Heavey, Dennis J., Barrow, Susan E., Hickling, Nicola E., and Ritter, James M.
Nature; November 1985, Vol. 318 Issue: 6042 p186-188, 3p
Acetylsalicyclic acid (aspirin) inhibits prostanoid synthesis1,2by irreversible acetylation of fatty acid cyclooxygenase (EC 126.96.36.199)3. It thereby inhibits synthesis of pro-aggregatory thromboxane A2(TXA2) by platelets2,4and is widely used in the treatment and prophylaxis of vascular disease. Its efficacy, however, may be reduced since it also inhibits formation of prostacyclin (PGI2)5,6which is a vasodilator and anti-aggregatory agent7,8. There is uncertainty over the optimum dose regimen for aspirin since although it inhibits platelet thromboxane production for many days4, the magnitude and duration of its effect on PGI2production by vascular endothelium in vivo is unknown. Resting plasma concentrations of PGI2(measured as the stable hydrolysis product 6-oxo-PGF1α) are at or below the limit of sensitivity of the most sensitive assays9and cannot therefore be used to demonstrate a reduction in production. Bradykinin stimulates PGI2synthesis by cultured human vascular endothelial cells10and we have shown that it stimulates PGI2production by man in vivo11. We report here that an oral dose of aspirin (600 mg) causes rapid and substantial inhibition of bradykinin-stimulated PGI2production, but recovery occurs within 6 hours; this implies that endothelial PGI2synthesis would be spared most of the time during dosing once daily with even this relatively large dose of aspirin.
Chemical Engineering Communications; January 1991, Vol. 108 Issue: 1 p289-305, 17p
Air purification and vapor recovery by pressure swing adsorption (PSA) were experimentally investigated using the silicalite-DMMP-air system. The results from several cyclic steady-state PSA runs were compared at constant throughput with those from a previous study on the BPL activated carbon-DMMP-air system. The performance of BPL activated carbon was superior to that of silicalite because it demonstrated complete cleanup of the product effluent when starting from a saturated column, whereas, at similar process conditions, silicalite was able to cleanup only a portion of the product effluent. Nevertheless, both silicalite and BPL activated carbon respectively demonstrated enrichments (Ye/Yf) of 12 and 15 using only moderate vacuum. However, there were significant differences in the shapes of the cyclic steady-state product and exhaust profiles which were attributed to differences in the 1) mass transfer rates, 2) adsorption capacities, or 3) possibly shapes of the adsorption isotherms.
Chemical Engineering Science; January 1991, Vol. 46 Issue: 2 p563-574, 12p
The effect of hysteresis on fixed-bed desorption was investigated, based on local equilibrium theory. According to the geometric condition: ∂q/∂y= Δq/Δy, different type isotherms were analyzed to determine the locations of simple waves, shcok waves and/or combined waves. The characteristics, bed profiles and elution curves were constructed for three systems which collectively exhibited isotherm Types I, II, IV and V, and hysteresis loop Types H1, H2 and H4. The effect of hysteresis was ascertained by comparing elution along the adsorption branch of the isotherm with that along the desorption branch. The results showed that hysteresis can increase significantly the time associated with desorption. To attain complete desorption, 10–30% delay was realized by three systems investigated. Moreover, because the elution curves were, in some cases, significantly different, an even greater effect of hysteresis can be realized when complete regeneration is not advantageous.
Chowienczyk, Philip J, Watts, Gerald F, Wierzbicki, Anthony S, Cockcroft, John R, Brett, Sally E, and Ritter, James M
Journal of the American College of Cardiology; April 1997, Vol. 29 Issue: 5 p964-968, 5p
Objectives. We sought to determine whether hypertriglyceridemia in patients with lipoprotein lipase (LPL) dysfunction is associated with endothelial dysfunction in resistance vessels of the forearm vasculature.
