberta Ingenuity Fund, now Alberta Innovates, via a Scholar award provided to Dr. Pereira-Almao. The authors acknowledge Dr. C. Scott and L. Carbognani for support. Support from and several members of the Catalysis for Bitumen Upgrading and Hydrogen Production (CBUHP) group from the University of Calgary, in analytical and technical matters is also acknowledged.


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Supporting Information Available: Experimental and model predicted compositions of thermal cracking and AQP at different experimental conditions (xlsx). This information is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1. Speight, J. G.; Özüm, B., Petroleum refining processes. Marcel Dekker: New York, 2002. 2. Pereira, P.; Marzin, R.; Zacarias, L.; Trosell, I.; Hernandez, F.; Cordova, J.; Szeoke, J.; Flores, C.; Duque, J.; Solari, B., AQUACONVERSION (TM): A New Option for Residue Conversion and Heavy Oil Upgrading. Vision Tecnol. 1998, 6, 5. 3. Le Page, J.-F; Chatila., S.; Davidson, M., Resid and Heavy Oil Processing. Editions Technip: Paris, 1990. 4. Fathi, M. M.; Pereira-Almao, P., Catalytic Aquaprocessing of Arab Light Vacuum Residue via Short Space Times. Energy Fuels 2011, 25 (11), 4867-4877. 5. Pereira, P. M., R.; Zacarias, L.; Cordova, J.; Carrazza, J.; Marino, M. Steam conversion process and catalyst. U.S. Patent No.5885441, March 23, 1999. 6. Fahim, M. A.; Al-Sahhaf, T. A.; Elkilani, A., Fundamentals of Petroleum Refining. Elsevier Science Limited: Amsterdam, 2010; p 133. 7. Joshi, J. B.; Pandit, A. B.; Kataria, K. L.; Kulkarni, R. P.; Sawarkar, A. N.; Tandon, D.; Ram, Y.; Kumar, M. M., Petroleum Residue Upgradation via Visbreaking: A Review. Ind. Eng. Chem. Res. 2008, 47 (23), 8960-8988. 8. Kataria, K. L.; Kulkarni, R. P.; Pandit, A. B.; Joshi, J. B.; Kumar, M., Kinetic Studies of Low Severity Visbreaking. Ind. Eng. Chem. Res. 2004, 43 (6), 1373-1387. 9. Singh, J.; Kumar, M. M.; Saxena, A. K.; Kumar, S., Reaction pathways and product yields in mild thermal cracking of vacuum residues: A multi-lump kinetic model. Chem. Eng. J. 2005, 108 (3), 239-248. 10. Petroleum, I. o., Modern Petroleum Technology, Volume 2 Downstream, 6th Edition 6ed.; John Wiley & Sons: 2000. 11. Yasar, M.; Trauth, D. M.; Klein, M. T., Asphaltene and Resid Pyrolysis. 2. The Effect of Reaction Environment on Pathways and Selectivities. Energy Fuels 2001, 15 (3), 504-509. 12. Banerjee, D. K.; Laidler, K. J.; Nandi, B. N.; Patmore, D. J., Kinetic studies of coke formation in hydrocarbon fractions of heavy crudes. Fuel 1986, 65 (4), 480-484. 13. Gray, M. R., Through a glass, darkly: Kinetics and reactors for complex mixtures syncrude innovation award lecture. Can. J. Chem. Eng. 1997, 75 (3), 481-493. 14. Weekman, V. W., Lumps, models, and kinetics in practice. AIChE: 1979. 15. Carbognani, L.; Lubkowitz, J.; Gonzalez, M. F.; Pereira-Almao, P., High Temperature Simulated Distillation of Athabasca Vacuum Residue Fractions. Bimodal Distributions and Evidence for Secondary “On-Column” Cracking of Heavy Hydrocarbons. Energy Fuels 2007, 21 (5), 2831-2839. 16. Di Carlo, S.; Janis, B., Composition and visbreakability of petroleum residues. Chem. Eng. Sci. 1992, 47 (9-11), 2695-2700. 17. Reyniers, M. F.; Beirnaert, H.; Marin, G. B., Influence of coke formation on the conversion of hydrocarbons: I. Alkanes on a USY-zeolite. Appl. Catal., A 2000, 202 (1), 49-63. 18. Ayasse, A. R.; Nagaishi, H.; Chan, E. W.; Gray, M. R., Lumped kinetics of hydrocracking of bitumen. Fuel 1997, 76 (11), 1025-1033. 19. Meng, X.; Xu, C.; Gao, J.; Li, L., Seven-lump kinetic model for catalytic pyrolysis of heavy oil. Catal. Commun 2007, 8 (8), 1197-1201. 20. Raseev, S., Thermal and Catalytic Processes in Petroleum Refining. Marcel Dekker: New York, ACS Paragon Plus Environment

