Article pubs.acs.org/IECR
Construction and Commissioning of a Continuous Reactor for Hydrothermal Liquefaction Anders Juul Mørup, Jacob Becker,* Per Sigaard Christensen, Kasper Houlberg, Elpiniki Lappa, Maika Klemmer, René Bjerregaard Madsen, Marianne Glasius, and Bo Brummerstedt Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, 8000 Aarhus, Denmark S Supporting Information *
ABSTRACT: The purpose of this paper is to give a comprehensive description of the construction and commissioning of a continuous reactor system for hydrothermal liquefaction of biomass. The basis is a newly established facility at Aarhus University. It is capable of handling viscous biomass slurries and features a novel induction-based heating method that facilitates well-defined reaction environments. Carbon balance closure is obtained as all product fractions are recovered and positively quantified. The paper includes a residence time distribution measurement and a 24 h proof-of-concept experiment conducted at 350 °C, 250 bar, and 15 min reaction time. It is based on the biomass dried distillers grains with solubles, a waste product of the bioethanol industry. The experiment seeks to determine the steady-state characteristics of the continuous reactor system for use in future experimental studies. It was found that steady state occurs within 6 h. Furthermore, data sampling windows of 2.1 h were found to mask the intrinsic variations of the system while still exposing trends. At steady state, the oil mass yield was found to be 38.9 ± 3.2% and the higher heating value was 35.3 ± 0.28 MJ kg−1. between an autoclave and a flow system.9 Here, the biomass is actively fed into a preheated autoclave, and the product flushed from it at the end of the experiment. The result is instant heating and the possibility of an appreciable volume. One issue with this kind of system may be cross contamination between products and unreacted biomass in the tubing leading to and from the reactor.9 Another alternative is the semicontinuous reactor, where biomass is preloaded into the reactor. The reactor is then heated, and the liquid products are eluted by injecting water and a catalyst solution.10 This reactor is, however, more suited for decomposition studies, as cross reactions are prevented due to the continuous elution. Continuous reactor systems (CRSs) for HTL have the advantages of improved throughput and the possibility of regular product sampling. This enables different statistical analyses, for instance of oil yields. Heating in a CRS is less problematic than with batch reactors because the system can be preheated and the actual feedstock mass flow is small. In terms of power, this is much less demanding and may yield a more well-defined experiment. There are several types of CRSs, which have been used for HTL.3 One is the continuous stirred tank reactor (CSTR) where the feedstock passes through a stirred, preheated holding chamber of appropriate volume leading to a certain mean residence time (i.e., actual time it takes to pass through the CRS).11 As a side effect, it does create a product that contains small amounts of unreacted biomass and intermediates in addition to fully formed oil. Another CRS is the plug-flow reactor (PFR) system which is built exclusively
1. INTRODUCTION Hydrothermal liquefaction (HTL) is the wet-chemical approach for converting biomass into a crude oil.1 As most biomasses on Earth have a high water content (typically 25− 50%), using water as a solvent and active agent for the process is highly advantageous. HTL is therefore able to convert asharvested biomasses or water-rich waste products from human society, which adds value to otherwise exhausted substances. Fundamentally, the HTL process yields four product fractions: oil, gas, solid residue (SR), and an aqueous phase. The oil may be separated from the aqueous fraction by simple centrifugation or alternatively using solvents. These may however change the oil composition and disturb yield calculations because the oil may react with the solvent and because all compounds cannot be dissolved by any single solvent.2 The oil is a drop-inreplacement of heavy fuel oil and can be upgraded to petroleum products.3 HTL has been under intense study during the past decade. While continuous flow HTL is relevant for the development of full scale plants, the use of batch reactors prevails due to low cost, simple constructs, and easy handling. Elliott et al. has recently published a review on the development from batch to continuous HTL.4 Typically, studies done in batch reactors have low heating and/or cooling rates.5 This results in a poorly defined experiment because reaction time and temperature are not decoupled parameters and leaves the question of what happens at the true process conditions. Indeed, heating rate has been shown to impact the conversion profile.6,7 To circumvent this problem, microbatch reactors and fluidized sand bath heating may be used.8 However, these reactors inherently possess small volumes and hold very little product, which limits the capacity for subsequent analyses and any post-treatment of the oil. An alternative is the stop-flow reactor, i.e., a hybrid © 2015 American Chemical Society
Received: Revised: Accepted: Published: 5935
February 18, 2015 May 14, 2015 May 15, 2015 May 15, 2015 DOI: 10.1021/acs.iecr.5b00683 Ind. Eng. Chem. Res. 2015, 54, 5935−5947
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Figure 1. Process flow diagram for the continuous reactor system.
