Partial Upgrading of Bitumen: Impact of Solvent Deasphalting and

Aug 24, 2017 - Although not all products were characterized, it was possible to calculate a hydrogen balance using data from other studies(21, 22) wit...
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Partial Upgrading of Bitumen: Impact of Solvent Deasphalting and Visbreaking Sequence Ashley Zachariah†,‡ and Arno de Klerk*,† †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada Praxair Canada Inc., 9020 24 Street NW, Edmonton, AB T6P 1X8, Canada

‡

ABSTRACT: Solvent deasphalting and visbreaking are two important technologies in the development of processes for partial upgrading of oilsands-derived bitumen to improve oil fluidity for pipeline transport. This work investigated the impact of the process sequence, solvent deasphalting followed by visbreaking (SDA-Vis) compared to visbreaking followed by solvent deasphalting (Vis-SDA). Thermal conversion during visbreaking was performed at 380 °C for 85 min, and solvent deasphalting was performed with n-pentane. Using this combination of processes in either sequence changed bitumen from a viscosity-limited fluid to a density-limited fluid with respect to pipeline specifications. The density and viscosity of the oil products from SDA-Vis and Vis-SDA were comparable. It was found that SDA-Vis achieved 2 wt % higher liquid yield than Vis-SDA. Conversely, VisSDA produced an oil product with higher hydrogen to carbon ratio compared to SDA-Vis. This difference could be explained in terms of hydrogen transfer during thermal conversion. Overall, the diluent requirements to meet pipeline fluidity requirements could be reduced by 40% relative to the Cold Lake bitumen feed. deasphalting first and visbreaking second (SDA-Vis) is that the onset of coking during thermal conversion can be delayed. Operating with a SDA-Vis sequence enables higher conversion of the residue fraction during visbreaking, because material that is more likely to form “coke” by phase segregation13 is removed beforehand. There are also changes in the nature of the product obtained by performing solvent deasphalting before thermal conversion.14 The potential advantage of performing visbreaking first and solvent deasphalting second (Vis-SDA) is that the upgraded oil has a lower asphaltenes content, which may have a stability benefit. Operating with a Vis-SDA sequence could potentially also retain more hydrogen in the product, because asphaltenes are hydrogen donors during thermal processing.15,16 Initial work on the sequencing of solvent deasphalting and visbreaking was inconclusive, 17 which was ascribed to insufficient visbreaking severity. The liquid yield from SDAVis was marginally higher, but the liquid product from Vis-SDA was of marginally better quality, with lower microcarbon residue content, lower refractive index, and higher aliphatic hydrogen content. In this study the impact of solvent deasphalting and visbreaking sequence was investigated at visbreaking conditions that approached onset of coking for visbreaking, which were established previously.18 The focus was on differentiating between SDA-Vis and Vis-SDA with respect to material balance and liquid product properties. The comparison was made for operation at similar solvent deasphalting and visbreaking process conditions. The outcome of the work is discussed in the context of partial upgrading and the implications for partial upgrader design.

1. INTRODUCTION A partial upgrader, or field upgrader, is a process facility for the conversion of bitumen and heavy oil at the least capital and operating cost to an oil product with sufficient fluidity to enable pipeline transport with little or no further dilution. The partially upgraded oil is converted primarily to decrease viscosity and density, while improvements in other oil properties are desirable, but incidental. Partial upgrading concepts and processes for such upgrading have been discussed for several decades,1−5 but apparently with limited industrial interest. With the sharp decrease in oil prices at the end of 2014, the situation changed and development of partial upgrading process technology to enable pipeline transport of bitumen and heavy oil gained momentum. Analysis of the literature on partial upgrading highlights the need for two processing steps that are common to most process concepts: thermal conversion and carbon rejection. The least severe thermal conversion technology and the technology of interest in this study is visbreaking. Visbreaking is a mild thermal cracking, or pyrolysis process, that is typically operated in the temperature range of 430−500 °C.6−9 As its name suggests, visbreaking was originally developed as a refining technology to reduce viscosity of heavy oil. On a capacity basis, it is also the least expensive of the thermal conversion technologies.10 Carbon rejection can take place in combination with thermal conversion, such as by coking, or it can be performed as a separate step, such as by solvent deasphalting. The carbon rejection technology of interest in this study is solvent deasphalting. Solvent deasphalting causes phase separation through the addition of a light hydrocarbon solvent to the feed material.11,12 The upgraded product is called deasphalted oil (DAO), and the carbon rejection product is asphalt. The sequencing of solvent deasphalting and visbreaking steps was of interest. The potential advantage of performing solvent © 2017 American Chemical Society

Received: July 11, 2017 Revised: August 4, 2017 Published: August 24, 2017 9374

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in deuterated chloroform before analysis. Viscosity was measured using an Anton Paar RheolabQC viscometer at 10 s−1 shear rate with a cylinder in cup configuration. Density was measured using an Anton Paar DMA 4500 M density meter. Microcarbon residue (MCR) was determined using a Mettler Toledo TGA/DSC1 with MX5 microbalance. The mass percentage of material remaining after heating in an alumina crucible at a heating rate of 10 °C/min from 25 to 600 °C under inert (N2) atmosphere was taken as the MCR value. Refractive index was measured using an Anton Paar Abbemat 200 refractometer.

