Microchannel Catalytic Processes for Converting ... - ACS Publications

Chapter 17

Downloaded by UNIV OF TENNESSEE KNOXVILLE on May 26, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch017

Microchannel Catalytic Processes for Converting Biomass-Derived Syngas to Transportation Fuels Chunshe Cao, Yong Wang*, Susanne B. Jones, Jianli Hu, X. Shari Li, Douglas C. Elliott, and Don J. Stevens Pacific Northwest National Laboratory, 902 Battelle Boulevard, MSIN K8-93, Richland, WA 99352

In this paper, a catalytic microchannel reactor integrated with highly efficient heat exchangers was used to demonstrate the significantly improved Fischer-Tropsch (FT) productivity by minimizing heat and mass transfer limitations in this three phase reaction system. A factor of up to 15 process intensification can be achieved for F T synthesis using microchannel reactors. In order to achieve high yields of naphtha and diesel range of hydrocarbons (C5-C19), we have developed a unique structured catalyst system suitable for the deployment in microchannel reactor applications. This engineered catalyst structure is based on metallic monolith supports, uniformly coated with an improved catalyst formula. Such an ordered catalyst structure has the advantages of higher thermal conductivity and more reduced mass transfer resistance compared to the conventional catalysts. By tailoring the mass transfer limitations, we have demonstrated that this engineered catalyst produces hydrocarbons with narrower carbon distributions (mainly < C ) than a conventional particulate catalyst at similar conversion and methane selectivity. In particular, majority of the synthesis products fall into the gasoline and diesel range. This unique product distribution, in turn, has a positive impact on process economics since a milder or no hydrocracker will be required in downstream product processing. 25

© 2005 American Chemical Society

Wang and Holladay; Microreactor Technology and Process Intensification ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Production of clean transportation fuels from renewable and sustainable biomass is receiving growing attention in recent years due to the concerns with climate change and the need for energy diversity (I). Biomass feedstock provides unique renewable features since carbon dioxide emissions from its use are absorbed by newly grown biomass (2,3). Conversion of biomass to fuels generally goes through one of the two routes of thermal upgrading processes: pyrolysis for direct production of liquid o i l or gasification for syngas production. A s shown in Figure 1, the biomass feedstock needs first be pretreated for conversion for both processes. The pretreatment includes the unit operations such as grinding and drying. Pyrolysis route

Fuels & Chemicals

ι

Fuel Synthesis: Gasification

Electricity

Methanol/DME Ethanol FT Gasoline/Diesel

Feedstock Handling

Heat & Power Generation Pyrolysis

Upgrading

Figure I. Biomass conversion to fuels and energy

typically requires that the pyrolysis oil is upgraded by hydrotreating to provide deoxygenated high heating value oil for heat and power generation. The syngas generated from the gasification route can be converted to liquid hydrocarbon fuels using technologies such as F T synthesis. Since various process options in the gasification can be chosen such as atmospheric and pressurized, air-blown and oxygen-blown, a wide range of syngas compositions with H / C O ratios varying from 0.45 to 2 can be produced. Consequently, to suit the F T synthesis, the H / C O ratio needs to be adjusted using gas conditioning processes such as water gas shift (WGS) and/or methane steam reforming (SMR). Prior to the gas conditioning, any contaminants such as H S , N H , dust and alkalis also need to 2

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275 be removed or significantly reduced to avoid catalyst poisoning. Another major gas cleaning step is the costly carbon dioxide removal from syngas mixtures prior to F T synthesis. A further economic constraint i n conversion o f biomass to fuels is associated with the distributed nature of biomass feedstocks. In another word, biomass feedstocks may not be near existing chemical or petrochemical plants and are not economic to centralize. Even the largest gasifiers can not process more than 1000 ton biomass/day which is equivalent to about 1100 bpd liquid F T fuels and about a factor of 20 smaller than the scale at which conventional gas-to-liquid (GTL) plants are economic (Figure 2) (4). Centralizing syngas from gasifiers is not an economic option since there is no existing pipeline in the U S through which the syngas can be transferred to large central facilities. Therefore, an enabling technology is needed for biomass conversion to fuels.