from tubing.12 Due to hydrodynamics, it will also exhibit a distribution of residence times although not to the same extent as the CSTR. Coke buildup can occur if the process conditions are unsuited for the selected biomass, and the CRS introduces the new and serious issue of clogging. It is a recurring issue which can arise from a number of factors. For instance, the raw slurry can dewater and solidify if it is not stable. This necessitates much higher requirements for feedstock preparation. Likewise, minimizing formation of SR is critical; this mandates prestudies to assess proper reaction conditions, catalyst loads, etc. Batch reactors heated in fluidized sand baths are useful in this regard. CRSs for HTL have been developed independently around the world, e.g., at the Pittsburgh Energy Research Center,13 Lawrence Berkeley Laboratory,14 Albany Biomass Liquefaction Experimental Facility,15 Shell Research Amsterdam,16 University of Illinois,11 Shinshu University,17 Karlsruhe Institute of Technology,18 University of Sydney,12 Pacific Northwest National Laboratory (PNNL),19 and Aalborg University.20 However, there are few in-depth descriptions of a flow-reactor constructs, and this hampers discussion on limitations and pitfalls. This is also true for commercial HTL CRSs; one company currently developing such solutions is Steeper Energy ApS, Denmark. This history of CRSs for HTL leaves a strong incentive to provide more details to the scientific community. The purpose of this paper is to give that detailed description of the construction of a continuous-flow reactor including the subsequent commissioning and a 24 h test experiment using a residue from the grain-based bioethanol industry. The paper is meant as an aid and source of input to other research groups which may be interested in developing continuous HTL facilities.
Figure 2. Picture of the continuous reactor system.
mL. Additionally, the tubing accounts for 110 mL, the filter 350 mL, and the separator 430 mL. The volume of these auxiliary parts should be kept to a minimum to reduce the start-up time. The CRS is made entirely from 316 stainless steel (SS316). The tube diameter and fitting type has been selected according to their intended use and is a compromise between maintaining appreciable flow rates, minimizing risks of clogging, and keeping the temperature and pressure ratings sufficiently high. The feedstock introduction system employs 3/8 in. O.D. (6.22 mm I.D.) tubing with Swagelok Gaugeable fittings. They are seamless and feature a constant inner diameter. This helps prevent clogs, e.g., in the form of orifice bridging or slurry dewatering. This type has a pressure rating of 448 bar at ambient temperature, which is satisfactory as the feedstock introduction system is not heated. Furthermore, it allows for easy construction but is not suited for multiple dismantlings. All hot sections and coolers use 3/8 in. O.D. (5.17 mm I.D.) medium pressure tubing with cone-and-thread fittings from Maximator. This type has a pressure rating of 1550 bar at ambient temperature and 350 bar at 650 °C; i.e., it is well suited for hot operation. The cone-and-thread fitting system allows for dismantling without excessive wear, thus allowing for cleaning or clog-removal. The letdown section after the coolers uses 1/4 in. O.D. (4.00 mm I.D.) tubing with Swagelok Gaugeable fittings. Here, the feedstock has been liquefied and higher flow rates are necessary in order to avoid particulate precipitation and settling of the oil. The temperature is kept above 70 °C in
2. REACTOR SYSTEM DESIGN The typical HTL CRS consists of five sections, namely, (1) a feed introduction system, (2) a heating zone, (3) a reactor, (4) a letdown zone, and (5) a product separation zone. The present CRS is also constructed in this way. The system is fully computer controlled, running on a customized LabVIEW platform which offers an abundance of prudent safety measures and a convenient graphical user interface (GUI). Figure 1 shows the full process flow diagram of the present system, and the subsequent sections describe each of these individually. The CRS is depicted in Figure 2. The main reactor is interchangeable and can assume a volume of 608, 192, or 25 5936