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Page 34 of 37

2003. 21. Xiao, J.; Wang, L.; Chen, Q.; Wang, D., Modeling For Product Distribution in Thermal Conversion of Heavy Oil. Pet. Sci. Technol 2002, 20 (5/6), 605. 22. Sánchez, S.; Rodríguez, M. A.; Ancheyta, J., Kinetic Model for Moderate Hydrocracking of Heavy Oils. Ind. Eng. Chem. Res. 2005, 44 (25), 9409-9413. 23. Fylstra, D.; Leon, L.; Watson, J.; Waren, A., Design and Use of the Microsoft Excel Solver. Interfaces 1998, 28 (5), 29-55. 24. Liu, Y.; Bai, F.; Zhu, C. C.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K., Upgrading of residual oil in sub-and supercritical water: An experimental study. Fuel Process. Technol. 2012. 25. Fathi, M. M. Comparative Upgrading of Arab Light Vacuum Residuum via Aquaprocessing and Thermal Cracking. Dissertation, University of Calgary, Calgary, 2011. 26. Pitzer, K. S., Self-ionization of water at high temperature and the thermodynamic properties of the ions. J. Phys. Chem. 1982, 86 (24), 4704-4708. 27. Shu, W. R.; Venkatesan, V. N., Kinetics of Thermal Visbreaking of a Cold Lake Bitumen. J. Can. Pet. Technol 1984, 23 (2), 60-64. 28. Rahimi, P.; Gentzis, T., The Chemistry of Bitumen and Heavy Oil Processing. In Practical Advances in Petroleum Processing, Hsu, C.; Robinson, P., Eds. Springer New York: 2006; pp 597634. 29. McKetta Jr, J. J., Petroleum processing handbook. Marcel Dekker: New York, 1992. 30. Yang, J.; Wang, L.; Tian, C.; Xiao, J.; Yang, C., Improved method for kinetic parameters estimation of non-isothermal reaction: Application to residuum thermolysis. Fuel Process. Technol. 2012, 104 (0), 37-42. 31. Del Bianco, A.; Panariti, N.; Anelli, M.; Beltrame, P. L.; Carniti, P., Thermal cracking of petroleum residues: 1. Kinetic analysis of the reaction. Fuel 1993, 72 (1), 75-80. 32. Singh, J.; Kumar, S.; Garg, M. O., Kinetic modelling of thermal cracking of petroleum residues: A critique. Fuel Process. Technol. 2012, 94 (1), 131-144. 33. AlHumaidan, F.; Lababidi, H.; Al-Rabiah, H., Thermal cracking kinetics of Kuwaiti vacuum residues in Eureka process. Fuel 2012. 34. Sawarkar, A.; Pandit, A.; Joshi, J., Studies in coking of Arabian mix vacuum residue. Chem. Eng. Res. Des. 2007, 85 (4), 481-491. 35. Mohaddecy, S. R. S.; Sadighi, S., Simulation and kinetic modeling of vacuum residue soakervisbreaking. Pet. Coal 2011, 53 (1), 26-34.


