Vacuum reboiler

The large fraction of tube length used for sensible heating in vacuum reboilers leaves little density difference for thermal circulation. This fact, plus the frequent... 

Controlling heat input by a control valve in the reboiler condensate outlet line may adversely affect reboiler stability, especially in vacuum reboilers. This valve varies condensate level in the heating side of the reboiler. It therefore varies the point at which heating is first applied to the process side. This is equivalent to operating the process side at a fluctuating reboiler sump level. Further details of this and alternative control schemes are in Sec. 17.1.2. 

Installing a throttling valve in the reboiler inlet line (Fig. 15.4a). Throttling this valve reduces the length of the preheat zone. This technique is mainly effective in vacuum reboilers. 

Reducing the liquid level in the column. This technique can only be used if no baffle is present in the bottom compartment. This technique is most effective in vacuum reboilers, but it is also frequently applied in pressure reboilers. 

Beyond 340°C in the reboiler, the residue begins to crack thermally. If the distillation is stopped at this point, the residue is called the atmospheric residue. In order to continue, the distillation is conducted under a low pressure, vacuum", so as to reduce the temperature in the reboiler. 

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.

The reconcentration of dilute (50—60%) sulfuric acid is one of the more costly operations in the manufacture of ethanol by this process. An acid reboiler, followed by a two-stage vacuum evaporation system, raises acid concentration to about 90%. The 90% acid is then brought to 96—98% strength by fortification with 103% oleum (fuming sulfuric acid). 

Type 77 units are the condenser and reboiler designs. One side is spiral flow and the other side is in cross flow. These SHEs provide veiy stable designs for vacuum condensing and reboiling seiwices. A SHE can be fitted with special mounting connec tions for refliix-type vent-condenser apphcations. The vertically mounted SHE directly attaches on the column or tank. 

Batch distillation equipment can range from a free-standing column with a reboiler, condenser, receiver, and vacuum system, to the use of a jacketed reactor with a condenser. Distillation often involves the generation of combustible vapors in the process equipment. This necessitates the containment of the vapor within the equipment, and the exclusion of air from the equipment, to prevent the formation of combustible mixtures that could lead to fire or explosion. 

When higher lean glycol concentrations are required, stripping gas can be added to the reboiler, or the reboiler and still column can be operated at a vacuum. 

Natural circulation reboilers are effective and convenient units for process systems operating under pressure. They are usable in vacuum applications but must be applied with care, because the effect of pressure head (liquid leg) on the boiling point of the fluid must be considered. The temperature difference between the heating medium and boiling point of the fluid may be so small as to be impractical, regardless of the tube length in a vertical unit. 

Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosyphon effect. This will increase the cost of the column supporting-structure. Horizontal reboilers require less headroom than vertical, but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

The tube lengths used for vertical thermosyphon reboilers vary from 1.83 m (6 ft) for vacuum service to 3.66 m (12 ft) for pressure operation. A good size for general applications is 2.44 m (8 ft) by 25 mm internal diameter. Larger tube diameters, up to 50 mm, are used for fouling systems. 

Thermosyphon reboilers are usually cheapest but not suitable for high-viscosity liquids or vacuum operation. Further discussion of reboilers will be deferred until Chapter 15. In the preliminary phases of a design, it is best to assume that enough stages are available in the column itself to achieve the required separation. 

The mechanism by which nonkey components affect a given separation is more complex in practice than the broad arguments presented here. There are complex interrelationships between the volatility of the key and nonkey components, and so on. Also, it is often the case that the distillations system has constraints to prevent certain heat integration opportunities. Such constraints will often present themselves as constraints over which the pressure of the distillation columns will operate. For example, it is often the case that the maximum pressure of a distillation column is restricted to avoid decomposition of material in the reboiler. This is especially the case when reboiling high molar mass material. Distillation of high molar mass material is often constrained to operate under vacuum conditions. Clearly, if the pressure of the distillation column is constrained, then this restricts the heat integration opportunities. Another factor that can create... 

Vacuum operation should be avoided and the reboiler temperature kept as low as possible to minimize fouling,... 

By approaching the refinery design from a crude oil perspective, the advantage of preseparation by stepwise condensation after HTFT synthesis was reduced. The refinery design included primary separation steps typically found in crude oil refineries, namely, an atmospheric distillation unit (ADU) that is followed by a vacuum distillation unit (VDU). Despite the design intent, the operation of these units, out of necessity, had to be different. The reboiler temperature of the ADU was... 

Many refineries now use vacuum pumps and surface condensers in place of barometric condensers to eliminate generation of the wastewater stream and reduce energy consumption. Reboiled side-stripping towers rather than open steam stripping can also be utilized on the atmospheric tower to reduce the quantity of sour-water condensate. 

Barometric condenser systems can be a major source of contamination in plant effluents and can cause a particularly difficult problem by producing a high-volume, dilute waste stream [8]. Water reduction can be achieved by replacing barometric condensers with surface condensers. Vacuum pumps can replace steam jet eductors. Reboilers can be used instead of live steam reactor and floor washwater, surface runoff, scrubber effluents, and vacuum seal water can be reused. 

Fractionation columns in tar-acid refineries are generally operated under vacuum and heated by high pressure steam or circulating hot oil. Calandria in the reboilers, condensers, rundown lines, and receiving tanks are constructed of stainless steel, or, in the case of the condensers, of tin or nickel. 

The main distillation types include atmospheric, vacuum, steam, azeotropic, extractive, and pressure distillation [45]. AU of these distillation methods can be carried out in a batch or continuous marmer with the exception of extractive distillation, which is solely continuous by nature. Gomplex solvent systems often require the use of multiple distillation columns in series to purify certain solvents that are not easily separated. The energy consumption in distillation columns can therefore be quite large because of the continuous operation of condensers and reboilers over extended periods of time. In order to cut down on these costs, both vacuum and steam distillation can be employed ]45]. 

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