Fouling is generally defined as the deposition and accumulation of unwanted materials such as scale, algae, suspended solids and insoluble salts on the internal or external surfaces of processing equipment including boilers and heat exchangers (Fig 1). Heat exchangers are process equipment in which heat is continuously or semi-continuously transferred from a hot to a cold fluid directly or indirectly through a heat transfer surface that separates the two fluids. Heat exchangers consist primarily of bundles of pipes, tubes or plate coils.
Solution Manual Of Process Heat Transfer By D Q Kern Hit
Fouling on process equipment surfaces can have a significant, negative impact on the operational efficiency of the unit. On most industries today, a major economic drain may be caused by fouling.The total fouling related costs for major industrialised nations is estimated to exceed US$4.4 milliard annually. One estimate puts the losses due to fouling of heat exchangers in industrialised nations to be about 0.25% to 30% of their GDP [1, 2]. According to Pritchard and Thackery (Harwell Laboratories), about 15% of the maintenance costs of a process plant can be attributed to heat exchangers and boilers, and of this, half is probably caused by fouling. Costs associated with heat exchanger fouling include production losses due to efficiency deterioration and to loss of production during planned or unplanned shutdowns due to fouling, and maintenance costs resulting from the removal of fouling deposits with chemicals and/or mechanical antifouling devices or the replacement of corroded or plugged equipment. Typically, cleaning costs are in the range of $40,000 to $50,000 per heat exchanger per cleaning.
Major detrimental effects of fouling include loss of heat transfer as indicated by charge outlet temperature decrease and pressure drop increase. Other detrimental effects of fouling may also include blocked process pipes, under-deposit corrosion and pollution. Where the heat flux ishigh, as in steam generators, fouling can lead to local hot spots resulting ultimately in mechanical failure of the heat transfer surface. Such effects lead in most cases to production losses and increased maintenance costs.
Loss of heat transfer and subsequent charge outlet temperature decrease is a result of the low thermal conductivity of the fouling layer or layers which is generally lower than the thermal conductivity of the fluids or conduction wall. As a result of this lower thermal conductivity, the overall thermal resistance to heat transfer is increased and the effectiveness and thermal efficiency of heat exchangers are reduced. A simple way to monitor a heat transfer system is to plot the outlet temperature versus time. In one unit at an oil refinery, in Homs, Syria, fouling led to a feed temperature decrease from 210C to 170C. In order to bring the feed to the required temperature, the heat duty of the furnace may have to be increased with additional fuel required and resulting increased fuel cost. Alternatively, the heat exchanger surface area may have to be increased with consequent additional installation and maintenance costs. The required excess surface area may vary between 10-50%, with an average around 35%, and the additional extra costs involved may add up to a staggering 2.5 to 3.0 times the initial purchase price of the heat exchangers.
With the onset of fouling and the consequent build up of fouling layer or layers, the cross sectional area of tubes or flow channels is reduced. In addition, increased surface roughness due to fouling will increase frictional resistance to flow. Such effects inevitably lead to an increase in the pressure drop across the heat exchanger, which is required to maintain the flow rate through the exchanger, and may even lead to flow blocks. Experience with pressure drop monitoring has shown, however, that it is not usually as sensitive an indicator of the early onset of fouling when compared to heat transfer data; thus pressure drop is not commonly used for crude preheat monitoring. In situations where significant swings in flow rates are experienced, flow correction can be applied to both pressure drop and to heat transfer calculations to normalise the data to a standard flow.
Initiation or delay period. This is the clean surface period before dirt accumulation. The accumulation of relatively small amounts of deposit can even lead to improved heat transfer, relative to clean surface, and give an appearance of "negative" fouling rate and negative total fouling amount.
