Crude preheat

United States discoveries of, 78 597 Crude petroleum, 24 255 Crude preheat trains, 73 221-223 design methodology for, 73 223-224 Crudes... 

Air emissions from a petroleum distillation unit include emissions from the combustion of fuels in process heaters and boilers, fugitive emissions of volatile constituents in the crude oil and fractions, and emissions from process vents. The primary source of emissions is combustion of fuels in the crude preheat furnace and in boilers that produce steam for process heat and stripping. When operating in an optimum condition and burning cleaner fuels (e.g., natural gas, refinery gas), these heating units create relatively low emissions of sulfur oxides, (SO c), nitrogen oxides (NO c), carbon monoxide (CO), hydrogen sulfide (H2S), particulate... 

Tube velocity >1 m/s for crude preheat exchangers, tube velocity is 1 to 2 m/s for overhead water condensers, tube velocity is 1 to 3.5 m/s. To minimize fouling, keep tubeside velocities 3 to... 

The cost of treatment chemicals or substances may be high (in Chapter 3 for instance the cost of additives to reduce the fouling in crude preheat stream of 100,000 barrels per day oil refinery, was of the order of 1.6 x 10 in 1981). There is clearly an incentive to keep these costs low, otherwise they may approach the cost of the problem they are intended to alleviate. 

FIGURE 14.10. Effect of an antifouling agent on the heat transfer coefficient against operating time on a crude preheater [Haluska and Shaw 1975]... 

Fouling is a very serious issue for crude preheat train in oil refining plants and it is characterized by the following root causes ... 

Low Velocity Low velocity can promote accumulation of fouling deposits. The desirable velocity for heat exchangers for the crude preheat train is between 6 and 7 ft/s. 

For the crude preheat train in the refinery plant, three beneficial modifications are identified. Modifications 1 and 2 add a new shell to each of the feed preheat exchangers serviced by the diesel pump-around (PA). Modification 3 adds surface area to the diesel product run down heat exchanger. Before implementing these changes, it is crucial to make sure that the existing pump can handle the increased diesel PA flow and the increase can be tolerated in product cut points. These modifications can save 47 million Btu/h of fuel in the crude heaters, equating to cost savings of 2.3 million per year. With only a 2.2 million installed cost, this project provides a fast payback. 

Heat transfer in crude exchangers often declines because of tube-side fouling. The difference in crude preheat may be 50 "F for a dirty vs a clean exchanger train. 

Hot vapors, flowing up the fractionator from the flash zone, are partially condensed by contact with cooler pumparound liquid. The heat absorbed by the pumparound stream is used to preheat crude. As the pumparound circulation rale is increased, both heat removal from the fractionator and from crude preheat increase. This saves furnace fuel. 

A leak in the LVGO pan will have the same effect as letting the pan overflow, i.e., draw temperatures and hence crude preheat will decline. Tight trapout pans in vacuum towers almost always save energy. 

Tiny pigs (i.e., small brushes) are individually forced—one pig at a time—through each tube with a high-pressure water gun. One of my clients has cut open U bends to prove the effectiveness of this pigging. It s effective but expensive. About 10,000 (in 2012) for a crude preheat bundle (3000-4000 ft ). This is a good argument for a floating-head bundle, rather than a U-tube bundle. 

Crude preheat train, with crude on tube side or sometimes on shell side (see next section)... 

Twisted Tubes Are Used in Crude Preheat Mainiy with Crude on the Tube Side... 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign. ... 

In this case, the cause of refinery corrosion is the presence of contaminants in the crude oil as it is processed. Corrosive hydrogen chloride evolves in crude preheat furnaces from relatively harmless magnesium and calcium chlorides entrained in crude oil. In petrochemical plants, certain corrosives may have been introduced from upstream refinery and other process operations. Other corrosives can form from corrosion products after exposure to air during shut-down polythionic acids fall into this category. Corrosive contaminants are as follows ... 

Organic chloride in cmde oils wiU form varions amounts of hydrogen chloride at the elevated temperatures of crude preheat furnaces. Many crude oils contain small amounts of organic chlorides (5 to 50 ppm), but the major problem is contamination with chlorinated organic solvents during production. 

If the salt concentration of the crude oil is over (1 lb/1000 bbls) of crude, consideration needs to be given to the problem of chloride fouling and corrosion caused by hydrochloric acid resulting from the hydrolysis of MgClj and CaClj in the distillation-tower crude preheating exchangers, as follows ... 

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