The knowledge base for nanocellulose (NC) has grown exponentially over the past two decades and continues to expand with the increasing number of potential applications demonstrated in the literature and the patent space. NC has multiple forms depending on the starting cellulose source and the specific process used to produce it. Its high degree of surface reactivity makes it an ideal support structure for a wide variety of functional groups, leading to the ability to engineer materials for very specific applications. However, removing water from an NC suspension, e.g., dewatering and drying, while retaining the nanoscale properties of the NC remains a significant challenge to successful commercialization of NC materials. Processes for dewatering and drying of NC are desirable because of the high transport costs of shipping dilute aqueous suspensions, as well as end-use application requirements. Therefore, the development of nondestructive, cost-effective, scalable, and environmentally friendly dewatering and drying processes is important for commercial deployment of NC applications. This review addresses the current state of published knowledge on NC dewatering and drying and identifies research gaps that could be further explored in a precompetitive context to accelerate commercialization.

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We report a detailed study of graphene oxide (GO) membranes for concentration of Kraft black liquor (BL), which is a caustic (pH ∼ 12), hot (80–95 °C), and high-volume (∼500 gal/min in a typical pulp mill) byproduct of the papermaking process. Membrane-based concentration of BL is attractive as an energy-efficient alternative to thermally driven evaporation processes but challenging due to the harsh operating conditions and high fouling potential of BL (15–18 wt % solids). We fabricate thin (<300 nm) GO membranes supported on macroporous poly­(ethersulfone) (PES) supports by vacuum filtration techniques and discuss in detail their morphology, structure, thermomechanical stability, and chemical stability as characterized by several techniques. Furthermore, detailed permeation measurements at transmembrane pressures (TMPs) up to 50 bar and temperatures up to 85 °C show that the membranes have high performance in concentrating BL feeds containing high and low TS (total solids): high flux (in the range of 5–50 kg m–2 h–1), high lignin rejection (up to 98%), low fouling, and high stability throughout extended exposure (30 days) to BL at realistic operation conditions. The molecular weight cutoff (MWCO) of the membranes was determined to be ∼625 Da by means of dye rejection experiments. The present membranes are also expected to have low cost due to the use of relatively inexpensive functional membrane and substrate materials (GO and PES).

Black liquor (BL) concentration by multieffect evaporation is an extremely energy-intensive operation in the kraft pulping cycle. Membranes can significantly save energy in this process, but conventional membranes are strongly challenged by low solute rejections and poor stability in BL, which is a complex mixture containing dissolved lignin, other nonlignin organics, multiple inorganic salts at highly alkaline pH, and process temperatures of 70–85 °C. Here we describe in detail the fabrication, modification, and characterization of robust and high-performance graphene oxide (GO) nanofiltration membranes for BL concentration. We show that poly­(ether sulfone) (PES)-supported GO membranes prepared from chemically reduced GO, and then subjected to high-pressure hydraulic compaction, show excellent chemical and mechanical stability under real BL conditions in comparison to conventional GO membranes. These membranes (referred to as “GO-3” in this work) show near-perfect (>99%) lignin rejection, high total organic carbon (TOC) rejection (up to 93%), and greatly improved inorganic rejections especially for divalent anions that are predominant in BL. Finally, the GO-3 membranes are scaled up on larger PES sheets (∼660 cm2 in size) and are operated under realistic cross-flow conditions with real BL feed flow rates as high as 10 L/min at 70 °C. The GO-3 membranes show robust performance over more than 1500 h (60 days) of continuous operation in multiple cycles of 10–50 bar transmembrane pressures, attaining stable and sustained permeate fluxes as high as 25 LMH and excellent rejection performance equal to that obtained at smaller scale. The main results of this work have strong implications on the development of membrane processes for BL dewatering and more generally for processing of complex biorefinery feed streams.