2. EXPERIMENTAL SECTION 2.1. Materials. The investigation was conducted with oilsands bitumen from the Cold Lake region in the province of Alberta, Canada. Characterization of the bitumen was reported before17 and will be shown for comparative purposes with the results of this study in section 3. The solvents used were n-pentane (98%, Fisher Scientific) and methylene chloride (99.9%, Fisher Scientific). 2.2. Equipment and Procedure. All experiments were performed in triplicate. The results that are reported are the average (x) and sample standard deviation (s) of experiments in triplicate. Experimental values are reported in the format x ± s. 2.2.1. Solvent Deasphalting. Solvent deasphalting of bitumen and deasphalted oil was performed using the same procedure. The material to be deasphalted was mixed with n-pentane in a 1:40 ratio. The mixture was covered and stirred with a magnetic stirrer at room temperature (∼23 °C) for 1 day. The solids were allowed to settle before the solids were separated from the liquid by vacuum filtration through a 0.22 μm aperture filter and washed with n-pentane. The solids were dried overnight at 75 °C in a vacuum oven before the mass of the solids was determined. This is a more rigorous precipitation procedure than one would encounter in industrial practice.11 The reason for such a rigorous precipitation was to assist with interpretation and facilitate comparison of the n-pentane insoluble content in this work with that reported in the literature. The n-pentane was removed from the liquid product by rotary evaporation. It was not possible to remove all of the n-pentane by rotary evaporation. After rotary evaporation the product was left at ambient conditions until constant mass was obtained. This required several days (removal of n-pentane is transport-limited). Liquid product characterization and/or thermal conversion of the deasphalted oil was performed only subsequent to n-pentane removal. 2.2.2. Thermal Conversion. Thermal conversion of bitumen and deasphalted oil was performed using the same procedure. In this investigation thermal conversion was performed at 380 °C for 85 min; conditions previously determined to be close to onset of coking when visbreaking Cold Lake bitumen. The decision to perform thermal conversion at conditions milder than those practiced industrially was motivated by encouraging results previously reported.18 Performing the conversion at milder conditions also had other potential benefits for the development of partial upgrading technology, as will be discussed in Section 4.5. The thermal conversion was performed in a microbatch reactor manufactured from Swagelok half-inch tubing and fittings. In a typical experiment, ∼8 g of material was placed in the reactor, which was purged and pressurized with nitrogen to a pressure of 4 MPa. The sealed reactor was placed into a preheated fluidized sand bath heater. The internal temperature in the reactor was monitored by a thermocouple, and the start of the reaction time was taken when the internal temperature reached 380 °C. This temperature was reached 6−8 min after submerging the reactor in the hot fluidized sand. After 85 min at 380 °C, the reactor was removed from the hot fluidized sand and allowed to cool. The internal temperature decreased to 430 °C as is more typical of industrial practice. The milder conditions were justified based on previous work,18 from which it was anticipated that at these conditions high liquid yield could be obtained, as well as a significant permanent reduction in oil viscosity. These were desirable outcomes for partial upgrading, where the primary aim is fluidity improvement, rather than upgrading per se. Viscosity is not just dependent on cracking to lighter fractions. It was found that viscosity of converted oil could not be calculated from its boiling point distribution based on the viscosity of the distillation fractions of the feed.23 Thermal conversion changed the viscosity of boiling fractions. The main difference between SDA-Vis and Vis-SDA was the gas yield (Table 2), which was a consequence of cracking in combination with hydrogen transfer, as noted in section 4.1. The nature of the feed, DAO versus bitumen, had a direct

ρ = a1 + a 2 ·T ′

(1)

The temperature correlation coefficient a2 was reported for different bitumen samples, and a2 was in the range −0.612 to −0.602.25 Using a value of a2 = −0.607, values for a1, the reference density, were calculated using the data in Table 5. The temperature (T, K) dependence of viscosity (μ, mPa·s) was described by a correlation that was developed for bitumen (eq 2).27 log10 log10(μ + 0.7) = b1 + b2 · log10(T )

(2)