Biomass scale 90,000 80,000

c

Φ

% C

Β

Δ 3

ι • 1 1

70,000

_

• Sasol • Shell A Energy International • Exxon • Syncrude Technology

60,000 50,000

40,000 Ο => 30,000 υ 20,000 'δ 10,000 α

(Ο

ι ,

0

20,000

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40,000

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60,000

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1

100,000

GTL Plant Capacity, bbl/day

120,000

Figure 2. Conventional GTL plant capital investment MicroChannel reactors, which have been developed by P N N L for the past decade (5,6,7,8,9), are modular and less dependent on the economy of scale. MicroChannel reaction technology provides a potential breakthrough solution to the challenge of deploying biomass conversion to fuel processes. These reactors have a sandwich-like multi-layer structure consisting o f a large number o f closely spaced channels with a gap of less than 2mm, which reduces heat and mass transport distance and greatly enhances heat and mass transfer efficiency. Heat transfer is further enhanced due to the high surface-to-volume ratio achievable in microchannel reactors. Such high heat transfer efficiency permits the isothermal operation of highly exothermic F T synthesis at higher reaction temperatures without sacrificing the selectivity. The improved mass transfer



276 mitigates the mass transport problems typically associated with conventional fixed-bed and slurry reactors for F T synthesis. In addition to heat and mass transfer improvement, the unique layered sheet architecture of microchannel reactors allows the integration o f gas conditioning and reactant/product heat exchange within a single unit to improve the thermal efficiency and reduce the capital cost. Consequently, microchannel reactors potentially allow process intensification for F T synthesis. In this paper, a new type of highly active and selective cobalt based catalyst and its engineered structures based on metal substrates suitable for microchannel reactors have been developed. This study centers on the catalyst development and their unique performance i n microchannel reactors to meet the biomass conversion requirements. The potential elimination of costly C 0 separation and hydrocracking steps using microchannel reactors and the subsequent impact on the capital investment are evaluated. 2

Experimental

Catalyst Preparation. Acidic γ- A 1 0 (Sasol) with an average particle size of 45μιη was precalcined at 500°C in air for 2 hours prior to impregnation. This support has a spherical shape and uniform particle distribution. A n aqueous solution of cobalt nitrate hexahydrate ( C o ( N 0 ) . 6 H 0 ) (98% purity, Aldrich) and Perrhenic acid (HRe0 ) (Engelhard, 53.29wt% P.M.,) was impregnated onto the γ-Α1 0 support using a multi-step incipient-wetness impregnation method. After each impregnation, the catalysts were dried in air at 90°C for 8 hrs followed by calcination at 350°C for 3 hours. Five sequential impregnations were used to give final formulated catalyst with 30wt%Co and 4.5wt%Re on alumina. The synthesized powder catalyst has a surface area of 60m /g and pore volume of 0.14cm /g. 2

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A n engineered catalyst is based on an aluminum monolithic substrate due to its high thermal conductivity. The substrate was microstructured with doubleside alternative valleys and peaks as shown in Figure 3. The width of each valley is 254μηι, the valley depth is 635μηι, and the peak width is 254μηι. The overall dimensions of each monolith are 12.7mm x 1.5mm x 31.8mm. The surface of aluminum substrate was first oxidized in air at 550°C to enhance the adhesion to catalyst layers, and dip coated with a solution of P Q A 1 0 (Nyacol), pluronic F-127(HO(CH CH 0) 6(CH CH(CH3)0)7o(CH CH 0) H), and ethyl alcohol (Aldrich) mixture at 1:0.9:3.5 weight ratios. The aluminum substrate was 2

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277 then dried at 100°C for 20 minutes. The dipping and drying cycle was repeated to achieve the target alumina loading and coating thickness. Finally, the monolith was calcined at 450°C for 4 hours to remove F127. A n aqueous solution of cobalt nitrate hexahydrate ( C o ( N 0 ) . 6 H 0 ) (98% purity, Aldrich) and Perrhenic acid (HRe0 ) (Engelhard, 53.29wt% P.M.,) was co-impregnated onto the alumina layer to achieve desired Co and Re loadings, followed by drying in air at 90°C for 8 hrs and calcination at 350°C for 3 hours. A s a typical example, the monolith has a catalyst coating thickness of 15μιη, a surface area of 140m /gof active coating layer, an average pore size of 95 Â, and about 30 wt% Co (relative to active coating layer) with an atomic Co/Re ratio of 21. 3

