»
Fouling Focus
Heat exchanger fouling in practice – understand & mitigate
Part 2 – what ' s in the deposit ?
In this series of articles we will look at how heat exchangers foul , how to understand the root causes of fouling , and how to mitigate the impact of fouling . The material presented is based entirely on the author ’ s experience and analysis of operating situations in the Oil & Gas industry . However , many theories and varied experiences exist across the industry and amongst researchers .
About the author
Himanshu Joshi retired from Shell in 2021 after 34 combined years with ExxonMobil and Shell , during which he specialized in heat exchangers and fouling . He was part of a team that was granted a patent related to fouling deposit analysis at ExxonMobil , and led applied fouling R & D projects at both companies . He has made several presentations about the field aspects of fouling and fouling mitigation , and deployed many mitigation technologies in the field . He can be reached by email at alph . hmj @ gmail . com .
By Himanshu Joshi , Heat Exchanger Specialist
In Part 1 we saw that several precursor materials could be responsible for fouling , and flow and temperature conditions affect the rate of deposition and the conversion of precursors to the ultimate fouling deposit . To understand which fouling mechanism applies and what remedial actions can be taken , one critical bit of information is what ’ s in the deposit - is it dominated by inorganic material or organic , and what that material consists of , including a quantification of the elements / compounds in the deposit . In this article we will see how to accurately quantify the composition of a fouling deposit and in Part 3 how to relate it to the heat exchanger flow and temperature conditions to fully understand the fouling mechanism . The analysis procedure summarized in this article was described in detail in the Reference . It consists of a step of preparing the sample for analysis and conducting tests in a certain sequence to get all the relevant information , but without excessive testing . It is important to note that merely knowing which chemical elements or compounds are present in the deposit is usually not sufficiently useful information . It is critical to quantify each element and determine which , if any , d ominate the deposit ; or whether several types of solids are present in meaningful quantities .
Step 1 - Sample preparation To get an accurate analysis of the deposit , correct sample preparation is the most critical step . There are two aspects to preparing the sample : Removal of trapped fluid and homogenization of the solids . The specific steps mentioned below apply to deposits from petroleum oil and petrochemicals processing heat exchangers ( or other equipment ) but the concepts will apply to any process . All subsequent analyses should be done on the sample prepared as below . Process fluid removal - Deposits contain the process fluid which could be trapped within the deposit or just coating the samples . The deposit could also be contaminated with flushing solutions , water from a steam-out etc . The trapped fluids or flushing solutions are not part of the foulant and need to be removed before the deposit is analyzed . Toluene , methylene chloride or equivalent solvents should be used to fully extract and wash away such residual feeds and wash solutions . For the purpose of determining the root cause and potential mitigation techniques , we just want to know what ’ s in the solid part of the deposit , so including the process fluid and other liquids in the analysis will indicate the wrong composition . To remove the fluids , add one of the above-mentioned solvents to a sufficient quantity of the sample for the needed analyses . Typically , 5-10 grams of the prepped sample is sufficient for the first level of study . If the sample appears to contain significant levels of unwanted fluids , select a larger quantity such that 5-10 grams of solvent-washed foulant can be recovered . Soak the deposit at room temperature for several hours in a sealed container with approximately 10:1 amount of the solvent . Filter the mixture and wash with additional solvent at least three times or more until the final filtrate is colorless indicating that all the trapped fluid has been removed , discard the filtrate , and dry the remaining deposit in a vacuum oven at 100 ° C overnight . Homogenization - Fouling deposits are not uniform so we must mix the sample thoroughly to get an average composition and not be misled by analyzing a portion of the deposit which might happen to be rich in just one component . Homogenize the dried material by first grinding and sieving the entire sample to at least -60 mesh particle size , then mixing well .
Step 2 – TGA ( ThermoGravimetric analysis ) The recommended TGA test subjects the sample to heat under an inert atmosphere , increasing the temperature
Heat
Inert gas or air
Prepped Sample
Metal Pan , typically Pt
Scale
Fig . 1 . Representation of a TGA Instrument . www . heat-exchanger-world . com Heat Exchanger World September 2023
49