Adsorption; June 1997, Vol. 3 Issue: 2 p151-163, 13p
A two-level fractional factorial study was performed by computer simulation on the periodic state process performance of a pressure swing adsorption-solvent vapor recovery process (PSA-SVR). The goal was to investigate factor (parameter) interaction effects on the process performance, i.e., interaction effects that cannot be ascertained from the conventional “one-at-a-time” approach. Effects of seven factors, i.e., the purge to feed ratio, pressure level, pressure ratio, heat transfer coefficient, feed concentration, feed volumetric flow rate and bed length to diameter ratio, on the process performance were investigated. The results were judged in terms of the light product purity, heavy product enrichment (and relative enrichment) and recovery, and bed capacity factor. Only the purge to feed ratio, pressure ratio, and feed concentration had significant effects on the benzene vapor enrichment (and relative enrichment); and no two-factor and higher interactions were observed. The light product purity was affected by all seven factors; and the relative importance of the effect of each factor depended on the levels of the other factors, i.e., significant two-factor interaction effects existed. Two-factor interaction effects also existed on the benzene vapor recovery, although the effects of all seven factors and their interactions were relatively small. The bed capacity factor was affected mainly by the purge to feed ratio, the heat transfer coefficient and the feed concentration; two factor and higher order interaction effects were insignificant. Overall, this study demonstrated the utility of fractional factorial design for revealing factor interactions and their effects on the performance of a PSA-SVR process.
Bandosz, Teresa J., Lin, Chuan, and Ritter, James A.
Journal of Colloid and Interface Science; February 1998, Vol. 198 Issue: 2 p347-353, 7p
The structural and chemical properties of silica–alumina xerogels obtained using the sol–gel process were investigated. As methods of characterization, sorption of nitrogen, thermal analysis, and potentiometric titration were chosen. The results obtained showed a high contribution of micropores in the structure of the silica–alumina xerogels, which decreased with increasing content of aluminum. An investigation of the acidic–basic properties of the surfaces showed the presence of centers which have not been detected in commercial catalysts obtained in a more traditional way. These centers may be the result of synergy between our components and/or different coordinations between the cations.
Adsorption; March 1998, Vol. 4 Issue: 2 p159-172, 14p
Heat effects in the pressure swing adsorption (PSA)-n-butane vapor recovery process were investigated at the periodic state by computer simulation. The PSA process utilized a two-bed, four-step, vacuum swing cycle and BAX activated carbon as the adsorbent. The heat effects were manifested by varying the heat transfer coefficient (h) from isothermal to adiabatic, while simultaneously varying the adsorbed phase heat capacity (Cpa) from zero to that of the saturated liquid. In terms of the bed capacity factor (BCF), isothermal operation always resulted in the best performance, whereas adiabatic operation was not the worst; independent of Cpa, the worst performance occurred at an intermediate h. Cpaalso had a significant effect on the BCF, where a larger Cpa(i.e., a larger heat sink) always decreased the BCF and thus improved the process performance. A factorial analysis showed that the effect of Cpaon the BCF became even more pronounced as the cycle time increased. h and Cpahad essentially no effect on the solvent vapor enrichment under the conditions investigated. Overall, this study demonstrated that the effects of h and Cpaare uniquely coupled; thus knowing their magnitudes is paramount to obtaining accurate predictions from a PSA-solvent vapor recovery model.
Ebner, Armin D., Ritter, James A., and Popov, Branko N.
Journal of Colloid and Interface Science; July 1998, Vol. 203 Issue: 2 p488-492, 5p
A potentiostatically enhanced complexation model for open systems has been developed that describes the adsorption of heavy metal ions under different pH conditions and forcing electrostatic potentials. This model is based on the so-called complex formation models, and it assumes adsorption of only free ionic species, existence of one type of adsorption site, and limited adsorption site availability. The results show that potentially enhanced adsorption processes can be used for the adsorption of heavy metal ions at pH conditions where adsorption is weak, yet potentials are small enough to avoid reduction and plating, and also electrolysis of water. Moreover desorption can be achieved by turning off the applied potential or by reversing it, thereby avoiding the use of additional chemicals, e.g., a strong acid, to achieve regeneration.
