Recovery rate, compression

A general rule is that the compressive strength of plastics is greater than its tensile strength. However, this is not generally true for reinforced TSs. The compression testing of foamed plastics provides the designer with the useful recovery rate (see Fig. 3-19). 

The TaN was also synthesized by shock compression (62). Shock compression recovery experiments with a porous sample of tantalum nitride with a hexagonal structure were performed in the impact velocity range up to 1.5 km s . The recovery rate of the Bl-type phase increased with increasing porosity. Almost 100% recovery was achieved for a powder of 70% porosity on impact by a 2-mm-thick tungsten plate with velocities above 1.4 km s . The shock-synthesized Bl-type tantalum nitride has good stoichiometry compared with combustion-synthesized ones. 

Membrane systems are also successfully employed to recover VOCs from air streams. The membranes are used in a vapour recovery process, which combines compression-condensation and membrane vapour separation. The membrane separation step enhances the recovery possible with compression and compensation alone, allowing the process to operate at much higher recovery rates, or allowing the temperature and pressure conditions to be relaxed. This is a developing technology that shows great promise for resource recovery and pollution prevention. 

Keywords compressibility, primary-, secondary- and enhanced oil-recovery, drive mechanisms (solution gas-, gas cap-, water-drive), secondary gas cap, first production date, build-up period, plateau period, production decline, water cut, Darcy s law, recovery factor, sweep efficiency, by-passing of oil, residual oil, relative permeability, production forecasts, offtake rate, coning, cusping, horizontal wells, reservoir simulation, material balance, rate dependent processes, pre-drilling. 

Gas reservoirs are produced by expansion of the gas contained in the reservoir. The high compressibility of the gas relative to the water in the reservoir (either connate water or underlying aquifer) make the gas expansion the dominant drive mechanism. Relative to oil reservoirs, the material balance calculation for gas reservoirs is rather simple. A major challenge in gas field development is to ensure a long sustainable plateau (typically 10 years) to attain a good sales price for the gas the customer usually requires a reliable supply of gas at an agreed rate over many years. The recovery factor for gas reservoirs depends upon how low the abandonment pressure can be reduced, which is why compression facilities are often provided on surface. Typical recovery factors are In the range 50 to 80 percent. 

The flow coefficient Cv is determined by calibration with water, and it is not entirely satisfactory for predicting the flow rate of compressible fluids under choked flow conditions. This has to do with the fact that different valves exhibit different pressure recovery characteristics with gases and hence will choke at different pressure ratios, which does not apply to liquids. For this reason, another flow coefficient, Cg, is often used for gases. Cg is determined by calibration with air under critical flow conditions (Fisher Controls, 1977). The corresponding flow equation for gas flow is... 

When Zotefoam HDPE materials of density 98 kg m" were subjected to a single major compressive impact (419), after recovery at 50 °C for 1 hour, the performance, defined as the energy density absorbed before the compressive stress reached 2.5 MPa was back to 75% of the initial value. Further severe impacts caused a further deterioration of the performance of the recovered foam. Peak compressive strains of 80 to 90% caused some permanent buckling of the cell walls of HDPE foams. The recovery is much slower than the 0.1 second impact time, so is not a conventional linear viscoelastic response. It must be driven by the compressed air in internal cells in the gas, with some contribution from viscoelasticity of the polymer. Recovery of dimensions had slowed to a very low rate after 10 seconds at 20 °C or after 10 seconds at 50 °C. 

More modem versions of the Defo test have vacuum preparation of the test piece and computerised control but although they measure both the viscous and elastic components, it is still a compression test at low shear rate. Isayev et al27 described an instrument and method to discriminate between materials by measuring the elastic recovery at very short times. 

In the past, parallel plate compression plastimeters have been quite widely used for measuring rate of cure and methods have been standardised. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity or recovery or some combination of these, is then plotted against time of heating to give a scorch curve. An even more time consuming procedure was to measure tensile properties as a function of cure time. 

The test piece is compressed at 23 °C and then held at the test temperature for a set time, commonly 24 h. For elevated temperatures the test piece is either immediately removed and allowed to recover at 23 °C for 30 min before measurement or allowed to cool whilst still compressed and measured after a further 30 min recovery. The first procedure has been standard for many years but is not a particularly logical sequence as it allows recovery at an undefined cooling rate, which must differ between the two sizes of test piece. A third alternative, not included, is to allow recovery at the test temperature. Recovery is speeded up at an elevated temperature and slowed down if cooling takes place under compression. 

The rate at which a rubber recovers after compression can be an important consideration and Isayev et al36 devised an apparatus to measure recovery at very short times. For quality control, the time to complete a... 

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