Facilities & Laboratories

Hydrothermal Liquefaction (HTL) Process Overview

In the HTL process, wet biomass slurry is converted to biocrude at elevated temperatures (300-360°C) while the pressure is maintained above the vapor pressure of water (150 to 200 atm, at these temperatures) to facilitate a condensed-phase reaction medium. Compressed hot water has enhanced solvent properties that facilitate the formation of liquid oil products from biomass. Biomass is dissolved and liquefied in this process, and the major products are biocrude, water with dissolved organics, gas, and solids (primarily ash). HTL reactions involve fragmentation and condensation coupled with dehydration, decarbonylation, and decarboxylation. The chemistry is energy-neutral or mildly endothermic. For lignocellulosic feedstocks, buffering agents such as sodium carbonate are normally employed to reduce acid-catalyzed degradation reactions, which lead to excess polymer/char formation. Residence times at temperature typically range from 5 to 60 minutes.

The Pacific Northwest National Laboratory (PNNL) is a technology leader in HTL and upgrading of HTL biocrudes for fuel production. PNNL began research in the hydrothermal processing of biomass in the mid-1970s and has been operating several bench-scale continuous flow HTL systems since 2008. The bench scale systems, located at the Bioproducts, Sciences and Engineering Laboratory (BSEL), operate at typical flowrates of 2 to 4 L/h. The PDLW Modular Hydrothermal Liquefaction System (MHTLS) represents a 5X scale-up of the bench-scale system and also has additional features that are important for scale-up, including slurry feed preparation, integrated feed/product heat exchange, parallel in-line product filtration, and continuous oil/water separation.

PDLW MHTLS Overview

The PDLW MHTLS is an engineering-scale process system that is installed in the high-bay of the PDLW facility. The primary objectives of the system are to:

1. Provide technical and processing information that are important for system scale-up.
2. Generate sufficient product for detailed analyses and evaluation or for further upgrading and fractionation to fuels for detailed analyses and evaluation.

The system has been divided into three general operating areas, with each area located on a separate skid. Biomass feedstock is prepared and staged on skid 1. Hydrothermal liquefaction is performed on skid 2, with biomass being converted to liquid products and solids (mainly ash) being removed. On skid 3, the products are separated into biocrude, aqueous byproduct and gas (mainly CO₂). Skid 3 also includes tankage for staging products.

The PDLW MHTLS has a nominal feed flowrate of 12 L/h and was designed for 24-hr/day, attended operation. The system is capable of processing approximately 2,000-L of feed slurry in 1-wk of operation.

MHTLS Key Features

Feature	Attributes/Operating Parameters
General Operations	Designed for 24-hr/day, attended operation. The system is capable of processing approximately 2,000-L of feed slurry in 1-wk of operation.
Skid 1: Feed Preparation and Staging
Hockmeyer Immersion Mill	Ability to mill solids in aqueous slurry to prepare a pumpable slurry (typically, 99% of particle volume below 500 microns. Capable of milling a batch of feed (up to 120-L in less than 2 hours).
Prepared Feed Storage	Capacity = 570-L at a given time.
Skid 2: HTL Processing
Nominal Feed Flowrate	12 L/hr (slurries typically at approximately 20 wt% solids).
Nominal Operating Temperature	350°C
Nominal Operating Pressure	200 atm
Reactor Configurations	Tubular reactor (1/2" diameter [0.065" wall] tubing, 3-L capacity). Continuous stirred tank reactor (2-L capacity)
Inline Solids Removal	Settling + particle removal by filtration (rating of 98% at 5 µm).
Skid 3: Product Separations and Storage
Separations Systems	Gas/liquid Liquid/liquid (biocrude/water) Coalescer for removal of oil droplets from aqueous stream
Gas Conditioning/Measurement	Coalescer, in-line gas chromatograh, H2S scrubber, wet test meter.
Product Storage	Aqueous product storage capacity = 1,900-L, with ability for volumetric measurement. Biocrude product storage capacity = 60-L/tank (multiple tanks), with weighing scales.