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injectors is accomplished in seconds. Thus, a continuous flow is preserved. The injection HPLC pump can deliver pressures up to 410 bar (6000 psi), which is more than adequate for HTL (typically requiring 200−350 bar). The flow range is 0.01−24.0 mL/min, though flow rates below 6 mL/min are impractical. The maximum flow rate of 24 mL/min means a process capacity of 1.44 L/h. When yields in the 30−50% range at feedstock drymatter load of 20% are assumed (typical values when reviewing literature), there is an oil production capacity of 90−150 g/h. The reaction time is calculated as the space time (τ) from the mass flow and system volume while adjusting for density using the following equation:
the letdown and separator sections to keep the oil viscosity low. The CRS was designed in Autodesk Inventor before the actual construction began. This was necessary due to space constraints in the laboratory and provided a better spatial overview of the system. This allowed an increased focus on accessibility and simple disassembly (through virtual design iterations). 2.1. Feedstock Introduction. Acquiring a lab-scale high pressure slurry pump remains an issue for continuous HTL. A single commercial pumping system is available from Teledyne ISCO (500HV), but it is costly and has a max pressure of just 258 bar. The alternative is customized pumping solutions such as the hydraulic dual piston system employed at the Paul Scherrer Institute, Switzerland.21 A recent review and book contribution by Elliott et al. provides an overview.3,4 For the CRS presented here, a dual injector system has been constructed to introduce biomass slurries, Figure 3. The
Lρπr 2 (1) ṁ where L is the reactor length, ρ is the density at reaction conditions, r is the inner reactor radius, and ṁ is the mass flow rate. This calculation necessitates a constant and well-defined reaction temperature and pressure, along with an accurate mass flow. The injection HPLC pump calculates the flow rate based on the speed of its dual pistons, which in our experience causes an offset of 5−10%. Therefore, the water container which feeds the pump is placed on a digital scale (KERN FCB 8K0.1, resolution 0.1 g) which ensures accurate measurement of mass flow based on linear regression between 0.5 Hz measurements. One of the issues of both lab and commercial scale continuous HTL is forming stable and pumpable slurries of high dry matter content.3 Algae work well, although macroalgae require a size-reducing pretreatment.22 Lignocellulosic biomasses are more challenging as they tend to sediment or dewater which creates blockages. Lignin is a problem in itself.4 It is possible to improve the pumpability and stability of biomass slurries by milling, thermal pretreatment, or adding a stabilizer such as carboxymethyl cellulose. All of which have been successfully tested in-house. It is useful to assess the pumpability prior to using a CRS and risk clogging. An approximate method is filling the slurry into a 50 mL syringe and slowly pushing it through the orifice. If dewatering occurs, further pretreatment is necessary. 2.2. Heating. Since heating rates are of great importance to HTL,6,7,23 much effort has been invested in optimized heating on this CRS. The heating is 2-fold consisting of an optional tubular preheater and a 10 KW induction main heater (Ambrell EasyHeat 8310, 190 kHz). The preheater is designed to heat the feedstock to 150 °C. The main heater raises the temperature to the final set point, for HTL typically 300− 450 °C. The normal method in typical CRSs is to heat a stainless steel tube either using a heating jacket or by submerging in an oil bath. This places an upper limit on heating efficiency due to the thermal conductivity of steel. Thinner tubes with thinner walls can be used to mitigate this, though at an increased risk of clogging. Above pilot scale, the large throughput required (i.e., large I.D. tubing, thick walls) places a serious limitation on heating rates within this paradigm. The advantage of induction is that the heating is applied into the material. This is defined by the skin depth (δ), which is the depth at which the current density has fallen to 1/e (0.37). δ is frequency and material dependent and has a value of 1.1 mm for SS316. As the tube wall thickness is 2.2 mm, significant heating thus emanates within the steel itself rather than relying on thermal conductivity from the outer surface. This enables a very rapid heating even within a 3/8 in. tube. Heating of the τ=
Figure 3. Dual injector system with drain valve and remote control. Outlet valves are placed directly beneath the table.