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Table 1. AQP Space Times and Temperatures. *ti(hr)

435 oC

440 oC

445 oC

t0

0.000

0.000

0.000

t1

0.133

0.118

0.095

t2

0.143

0.125

0.100

t3

0.154

0.133

0.105

t4

0.167

0.143

0.111

t5

0.182

0.154

0.118

t6

0.200

0.167

0.125

*i: Number of experimental points

Table 2. SARA and API Gravities of Unconverted ALVR and Cold Lake Bitumen. Property

Original ALVR4

Cold Lake Bitumen28

Saturates wt%

6.1

20.74

Aromatics wt%

60.3

39.2

Resins wt%

23.5

24.81

Asphaltenes wt%

10.1

15.25

6.1

10.71

o

API


24


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Page 36 of 37

Table 3. ALVR Experimental and Model Predicted Upgrading Kinetic Parameters and Proximities. AQP Kinetics T (oC)

435

440

445

-1

-1

-1

hr

hr

hr

Ea KJ/mol

Thermal Cracking Kinetics R2

400

405

408

435

440

445

-1

-1

-1

-1

-1

-1

hr

hr

hr

hr

hr

hr

Ea KJ/mol

R2

k1

0.810 1.000 1.161

152

0.991

0.171

0.204

0.235

0.638

0.761

0.906

148

0.992

k2

0.407 0.515 0.624

180

0.997

0.028

0.035

0.042

0.142

0.177

0.219

183

0.997

k3

0.230 0.288 0.359

188

1.000 0.0003 0.0004 0.0005 0.0016 0.0020 0.0025

190

0.997

k4

0.513 0.612 0.762

167

0.995

0.114

0.144

0.173

0.650

0.821

1.034

196

0.994

k5

6.080 7.044 7.726

101

0.984

1.230

1.436

1.624

3.922

4.584

5.346

131

0.993

k6

0.047 0.060 0.083

245

0.993

0.091

0.130

0.153

0.825

1.112

1.493

250

0.996

k7

0.366 0.440 0.521

150

1.000

0.064

0.080

0.091

0.282

0.345

0.420

169

1.000

Calculated k(540C+ ALVR)

1.96

2.42

2.91

172

0.31

0.38

0.45

1.43

1.76

2.16

179

Global k(540C+ ALVR)

1.95

2.41

2.91

168

0.31

0.38

0.45

1.44

1.76

2.16

173

*APE

0.5

0.1

0.0

2.0

0.64

0.14

0.33

0.5

0.1

0.0

3.4

**MAPE

2.6

5.4

3.3

5.1

3.1

2.2

•

•

NA

k1+ k2+ k3+ k4

* Absolute Percentage Errors ** Mean Average Percentage Error


25

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Table 4. Conversions and Products P-values & Compositions as Functions of LHSV for AQP at 440 oC. Space % Time Conversion (hr)

P-Value

540oC+ ALVR

% VGO

% Distillates

% Naphtha

% Gas

0.118

20.2

1.55

61.8

13.9

16.6

2.4

5.4

0.125

21.4

1.45

61.0

13.6

16.9

2.7

5.8

0.133

23.1

1.40

59.5

13.4

18.0

2.9

6.2

0.143

27.3

1.35

58.1

13.1

19.5

3.0

6.3

0.154

33.5

1.20

55.8

13.1

21.0

3.4

6.7

0.167

35.1

1.00

54.4

12.5

22.1

3.6

7.3

Table 5. Conversions and Products P-values & Compositions as Functions of LHSV for TC at 400 oC. Space % Time Conversion (hr)

P-Value

540oC+ ALVR

% VGO

% Distillates

% Naphtha

% Gas

0

Feed

2.90

81.7

18.3

0.0

0.0

0.0

0.4

18.1

1.70

73.0

13.5

8.5

0.9

4.1

0.5

21.7

1.55

69.3

14.0

11.4

1.1

4.2


26

Kinetic Modeling of Arab Light Vacuum Residue Upgrading by

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