Skid 1: Feed Preparation and Staging

Skid 1 includes equipment for grinding, mixing, and feed staging. Biomass feedstock for HTL may be received at the PDLW highbay as low-moisture particulate solids (e.g., wood chips, corn stover, wheat straw, dried algae), high-moisture solids (e.g., wastewater treatment sludges, sun-dried algae cakes, grape pomace), or homogeneous slurries (e.g., algae slurries). For HTL processing in the PDLW MTHLS, the feedstocks are formatted to create pumpable slurries (typically 15 to 25% solids) with a volume mean particle size of 20 to 100 µm. The extent of formatting/milling will depend on the characteristics of incoming feedstock.

The workhorse of skid 1 is the Hockmeyer Model HCPS-2.5 Counter Peg Immersion Mill. The Hockmeyer Mill is designed to mill solids in aqueous slurry down to 99% less than 500 micron with a typical volume mean particle size of 20-100 microns. Milling a batch of feed (up to 120-L) can be accomplished in less than 2 hours of operating time. Once a batch is completed, the milled slurry can be pumped to a 950-L agitated staging tank on skid 1 or to a day tank on skid 2. Equipment on skid 1 can be utilized to support the downstream MHTLS operations or can be used to prepare feed for other operations (e.g., for the bench-scale continuous HTL systems).

Skid 2: HTL Processing

All high pressure/high temperature operations are confined to skid 2. On this skid, biomass slurry feedstock is pressurized and heated in a series of reactors to HTL conditions (nominally 350°C and 200 atm) and converted to biocrude and byproducts. The skid can be operated in one of two general configurations:

• Configuration 1, in a tubular reactor configuration with integrated feed/product heat exchange; and
• Configuration 2, with a continuous stirred tank reactor (CSTR) preceding the tubular reactor configuration and without heat integration. The nominal feed flowrate to the system is 12 L/h.

Downstream of the reactors, solids (predominately ash) are removed by settling and filtration at temperatures and pressures close to that of the reactor. Effective solids removal minimizes emulsion formation and results in more efficient downstream separation of the biocrude phase from the aqueous fraction. Two filtration systems are available and can be operated in parallel. When the pressure drop across a filter reaches a predetermined value, an online blowdown sequence is initiated to clear the filter element. Filter elements with a removal rating of 98% at 5 µm are used.

In Configuration 1, after solids removal, the product is cooled while passing through the product-side of a heat exchanger which is used to preheat the incoming feed. Additional cooling of the product can be performed using a tube-in-tube coil with water in the outer tube. The cooled product is depressurized as it passes through a pressure control valve before product separations on skid 3. In Configuration 2, after solids removal, the product is cooled in a finned air cooled tube exchanger (no heat exchange with the incoming feed). The product stream is then depressurized as it as it passes through a pressure control valve before product separations operation on Skid 3. To provide process robustness, there are three parallel pressure control valves that can be operated independently.

Skid 3: Product Separations and Storage

After completion of the HTL reactions, the liquefied product consists of a mixture of substantially non-polar biocrude liquids and an aqueous phase. In addition to water, the aqueous phase contains dissolved salts, polar organics (e.g., alcohols, organic acids), and sparingly soluble larger organic compounds (up to their solubility limits). Upon depressurization—at the pressure letdown device located on skid 2—carbon dioxide gas is released from the liquid product solution.

There are two separators on skid 3 that function to divide the product stream into the separate aqueous byproduct, biocrude, and non-condensable gas phases. The first separator is a gas-liquid separator with a demister. The liquid product then flows to the second separator, where biocrude/aqueous separation takes place. The biocrude and aqueous phases are then collected in separate storage tanks.

After leaving the first separator—the gas-liquid separator—the gas phase passes through a condenser to remove condensable components (i.e., oil mists, water, ethanol, and methanol). After the condenser, the offgas is sampled by an online gas chromatograph to analyze the composition of the non-condensable gas. The offgas then passes through a wet test meter to measure the volumetric gas flow. Next, the process offgas is routed to a scrubber to remove H₂S that may be present in the stream before it is directed to the ventilation system.

Catalysis Capabilities Overview

PNNL is a technical leader in catalysis. The PNNL Institute for Integrated Catalysis (IIC) enables a collaborative environment for more than 120 scientists and engineers from different organizations within PNNL. The catalyst efforts span from basic to applied research, development and application. The cross-disciplinary collaboration produces a unique creative environment, which is a prerequisite for transformative research.

