13 research outputs found
Efficient Retrofitting Approach for Improving Heat Recovery in Heat Exchanger Networks with Heat Transfer Intensification
In this paper, an efficient design
method is presented to implement
heat transfer intensification technologies for increasing energy saving
in industrial scale heat exchanger networks (HENs). First of all,
the detailed models of shell-and-tube heat exchangers are utilized
to estimate exchanger performances under normal and intensified conditions,
and an optimization method based on simulated annealing is proposed
to find appropriate retrofitting options in the existing HENs. Then,
a novel retrofit approach based on the exchanger models and optimization
method is introduced to retrofit HENs with several possible retrofit
strategies, including exchanger relocations, heat transfer intensification,
additional area, and stream repiping. Moreover, a new efficient software
package (int-HEAT) is developed for the addressed retrofit problems.
The interface of int-HEAT is designed to focus on a clear presentation
of HEN retrofit procedure, which allows users to easily control all
the retrofit stages. An industrial problem of preheat train for crude
oil distillation is solved with the use of the new developed software
through all the main steps of the proposed retrofit procedure, namely
reducing energy consumption, identifying retrofitted exchangers, selecting
retrofit strategies, and implementing heat transfer intensification
techniques. The case study demonstrates the validity and efficiency
of the developed software tool and the proposed approach
Methanol O–H Bond Dissociation on H‑Precovered Gold Originating from a Structure with a Wide Range of Surface Stability
Gold has been shown to exhibit promising catalytic activity,
and
understanding the fundamental interactions of reactants and hydrogen
atoms on a gold surface is key to gaining insight into hydrogenation
reaction mechanisms. In this paper, we report that the adsorption
of methanol onto a H-precovered Au(111) surface induces an adsorbate
structure, or set of structures, on the surface involving both methanol
and hydrogen adatoms with a wide range of stability on the surface.
Coadsorption of H/MeOD or D/MeOH indicates H/D exchange between the
two surface species, providing evidence that the H-precovered gold
surface can dissociate the methanol O–H bond at low temperature
(<120 K). These isotopic experiments also demonstrate that hydrogen/deuterium
atoms released from a methanol molecule desorb at higher temperatures
than hydrogen/deuterium atoms originating from the surface, providing
insight into the adsorbate structure(s) present. Additionally, the
presence of MeOH on the surface is shown to inhibit the ability of
adsorbed MeOD to undergo hydrogen exchange, providing additional clues
regarding the exchange reaction mechanism. These phenomena are also
shown to exist for ethanol on H-precovered Au(111), suggesting that
this behavior may be common to alcohols or species with an O–H
functional group in general. These observations give insight into
the behavior of the O–H group on a gold surface, which can
aid in determining reaction mechanisms and directing future catalytic
research
Low-Temperature Hydrogenation of Acetaldehyde to Ethanol on H-Precovered Au(111)
Gold-based classical high surface area catalysts have been widely investigated for hydrogenation reactions, but fundamental studies on model catalysts are lacking. We present experimental measurements of the reaction of hydrogen adatoms and adsorbed acetaldehyde on the Au(111) surface employing temperature-programmed desorption. Here, we show that chemisorbed hydrogen adatoms bind weakly with desorption peaks at ∼110 K, indicating an activation energy for recombinative desorption of ∼28 kJ/mol. We further demonstrate that acetaldehyde (CH<sub>3</sub>CHO) can be hydrogenated to ethanol (CH<sub>3</sub>CH<sub>2</sub>OH) on the H-atom-precovered Au(111) surface at cryogenic temperatures. Isotopic experiments employing D atoms indicate a lower hydrogenation reactivity
Catalyst-Substrate Helical Character Matching Determines the Enantioselectivity in the Ishihara-Type Iodoarenes Catalyzed Asymmetric Kita-Dearomative Spirolactonization
Catalyst design has traditionally focused on rigid structural
elements
to prevent conformational flexibility. Ishihara’s elegant design
of conformationally flexible C2-symmetric
iodoarenes, a new class of privileged organocatalysts, for the catalytic
asymmetric dearomatization (CADA) of naphthols is a notable exception.
