Selection of Inhibitors

A system must be carefully examined before a program of corrosion inhibition can be planned effectively. The examination must include a survey of any adverse effects an inhibitor may have on the process. The most likely adverse effects are foaming, the formation of an emulsion, and loosening of scale. 

The test for foaming should be performed. Most corrosion inhibitors cease to function at a pH below 3. Normal fihn-forming organic inhibitors of the water-soluble type have an upper temperature limit of 140 °C (300 °F), while oil-soluble inhibitors have a limit of 196°C(385°F), when cooled the inhibitor is active again, so it is not destroyed if the temperature is not too high. 

Watch for inhibitors that polymerize at higher temperatures. Also be aware that the evaporation of solvent carrying the inhibitor can leave the inhibitor as a gunk in the well. In treating dry gas wells, this can be minimized by using a solvent that has a considerably higher boiling point than the condensate produced by the well. 

For most applications it is desirable to use an inhibitor that is insoluble, but dispersible, at a 10 to 25% concentration in the hydrocarbon diluent, which may be distillate, aromatic solvent, crude oil etc. The inhibitor will film from the liquid onto the metal surface. Care must be taken that the inhibitor is not tied up as the inner phase of a water emulsion. The inhibitor has difficulty breaking from a dispersion of this type. 

Remember that a corrosion inhibitor program is basically a coating treatment. The amount of inhibitor required depends upon the amount of metal to be protected, not upon the volume of fluid produced by the well. The amount of fluid produced determines the frequency of treatment, although it is probable that no well should go longer than three months between corrosion treatments. 

In a typical refrigeration unit, ethylene glycol is injected into the system to prevent hydrate formation and to absorb the liquid water that forms. For acid gas applications, some have chosen to inject methanol. The acid gas components are slightly less soluble in methanol than in glycol. 

Zabcik and Frazier (1984) describe in some detail two C02 dehydration units. The details of these two dehydration units are given in table 7.4. 

The glycol circulation rate for Case 1 is equivalent to 9.7 gal of TEG per lb of water removed and in Case 2 it is 5.9 USgal/lb. These are well in excess of the typical 2 to 4 USgal/lb typically recommended for a natural gas dehydration unit. Zabcik and Frazier (1984) do not offer an explanation as to why the glycol circulation rate is so high. 

The water contents given in table 7.4 come from the analysis from the literature data by Zabcik and Frazier (1984). The values predicted by AQUAlibrium are 113 lb/MMSCF for Case 1 and 111 for Case 2, which agree well with the data in table 7.4 differences of only 2.7% and 5.7%. 

In Case 1, the TEG is preheated before the flash tank, and that is why the temperature seems a little high. In Case 2, the temperature increase is due merely to the absorption of the gaseous components into the glycol. 

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A comprehensive list of P-gp modulators or inhibitors, classified according to their chemical structures, has been published recently [87]. This shows that the structures of inhibitors are almost as heterogeneous as those of the substrates. A small but representative selection of inhibitors is shown in Fig. 20.12 and Table 20.1. In an attempt to clarify the different mechanisms of P-gp modulation or inhibition, the H-bonding concept discussed above is applied. To this end, the modulators or inhibitors in Table 20.1 were ordered according to their H-bond acceptor potential and divided in three groups comprising compounds with (i) a low EUh (<2 i.e., not transported) (ii) an intermediate EUh (— 3—6) and (iii) a high H-bond acceptor potential ( EUh > 10 i.e., transported slowly). 

Preparation of Krebs-2 translation extracts Krebs-2 extracts are an ideal system to screen for compounds that inhibit translation because they faithfully recapitulate the cap dependency and the cap-poly(A) synergism associated with eukaryotic mRNA translation (Svitkin and Sonenberg, 2004), unlike standard rabbit reticulocyte lysates (RRL) (Borman et al., 2000). Furthermore, the translation of many types of IRESes is supported in Krebs-2 extracts. The use of commercially available translation competent extracts prepared from RRL, wheat germ, and E. coli is extremely useful in assessing selectivity of inhibitors identified in primary screens. 

