Synthesis of Mixed Refrigerant Cascade Cycles

This paper considers the synthesis of refrigeration systems with refrigerant mixtures as working fluids. The employed refrigeration topology encompasses features from industrial liquefaction of natural gas (LNG) systems. This configuration consists of a combination of horizontal and vertical cascades generalizing the vertical cascade of pure refrigerant systems. The key features of mixtures exploited here are their ability to evaporate/condense over a temperature range and their potential to generate streams with different compositions through partial condensation. The synthesis problem is formulated and solved as a nonlinear program. The proposed methodology is illustrated using an example of cooling a methane-rich stream.     

Nonlinear decoupling control in the presence of sensor and actuator deadtimes

This work focuses on synthesizing nonlinear decoupling controllers for multivariable nonlinear systems represented by a state-space model, in the presence of deadtimes. The deadtimes appear in both the inputs and the outputs, but not in the states, and are physically associated with sensors and actuators. Simple sufficient conditions for feasibility of closed-loop deadtimes are derived, which rely only on the structural properties of the system. A control law is then derived so that the closed-loop system is input/output linear and decoupled, with deadtimes equal to the smallest ones that satisfy the feasibility conditions. The proposed method is applied to a chemical process. Its performance is evaluated through simulation in the presence of set-point and disturbance changes.     

Synthesis of Mixed Refrigerant Cascade Cycles

This paper considers the synthesis of refrigeration systems with refrigerant mixtures as working fluids. The employed refrigeration topology encompasses features from industrial liquefaction of natural gas (LNG) systems. This configuration consists of a combination of horizontal and vertical cascades generalizing the vertical cascade of pure refrigerant systems. The key features of mixtures exploited here are their ability to evaporate/condense over a temperature range and their potential to generate streams with different compositions through partial condensation. The synthesis problem is formulated and solved as a nonlinear program. The proposed methodology is illustrated using an example of cooling a methane-rich stream.     

Fe(PyTACN)‐Catalyzed cis‐Dihydroxylation of Olefins with Hydrogen Peroxide

A family of iron complexes with general formula [Fe(II)(^R,Y,XPyTACN)(CF₃SO₃)₂], where ^R,Y,XPyTACN=1‐[2′‐(4‐Y‐6‐X‐pyridyl)methyl]‐4,7‐dialkyl‐1,4,7‐triazacyclononane, X and Y refer to the groups at positions 4 and 6 of the pyridine, respectively, and R refers to the alkyl substitution at N‐4 and N‐7 of the triazacyclononane ring, are shown to be catalysts for efficient and selective alkene oxidation (epoxidation and cis‐dihydroxylation) employing hydrogen peroxide as oxidant. Complex [Fe(II)(^Me,Me,HPyTACN)(CF₃SO₃)₂] ( 7 ), was identified as the most efficient and selective cis‐dihydroxylation catalyst among the family. The high activity of 7 allows the oxidation of alkenes to proceed rapidly (30 min) at room temperature and under conditions where the olefin is not used in large amounts but instead is the limiting reagent. In the presence of 3 mol% of 7 , 2 equiv. of H₂O₂ as oxidant and 15 equiv. of water, in acetonitrile solution, alkenes are cis‐dihydroxylated reaching yields that might be interesting for synthetic purposes. Competition experiments show that 7 exhibits preferential selectivity towards the oxidation of cis olefins over the trans analogues, and also affords better yields and high [syn‐diol]/[epoxide] ratios when cis olefins are oxidized. For aliphatic substrates, reaction yields attained with the present system compare favourably with state of the art Fe‐catalyzed cis‐dihydroxylation systems, and it can be regarded as an attractive complement to the iron and manganese systems described recently and which show optimum activity against electron‐deficient and aromatic olefins.     

Interfacial instabilities in coextrusion flows of low‐density polyethylenes: Experimental studies

A fundamental investigation into the interfacial instability phenomenon was performed. Coextrusion experiments were carried out using well‐characterized low‐density (LDPE) resins in an effort to gain a better understanding of interfacial instability phenomena. The resins used were chosen carefully and included materials of high and low viscosity as well as broad and narrow molecular weight distributions (MWD). The experiments involved the coextrusion of either the same material in both layers or various combinations of the four materials and the focus of the work was to elucidate the effects of flow rates, molecular weight (MW) and MWD on interfacial instability. The effect of the geometry at the point where the materials merged was also investigated. It was concluded that there are essentially two types of interfacial instabilities and that the MW had the strongest effect on the occurrence of the “zig‐zag” instability due to high interfacial stress while the breadth of the MWD had a strong effect on the appearance of the “wave” instability. Broad MWD materials had a greater tendency to exhibit interfacial instability, which is more due to layer ratio than processing conditions or die geometries. The results suggest that the origin of the “wave” type of interfacial instability is due to an extreme extensional deformation of the minor layer at the merge point and that the viscoelastic properties of adjacent layers determine the instability development.     

