Testing long-term

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

The accelerated aging test should take into account the associated materials as well as the atmosphere that will be encountered in actual use, since they are also controlling factors. It is helpful to include materials of known performance against which to rate the new material, since this allows a check of controlling factors and further validates the extrapolation. Thus, existing data from long term tests may be of considerable value. 

The findings from two long term test runs in the SASOL plant relevant to catalyst life under design conditions in a commercial methane synthesis plant have already been published (3). This paper reports further test results from both demonstration units concerning the effect of certain reaction parameters which are the basis for flexibility and operability of the Lurgi methanation scheme. 

The SASOL plant was operated with a surplus of C02 during a long term test of 4000 hrs. Of the C02 in the synthesis gas, 33.4% was metha-nated while the remaining 66.6% left the reaction system unconverted. Product gas from final methanation yielded specification grade SNG containing residual hydrogen of 0.7 vol % and residual CO of less than 0.1 vol %. The heating value was 973 Btu/standard cubic foot (scf) after C02 removal to 0.5 vol % (calc.). 

The Schwechat plant was operated with a stoichiometric synthesis gas in a long term test of 5000 hrs. The residual hydrogen content could be decreased to 2.2 vol % which resulted in a heating value of 950 Btu/scf when about 1 vol % nitrogen was present in the synthesis gas. 

The long term tests in the SASOL plant as well as in the Schwechat plant were run with outlet temperatures of 450°C, but both plants were also operated with higher loads that caused reactor outlet temperatures of 470°C or even higher. In comparison with the test run at 450°C, only a slight increase in deactivation rate was detectable which demonstrates the thermostability of the catalyst. From the aspect of thermostability, outlet temperatures of 450°-470°C are acceptable. Further considerations including the possibility of overload operation, the SNG specification to be achieved in final methanation, end-of-run conditions, and cost of reactor material will affect the selection of optimum outlet temperature. 

Monomethyltirf. Acute toxicity studies were identified for monomethyltin for algae, invertebrates, and fish. Chronic NOECs were available for algae and invertebrates. A chronic NOEC of 0.007 mg/1 for monomethyltin chloride in Scenedesmus subspica-tus was the lowest reported result. Since there were no long-term test results available for fish, it was necessary to apply an uncertainty factor of 50 to the critical study. 

Monobutyltirr. Four acute toxicity studies were identified for monobutyltin chloride. The critical study was an acute EC50, based on immobilization, for Daphnia magna at a concentration of 25 mg/1. All four tests were acute, and, in the absence of long-term tests, it was decided to apply an uncertainty factor of 1000. 

Dibutyltin. A larger data set exists for dibutyltin, including both acute and long-term test results. The lowest concentration identified was a chronic NOEC of 0.015 mg/1 for Daphnia magna exposure to dibutyltin chloride. Long-term values were available across three trophic levels, and, therefore, an uncertainty factor of 10 was considered appropriate. 

Organism Acute test Long-term test... 

Guidance on whether the values from the acute or the long-term test should be used is given in the EU-Guideline 8075AT/97. Normally, the values of the long-term test are relevant for residue analytical purposes. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

FIGURE 3.20 Cell voltage drop calculated from 1-V curves at 0.3 Acm 2 before and after tested for 1000 h. Operating temperature temperature during the long-term test characterization temperature temperature at which the I-Vcurves before and after the long-term tests were measured. (From Mai, A. et al., Solid State Ionics, 177 1965-1968, 2006. With permission.)... 

Cell membrane failures not predicted by tests of cell membrane life and lack of long-term testing on feeds containing Cl and F. 

Stability. Producers must state the length of the reference material s useable life, since they can be sensitive to light, humidity, microbial activity, temperature, time, etc. Long-term testing is required to validate the stability of a material under a variety of storage and transport conditions. 

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