For Cold Lake bitumen, the correlation coefficients b1 and b2 were found to be b1 = 9.15 and b2 = −3.43,26 but it was also noted that the numeric values for subfractions of the bitumen were different. Using a value of b2 = −3.43, values for b1 were calculated using the data in Table 5. The change in viscosity with addition of a low-viscosity solvent (diluent) was calculated using a correlation that was developed specifically for bitumen and diluent mixtures (eqs 3−5).27 9377

DOI: 10.1021/acs.energyfuels.7b02004 Energy Fuels 2017, 31, 9374−9380

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Energy & Fuels ln(υm) = exp[α ·(1 − x Dn)] + ln(υD) − 1

(3)

exp(α) = ln(υB) − ln(υD) + 1

(4)

n = υD·[0.9029·υD + 0.1351]−1

(5)

7. Solvent deasphalting in combination with mild visbreaking reduces the diluent requirements compared to direct bitumen Table 7. Diluent Addition to Meet Pipeline Viscosity and Density Specifications and the Associated Volume Contraction Incurred

The kinematic viscosity of the mixture (υm, cSt) is related to the kinematic viscosities of the bitumen (υB, cSt) and diluent (υD, cSt) through a viscosity interaction parameter (α) and a viscosity reduction parameter (n) as a function of the mass fraction of diluent (xD) in the mixture. The density of bitumen and diluent mixtures is affected by volume contraction. This effect becomes more pronounced with increase in temperature and solvent mass fraction up to 50 wt %. The density of the mixture (ρm, kg/m3) could be expressed in terms of the density of the bitumen (ρB, kg/m3) and the diluent (ρD, kg/m3) as a function of the mass fraction of diluent (xD) in the mixture and a binary interaction parameter (βB,D) through eq 6.28 ⎛1 1 − xD x 1 1 ⎞ ⎟⎟β = + D − x D(1 − x D)⎜⎜ + ρm ρB ρD ρD ⎠ B,D ⎝ ρB

description viscosity specification diluent mass fraction volume contractiona density specification diluent mass fraction volume contractiona

bitumen

SDA-Vis oil

Vis-SDA oil

0.236 0.0133

0.092 0.0034

0.123 0.0042

0.204 0.0117

0.147 0.0054

0.138 0.0047

Volume contraction = (ideal volume − actual volume)/(ideal volume).

a

dilution by around 40%, i.e., from 24 wt % to 14−15 wt %. The diluent requirement for both of the partially upgraded oils depended on the density specification and not the viscosity specification. However, for Cold Lake bitumen the diluent requirement depended on viscosity. 4.4. Cost Implications. A comparison of the cost implications was made using the process economic data from Maples.10 The comparison was performed on a basis of 25 000 barrels per calendar day Cold Lake bitumen feed (Table 8). It is

(6)

For Athabasca bitumen and pentane, the interaction parameter was βB,D = 0.0556, and for an equal mass mixture of bitumen and pentane at ambient conditions, about 1% volume contraction was reported.28 (For aromatic solvents, βB,D ≈ 0.29) The diluent properties matter in any calculation of the diluent requirements to meet pipeline viscosity and density specifications. For the calculations shown here (Figure 3), a 1:1

Table 8. Relative Comparison of Costs for SDA-Vis and VisSDA Processes on a 25 000 bbl/day Basis description

SDA-Vis

Vis-SDA

29.9 21.6 51.5

29.1 24.0 53.1

60.8 −396.1 3930

60.6 −595.5 4140

a

capital cost ($ million) solvent deasphalting visbreaking total utilitiesb electricity (MWh/day) low-pressure steam (ton/day)c fuel gas (GJ/day) a

Costs in 1991 U.S. dollars for a U.S. Gulf Coast location. bAir cooling, no cooling water use. cNegative value indicates production of steam, not consumption.

a relative comparison, and the capital costs were not adjusted for location, time, or currency relative to the published data. More recent economic data can be found in the discussion on heavy oil upgrading by Carrillo and Corredor.4 From a capital cost perspective, the SDA-Vis sequence is slightly cheaper than Vis-SDA. In terms of utilities, SDA-Vis requires less fuel gas than Vis-SDA because of the lower capacity visbreaker, but it also produces less low-pressure (0.4 MPa gauge) steam during heat recovery. One aspect that was not captured by this simple comparison is the potential impact of the feed properties on the solvent deasphalting unit. Maples10 points out that the capital cost of a solvent deasphalting unit is more dependent on solvent type

Figure 3. Calculated kinematic viscosity (solid symbols) and density of mixtures of bitumen (●, ○), SDA-Vis oil product (■, □), and VisSDA oil product (▲ ,Δ) with different mass fractions of diluent.

molar ratio of n-pentane and n-hexane was employed in lieu of data for a specific light straight run naphtha or natural gas condensate. To facilitate comparison, the values for diluted Cold Lake bitumen are also provided. Key values used in the calculations are summarized in Table 6. The threshold values for diluent addition necessary to meet pipeline viscosity and density specifications are shown in Table

