Reboiled water strippers

Many water strippers are initially designed with steam reboilers, rather than with open stripping steam. The amount of steam required is the same in either case. The great advantage of the reboiled stripper is that the steam condensate is recovered, and recycled back to the boilers. When open stripping steam is used, the steam condensate is added to the stripped water, thus increasing the plant s water effluent. Hence, the use of open stripping steam is environmentally unfriendly. 

Type 430 stainless is resistant to chloride stress corrosion cracking and elevated sulfide attack. Applications are found in nitric acid services, water and food processing, automobile trim, heat exchangers in petroleum and chemical processing industries, reboilers for desulfurized naphtha, heat exchangers in sour-water strippers and hydrogen plant efluent coolers. The compatibility of type 430 stainless steel with selected corrodents is provided in Table 7.4, which is taken from Reference [1]. 

That is, the old back issues of the NPRA Q A (i.e.. The National Petroleum Refiners Associated, Question and Answer) sessions. With this authoritative source as support, 1 asked the operator to stop the NaOH addition to his sour water stripper feed. Two hours later, at the same reboiler duty, the NHj content of the stripped sour water had gone from 0 to about 5 ppm. And the emulsion layer in the crude unit desalter had been reduced enough so that brine carry-over was no longer a problem. The 5 ppm of ammonia was quite acceptable. 

One of the questions that 1 am often asked is what can be done to mitigate reboiler tube failures in sour water strippers and the consequent steam leaks into the stripped sour water ... 

Referring to Fig. 47.3, for many refinery operations, the circulating thermosyphon reboiler shown can be eliminated and replaced with the same quantity of stripping steam, thus further simplifying the design of your waste water stripper installation. 

For many waste water strippers used in refinery services, rapid rates of reboiler fouling are experienced, not due to corrosion, but as a consequence of heavy hydrocarbon sludge, that settles out in the bottom of the sour water stripper feed tank. In particular, slurry oil that is produced in a refinery catalytic cracking unit can accumulate in the stripper s feed tank. This slurry oil, being denser than water, settles out in the tank and eventually is drawn into the suction of the sour water stripper feed pump. The heavy hydrocarbon phase then accumulates in the stripper s reboiler, and with time and temperature fouls the reboiler s tubes. 

Fig. 38. Caustic purification system a, 50% caustic feed tank b, 50% caustic feed pumps c, caustic feed preheater d, amonia feed pumps e, ammonia feed preheater f, extractor g, trim heater h, ammonia subcooler i, stripper condenser j, anhydrous ammonia storage tank k, primary flash tank 1, evaporator reboiler m, evaporator n, caustic product transfer pumps o, purified caustic product cooler p, purified caustic storage tank q, ammonia stripper r, purified caustic transfer pumps t, overheads condenser u, evaporator v, evaporator vacuum pump w, aqueous storage ammonia tank x, ammonia scmbber y, scmbber condenser 2, ammonia recirculating pump aa, ammonia recycle pump. CW stands for chilled water.

When MEA is used in the presence of COS and CS2, they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper. The heat-stable salts remain in the reclaimer until the reclaimer is full. Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. 

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

The debutanized gasoline is cooled, first by supplying heat to the stripper reboiler or preheating the debutanizer feed. This is followed by a set of air or water coolers. A portion of the debutanizer bottoms is pumped back to the presaturator or to the primary absorber as lean oil. The balance is treated for sulfur and blended into the refinery gasoline pool. 

Ammonia absorption refrigeration is particularly applicable when low level heat is available for operation of the stripper reboiler and power costs are high. Steam jet refrigeration is the large scale system of choice when chilled water is cold enough, that is above 40T or so. 

The most widely used is ammonia absorption in water. A flowsketch of the process is in Figure 8.27. Liquid ammonia at a high pressure is obtained overhead in a stripper, and then is expanded through a valve and becomes the low temperature vapor-liquid mixture that functions as the refrigerant. The low pressure vapor is absorbed in weak liquor from the bottom of the stripper. Energy input to the refrigeration system is primarily that of the steam to the stripper reboiler and a minor amount of power to the pump and the cooling water circulation. 

In the subsequent product stripping section, the product is stripped free of benzene and water using a live steam stripper or a reboiler column. The effluent discharged from the washing area is steam-stripped to recuperate MNB. 

If n-pentane is selected as the entrainer for an azeotropic distillation scheme, an ethanol product containing less water than that obtained in the extractive distillation method is easily obtained with entrainer-etha-nol ratios of 2.S-3.5, mole basis (10). For a ratio of 3.214, the water content of the ethanol is less than 3 ppm. Only 18 equilibrium trays are required in a column of less than 5 feet diameter. The heat loads in millions Btu/hour are about 10.7 for the reboiler and 11.3 for the condenser. A stripper is used to recover n-pentane and ethanol from the aqueous phase. The recovered n-pentane and ethanol can be recycled either to the feed or to the reflux stream of the azeotropic distillation column. 

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