Despite the widespread use of the Ishihara catalysts for CADAs, the
reaction mechanism remains the subject of debate, and the mode of
asymmetric induction has not been well established. Here, we report
an in-depth computational investigation of three possible mechanisms
in the literature. Our results, however, reveal that this reaction
is best rationalized by a fourth mechanism called “proton-transfer-coupled-dearomatization
(PTCD)”, which is predicted to be strongly favored over other
competing pathways. The PTCD mechanism is consistent with a control
experiment and further validated by applying it to rationalize the
enantioselectivities. Oxidation of the flexible I(I) catalyst to catalytic
active I(III) species induces a defined C2-symmetric helical chiral environment with a delicate balance between
flexibility and rigidity. A match/mismatch effect between the active
catalyst and the substrate’s helical shape in the dearomatization
transition states was observed. The helical shape match allows the
active catalyst to adapt its conformation to maximize attractive noncovalent
interactions, including I(III)···O halogen bond, N–H···O
hydrogen bond, and π···π stacking, to stabilize
the favored transition state. A stereochemical model capable of rationalizing
the effect of catalyst structural variation on the enantioselectivities
is developed. The present study enriches our understanding of how
flexible catalysts achieve high stereoinduction and may serve as an
inspiration for the future exploration of conformational flexibility
for new catalyst designs
Structure Revealing H/D Exchange with Co-Adsorbed Hydrogen and Water on Gold
A fundamental understanding of the interactions between
coadsorbed
water and hydrogen on metallic surfaces is critical to many chemical
processes including catalysis and electrochemistry. Here, we report
on the strong and intricate interactions between coadsorbed H/D and
water on the close-packed (111) surface of gold. Deuterium isotopic
labeling shows H/D exchange in H–D<sub>2</sub>O and D–H<sub>2</sub>O systems, indicating water dissociation and suggesting a
nonrandom scrambling process by revealing the origin of hydrogen evolution
(from surface H atoms or from water molecules) during annealing. In
this reaction, the protonation of the H-bonding ice network (i.e.,
the formation of (H<sub>2</sub>O)<sub><i>n</i></sub>H<sup>+</sup>) is energetically favorable and is responsible for water
dissociation. Density functional theory (DFT) modeling suggests that
the thermodynamics and structure of the protonated clusters are predominant
factors for yielding the traceable H<sub>2</sub> desorption features
from the surface interaction with H atoms, providing insights into
reaction mechanisms
Optimal Synthesis of Water Networks for Addressing High-Concentration Wastewater in Coal-Based Chemical Plants
This
paper outlines the development of an optimization-based method
for synthesizing a water network, which incorporates various treatment
technologies to address the high-concentration wastewater in coal-based
chemical plants. One important feature of the proposed approach is
that it associates a multistep wastewater treatment design within
a source–regeneration–sink superstructure. This design
can enforce certain design and structural specifications to tighten
the model formulation and enhance solution convergence. A mixed integer
nonlinear programming problem is formulated based on the proposed
superstructure, which involves unit-specific shortcut models instead
of the fixed impurities removal model to describe it accurately. The
proposed method for water network synthesis is demonstrated using
two case studies, which determine the effect of streams composition
and wastewater treatment technologies on the total network cost, freshwater
consumption, and water network design. The results highlight the ability
of the proposed model for the developed water network synthesis by
computing quickly and realizing the goals of cost savings and discharge
reduction
Highly Selective, Facile NO<sub>2</sub> Reduction to NO at Cryogenic Temperatures on Hydrogen Precovered Gold
We have discovered that NO<sub>2</sub> is reduced to
NO at 77 K
by hydrogen precovered gold in vacuum. Here, we investigate the partial
reduction of NO<sub>2</sub> to NO on an atomic-hydrogen populated
model gold catalyst for a more fundamental understanding of the surface
chemistry of hydrogenation. Gold-based catalysts have been found to
be active for many hydrogenation reactions, but few related fundamental
studies have been conducted. Our experimental results reveal a high
catalytic activity for gold: indeed, NO<sub>2</sub> is reduced to
NO with 100% conversion and 100% selectivity at temperatures lower
than 120 K. Density functional theory calculations and reflection–absorption
infrared spectroscopy measurements indicate that HNO<sub>2</sub> and
N<sub>2</sub>O<sub>3</sub> are intermediates which are highly dependent
on surface hydrogen concentrations; subsequent hydrogenation of HNO<sub>2</sub> and dissociation of N<sub>2</sub>O<sub>3</sub> upon annealing
induces the production of NO and H<sub>2</sub>O
Encapsulation of Single Nanoparticle in Fast-Evaporating Micro-droplets Prevents Particle Agglomeration in Nanocomposites
This work describes
the use of fast-evaporating micro-droplets to finely disperse nanoparticles
(NPs) in a polymer matrix for the fabrication of nanocomposites. Agglomeration
of particles is a key obstacle for broad applications of nanocomposites.