Formation and behavior of composite CO2 hydrate particles in a high-pressure water tunnel facility

Sinking CO₂ composite particles consisting of seawater, liquid CO₂, and CO₂ hydrate were produced by a coaxial flow injector fed with liquid CO₂ and artificial seawater. The particles were injected into a high-pressure water tunnel facility to permit determination of their settling velocities and dissolution rates. Injections were performed at fixed pressures approximately equivalent to 1200-m, 1500-m, and 1800-m depths and at temperatures varying from approximately 2 to 5 °C. Immediately after injection, the cylindrical particles were observed to break away from the injector tip and often aggregated into sinking clusters. The seawater flow in the tunnel was then adjusted in a countercurrent flow mode to suspend the particles in an observation window so that images of the particles could be recorded for later analysis. The flow would often break or cause rearrangement of some of the clusters. Selected individual particles and some clusters were studied until they became too hydrodynamically unstable to follow. In general, the flow required to suspend clusters or individual particles decreased with time as the particles dissolved. For example, one particle was produced and observed for over 6 min at an average pressure of 15.022 MPa and an average temperature of 5.1 °C. Its sinking rate, determined from the flow required for stabilization, changed from 37.2 to 3.3 mm/s over this time. Particle sinking rates were compared to correlations from the literature for uniform cylindrical objects. Reasonable agreement was observed for short times; however, the observed decrease in sinking velocity with time was greater than that predicted by the correlations for longer times. Particle dissolution rates, based on changes in diameter, were also determined and varied from 5 to . A pseudo-homogeneous mass transfer model was used to predict single-particle dissolution rates. Good agreement was achieved between experimental dissolution data and the modeling results.     

Prediction of Viscoelastic Properties of Enzymatically Crosslinkable Tyramine–Modified Hyaluronic Acid Solutions Using a Dynamic Monte Carlo Kinetic Approach

The present study deals with the mathematical modeling of crosslinking kinetics of polymer–phenol conjugates mediated by the Horseradish Peroxidase (HRP)-hydrogen peroxide (H₂O₂) initiation system. More specifically, a dynamic Monte Carlo (MC) kinetic model is developed to quantify the effects of crosslinking conditions (i.e., polymer concentration, degree of phenol substitution and HRP and H₂O₂ concentrations) on the gelation onset time; evolution of molecular weight distribution and number and weight average molecular weights of the crosslinkable polymer chains and gel fraction. It is shown that the MC kinetic model can faithfully describe the crosslinking kinetics of a finite sample of crosslinkable polymer chains with time, providing detailed molecular information for the crosslinkable system before and after the gelation point. The MC model is validated using experimental measurements on the crosslinking of a tyramine modified Hyaluronic Acid (HA-Tyr) polymer solution reported in the literature. Based on the rubber elasticity theory and the MC results, the dynamic evolution of hydrogel viscoelastic and molecular properties (i.e., number average molecular weight between crosslinks, M_c, and hydrogel mesh size, ξ) are calculated.     

Analysis of mixing during melt-melt blending in twin screw extruders using reactive polymer tracers

Mixing during melt-melt blending of segregated polypropylene melt streams in a co-rotating twin screw extruder was experimentally investigated. The mixing limited reaction between two polymer reactive tracers, which are terminally functionalized polyolefin oligomers, was used to determine the mixing performance of a kneading block section. The selected functional groups were succinic anhydride and a primary amine, and Fourier-Transform Infrared Spectrometry (FT-IR) was used to determine the anhydride conversion. In the absence of interfacial tension, the reaction conversion was directly related to the amount of interfacial area generated. Experiments were completed to study the effects of operating conditions, kneading block design, and polymer material properties. The screw speed effect was observed to be non-linear because of competing contributions from shear rate, residence time, channel fill, and viscous heating. The mixing performance of kneading blocks backed by a reverse conveying element was observed to follow the trend of: forward > reverse > neutral. For each kneading block design, the mixing performance decreased with an increase in polymer viscosity.     

Price dynamics in European petroleum markets

This paper analyses horizontal and vertical price dynamics in the EU petroleum markets. The results indicate that the cross-country price differentials have significant impact on the local price adjustments. We investigate the cross-national price spill-overs and find that the extent of the welfare transfer due to asymmetric price transmission, when analysed in a cross-country setting, is less pronounced than claimed in previous contributions in this area. We also find empirical evidence, although indirect, for the politically charged concept of “fuel tourism”, using a pan-European cross-product time series dataset.