Fouling exchangers

The values of h, and AP, calculated by this procedure are for clean exchangers and are intended to be as accurate as possible, not conservative. A fouled exchanger will generally give lower heat-transfer rates, as reflected by the dirt resistances incorporated into Eq. (11-2), and higher pressure drops. Some estimate of fouling effects on pres-... 

Solvent feed temperature too hot] fouled exchanger/undersized heat exchanger/ ambient temperature too hot. 

Inadequate heat transfer] [maldistribution] 7insufficient heat exchanger area/ design error/fouled exchanger. See Section 3.3. 

Offline cleaning. In some applications, flows are diverted in a bypass pipe or exchanger, and then the fouled exchanger is cleaned offline. The following methods are frequently used ... 

To remove the waxy deposits online, block in and bypass the tube side of the fouled exchanger, while continuing sheUside flow, which heats the tubes. After a while, waxy deposits are melted off and harder deposits are spalled off due to thermal shock. After 20 min, restore normal flow. According to Lieberman (1997), this method works well in several cases. 

Typieal fouled exehanger pressure drops are 1.3—2 times that of elean exchangers (Barletta, 1998). For extreme cases, fouled exchanger pressure drops are much higher than that of clean exchangers. 

In the second case, the value of k is reduced to 0.15 [°C/day] because heavy fouling exchangers were replaced with more advanced heat exchangers. At a lower fouling rate k, three effects can be observed ... 

As a remark, the above model can be adjusted to reflect different simations (i) where only the most fouled exchangers are selected to clean (ii) where time variant margin is estimated instead of average margin and (iii) other significant costs are introduced. 

Block in and bypass the tube side of the fouled exchanger. 

The reactor product cooling would cause excessive fouling in a conventional exchanger. 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

A, 5A, and 13X zeoHtes are the predorninant adsorbents for CO2 removal by temperature-swing processes. The air fed to an air separation plant must be H2O- and C02-ftee to prevent fouling of heat exchangers at cryogenic temperatures 13X is typically used here. Another appHcation for 4A-type zeoHte is for CO2 removal from baseload and peak-shaving natural gas Hquefaction faciHties. 

Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for Hquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries. 

Pretreatment of aqueous streams may be required prior to using ion exchange. Suspended soHds that can plug an ion-exchange unit should be reduced to the 10 p.m level. Organics that can foul resins can be removed by carbon adsorption. Iron [7439-89-6] and manganese [7439-96-5], commonly present in ground waters, should be removed because they precipitate on the resin. 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

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