PNNL catalyst capabilities include theoretical modeling, discovery, synthesis, characterization and testing. The Chemical & Biological Process Development (C&BPD) Group is focused primarily on catalytic processing as it applies to producing bio-derived chemicals and fuels. The C&BPD Group has catalyst synthesis, characterization, and a variety catalyst testing capabilities. The catalyst testing capabilities include batch and continuous flow high throughput testing, continuous flow reactor systems, ranging from 0.5-mL to 20-L. These reactor systems operate in a variety of configurations, including trickle bed, upflow and ebullated bed.

The PDLW 20-L Catalytic Reactor System represents the largest catalytic system at PNNL. This system is used to obtain scale-up information to some degree, however, the primary role of the system is to produce significant amounts of product (chemicals or fuels) for evaluation and testing purposes. The design liquid feed flowrate is 2.5 L/hr, enabling up to 420-L of liquid feed to be processed in a 7-day, 24-hr/day operation.

The PDLW 20-L Catalytic Reactor System is described in more detail below.

PDLW 20-Liter Catalytic Reactor System Overview

The PDLW 20-Liter Catalytic Reactor System is an engineering-scale process system that is installed in the high-bay of the Process Development Laboratory - West (PDLW) facility. This system, along with the PDLW Distillation Column System are housed within a Class 1, Division 2 rated enclosure to enable the processing of flammable materials.

The primary objective of the system is to generate sufficient quantities (e.g., 200 to 400-L) of product for evaluation and testing purposes.

The PDLW 20-Liter Catalytic Reactor System has a nominal liquid feed flowrate of 2.5 L/hr and was designed for 24-hr/day, attended operation. The system is capable of processing approximately 420-L of liquid feed in 1-wk of operation. Other key features and operating parameters for the PDLW 20-Liter Catalytic Reactor System are summarized in Table 2.

20-Liter Catalytic Reactor System Key Features

Feature	Attributes/Operating Parameters
General Operations	Designed for 24-hr/day, attended operation. The system is capable of processing approximately 420-L of liquid feed in 1-wk of operation.
Feed Preparation and Staging
Liquid Feed Storage	Capacity 83-L with ability to replenish during a run.
Liquid Feed Filtration	Ability to remove solid particles (current configuration down to 25 µm in size).
Catalytic Processing
Nominal Liquid Feed Flowrate	2.5 L/hr
Nominal Gas Feed Flowrate	5,000 (standard) L/hr (e.g., hydrogen, nitrogen)
Nominal Operating Temperature	425°C
Nominal Operating Pressure	140 atm
Reactor Configuration	Tubular reactor (4" inner diameter), 20-L capacity. (Trickle-bed configuration).
Product Separations and Storage
Separations Systems	Gas/liquid Liquid/liquid (organic/water)
Gas Conditioning/Measurement	In-line gas chromatograph, H2S scrubber, wet test meter.
Product Storage	44-L each of organic and aqueous, with ability for volumetric and/or weight measurement, and ability for transfer of product to other storage vessels or distillation storage during a run.

The feed is staged next to the system in a 55-gallon drum. If needed, there are two filter systems—in parallel—that can be used to remove solids down to approximately 25 µm in size (current configuration). The filtered solids are then pumped to a feed tank. A high pressure pump is used to deliver the liquid feed to the top of the reactor, where a feed gas (as needed) is also fed to the reactor. Current options for feed gas are H₂ and N₂. The nominal flowrates for the liquid and gas are 2.5 L/hr and 5,000 (standard) L/hr, respectively. The liquid and gas flow downward (trickle bed) through the reactor.

The reactor is constructed of Hastelloy C-276, has an internal diameter of 4" and is approximately 9' long, giving it a maximum bed volume of 20-L. The reactor is heated externally via a vertical tube furnace. There are eight separate, independently controlled heating zones along the length of the reactor to enable for operation at different temperatures within the reactor if desired. The nominal operating temperature and pressure of the reactor system are 425°C and 140 atm.