injectors are tubular vessels with internal pistons (High Pressure Equipment Company TOC11-20) that run asynchronously to ensure a continuous flow. The biomass is displaced into the reactor by pumping water at excess pressure behind the pistons. Two Scientific Systems Prep 24 HPLC pumps are used to, respectively, inject biomass slurry and to prepressurize the injectors before activation. Switching between injectors is accomplished by two Tescom VT6 valves, which direct water into the appropriate injectors, while actuated Swagelok SS-83KS6-A15 full bore ball valves admit feedstock to the system. 3/8 in. Swagelok VCO O-ring fittings are used for the immediate connection to the injectors in order to make them easy to mount and dismount. The injector system can be bypassed to flush the reactor with water or gas. When shifting to a new injector, it must be prepressurized to avoid backflow. Because the injectors are prepressurized, switching between 5937
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different frequency intervals, making the set cover an appropriate range (130−250 kHz). Furthermore, a thick grounding wire is clamped onto the tube immediately downstream of the ferrite cores. 2.3. Reactor. The reactor unit is of the tubular-flow type. It is interchangeable between three dimensions to access a wider parameter space and ease accessibility for cleaning purposes. Two annealed SS316 reactors have been commissioned from High Pressure Equipment Company (USA). One is a 1.2 m × 1 in. I.D. confined gasket closure reactor (TOC), the other a 1.2 m × 9/16 in. I.D. micro reactor (MS). The smaller reactor is acquired for subcritical HTL, i.e., 300−370 °C, 200−350 bar, and 5−45 min reaction time. The larger reactor allows the same reaction times and pressures to be investigated while at 370− 450 °C. A third 1.2 m × 5.17 mm I.D. reactor has been constructed for short (10 L of gas, it is readily obtainable. The risk of contaminating the surrounding laboratory environment is correspondingly increased. A well-ventilated environment is therefore recommended. This CRS is placed inside a 14 m2 walk-in fume hood with an air exchange up to ∼1000 m3/h. Volustats on the air supply and exhausts enable this entire
Figure 5. Letdown section with coolers, filter, and back pressure regulating valve.
the effluent velocity so that particles settle in the bottom of the vessel and liquids pass overhead through the filter. This permits prolonged runs on the reactor system without filling the filter housing or blocking the filter. The trade-off is a prolonged startup period of the reactor as it takes more than an hour to saturate the filter. This reduces system flexibility and it is possible that a smaller filter could suffice, e.g., Tescom tee type filters. Swagelok tee type filters have been tested unsuccessfully as they fill in about an hour. The process pressure is kept constant by a dome-loaded Tescom 26-1762 back pressure regulator valve (BPRV) with a Tescom ER3000 pressure controller. This style of regulator releases product only when the reactor pressure exceeds a preset value. Together with the constant flow of a HPLC pump, this ensures reaction pressures within ±5 bar. The system has an alternate Swagelok spring loaded pressure relief valve (PRV) outlet that can be used if the BPRV fails or the filter clogs. Upstream filtering is essential for any pressure regulating valve to avoid abrasion and premature failure. All valves should employ Kalrez O-rings due to their excellent chemical and thermal resistance. The BPRV is kept at 70−80 °C using insulated heating wire (Omega FGR Series). 2.5. Product Separation. Gas/liquid separation is facilitated in a custom-made 316 SS vessel (Ø 7.6 × 18 cm), Figure 6. The top has three ports: (1) reactor effluent inlet, (2)
Figure 6. Separator vessel and valve. 5939
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Figure 7. Graphical user interface.
Figure 8. Temperature profile obtained at 350 and 400 °C of the hot sections of the CRS. Heat: induction heater; Inlet: trace heated inlet tubing; Cap: reactor mount heating jacket; Reactor: reactor heating jacket; Outlet: insulated outlet tubing; Cooler: tempered effluent cooler.
corresponding to the midpoint between ambient and reaction temperature, i.e., linear heating profile. In reality, the heating is faster initially due to a larger temperature difference between the slurry and the induction tube. The initial heating further benefits from a high water density, which improves the heat transfer. Consequently, the actual heating profile is not linear but rather logarithmic. Overall, the temperature is very well-defined for a CRS. Elliott et al. uses a CSTR as preheater running at reaction temperature, hereby providing instantaneous heating.19 The downside of this method is that very low reaction times are practically unobtainable. On the other hand, it is easily scalable; this is a challenging prospect for the inductive method. Here, it has been selected due to the flexibility it proves for scientific studies at the present scale. 3.2. Residence Time Determination. The residence time distribution (RTD) has been characterized experimentally to determine how the actual residence time deviates from the ideal space time, τ. There are a few examples of RTD measurements for hydrothermal reactor systems in the literature.26−29 A common theme is the issue of finding an inert tracer compound that is chemically stable, soluble, and noncorrosive in the suband supercritical water. This is illustrated by their choices of tracers: NaCl, dye rodamin B, phenol, and tetralin. We used 5 mL of 0.3 M NaOH as it is stable and noncorrosive toward
compartment to be kept at a slightly reduced pressure in order to confine volatiles. However, the main confinement is done via a PETG box which encases the entire CRS liquid outlet section and is ventilated at ∼100 m3/h.
