Resistance modeling

Fig. 9. Uptake curves for N2 in two samples of carbon molecular sieve showing conformity with diffusion model (eq. 24) for sample 1 (A), and with surface resistance model (eq. 26) for example 2 (0)j LDF = linear driving force. Data from ref. 18.

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

Although the status of many 3D codes makes it possible to carry out detailed scenario calculations, further work is needed. This is particularly so for 1) development and verification of the porosity/distributed resistance model for explosion propagation in high density obstacle fields 2) improvement of the turbulent combustion model, and 3) development of a model for deflagration to detonation transition. More data are needed to enable verification of the model in high density geometries. This is particularly needed for onshore process plant geometries. 

Gl. Gadina, M., Bertini, R., Mengozzi, M., Zandalasini, M., and Ghezzi, P., Protective effect of chlorpromazine on endotoxin toxicity and TNF production in glucocorticoid-sensitive and glucocorticoid-resistant models of endotoxic shock. J. Exp. Med. 173, 1305-1310 (1991). 

K Dackson, JA Stone, KJ Palin, WN Charman. Evaluation of the mass balance assumption with respect to the two-resistance model of intestinal absorption by using in situ single-pass intestinal perfusion of theophylline in rats. J Pharm Sci 81 321 — 325, 1992. 

Commonly Employed Experimental Disease Resistance Models. ... 

Selgrade, M.K., Daniels, M.J. and Dean, J.H., Correlation between chemical suppression of natural killer cell activity in mice and susceptibility to cytomegalovirus rationale for applying murine cytomegalovirus as a host resistance model and for interpreting immunotoxicity testing in terms of risk of disease, J. Toxicol. Environ. Flealth, 37, 123, 1992. 

Wilson, S.D. et al., Correlation of suppressed natural killer cell activity with altered host resistance models in B6C3F1 mice, Toxicol. Appl. Pharmacol., 177, 208, 2001... 

Further evidence to support the notion of a role for immunotoxic environmental contaminants in the 1988 outbreak came from two studies of laboratory rats carried out in tandem with the seal studies. PVG rats were fed the same two batches of herring used in the seal study, with a similar pattern of effects observed in the seals [63,64], However, there were additional indications of immunotoxicity that could not be evaluated in seals for ethical or technical reasons, including increased virus titers in a rat cytomegalovirus (RCMV) host resistance model, and reduced thymus cellularity in the rats fed Baltic Sea herring. A positive control group of rats in one of the studies was exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin, thereafter exhibiting an amplified pattern of the effects that had been observed in the Baltic group. The collective results from the captive seal studies and the laboratory animal studies were seen to implicate an AhR-mediated immunotoxicity, in which dioxin-like PCBs played a dominant role [64, 65],... 

A glucocorticoid-resistance model has been proposed to provide an explanation for how stress might influence diseases in which excessive inflammation is observed (e.g., allergies, autoimmune diseases, rheumatoid arthritis, and cardiovascular disease). In these cases, chronic stress diminishes the immune system s sensitivity to glucocorticoids that normally terminate the inflammatory response. For example, in a study of a group of 50 parents caring for a child undergoing treatment for pediatric cancer, whole blood of parents of cancer patients exhibited a lesser dexamethasone-dependent suppression of IL-6 production in vitro compared to parents of medically healthy children.94... 

Miller, G.E., Cohen, S., and Ritchey, A.K., Chronic psychological stress and the regulation of pro-inflammatory cytokines A glucocorticoid-resistance model, Health Psychol. 21, 531,2002. 

Schneider and Klein(5) have pointed out that the early stages of cross-flow microfiltration often follow such a pattern although the growth of the cake is limited by the cross-flow of the process liquid. There are a number of ways of accounting for the control of cake growth. A useful method is to rewrite the resistance model to allow for the dynamics of polarisation in the film layer as discussed by Fane 6. Equation 8.3 is then written as ... 