In Table 2 analytical results are shown from deposits obtained from the four chain feed/effluent heat exchangers in which the hot product effluentis used for pre-heating the cold naphtha feedstock for a naphtha hydrotreater plant at the Homs Oil Refinery [7]. This plant is one of the most important units at the Homs Refinery, with an annual capacity of 480,000 tons/yr. It is used to remove impurities such as sulphur, nitrogen, oxygen, halides and trace metal impurities that may deactivate reforming catalysts. Furthermore, the quality of the naphtha fractions is also upgraded by reducing potential gum formation as a result of the conversion of olefins and diolefins into paraffins. The process utilises a catalyst (Hydrobon) in the presence of substantial amounts of hydrogen under high pressures (50 bars) and temperatures (320C) (Fig. 2). A major fouling problem was encountered early on in the heat exchangers, indicated by an increased pressure drop, decreased flow rate and lower temperatures at the heat exchangers outlet.
Particulate fouling, which is the most common form of fouling, can be defined as the process in which particlesin the process stream deposit onto heat exchanger surfaces. These particles include particles originally carried by the feed stream before entering the heat exchanger and particles formed in the heat exchanger itself as a result of various reactions, aggregation and flocculation. Particulate fouling increases with particle concentration, and typically particles greater than 1 ppm lead to significant fouling problems.
Chemical particle formation is the basic mechanism of particle formation in heat exchangers fluid streams, although organic material growth and biological particle formation, or biofouling, may occur in sea water systems and in types of waste treatment systems. Biofouling may be of two kinds: microbial fouling, due to microorganisms (bacteria, algae, and fungi) and their products, and macrobial fouling, due to the growth of macroorganisms such as barnacles, sponges, seaweeds or mussels. On contact with heat-transfer surfaces, these organisms can attach and breed, reducing thereby both flow and heat transfer to an absolute minimum and sometimes completely clogging the fluid passages. Such organisms may also entrap silt or other suspended solids and give rise to deposit corrosion. Corrosion due to biological attachment to heat transfer surfaces is known as microbiologically influenced corrosion. For open recirculating systems, bacteria concentrations of the order of 1 x 105 cells/ml and fungi of 1 x 103 cells/ml may be regarded as limiting values [10].
Corrosion fouling is fouling deposit formation as a result of the corrosion of the substrate metal of heat transfer surfaces. This type of corrosion should not be confused, however, with the under-deposit corrosion, referred to earlier, which is one of the aftereffects of fouling.
Coking and Polymerisation are major causes of fouling in heat exchangers. Decomposition of organic products can lead to the formation of very viscous tar or solid coke particles at high temperatures and polymerisation involves the formation of undesirable organic sediments or polymers. The coke particles and polymers formed in the heat exchanger may grow to such a large size that they drop out of solution and deposit on the process equipment. Such deposits can be extremely tenacious and may require burning off the deposit to return the heat exchanger to satisfactory operation.
Separation of solid particles from fluid stream and their eventual deposition onto heat exchanger surfaces may be a result of many physical processes including condensation from gas phase, gravitational settling, crystallisation and electro-kinetic effect.
Suspended particles such as sand, silt, clay, and non-oxides may become too large to remain entrained in the flowing fluid stream. If the particles are sufficiently large and/or heavy that gravity controls the deposition process, we then have what is known as sedimentation fouling, which can often be prevented with relative ease by pre-filtration or pre-sedimentation of the offending particles. Sedimentation fouling is strongly affected by fluid velocity, and suspended particles in the process fluids will deposit in low-velocity regions, particularly where the velocity changes quickly, as in heat exchanger water boxes and on the shell side [17]. Wall temperature, on the other hand, may have less effect in general on sedimentation fouling, although a hot wall may cause a deposit to "bake on" and become very hard to remove.
The dependence of salt solubility on temperature is often the driving force for precipitation fouling. This temperature dependence may be different for different salts, with salt solubility increasing or decreasing with increasing temperature so that different salts may foul the cooling or heating surfaces depending on their solubility temperature dependence. While for most salts the solubility gets higher with increasing temperatures, there are salts such as calcium sulphate which have retrograde solubility dependence and are therefore less soluble in warm streams. Such salts will crystallise on heat transfer surfaces if the streams encounter a surface at a temperature higher than the saturation temperature of these salts.The calcium sulphate scale is hard and adherent and usually requires vigorous mechanical or chemical treatment to remove it. Other typical scaling problems are calcium and magnesium carbonates and silica deposits. 2ff7e9595c
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