Fouling Solutions: Is Suspended Solids Filtration the Right Answer? (full
Fouling, or the accumulation of solid material on a surface, is an increasingly prevalent and challenging problem in most processing plants across many industries. The costs associated with fouling problems in 2012 have been estimated at over $4 billion in the United States alone. Filter plugging and reduced lifetime, reduced heat transfer in heat exchangers, column packing deterioration, and reduced throughput are just a few of the harmful effects caused by fouling (Figure 1).
Suspended solids contamination is often thought to be the main contributor to fouling challenges, and feed filtration is commonly used to protect important process equipment. While suspended solids are often associated with fouling tendency, fouling can occur by a number of mechanisms including salts precipitation, chemical reaction, and biofouling among others; in these cases, solutions other than filtration must be employed. In order to properly eliminate or reduce fouling in process streams, the correct mitigation technique should be chosen based on the mechanism of fouling.
The tendency of a fluid and its mechanism to foul is related to many factors, and predicting this based on process conditions and stream quality is a speculative and inaccurate science. The most accurate method for determining fouling tendencies is a representative simulation of the process conditions. Unique and advanced fouling analysis and evaluation capabilities are now being used by plants in order to proactively diagnose and resolve fouling issues before damage to the process can occur. One of the most accurate and representative laboratory tests used to determine fouling tendencies of a process fluid is called the Hot Liquid Process Simulator (HLPS). By measuring fouling tendency of a fluid while applying different treatment methods using the HLPS, the mechanism of fouling can be revealed, and the correct mitigation technique can be appropriately chosen.
The HLPS is a small-scale dynamic process simulation used to mimic fouling in a laboratory setting. The system consists of a reservoir charged with the sample fluid that is pumped through a test section at a fixed flow rate by a downstream pump. An electrically heated element is positioned vertically in a tube forming an annulus through which the fluid flows, and the temperature of the element is precisely controlled by a thermocouple located in its interior. Process conditions are reproduced by adjusting the flow rate of the fluid and the temperature of the element. The temperature of the element can be adjusted up to 650ºC and the pressure up to 600 psi. As foulant material deposits on the heated element, heat transfer from the element to the fluid deteriorates and pressure drop increases, and a subsequent decrease in the outlet temperature or pressure can be recorded.
In relation to filtration and other separation processes, fouling tendency analysis is useful for determining the correct treatment method for fouling problems. As mentioned, the system can be used to determine fouling tendencies of streams before and after treatment methods are applied. In one study the HLPS was used to correctly identify hydrocarbon liquids as the major contributor to fouling, and a more effective solution was designed rather than conventional filtration techniques.
The HLPS was successfully used to determine the fouling tendency of an aqueous stream with high amounts of solids and hydrocarbons contamination. An unaltered sample was analyzed in addition to a filtered sample and a filtered and hydrocarbon-free sample. The study shows that differential temperature was greatly reduced after hydrocarbon removal (Figure 2). Differential temperature was not significantly reduced in testing of the filtered sample, suggesting that contrary to original beliefs, the hydrocarbon contamination was primarily responsible for fouling issues at the plant. Visual inspection of the test elements support the measured results as hydrocarbon deposits were observed on element used when non-extracted samples were processed (Figure 3). With this analysis, plant engineers were able to proceed with a better solution for fouling reduction by improving hydrocarbon removal in addition to solids filtration.
Fouling tendency analysis with HLPS technology has been successfully used in many projects to make proactive and informed decisions. With precise simulation of plant conditions using actual process fluids, the HLPS produces accurate and invaluable information related to liquids processing, fouling tendencies and potential mitigation strategies. In this case, the relation between hydrocarbon liquids and fouling was revealed to be important, and hydrocarbon recovery technologies were selected as a necessary solution in addition to filtration. The case also revealed the importance of contaminant characterization in process streams as dissolved, emulsified and free liquid contaminants as well as suspended and dissolved solids can originate a multitude of fouling mechanisms.
This above case demonstrates a valuable lesson: suspended solids are not always the main contributor to fouling episodes, and filtration is not always the proper solution to prevent fouling. Several tools can be utilized to find the true source of fouling in a water or process stream, and more effective solutions can be developed once the source is identified. In certain cases, such as the one in this paper involves hydrocarbon contamination as the main source of fouling. In other cases, dissolved species and/or certain process conditions can create an entirely different mechanism of fouling.
This is also the case for many sour water plants that experienced heavy fouling due to suspended solids, emulsifies and also dissolved species. The change in process conditions as feed water enters the stripper creates an environment conducive to fouling and deposition. Feed filtration is typically instrumental in reducing fouling to some extent, but emulsified and dissolved species remain in the aqueous phase in the feed stream and cannot be filtered. More effective solutions such as coalescers, activated carbon adsorption or centrifugation can be applied to remove these contaminants. However, understanding of the fouling mechanism must be identified before implementing such options. This is just one example of the many units experiencing fouling associated with contaminants other than suspended solids.
Fouling is a complex and variable phenomena, and a meticulous and fundamental process investigation is needed to resolve fouling challenges. Revealing the mechanism of fouling and its contributors is key to producing the right solution for every individual application. With proactive and fundamental analysis, plant engineers and operators can better understand mechanisms of fouling other than suspended solids deposition and choose the most appropriate alternative for their mitigation.