Table 6. Key Physical Properties Used in Calculations to Determine Diluent Requirements to Meet Pipeline Specifications description

diluent

bitumen

SDA-Vis oil

Vis-SDA oil

density at 7.5 °C (kg/m3) viscosity at 7.5 °C (mPa·s) kinematic viscosity at 7.5 °C (cSt)

655.9 2.99 × 10−1 4.56 × 10−1

1026.7 1.47 × 107 1.43 × 107

994.7 6.47 × 103 6.50 × 103

990.7 2.09 × 104 2.11 × 104

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from SDA-Vis and Vis-SDA are comparable (Table 5), the same is not true of their H/C ratio (Table 3), aliphatic to aromatic hydrogen content (Table 4), and microcarbon residue values (Table 5).

and solvent-to-feed ratio than capacity. In the experimental work these variables were controlled to be the same, but in practice much lower solvent-to-feed ratios will be employed, and one should anticipate that operation with viscous bitumen and a less viscous visbroken bitumen would be different. The cost per capacity of a solvent deasphalting unit processing visbroken bitumen is potentially lower than that of a solvent deasphalting unit processing bitumen. 4.5. Relevance to Partial Upgrader Design. The purpose of the study was to compare SDA-Vis and Vis-SDA process sequences, but considering the application, it is worthwhile to include some observations on this approach for partial upgrading. Ideally, a partial upgrading process would produce an oil product that meets pipeline specifications without diluent addition. It was shown that visbreaking at 380 °C changed the bitumen from a viscosity-constrained to a density-constrained material (Table 7). It resulted in limited cracking and had a low gas yield. Low gas yield, in addition to the benefit of keeping the oil hydrogen-rich and limiting loss of liquid product, has another advantage. As the temperature of thermal treatment is increased, the rate of hydrogen sulfide (H2S) evolution is increased.30 Although H2S content in the gaseous product was not tracked in the present study, it affects gas cleaning requirements. When the H2S content is low and the gas yield is low, it might be possible to use the gaseous product from visbreaking as fuel gas and remain within the SOx emission limit without stack gas treatment. Visbreakers are usually operated as once-through units.7 The reason for this is that thermal conversion before the onset of “coke” formation is limited by the stoichiometry of hydrogen and carbon because of the loss of hydrogen-rich material as gas. However, when visbreaking is used in combination with solvent deasphalting, the process provides a method for removing carbon-rich material as asphalt. This could enable recycling of part of the product to increase conversion. The processing sequence affects the design approach to increase conversion. In the case of SDA-Vis, the visbreaker can be operated at higher conversion with DAO as feed than would be possible with bitumen as feed. This type of operation was studied by Cabrales-Navarro and Pereira-Almao,31 and it was found that DAO from a once-through operation was more reactive than the DAO produced when recycling the heavy fraction from SDA-Vis back to the solvent deasphalting step. The present study found that the oil product from SDA-Vis has a H/C ratio (Table 3) lower than that from Vis-SDA and potentially contains more olefinic material. When recycling a lower H/C ratio oil product, it would have to be recycled to the solvent deasphalting step, otherwise it would accelerate the onset of coking, because new n-pentane insoluble material was formed during DAO visbreaking (Table 1). These observations are consistent with what was reported for thermal conversion of once-through DAO in comparison with DAO from the recycle operation.31 During recycle operation, when the oil products would have to pass through solvent deasphalting again, the new n-pentane insoluble material that was formed during visbreaking would also be rejected as asphalt, thereby eroding the liquid yield gain of once-through SDA-Vis compared to once-through Vis-SDA (Figure 1). Recycle operation was not evaluated in this work, but the experimental data indicated that the sequencing of solvent deasphalting and visbreaking would also affect recycle designs. Although the density and viscosity of the products

5. CONCLUSIONS The impact of the process sequence of solvent deasphalting and mild visbreaking for partial upgrading of bitumen was investigated. Although the density and viscosity of the oil products were comparable, there was a definite trade-off in yield and chemical nature of the products. Operating with SDA-Vis sequence resulted in a 2 wt % higher liquid yield and slightly lower capital cost than Vis-SDA. Operating with Vis-SDA resulted in a more hydrogen-rich oil product. When the products were evaluated in terms of diluent requirements, the oil from Vis-SDA required slightly less diluent. Other implications of the sequencing of these two process steps for the design of a partial upgrader were also noted. The study made it clear that the sequencing mattered.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +1 780-248-1903. Fax: +1 780-492-2881. E-mail: [email protected]. ORCID

Arno de Klerk: 0000-0002-8146-9024 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was performed with financial support by the Helmholtz-Alberta Initiative and the Eco-ETI program of Natural Resources Canada.

■

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