3. COMMISSIONING 3.1. Temperature Profile. The temperature stability in the hot sections of the reactor system has been determined by measuring the temperature at key points along the reactor and tubing axis. The temperature profile has been recorded at 350 and 400 °C while keeping the pressure at 250 bar and calculated reaction time at 15 min (time at reaction conditions). The medium is pure water. The plotted results in Figure 8 show good temperature stability for both set points with minimal variance. This has been achieved by dividing the temperature control into four individually controllable zones, each controlled by a Eurotherm 3216 controller: (1) induction heater, (2) inlet tubing, (3) reactor cap, and (4) reactor. The temperature starts dropping at the outlet tubing as this is merely insulated. This is inconsequential as the time spent in the outlet tubing is negligible due to the small cross-sectional area (20 mm2 versus 160 mm2 in the reactor). The feedstock stream is heated from ambient to 350 °C in approximately 23 s. This value is calculated using eq 1 and with a density 5940
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conducted in a smaller 6 m × 2.1 mm I.D. (21 mL) reactor with space times of 33.3−87.0 s. The lower space times used in these studies reduce the time available for dispersion effects to progress. Consequently, it should be possible to reach plug-flow behavior for the present CRS by decreasing the space time through an increased mass flow. The overall broadening or dispersion of the RTD is caused by diffusion, which becomes significant due to the relatively long space time. Furthermore, the distribution is affected by two opposite-acting effects along with a less significant third effect. First, in a laminar flow, there is a heightened velocity at the reactor center streamline, which let tracer ions begin to exit the reactor at t = τ/2. This lets the tracer exit the reactor before it is expected. Second, some tracer ions will diffuse out toward the reactor walls and be withheld, as the flow at this outer streamline approaches zero. The withheld ions will slowly be released as the tracer ion concentration drops near the center streamline. This diffusion is expected to cause tailing. Indeed, the Reynolds number for the present reactor at reaction conditions is 187, corresponding to a laminar flow. Third, a tailing effect could arise from the reduced solubility of NaOH near the critical point. As the solubility of ions is reduced due to reduced polarity in subcritical water, it can be speculated that the Na+ and OH− ions adhere preferentially to the hydrophilic reactor walls in order to stabilize their charge. This would create a “chromatographic” separation of solvent and tracer where the reactor acts as the stationary phase. However, the reactor surface-area-to-volume ratio suggests that this effect should be minor. RTD measurements have been conducted at 20, 100, 200, 300, and 350 °C to test this solubility afforded chromatographic separation. No meaningful difference was observed from 100−350 °C, and the NaOH tracer is therefore deemed viable for subcritical RTD studies. The tracer onset is consistently detected after a relative retention time of t/tm = 0.5. This suggests a highly laminar flow, where a significant portion of the tracer flows at the center streamline. The data are shown in Figures S2 and S3 in the Supporting Information. Finally, some of the broadening is due to the injection procedure. For the present CRS, it is not possible obtain a perfect input pulse of tracer, because it takes a finite time to inject the tracer. Here, it lasts 32 s to ensure the tracer concentration is correct. The reason is the inlet tubing design: while the tracer inlet channel is unused, it effectively acts as a dead volume where some uncontrolled diffusion takes place. The interim conclusion is that the experimental mean residence time may differ considerably from the theoretical space time, as calculated from system volume and flow velocity. However, this difference must be an artifact of dispersion effects, as mass balances dictate that such holdups are impossible at steady state. These various time definitions are a marked difference from batch reactors, especially sand bath micro reactors, where reaction time may be easier to define. However, the ionic tracers used for this commissioning likely bring about a “worst case” scenario due to retention effects of the reactor walls. This will not be the case with oil species, especially in a steady-state situation where the entire system has been running extensively at set point conditions with feed input. The experimental data collected so far on this system (using NaOH tracer) is therefore considered to indicate acceptable residence time characteristics. 3.3. Cleaning. It is necessary to clean the CRS between experiments in order to avoid cross contamination and prevent any ash buildup. The objective is to be able to clean the system
SS316 and has an acceptable solubility under subcritical conditions. The tracer solution was injected via the injector system, and the concentration in the effluent was measured using a conductivity meter (Radiometer CDM210 with CDC641T conductivity cell) placed in a custom flow cell. The flow cell is shown in Figure S1 in the Supporting Information. The reactor fluid was demineralized water. The filter and BPRV outlet were bypassed in order to reduce the dead volume; instead, the PRV outlet immediately downstream of the cooler was used. It must be emphasized that this method characterizes the entire system, i.e., the reactor and the auxiliary sections (in- and outlet tubing, heaters, coolers, and PRV). The exit age distribution, E(t), mean residence time, tm, variance, σ, and Peclet number, Pe, are obtained from the following expressions:30 E (t ) =
c(t ) ∞
∫0 c(t )dt
(2)
0
tm =
∫∞ tE(t )dt
σ2 =
∫∞ (t − tm)2 E(t )dt
(3)
0
σ2 2 8 = + 2 2 Pe tm Pe
(4)
(5)
where c(t) is the concentration at time t. Figure 9 illustrates these entities in a set of experimental data obtained at 350 °C,
Figure 9. Residence time distribution.
