Chemical valving effect

It must be recognized that these channels are not empty but have poly(acrylic acid) (PAA) chains tethered to their inner walls. However, since the relative volume occupied by these chains is significantly lower than the parent PtBA, these channels are rendered substantially porous. A very interesting feature of these porous membranes is the chemical valving effect , wherein the relative permeability of the membranes was shown to vary by almost two orders of magnitude as a function of pH, with the lowest permeability being witnessed at a pH of 3 [41]. The explana-... 

Figure 16 also shows the effect of a chemomechanical contraction of the PVA-PPA membrane on water permeation when 6.5 V DC was applied in alternate on and off cycles [46]. It can be seen that the chemical valve membrane can increase and decrease the water permeability many times on electrical stimulation. Water permeability increased in proportion to the DC current. This makes it possible to use the membrane as a permeation-selective membrane continuously separating solute mixtures with different molecular sizes. This type of electrically activated chemical valve membrane exhibited long-term stability. 

A modulation of swelling forces in gels by chemical or physical stimuli enables dynamic control of the gel hydration and, thereby, effective diffusion and permeability of solutes can be obtained. Drug delivery systems (DDS), permselective membranes for selective extraction, and chemical valves are examples of stimuli-responsive polymer gel applications. 

Safety Showers. Safety showers and eyewash fountains or hoses should be installed where corrosive or toxic materials are handled. A large-volume, low velocity discharge from directly overhead should effect continuous drenching, ie, a minimum flow of 20 L/min (50 gal /min). Water to outside showers may be heated to a maximum temperature of 27°C by an electric heating cable. The valves for all safety showers should be at the same height and relative position to the shower head, and they should operate in the same way and direction. The shower station should be identified by paint of a bright, contrasting color. In areas where chemicals harmful to the eyes may be encountered, an eyewash fountain or spray should be available in case of splash accidents. 

Opening a manual valve. Manual valves which are normally closed to isolate two or more pieces of equipment or process streams can be inadvertently opened, causing the release of a high-pressure stream or resulting in vacuum conditions. Other effects may include the development of critical flows, flashing of liquids, or the generation of a runaway chemical reaction. 

Figure 12-17B. Effects of clearance volume on performance efficiency of reciprocating compressor cylinder (valve design effect). (Used by permission Livingston, E. H. Chemical Engineering Progress, V. 89, No. 2, 1993. American Institute of Chemical Engineers, Inc. All rights reserved.)...

When spills and releases of hazardous gases or liquids occur, the concentration of the hazardous material in the vicinity of the release is often the greatest concern, since potential health effects on those nearby will be determined by the concentration of the substance at the time of the acute exposure. There are many models of routine continuous discharges (e.g., discharges arising from leaky valves in chemical plants), but these carmot be applied to single episodic events. Research on the ambient behavior of short-term environmental releases and the development of models for concentration profiles in episodic releases are cmcial if we are to plan appropriate safety and abatement measures. 

The HP 6890 series GC gas-samphng valve technology, in conjunction with programmed interval sampling, makes this type of GC-based quahty-control automation a reahty. The GC can control and sense the position of multi-position valves to make it possible to automate sample-stream selection. Each sample, which is downloaded automatically, can then be analysed by a different method. Automated analysis of multiple process streams is especially attractive and cost-effective for complex chemical and petrochemical operations in which raw materials for product streams need to be assessed at regular intervals. 

Post Indicator Valve (PIV) A post-type valve diat provides a visual means of indicating "open" or "shut" position. It is found on the supply main of installed tire protection systems. Potentiation The effect of a chemical which does not itself have an adverse effect but which enhances the toxicity of another chemical. 

Studies of the formation, chemical composition, and properties of deposits have shown that they consist of partially oxidized organic material, including more or less nitrogen, sulfur, and phosphorus. Compounds of iron, silicon, calcium, and other metals are present in small quantity, together with substantial amounts of lead oxides, sulfates, and halides from combustion of the antiknock fluid. The effects of these deposits are both physical and chemical in nature they may physically interfere with lubrication, heat transfer, gas flow, operation of valves and spark plugs chemically, they may bring about corrosion and oxidation. 

While no chemical application has been necessary to demonstrate the impressive performance currently achievable at medium to high pressures, chemicals may help alleviate some of the detrimental effects of pumps and valves on low-pressure systems. 

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