Heat Exchanger World Magazine September 2023 | Page 50

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Fouling Focus

Wt %

120

100

80

60

40

20

0 0

Volatile material ( usually organic )

TGA Response Graph

Ash ( inorganic )

Temperature [ C ]

Coke-Like Material

200 400 600 800 1000

For all the above , standard tests ( such as from the ASTM Standards ) are available and are relatively low cost with a turnaround time of the order of a week . An example of results from analytical testing is shown in Fig . 3 . It shows a roughly 45 % organic component , amd inorganics which are mostly salt ( NaCl ), corrosion products ( FeS ), and silica ( SiO2 ). In the next article ( Part 3 ) we will see how to combine this information with process conditions to understand the fouling mechanism and ways to mitigate it . Two important aspects of the information in Fig . 3 are : ( 1 ) Mass balance - the wt % of all detected elements should ideally add to 100 %. In practice a value of 90 % or greater is satisfactory . And many times , the deficit can be attributed to oxygen , such as here where the silicon must have oxygen associated with it . ( 2 ) The H / C atomic ratio . This value can be used to determine the temperature at which the coke-like material in the deposit was formed – more on this in Part 3 .

Fig . 2 . Wt .% Loss as a Function of Temperature in a TGA Test

from ambient to 800 C , and continuously weighing the sample to measure weight loss . At 800 C air replaces the inert atmosphere so that the remaining sample can combust due to the presence of oxygen . At the end of combustion some solid will remain – it is called the ash . The test is shown conceptually in Fig . 1 , but individual instruments available from suppliers will vary in detail . The test is quick (~ 3 hrs ), inexpensive , and gives plenty of useful information . Most in-house analytical labs have a TGA instrument . In most cases , the material lost in the heat-up to 800 C is organic material which is still volatile . When air is introduced the weight loss due to combustion indicates coke ( or coke-like material , see Part 1 ). Ash is the inorganic portion of the deposit . TGA results give a good quantification of how much of the deposit is organic vs inorganic , and how much of the organic portion is coke . This helps guide further testing and eliminates unnecessary tests . A typical weight vs . time graph for a TGA is shown in Fig . 2 . The vertical axis shows the percentage of the starting weight going from 100 % to the wt % ash . The chart can be interpreted as follows :

• Approximately 12 % of the material is volatile organics ( 88 wt % remaining at 800 C ), possibly polymeric material plus leftover fluids from incomplete sample preparation .

• Approximately 33 % of the material is coke or cokelike ( 55 % remaining after combustion , [ 88-55 ] = 33 ). This makes the total organic portion 45 %.

• Approximately 55 % of the material is ash which is the inorganic component of the deposit .

Step 3 – Elemental analysis Elemental analysis involves three major tests – Carbon , Hydrogen , Nitrogen ( CHN ); Sulfur ; and Metals and other inorgaincs . Which of these tests are necessary should be determined by the TGA results . For example , if the TGA shows 90 % ash , the deposit is 90 % inorganic and there is no need to quantify the carbon and hydrogen . On the other hand , a deposit could be 90 % organic or a mixture , such as in Fig . 2 . In the latter case all tests are required .

Microscopy Microscopy is sometimes critical to understand the mechanism and identify the precursors . Additionally , it helps to confirm the analytical results . Two types of microscopies can be used : ( 1 ) SEM ( Scanning Electron Microscopy ) to understand the association of elements ( Fig . 4 ) or to visually see how layers are formed on a surface , and ( 2 ) Cross-Polarized Light Optical microscopy to understand the nature of coke in the deposit . Details of both are in the Reference .

Second-level testing If the two testing steps above ( plus microscopy ) leave some uncertainty , or you need to identify specific compounds , or the information is not sufficient to determine the whole

Wt %

Fig . 3 . Example of Elemental Analysis Results

50 Heat Exchanger World September 2023 www . heat-exchanger-world . com