250 bar, and 15 min reaction time (time at reaction conditions). The total space time is included for reference. It is calculated from eq 1 and includes the reactor and auxiliary sections. This discrete calculation accounts for the changing density of each section in accordance to the temperature profile in Figure 8. The mean residence time was 33.4 min, and the variance was 9.3 min. The expected space time was 24.2 min. This corresponds to an offset of 38.0%. This offset is primarily due to tailing, which extends to after 65 min. The RTD peak occurs at 26.9 min. The Peclet number expresses the rate-oftransport ratio of convection relative to dispersion. For the present CRS, the Peclet number is 28.02, which corresponds to an intermediate amount of dispersion and thus not plug-flow behavior. Kruse and Lietz conducted a similar study, which showed offsets of 7.59−21.9%.27 These studies were, however, 5941
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prepared by mixing 20% DDGS (∼250 μm grain size), 2% K2CO3, and 78% demineralized water. Syringe tests of the slurry were performed and passed. The mass flow corresponding to the desired space time was calculated using eq 1. The slurry was then loaded into an injector and mounted in the CRS. The injectors were prepressurized to 50 bar above system pressure. Subsequent injectors were filled no earlier than 10 min before activation in order to reduce the risk of sedimentation. Injections were continuous from this point onward. Product effluents were collected immediately following the first injection in order to monitor the development toward steady-state operation. Liquid products were collected in 500 mL centrifuge tubes and stored at 5 °C until separation. Each centrifuge tube corresponds to one injector as they were replaced simultaneously. Over the course of 23.34 h of operation, the injectors were filled 33 times (42.5 min/injector) corresponding to a total volume of 13 200 mL of processed slurry. The gas product volume was measured continuously using a flow meter. A gas sample was collected for every injector in 1 L gas sampling bags. After 5.7 h of operation, the flow meter malfunctioned due to condensed water accumulating in the sensor. Subsequently, the gas product was therefore determined by collecting the gas produced over 10 min in a 3 L gas bag and measuring the volume by emptying the bag with a large syringe. This was extrapolated to 42.5 min, corresponding to one injector. The gas volumes determined with this method were reproducible but 10% below those measured with the flow meter. With this method, the gas volume was determined for every third injector due to time constraints. SR was retained in the filter. It had a gray color but contained a considerable amount of oil as well. It was recovered after the experiment had ended by rinsing the filter and filter housing with ethanol in an ultrasonic bath. The ethanol was subsequently removed by vacuum filtration. Finally, the SR was washed with acetone to remove residual oil. The residual oil lost in the solvent rinse is discarded and considered part of the start-up phase. The liquid product consisted of a distinct oil phase and a water phase. These were separated mechanically using a Sigma 6-16 centrifuge running at 9775 relative centrifugal force (RCF) for 5 min. The centrifugation was performed at 0 °C to increase the viscosity of the oil. This helped retain the oil in the centrifuge tube while the water phase was decanted. The advantage of this method relative to solvent extraction is that the oil is not changed or contaminated in any way. A similar method is employed by PNNL.19 The water dissolved in the oil was determined through Karl Fischer titration using a Radiometer TitraLab 585 volumetric Karl Fischer titrator with triplicate measurements. Hydranal solvent CM was used as a work medium, and Hydranal titrant 5 was used as Karl Fischer reagent. Prior to titration, 1.000 g of oil was dissolved to a total volume of 5.000 mL using the work medium as a solvent. Viscosity was measured using a Brookfield DV-II+ Pro rotational viscometer with a SC 21 spindle at 25, 40, and 60 °C. The organic content in the aqueous phase was determined through room-temperature evaporation until the weight stabilized, circa 6 days. The analysis was carried out on three 50 mL samples. Elemental analysis was performed on the separated oil for each sample container and collectively for the SR. CHNS and O were determined using an Elementar CHNS-O Vario