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Figure 3. Engineered catalyst tailored for a microchannel reactor

Activity Testing Experiments were carried out in a microchannel reactor system to maintain isothermal conditions for the highly exothermic F T synthesis reactions. The details of this reactor are described else where (10). The channel gap of the reactor is 1.5mm wide and the channel width is 12.7mm. Powdered catalysts were packed into the channel. For engineered catalyst testing, two pieces of the mini-structured catalysts described above were snugly inserted in the microreactor providing the total length of the catalyst bed of 63.6mm. There was essential no gap between the monolithic catalyst pieces and the reactor walls. Four thermocouples along the catalyst bed indicated that the temperature gradient within the catalyst bed is less than 1 °C under the targeted reaction conditions. After catalyst was activated i n hydrogen at about 400°C for 8 hours, a syngas feed with H / C O ratio of 2 was introduced and F T synthesis was conducted at pressures from 10 to 40atm. Hydrocarbons and water in the effluent were condensed in a chilled vessel. Non-condensed gases were analyzed using an on-line gas chromatography (Agilent Q U A D H G2891A with Molsieve 5A, PoraPlotQ columns) to determine C O conversion and light product selectivity. 2


278 Condensed liquid wax products were analyzed with a HP5890 G C with a DB-5 column (15m long, 032mm i.d., 0.25um film thickness) connected to an FID detector.


Results and Discussion

Intensification of F T Process with an Improved Catalyst To demonstrate the potential o f F T process intensification achievable in microchannel reactors, as-synthesized powder catalyst was packed i n the microchannel reactor described in the Experimental section. The reaction temperature and gas flowrate were increased while maintaining a single-path C O conversion o f 63% and methane selectivity of less than 10%. A t 20atm and 235°C with H /CO=2, the highest G H S V achieved was 60,000 h r which corresponds to a contact time of 0.06 second (contact time is defined as catalyst bed volume divided by the feed gas flowrate at standard temperature and pressure). Under these reaction conditions, steady catalyst performance was observed over a time-on-stream of more than 100 hours. Table 1 summarizes comparison with the best performances in conventional fixed bed and slurry bed reactors reported in the open literature (11). Apparently, G H S V is about 15 times faster and the C5+ productivity is about 10 times higher than the conventional tubular fixed bed reactor. 1

2

Table L Comparison

of productivity

Slurry (10)

GHSV, v/v/hr 3

Cat Productivity kg/m /hr

enhancement

Tubular Fixed Bed (10)

MicroChannel*

750-1,000

3,000-4,000

60,000

350

700-800

7400

* P=20atm, T=235°C, H /CO=2, CO conversion: 63%, CH4 selectivity: 10% 2

The superior catalyst performance results from the effective heat and mass transfer characteristics of the microchannel reactor. The microcrochannel reactor with active heat removal provides near-isothermal environment, allowing the catalyst being operated at higher average temperature, which leads to high single-pass conversion at high G H S V . The efficient heat removal also minimizes the presence of hot spots in the catalyst bed, leading to low methane selectivity. A reactor modeling described else where (10,12) showed that a conventional


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fixed bed reactor exhibits significant temperature gradients. Under similar reaction conditions, the temperature gradient in a fixed-bed micro-tubular reactor (4mm ID) is as high as 30°C, which could significant promote the methanation reaction. In contrast, the microchannel reactor has a near isothermal temperature distribution with temperature gradient less than 1.5°C. Such an excellent temperature control also allows deployment of more active catalyst to improve space-time yield.

The Effect of C 0

2

Since the syngas generated via biomass gasification process contains significant amount of C 0 , it is important to evaluate the effect of C 0 on the F T synthesis performance. In particular, catalyst tolerance to C 0 during the F T synthesis needs to be evaluated to justify the necessity of costly C 0 removal. In a series of controlled experiments, syngas partial pressure was kept at 21 atm with a H / C O ratio of 2 while C 0 partial pressure was systematically increased from 0, 2 and to 5.6 atm, which corresponds to C 0 composition of 0%, 8.3%, and 20% in the feed gas (note that N with partial pressure of 0.85atm was used as the G C internal standard in the feed mixture). As can be seen in 2

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Microchannel Catalytic Processes for Converting ... - ACS Publications

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