The Brunauer type I is the characteristic shape that arises from uniform micro-porous sorbents such as zeolite molecular sieves. It must be admitted though that there are indeed some deviations from pure Brunauer type I behavior in zeoHtes. From this we derive the concept of the favorable versus an unfavorable isotherm for adsorption. The computation of mass transfer coefficients can be accompHshed through the construction of a multiple mass transfer resistance model. Resistance modehng utilizes the analogy between electrical current flow and transport of molecular species. In electrical current flow voltage difference represents the driving force and current flow represents the transport In mass transport the driving force is typically concentration difference and the flux of the species into the sorbent is resisted by various mechanisms. 

The moments of the solutions thus obtained are then related to the individual mass transport diffusion mechanisms, dispersion mechanisms and the capacity of the adsorbent. The equation that results from this process is the model widely referred to as the three resistance model. It is written specifically for a gas phase driving force. Haynes and Sarma included axial diffusion, hence they were solving the equivalent of Eq. (9.10) with an axial diffusion term. Their results cast in the consistent nomenclature of Ruthven first for the actual coefficient responsible for sorption kinetics as ... 

Currently there are few methods for specific investigation of immunotoxic effects, which are regarded as sufficiently validated for routine use (EC 2003). The plaque forming assay or the equivalent using the ELISA method (Enzyme-linked Immunosorbent Assay) are recommended to identify altered T-cell-dependent humoral responses. Of particular value for hazard assessment are the so-called host resistance models, in which the clinical relevance of immunotoxicity can be evaluated. Other methods may also be of value to provide information on the mode of immunotoxic action, e.g., mitogen stimulation tests and leucocyte phenotyping. However, further work is needed on standardization and validation of these test methods. 

Eicher C, Dewerth A, Kirchner B et al (2011) Development of a drug resistance model for hepatoblastoma. Int J Oncol 38 447 54... 

The physical and chemical processes occurring in a gas-liquid system are often treated in terms of a resistance model described in Box 5.2. As discussed there, the net uptake of gas (yIK.t) can be treated under some conditions in terms of conductances, T, normalized to the rate of gas-surface collisions. Individual conductances are associated with gas-phase diffusion to the surface (Tg), mass accommodation across the interface (a), solubility (rsol), and finally, reaction in the bulk aqueous phase (Tlxn). This leads to Eq. (QQ) ... 

FIGURE 5.16 Schematic of resistance model for diffusion, uptake, and reaction of gases with liquids. Tg represents the transport of gases to the surface of the particle, a the mass accommodation coefficient for transfer across the interface, rso, the solubilization and diffusion in the liquid phase, riM the bulk liquid-phase reaction, and rinlcrl.ll c the reaction of the gas at the interface. 

Hu et al. (1995) interpret this as evidence for an additional reaction channel corresponding to a rapid reaction at the interface itself. Several different approaches have been taken to introduce this additional reaction dimension into the resistance model. For example, Hu et al. (1995) add another resistance term to Fig. 5.16, l/rinturra(.u, shown by the dashed lines in that figure. Their data suggest that l/rintcrraa, is of the form [1 + k"ax /(k"axps), where k" is a measure of the... 

The measured uptake coefficient y is a convolution of the processes which affect the rate of gas uptake. Uptake coefficients have been measured for various PSC surfaces and sulfuric acid solutions using a variety of experimental methods, and have been interpreted in tenns of physico-chemical models to yield reactivities and solubilities of gaseous species. More most cases the total experimental uptake coefficient ym can be represented in tenns of a resistance model, whereby the resistances l/ym and 1/y are associated with reaction and liquid solubility [42] Quantitative values for y will be discussed later in section 2.3.4. 

It Is seen that the curves agree only up to the initial 15 minutes of the study. The two-resistance model thus provides a much better representation of the adsorption rates. Furusawa and Smith (25) have found this to be true for even small particle sizes, ranging from 200 to 900 microns In diameter. 

---