without opening any tubing or the reactor in order to minimize the work load and oil exposure. The present cleaning procedure relies on a strong alkaline soap. An alternative is using acetone, but this emphatically requires every O-ring in the CRS to be of Kalrez quality. The acetone-based cleaning is not used here as there is a wish to limit the use of strong and flammable organic solvents. The soap is furthermore able to remove odors from steel parts and glassware that acetone cannot. The degree of cleanliness can be followed from TOC or conductivity measurements of the reactor effluent. The current soap-based cleaning procedure is as follows: 1. The system is flushed with water at reaction conditions for 4 h to elute the majority of the organic content in the reactor. 2. 400 mL of 12.5% alkaline soap water solution (NaOH/ EDTA/2-(2-butoxyethoxy)ethanol/sodiumdodecylbenzenesulfonate) is injected. 3. The system is flushed with water for 2 h. 4. Steps 2 and 3 are repeated until no organic material can be seen in the effluent. Usually, this requires three cycles. At this stage, the system is clean enough to be odorless if opened. However, the soap leaves a thin ∼0.2 mm layer of ash on the reactor walls. This ash layer must be removed. 5. The temperature is set to 400 °C, and the flow is set as high as possible to create a turbulent reaction environment in order to loosen the ash layer. 6. The system is cooled to 180 °C and 20 bar, where after the pressure is sharply dropped to 0 bar through the PRV downstream of the cooler. This causes the water to partly flash into gas whereby it is rapidly expelled from the reactor. While care must be taken, this is highly efficient for removal of ash particles. This is done twice. It is strongly recommended that direct, manual cleaning of the reactor system should not be attempted before all oil has been dissolved and carefully disposed. Pressure relief and back pressure valves need regular maintenance to function optimally and must therefore still be disassembled. To avoid manual cleaning of the valves, the stainless steel parts are subjected to ultrasonication in 12.5% alkaline soap solution at 60 °C for 1 h.
4. EXPERIMENTAL SECTION The CRS was run at steady state for 24 h in order to test the stability of the system. The test employed the second generation biomass distiller’s dried grains with solubles (DDGS), a waste product from the grain-based bioethanol industry composed primarily of fat, protein, fibers, and minerals. A characterization is available in Supporting Information Table S1. DDGS slurries are easily pumped, and they convert fairly well; both features are essential to reliable HTL processing in a CRS. Being a processed substance (as opposed to a freshly harvested biomass), DDGS is welldescribed, homogeneous, and abundant. This makes it a handy benchmark substance poised at a useful position between pure model compounds (albumin, microcrystalline cellulose, glucose, etc.) and an as-harvested biomass. The experiment was performed at 350 °C, 250 bar, and 15 min reaction time (calculated space time at reaction conditions). The CRS was filled and pressurized with demineralized water and then heated to reaction temperature. Downstream tubing, fittings, valves, and the filter were heated to 80−100 °C. The system was then allowed to stabilize, circa 3 h. Feed slurry was 5942
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Industrial & Engineering Chemistry Research
g of SR was obtained or 1.17% when divided by the total mass of injected biomass. The oil and gas product yields from the 24 h experiment are shown in Figure 11. Gas production could be detected after
MACRO cube on 30 mg (CHNS) and 5 mg (O) samples with triplicate measurements. Higher heating values (HHV) were calculated using Dulong-Berthelot’s empirical formula.31 Elemental analysis on SR was carried out with a Spectro Xepos-II X-ray spectrometer operating at 50 keV using four different secondary targets and TurboQuant software (discerning elements from Z = 11 or heavier). The total organic carbon (TOC) and total nitrogen (TN) contents of the aqueous phase were measured in duplicates using a Hach & Lange DR2800 spectrophotometer with TOCkit LCK387 and TN-kit LCK338. The pH was measured using a Radiometer PHM 92 pH-meter. Gas samples were analyzed using an Agilent 7980A gas chromatograph equipped with a flame ionization detector (FID) for CH4 and C2H6, and a thermal conductivity detector (TCD) for H2, CO, and CO2. The detectors had separate inlets and columns. The TCD was connected to a CP-molsieve 5A column (30 m × 0.53 mm × 15 μm). The column flow was 6 mL/min using argon carrier gas and sample injection volume of 300 μL with a split ratio of 1:30. The FID detector was connected to a PoraPLOT Q column (25 m × 0.32 mm × 15 μm). The column flow was 2 mL/min using helium carrier gas and sample injection volume of 300 μL with a split ratio of 1:20. The temperature program was as follows: hold 50 °C for 2.5 min, ramp 230 °C at 50 °C/min, and hold 230 °C for 3 min. A summary of the product separation and analyses is given in Figure 10.
Figure 11. Mass yields of oil and gas plotted against reactor run time.
Figure 10. Product separation and analysis summary.
