Testing anti-foulants

Due to the cost, as well as time, in testing anti-foulants in plant applications, some laboratory test methods have evolved to determine the fouling potential of process streams and to evaluate the effects of alternative additives and treatment levels. These laboratory tests are always of relatively short duration - from several minutes to several days - and require intensification of the causative factors to increase fouling rates, and thus provide measurable changes in the system parameters during the test times, which are short relative to the weeks or months of acmal field fouling problems. 

The new Biocidal Products Directive is examined, which closes the gap in European legislation for products such as disinfectants and anti-foulants where there has not, until now, been a Europe-wide standard. The basic goals of the directive are to simplify and harmonise Europe s regulatory framework for biocides, and to remove trade barriers within the EU, and to improve the protection of both human health and the environment. The new directive must be enforced as national legislation in all member countries by 14 May 2000. Details are given of products defined within the directive, and ways in which the directive will be put into practice in the UK are discussed. The implications of the legislation on the industry in terms of time and money spent on testing for each active substance involved are examined. 

Oil-soluble dispersants are widely used to alleviate both organic and inorganic fouling problems. The object is not to prevent the initial formation of coke nuclei and other insoluble particles in the stream, but to reduce their tendencies to agglomerate into larger precipitates that can settle out of the process stream and deposit on and in various places in the equipment. A test for effectiveness of materials as anti-foulants, based on their ability to disperse carbon black in hydrocarbons can be established. 

Commercial materials recommended for use as anti-foulants in processing industries contain combinations of dispersants, anti-oxidants, metal deactivators, and/or corrosion inhibitors. The choice of the best material for a given application should be determined by effectiveness and cost. Screening tests to differentiate between alternative materials will be described below. Because of the wide variety of streams requiring treatment, many commercial anti-foulants have been developed for different applications. The situation is similar to that of corrosion inhibition and no universal remedy is available. 

Accordingly, the screening test should be used only to obtain preliminary information on materials that appear promising on a cost-performance basis. Promising materials should then be evaluated in the field for optimization of the anti-foulant treatment. 

Better correlation between test results and refinery experience with anti-foulants is claimed with data from the Jet-Fuel Thermal Oxidation Tester (JFTOT) developed by a San Antonio, Texas firm. The device operates on the same principles as the Erdco coker developed in 1965 by Amoco, and according to ASTM D-1660. One of the main advantages of the JFTOT tester is that it uses only one quart of fuel. 

Other methods of rapid evaluation in laboratory and/or field are proposed from time to time because of the need for a guide and an accurate screening method for anti-foulants. These methods should be considered as to their ability to measure true fouling rates or fouling potentials, or some other physical or chemical property purported to be related to the desired property. When extrapolating the test conditions to the field conditions, it should be remembered that the dangers in such extrapolations increase as the conditions between actual and test conditions diverge. 

An additional concept in the evaluation of anti-foulants by laboratory screening devices has been pointed out by Nathan and Dulaney. This concept considers the wide fluctuation in reproducibility of test data obtained at intermediate efficiency values of additive applications. At low efficiencies, such as those obtained at low treatment levels, or at high efficiencies, such as those obtained at high treatment levels, rephcate tests have good reproducibility. 

However, poor reproducibility at intermediate concentrations and efficiencies limits the ability to differentiate between the cost-effectiveness of alternative additives. Similar difficulties have been reported with respect to the evaluation of corrosion inhibitors in refinery processes and other applications and in testing the effect of surfactants employed as corrosion inhibitors and/or anti-foulants on the water tolerance of jet fuels (WSIM test). The limitations of screening tests emphasize the inadvisability of undue reliance on them and the need for following such tests with careful plant studies to obtain reliable technical and economic data on anti-foulant applications. 

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