Aerosols formed during combustion of black liquor cause a significant fire-side fouling problem in pulp mill recovery boilers. The ash deposits reduce heat transfer effectiveness, plug gas passages, and contribute to corrosion. Both. vapors and condensation aerosols lead to the formation of such deposits. The high ash content of the fuel and the low dew point of the condensate salts lead to a high aerosol and vapor concentration in most boilers. In situ measurements of the chemical composition of these deposits is an important step in gaining a fundamental understanding of the deposition process. Mared emission spectroscopy is used to characterize the composition of thin film deposits resulting fkom the combustion of black liquor and the deposition of submicron aerosols and vapors. New reference spectra of Na2S04, K2SO4, Na2C03 and KZC03 pure component films were recorded and compared with the spectra of the black liquor deposit. All of the black liquor emission bands were-identified using a combination of literature data and ab initio calculations. Ab initio calculations also predict the locations and intensities of bands for the alkali vapors of interest.This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately'owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. , .-'. AbstractAerosols formed during combustion of black liquor cause a significant fire-side fouling problem in pulp mill recovery boilers. The ash deposits reduce heat transfer effectiveness, plug gas passages, and contribute to corrosion. Both vapors and condensation aerosols lead to the formation of such deposits. The high ash content of the fuel and the low dew point of the condensate salts lead to a high aerosol and vapor concentration in most boilers. In situ measurements of the chemical composition of these deposits is an important step in gaining a fundamental understanding of the deposition process. Infrared emission spectroscopy is used to characterize the composition of thin film deposits resulting from the combustion of black liquor and the deposition of submicron aerosols and vapors. New reference spectra of Na2SO4, K2S04, Na2C03 and K2CO3 pure component films were recorded and compared with the spectra of the black liquor deposit. All of the black liquor emission bands were iden...

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Different alternatives to the conventional recovery of spent pulping liquors in the kraft pulping process have been studied by equilibrium calculations. These calculations were performed using FactSage. Three different black liquor gasification systems with addition of TiO 2 for direct causticization were studied: (i) MTCI conditions (600 °C and 0.3 MPa), (ii) Chemrec atmospheric booster conditions (950 °C and 0.1 MPa), and (iii) Chemrec pressurized system conditions (950 °C and 3 MPa). The studies of the influence of the molar ratio TiO 2 /Na 2 O showed that a TiO 2 / Na 2 O molar ratio of 0.5 is needed to capture all of the sodium in the condensed phase as sodium titanates (i.e., achieve complete direct causticization). Furthermore, at these conditions all of the sulfur is released to the gas phase. Different forms of sodium titanates are formed depending on the TiO 2 /Na 2 O molar ratio, and conceptually all of them could be used to produce NaOH in the consecutive leaching step.

Kraft black liquor (BL) is a complex multicomponent byproduct from wood pulping processes. After lignin, carbohydrate-derived carboxylic acids are the second most abundant class of organics in kraft BL and comprise a potential biomass-derived feedstock of more than 50 million tons/year globally. These acids can be valorized in biofuels and biobased chemicals production or further fractionated to produce specific high-value carboxylic acids from the mixture. However, both of these routes require a viable process to separate carboxylic acids from BL. In this work, an adsorption-based process was developed to address this challenging issue. The kraft BL stream was first pretreated to remove most of the lignin by membrane nanofiltration (NF) combined with lignin-selective adsorption. To separate organic acids from this pretreated BL, organophilic/hydrophobic granulated activated carbon (GAC) was identified as a promising adsorbent with excellent organic/inorganic selectivity and adsorption capacity (>100 mg/g GAC). We present comprehensive adsorption measurements and analysis to characterize the adsorbent and develop a viable cyclic operation scheme for the adsorption process. The GAC adsorbents show robust separation performance over 20+ cycles. We have successfully obtained concentrated aqueous organic acid mixtures with ∼95% purity from kraft BL. We propose a modified kraft process with energyefficient BL dewatering by NF and organic acids recovery for valorization.