Adsorption; September 1998, Vol. 4 Issue: 3-4 p337-344, 8p
Three different finite-difference routines were compared for solving the nonlinear, coupled, partial differential and algebraic equations that describe pressure swing adsorption processes. A successive substitution method (SS), a block LU decomposition procedure (BLUD), and the method of lines approach with adaptive time stepping (DASSL) were used to simulate and compare the computation times required to reach the periodic state for two different PSA systems: PSA-air drying and PSA-solvent vapor recovery. For both systems, the results showed that DASSL was nearly twice as fast as BLUD, whereas SS was nearly an order of magnitude slower than BLUD. DASSL and BLUD were also very robust and accurate, as nearly identical bed profiles were obtained from both methods under both transient and periodic state conditions.
Ferro, Albert, Queen, Lindsay R, Priest, Rachel M, Xu, Biao, Ritter, James M, Poston, Lucilla, and Ward, Jeremy P T
British Journal of Pharmacology; April 1999, Vol. 126 Issue: 8 p1872-1880, 9p
Some animal studies suggest that β‐adrenoceptor‐mediated vasorelaxation is in part mediated through nitric oxide (NO) release. Furthermore, in humans, we have recently shown that forearm blood flow is increased by infusion of β2‐adrenergic agonists into the brachial artery, and the nitric oxide synthase (NOS) inhibitor NG‐monomethyl‐L‐arginine (L‐NMMA) inhibits this response.The purpose of the present study was to determine whether stimulation of human umbilical vein endothelial β‐adrenoceptors causes vasorelaxation and nitric oxide generation, and whether this might be mediated by cyclic adenosine‐3′,5′‐monophosphate (cyclic AMP).Vasorelaxant responses were determined in umbilical vein rings to the nonselective β‐adrenergic agonist isoprenaline and to the cyclic AMP analogue dibutyryl cyclic AMP, following precontraction with prostaglandin F2α.NOS activity was measured in cultured human umbilical vein endothelial cells (HUVEC) by the conversion of [3H]‐L‐arginine to [3H]‐L‐citrulline, and adenylyl cyclase activity by the conversion of [α‐32P]‐ATP to [32P]‐cyclic AMP.Isoprenaline relaxed umbilical vein rings, and this vasorelaxation was abolished by β2‐ (but not β1‐) adrenergic blockage, and by endothelium removal or 1 mML‐NMMA. In addition, vasorelaxant responses to dibutyryl cyclic AMP were inhibited by 1 mML‐NMMA, with a reduction in Emaxfrom 90.0±9.3% to 50.5±9.9% (P<0.05).Isoprenaline 1 μMincreased NOS activity in HUVEC (34.0±5.9% above basal, P<0.001). Furthermore, isoprenaline increased adenylyl cyclase activity in a concentration‐dependent manner; this response was inhibited by β2(but not β1‐) adrenergic blockade. Forskolin 1 μMand dibutyryl cyclic AMP 1 mMeach increased NOS activity in HUVEC, to a degree similar to isoprenaline 1 μM. The increase in L‐arginine to L‐citrulline conversion observed with each agent was abolished by co‐incubation with NOS inhibitors.These results indicate that endothelial β2‐adrenergic stimulation and cyclic AMP elevation activate the L‐arginine/NO system, and give rise to vasorelaxation, in human umbilical vein.
Shen, Jiacheng, Ebner, Armin D., and Ritter, James A.
Journal of Colloid and Interface Science; June 1999, Vol. 214 Issue: 2 p333-343, 11p
A series of magnetic adsorbents (silica–magnetite composite oxides) containing 0, 20, 40, 60, 80, and 100 wt% magnetite were prepared via the sol–gel process, where 0.1-μm-radius magnetite particles were encased in a mesoporous silica matrix. The experimental pHpzcfor each of the pure oxides and their mixtures was tested against two applicable models in the literature; neither model explained the observed behavior. One of the models did not account for the heterogeneous nature of the two distinct surfaces present in these silica–magnetite composite oxides, and neither model accounted for the possibility of the silica locally altering the behavior of the magnetite functional groups. The behavior was corroborated independently, however, based on an analysis carried out with the measured surface acidity constants. This work also showed that the total exchange capacity of each of the mixed oxides was dominated by the high surface area of the silica, and that the electrolyte species, Na+or NO−3, did not completely complex with the surface of the oxides, as expected for weak binding ions. The chemical component of the free energy of adsorption of protons for the acid–base reactions also dominated the solvation and coulombic contributions for each of the pure oxides and their mixtures.