The classical approach to ensure the dispersibility of NPs is to modify
the surface chemistry of NPs with ligands. The surface properties
of NPs are inevitably altered, however. To overcome the trade-off
between dispersibility and surface-functionality of NPs, we develop
a new approach by dispersing NPs in a volatile solvent, followed by
mixing with uncured polymer precursors to form micro-droplet emulsions.
Most of these micro-droplets contain no more than one NP per drop,
and they evaporate rapidly to prevent the agglomeration of NPs during
the polymer curing process. As a proof of concept, we demonstrate
the design and fabrication of TiO<sub>2</sub> NP@PDMS nanocomposites
for solar fuel generation reactions with high photocatalytic efficiency
and recyclability arising from the fine dispersion of TiO<sub>2</sub>. Our simple method eliminates the need for surface functionalization
of NPs. Our approach is applicable to prepare nanocomposites comprising
a wide range of polymers embedded with NPs of different composition,
sizes, and shapes. It has the potential for creating nanocomposites
with novel functions
Fluorinated Pickering Emulsions Impede Interfacial Transport and Form Rigid Interface for the Growth of Anchorage-Dependent Cells
This study describes the design and
synthesis of amphiphilic silica nanoparticles for the stabilization
of aqueous drops in fluorinated oils for applications in droplet microfluidics.
The success of droplet microfluidics has thus far relied on one type
of surfactant for the stabilization of drops. However, surfactants
are known to have two key limitations: (1) interdrop molecular transport
leads to cross-contamination of droplet contents, and (2) the incompatibility
with the growth of adherent mammalian cells as the liquid–liquid
interface is too soft for cell adhesion. The use of nanoparticles
as emulsifiers overcomes these two limitations. Particles are effective
in mitigating undesirable interdrop molecular transport as they are
irreversibly adsorbed to the liquid–liquid interface. They
do not form micelles as surfactants do, and thus, a major pathway
for interdrop transport is eliminated. In addition, particles
at the droplet interface provide a rigid solid-like interface to which
cells could adhere and spread, and are thus compatible with the proliferation
of adherent mammalian cells such as fibroblasts and breast cancer
cells. The particles described in this work can enable new applications
for high-fidelity assays and for the culture of anchorage-dependent
cells in droplet microfluidics, and they have the potential to become
a competitive alternative to current surfactant systems for the stabilization
of drops critical for the success of the technology
Conversion of Dimethyl Ether to 2,2,3-Trimethylbutane over a Cu/BEA Catalyst: Role of Cu Sites in Hydrogen Incorporation
Recently,
it has been demonstrated that methanol and/or dimethyl ether can be
converted into branched alkanes at low temperatures and pressures
over large-pore acidic zeolites such as H-BEA. This process achieves
high selectivity to branched C<sub>4</sub> (e.g., isobutane) and C<sub>7</sub> (e.g., 2,2,3-trimethylbutane) hydrocarbons. However, the
direct homologation of methanol or dimethyl ether into alkanes and
water is hydrogen-deficient, resulting in the formation of unsaturated
alkylated aromatic residues, which reduce yield and can contribute
to catalyst deactivation. In this paper we describe a Cu-modified
H-BEA catalyst that is able to incorporate hydrogen from gas-phase
H<sub>2</sub> cofed with dimethyl ether into the desired branched
alkane products while maintaining the high C<sub>4</sub> and C<sub>7</sub> carbon selectivity of the parent H-BEA. This hydrogen incorporation
is achieved through the combination of metallic Cu nanoparticles present
on the external surface of the zeolite, which perform H<sub>2</sub> activation and olefin hydrogenation, and Lewis acidic ion-exchanged
cationic Cu present within the H-BEA pores, which promotes hydrogen
transfer. With cofed H<sub>2</sub>, this multifunctional catalyst
achieved a 2-fold increase in hydrocarbon productivity in comparison
to H-BEA and shifted selectivity toward products favored by the olefin
catalytic cycle over the aromatic catalytic cycle