0.75 h and increased steadily until 4.25 h where it appeared to level out. From 5 h, the yield appears to increase discontinuously and spike at 7 h. After this, the gas flow meter failed as the sensor got blocked by condensed water and the following measurements were obtained manually as described in the Experimental Section. It is possible that the spike and discontinuous increase are artifacts of the gas flow meter beginning to malfunction. It is believed that the condenser had too small of an inner diameter (1.9 mm), which allowed the condensed water droplets to create small “plugs” that were carried to the sensor by the gas. The oil production appears to reach steady state after 5.7 h, although the yield continues to fluctuate throughout the experiment. Part of this is due to the operation of the separator system, as its frequency of releasing liquid product does not align ideally with the sampling frequency, i.e., centrifuge bottle replacement. This frequency misalignment will be corrected in the control software for future experiments. For the present study, it can be corrected by normalizing the oil yield relative to the liquid product mass, as shown on Figure 11. While this gives a more accurate measure of the oil yield, there are still inherent fluctuations, which stem from oil retention in the filter and separator. 5.2. Sampling Frequency. The extent of retention can be seen from carbon balance on Figure 12. The water phase
5. RESULTS AND DISCUSSION The product fraction yields and analyses were determined per injector corresponding to a data window of 42.5 min during which 400 mL of slurry or 80 g of biomass was injected. This base sampling rate is inherited from the volume of the centrifuge tubes used to collect all liquid products. In this way, one injector corresponds to one centrifuge tube. 5.1. Yields. The oil yield was calculated from the mass of oil in one centrifuge sample, while the gas yield was calculated by applying the GC-TCD/FID data and flow meter volume measurements to calculate the gas mass. The method is equal to that employed by Christensen et al.8 The SR yield was determined from the residue recovered in the filter after the CRS was cooled and depressurized. A cumulative mass of 31.02
Figure 12. Carbon balance of the four product fractions plotted against reactor run time. 5943
DOI: 10.1021/acs.iecr.5b00683 Ind. Eng. Chem. Res. 2015, 54, 5935−5947
Article
Industrial & Engineering Chemistry Research
important to stress when doing HTL parameter studies on a CRS. The mass closure of 98.6 ± 3.3% and carbon closure of 103.5 ± 5.7% show that the obtained results are at their true values. This supports the credibility of this and, by proxy, future parametric studies. The carbon content in the aqueous phase may likely be reduced either via further optimization of reaction conditions or by recycling the water phase to suspend new biomass. Some studies suggest that recycling a saturated aqueous phase may increase the oil yield and reduce the viscosity.22,32,33 Elemental analysis of the oil reveals that the nitrogen content keeps increasing over the course of the experiment, Figure 13.
reaches a steady-state carbon yield after 2.5 h, while it takes another 3 h for the oil, as it must first saturate the inner surfaces of letdown-tubing, filter, and separator. The extent of retention is likely affected by the temperature in these sections, as it changes oil viscosity and solubility. Some of the carbon retained in the start-up phase may be released during the spike at 9.2 h, although the variance makes this observation inconclusive. The size of the separator will be reduced for future experiments. With batch reactors, one sample naturally equals the product of one experimental run with that reactor. With a CRS, what constitutes a “sample” hinges entirely on the time window of data collection. In any CRS, variation in reactor output is inevitable (reflected in Figure 11); this renders the most appropriate time window, a system/hardware specific entity which must be identified. The base concern is that the complexities of sample handling and analyses all increase with sampling frequency while gains in accuracy will be lost in output fluctuations at some point. To illustrate this, Table 1 Table 1. Oil Yield Average and Standard Deviation Calculated for Different Data Windows samples (#)
duration (h)
average (%)
deviation (%)
1 2 3 4 5
0.7 1.4 2.1 2.8 3.5
39.0 39.0 38.9 38.8 39.2
5.4 3.9 3.2 3.1 2.1
Figure 13. CHNS-O elemental composition of the oil product fraction plotted against reactor run time.
shows the average oil yield and standard deviation calculated for a data window of 1 to 5 samples while at steady state (5.7 h and forward). For the CRS on commissioning here, variances indicate that a data window of three samples corresponding to 2.1 h may minimize sample handling complexities while preserving sufficient resolution to reveal trends. It follows that the base sampling frequency of 42.5 min is certainly high enough to deliver credible results from this CRS. By way of reference, an entire experiment may be used for a (single) data window; this is done, e.g., by Elliott et al. (0.5−12 h run time)22 and Jazrawi et al. (
