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Apr-2012

Accurate quality checks in catalyst regeneration

A method based on density measurement delivers fast and repeatable analysis of chlorination agents used in catalyst regeneration

Atul Dave
Consultant

Viewed : 3919


Article Summary

A blend of perchloroethylene and isopropyl alcohol is used for the chlorination of regenerated catalyst by petroleum refining and other chemical industries. The blend is used to replenish chloride sites during the regeneration of catalysts, which lose their acidic sites as a result of the high-temperature regeneration process. Both of these chemicals are purchased in the purest form (>99 %) and subsequent blending is carried out at the plant’s battery limit. Once blending is done, a sample is sent to the laboratory to check the perchloroethylene concentration. Laboratories generally test such blend samples by high-temperature combustion, coulometric chloride titration and by back calculating the perchloroethylene concentration. This procedure is rather time consuming and has its limitations.

However, the concentration of perchloroethylene or isopropyl alcohol can also be calculated by measuring physical characteristics. Density is one such parameter that has a linear additive property and can also be accurately measured at different temperatures using an automatic density meter. It is assumed that, since perchloroethylene and isopropyl alcohol are used for a vital activity such as the catalyst regeneration process, a refinery usually procures both of the chemicals in the purest possible form (purity >99.5%). The technique mentioned here is based on this fact, as it is derived from the linear relationship between density and component concentration for any binary homogeneous liquid system.

Coulometry, being an instrumental method, requires chemicals and instrument stabilisation and occupies a great deal of time, whereas the method based on density measurement is simple and time saving and also has greater precision, especially because it is very suitable for a binary sample system such as an isopropyl alcohol/perchloroethylene blend. It is also free from chemical interference. However, it is essential that the user correctly follows the method based on density measurement. Any error in measuring the density of a blend sample will directly affect the final test result. Secondly, any change in base density due to the low purity of raw materials would affect the calculation of the percentage of perchloroethylene. The method based on density measurement is an absolute method based on the fundamental principles of chemistry. Hence, reference to any other standard is not required. Analysis time is just five minutes using a digital density meter. Some of the advantages include the high precision of digital density measurement, repeatability to 0.0002 gm/cm2 and the easy conversion of density to concentration using a simple mathematical formula.

In the case of coulometric titration of chloride, a fixed volume of sample is injected into a high-temperature furnace, where combustion occurs in the presence of oxygen. Organic halides are converted to HX, which is then coulometrically titrated. Most coulometers provide results in the parts per million range. Pure perchloroethylene contains 85.6% chloride. Such a high chloride concentration cannot be injected directly into a coulometer. Dilution of the sample is an essential step. For a pure perchloroethylene sample, approximately 1 gm of sample is diluted 10 000 times with isooctane to give approximately 85.6 ppm chloride. Next, 50 ml of the diluted sample is injected into the furnace. The test result observed in parts per million is then converted to a percentage by applying the dilution factor. Since the sample is diluted 10 000 times, for a pure perchloroethylene sample 1 ppm chloride represents 1% chloride.

A dilution error will adversely affect the test result. Due to such heavy dilution, an error of just 1 ppm would lead to an error of 1%. The precision limits calculated during this exercise were found to be +3 ppm @ 85 ppm chloride concentration. For a diluted perchloroethylene/isopropyl alcohol blend sample, this can translate into an error of up to +/-3% in the analysis, which is very high. A stand-by instrument may also be required to avoid any delay in the event of malfunction of the instrument. Analysis time is approx 30 minutes to one hour if the instrument is stable.

However, the method based on density measurement takes just five minutes. Density is an additive property, which means that for any multi-component system the density of the system will be a summation of the portions of measured density of each component represented within the system. For binary systems such as a perchloroethylene/isopropyl alcohol blend, the density of the blend will the summation of the density portion contributed by each component. For binary systems, this offers a great advantage. If we know the density of a blend, we can very precisely calculate the percentage composition of each component. The same principle can be used to calculate the concentration of perchloroethylene and isopropyl alcohol in a blend. The literature gives the density of pure (>99.9%) perchloroethylene as 1.621 gm/cm2 at 20°C and 1.6310 gm/cm2 at 15°C. The equivalent figures for isopropyl alcohol are 0.7846 gm/cm2 at 20°C and 0.7894 gm/cm2 at 15°C.

Density is a linear additive property at a constant temperature. Considering a blend of perchloroethylene/isopropyl alcohol with variable proportions, the following equation can be written (PCE = perchloroethylene and IPA = isopropyl alcohol):

Density of blend (D) = Density of PCE * (X) + Density of IPA * (Y)

X = % of PCE in blend and Y = % of IPA in blend. Now Y = 100-X, so the equation can be rewritten as:

Density of blend (D) = Density of PCE (X) + Density of IPA (100-X)
Is equal to D = 1.6310 (X) + 0.7894 (100-X)
Is equal to D = 1.6310X + 78.94 - 0.7894 X
Is equal to D = 0.8416X + 78.94

So % of PCE = (D - 78.94)/0.8416, where D = Observed density at 15°C.

If we know the density of blend (D), the percentage of perchloroethylene (X) can be calculated. But Y = 100-X. Since the value of X is known, the percentage of IPA (Y) can also be calculated.

By measuring the density of the blend (IPA/PCE), we can easily calculate the percentage composition of each component. In this study, the method based on density measurement was compared with the results obtained by coulometric measurement.


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