The selection of inhibitors considered in this section is complete in terms of the scissile bond surrogate functionalities that have been investigated, and representative in terms of residue substitutions, but it is not exhaustive... 

Selectivity of inhibitors Presence or absence of the target in eukaryotes can be assessed through bioinformatics however, certain prokaryotic enzymes with clear orthologs in eukaryotes are the targets of successful antibiotics trimethoprim [102] fluoroquinolones [59],... 

Several assays have been described by investigators in the field and they are used for different purposes. Assays involving purified Cox-1 and Cox-2 are used for convenience and speed or to obtain an estimate of the relative intrinsic potency and selectivity of inhibitors. Selectivity based on intrinsic affinity for the catalytic site permits the development of structure-potency relationships that one can interpret in terms of structural features of the inhibitors and the catalytic sites of the target enzymes (Bayly et al., 1999). Cell-based assays are used to determine whether the inhibitors can enter cells and inhibit Cox, as well as to determine inhibitor potency in a more biologically relevant environment than provided by purified enzyme assays. At Merck, CHO cell lines that overexpressed human Cox-1 or Cox-2 were developed and used for screening cell-based assays (Kargman et al., 1996). 

Selection of Inhibitor and Substrate Concentration, and Solvent Effects... 

The selectivity of inhibitors is usually achieved by fine-tuning of substituents around an inhibitory element (Figure 3.6.1). For efficiency, this fine-tuning requires a synthetic strategy that is robust, tolerates diverse functional groups, enables parallel synthesis on a small scale, automation, facilitated work-up, and, finally, isolation of the products. 

Novel theoretical method of selection of inhibitors (Corrosion, 50, 432,... 

In contrast to permeability tests, compounds are added not only to apical compartment (to determine flux A-B) but in an additional experiment to baso-lateral compartment (efflux B-A). As assay time 2h is proposed. At the same time, an inhibitor is added and results of permeability and efflux assays compared to results obtained without inhibitor. Additionally, a selection of inhibitors covering different efflux transporters is used. After a preincubation period used in many studies (15 minutes for example) test compounds are added and flux/efflux studies performed. 

Nasr G, Petit E, Supuran CT, Winum JY, Barboiu M (2009) Carbonic anhydrase II-induced selection of inhibitors from a dynamic combinatorial library of Schiff s bases. Bioorg Med Chem Lett 19 6014-6017... 

Fig. 7.3-5 Inhibitor discovery by ABPP. The potency and selectivity of inhibitors can be profiled in parallel by performing competitive ABPP reactions in proteomes. Complex proteomes are treated with a reversible inhibitor library and an activity-based probe, and subsequently...

Important results are achieved in this field when studying the oxidative transformations of organic substances inhibited by phenols [4-16], It was demonstrated with these examples that the reaction mechanisms of inhibited oxidation are nonlinear dynamical systems. In this situation they failed to obtain analytical solutions in the general case that would allow to select the key parameters of the reaction. Recommendations for the selection of inhibitor s structure and the operating conditions of materials on the basis of these parameters would be the next step. 

The selection of inhibitors may often depend on the methods available for discharge disposal. The most important regulations are the following ... 

Oils for Asphalt Precipitation Theory, Practice, and the Selection of Inhibitors," SPE PF10, 55-61. 

Ethylene glycol is moderately toxic. On ingestion, ethylene glycol is oxidized to glycolic acid which is, in turn, oxidized to oxalic acid which is toxic. It is critically important that additives do not significantly increase the toxicity or make the coolant hazardous to the environment. This to a great extent limits the selection of inhibitors. 

Sastri, V. S., Roberge, P. R., and Perumareddi, J. R., Selection of Inhibitors Based on Theoretical Considerations, in Roberge, P. R., Szklarz, K., and Sastri, S. (eds.). Material Performance Sulphur and Energy, Montreal, Canada, Canadian Institute of Mining, Metallurgy and Petroleum, 1992, pp. 45-54. 

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