Adsorption; October 1999, Vol. 5 Issue: 4 p373-380, 8p
A new approximate model for nonlinear adsorption (Langmuir model) and concentration dependent surface diffusion (HIO model) in a single particle was derived, based on a parabolic concentration profile assumption for the summation of the gas and adsorbed phases. The surface diffusivity was approximated with the adsorbed phase concentration evaluated at the surface of the particle, as the average of the adsorbed phase concentration, and as the average of the first two approximations. Overall, the approximate model based on the average of the first two approximations compared the best with the exact solution for a wide variety of systems and conditions.
AIChE Journal; March 2000, Vol. 46 Issue: 3 p540-551, 12p
A pressure swing adsorption (PSA)–solvent vapor recovery (SVR) process simulator was used to investigate new PSA cycle configurations designed for higher solvent vapor enrichment. These cycles were modifications of the four‐step Skarstrom cycle used commercially for PSA‐SVR and include the addition of a cocurrent blowdown step, and combinations of cocurrent blowdown and continuous/batch reflux steps. The recovery of gasoline vapor from tank filling operations was simulated with n‐butane, n‐heptane, and nitrogen as representatives of the light and heavy components in gasoline vapor, and carrier gas, respectively. Adding a cocurrent blowdown step increased the solvent vapor enrichment, depending mainly on the step ending pressure, not the step time. Both the continuous and batch reflux steps also increased the solvent vapor enrichment, but at the expense of an increased bed capacity factor. For similar increases in the solvent vapor enrichment, batch reflux led to significantly smaller bed capacity factors compared to continuous reflux and was thus superior for PSA‐SVR. Overall PSA‐SVR process performance improved markedly by adding cocurrent blowdown and batch reflux steps compared to the conventional four‐step cycle.
Ebner, Armin D., Ritter, James A., and Ploehn, Harry J.
Journal of Colloid and Interface Science; May 2000, Vol. 225 Issue: 1 p39-46, 8p
The feasibility of a high-gradient magnetic separation process, utilizing magnetite as the energizable element in lieu of stainless steel wool, is evaluated by means of an equilibrium, two-particle, magnetic hetero-flocculation model. The model calculates the net force, defined as the sum of the magnetic, electrostatic, and van der Waals forces, exerted on a paramagnetic nanoparticle that is in the proximity of a fixed magnetite particle. Since the nanoparticle–magnetite system is assumed to be in direct contact with the moving fluid, the influence of the hydrodynamic force on the magnetic attractive force between the two particles is also explored. This model clearly reveals the ranges and conditions over which each of these various forces contributes to the net force relative to Brownian (thermal) motion. The model also reveals the feasibility of using magnetite particles instead of stainless steel as the energizable element for high-gradient magnetic separation. Important variables investigated include the size and surface charge of the particles, the magnetic field, the flow velocity, the electrolyte concentration, and the magnetic susceptibility of the nanoparticle.
AIChE Journal; February 2001, Vol. 47 Issue: 2 p303-313, 11p
A new correlation between the capture cross‐section parameter (λ) and principal variables involved in high‐gradient magnetic separation (HGMS) was developed. It is based on the use of an alternative dimensionless parameter, β, in lieu of the traditional Um/UO, which facilitated the estimation of λ over a much broader spectrum of variables. This new correlation is far more versatile than correlations and approximate models in the current literature, which are only applicable to specific situations in HGMS systems. Overall, this new correlation should be extremely useful in the design, development, optimization and modeling of HGMS systems.