After exiting the bottom of the reactor, the liquid and gas are cooled and enter a gas-liquid separator, while still at operating pressure. The gas exits the top of the gas-liquid separator and the liquid exits the bottom. The pressure of the system is controlled by two pressure regulators in series on the gas-side of the system and a pressure regulator on the liquid-side of the system, just downstream of the gas-liquid separator. The offgas is sampled by an online gas chromatograph to analyze the composition of the non-condensable gas. The offgas then passes through a wet test meter to measure the volumetric gas flow. Next, if needed, the process offgas is routed through a scrubber to remove H₂S that may be present in the stream before it exits the system through the ventilation system.

The liquid exits the gas-liquid separator via a valve that is controlled based on the liquid level in the gas-liquid separator. Downstream of the gas-liquid separator is a gravity liquid-liquid separator that can be used to separate immiscible liquid phases (e.g., water and organic) if needed. The liquid products are then routed to product storage tanks that are equipped with scales so that the amount of product can be monitored throughout a run.

Distillation Capabilities Overview

Within the C&BPD Group, there are three different distillation systems that are utilized for fractionation of liquid feeds. The systems include:

1. 1.5-liter glass batch system capable of processing approximately 1.5-L of feed material/day. This system can operate under vacuum or at atmospheric pressure and is capable of operating at temperatures for recovery of various fuel fractions (e.g., gasoline, jet, and diesel).
2. Pope wiped film, glass, continuous distillation system capable of processing approximately 10 to 20 L/day. In its current configuration, the system is capable of operations up to 200°C, at atmospheric pressure or under vacuum. Replacement of the Teflon wiper blade assembly with carbon wiper blades would enable the system to be operated at temperatures up to 490°C.
3. PDLW Distillation Column System (stainless steel construction and stainless steel packing), capable of processing approximately 100-L over a 24-hr period. This system operates at atmospheric pressure and can operate at a reboiler temperature up to 425°C. This system is described in more detail below.

PDLW Distillation Column System Overview

The PDLW Distillation Column System is an engineering-scale process system that is installed in the high-bay of the Process Development Laboratory - West (PDLW) facility. This system, along with the PDLW 20-liter Catalytic Reactor System are housed within a Class 1, Division 2 rated enclosure to enable the processing of flammable materials.

The primary objective of the system is to generate sufficient quantities (e.g., 200 to 400-L) of product for evaluation and testing purposes.

The PDLW Distillation Column System has a nominal liquid feed flowrate of 4 L/h and was designed for 24-hr/day, attended operation. The system is capable of processing approximately 100-L of liquid feed in 24-hrs or operation.

Feature	Attributes/Operating Parameters
General Operations	Designed for 24-hr/day, attended operation. The system is capable of processing approximately 100-L of liquid feed in 24-hrs of operation.
Feed Preparation and Staging
Liquid Feed Storage	Capacity = 140-L, with ability to replenish during a run.
Distillation Processing
Nominal Liquid Feed Flowrate	4 L/hr
Maximum Reboiler Operating Temperature	425°C
Nominal Operating Pressure	Atmospheric
Column Features	Dimensions: 2" inner diameter x 126" tall, reboiler 4" x 30" tall. ~15-liter volume total volume. Packing Material: Sulzer BX Capability to reflux light (overhead) product back to distillation column.
Product Storage
Product Storage	83-L light (overhead) product, 83-L heavy (bottoms) product, and ability for transfer of product to other storage vessels during a run.

The feed to the system is stored in a 140-L feed tank and is pumped at a nominal feed flowrate of 4 L/hr to the feed stage of the distillation column. The distillation column is constructed of stainless steel (316), has an internal diameter of 2" and is approximately 126" long with a reboiler that has a 4" internal diameter and is 30" tall. The distillation column is packed with Sulzer BX packing material to allow for efficient fractionation. The reboiler is heated by an electric immersion heater, and can be operated at up to 425°C. The system is designed to operate at atmospheric pressure.

The system has the ability to reflux all or a portion of the light (overhead) product back to the top of the distillation column. The light (overhead) and heavy (bottoms) fractions are continuously removed from the distillation column and stored in separate product tanks that are equipped with scales so that the amount of product can be monitored during a run. Any offgas exits the system through the ventilation system.