Reboiler fouling

In the depropanizer tower the propane and lighter gases are taken overhead to become feed to the ethylene and propylene recovery facilities. Separation is accomplished at a relatively low overhead temperature of -25°F to minimize reboiler fouling by olefin polymerization. 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Heat transfer constraints Heat must be transferred into the liquid in the reboiler to boil off the vapor needed to provide the vapor-liquid contacting in the column. If the base temperature becomes too high and approaches the temperature of the heating source, the heat transfer rate will decrease and vapor boilup will drop. The same result occurs if the reboiler fouls and the heat transfer coefficient drops. In the condenser, heat must be transferred from the hot vapor into the coolant stream to remove the heat of condensation. If the column is operating at its maximum pressure, capacity maybe limited by condenser heat removal. 

To prevent reboiler fouling, and accumulation of solids in the column during steam-water operations, an adequate water blowdown must be purged out of the column base. 

Fouling. Reboiler fouling is common in services handling imstable or corrosive chemicals. Foulants either form scale on heat transfer surfaces, or plug tubes, or both. Increased fouling is often accompanied by increased degradation of material. 

Fouling is usually hindered by proper selection of materials of construction and the use of antifoulants and corrosion inhibitors. These mainly depend on the chemicals processed and are outside the scope of this text. Nonetheless, several design and operation features also help moderate reboiler fouling these are described below. 

Fouling. The downflow movement of liquid makes fouling a less severe problem with conventional internal circulation reboilers (Fig. 15. le) than with kettle reboilers. Fouling, however, can be severe if tubes are unflooded. The bathtub arrangement (Fig. 15.96) does not share the liquid downflow benefit, and fouling can be as much as a problem as with kettle reboilers. The isolating chamber forms a dirt trap, and must be blown down (e.g., by perforating the chamber floor). 

Instead of controlling flow to the reboiler (in Fig. 17.1c), one could use the pressm-e at the reboiler as the control parameter. Controlling reboiler pressure is not recommended because the relationship between pressure and condensing temperature, and therefore between boilup and pressure, is nonlinear. Further, the relationship between boilup and pressure changes as the reboiler fouls and when the heat transfer coefllcient varies. 

In steam-heated reboilers, the vapor inlet scheme minimizes the reboiler tube wall temperatures. This suppresses reboiler fouling (process side), and lowers thermal stresses at the reboiler heads. These thermal stresses often cause leakage at the channelhead to tubesheet gasket (234). 

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. 

Reboiler temperature increases with a limit often set by thermal decomposition of the material being vaporized, causing excessive fouling. 

For services in which fouling is high or in which downtime cannot be tolerated, two reboilers may be installed on the same distillation column. These reboilers may each be half sized so that downtime will be limited to a half-capacity operation each may be two-thirds sized or each may be a full 100% spare. The latter is, of course, the most expensive from an equipment investment standpoint but may pay for itself in uptime. 

Resistance of fouling and metal tube wall required for balanced operation of reboiler ... 

Figure 10-123. Vertical thermosiphon reboilers, DAT versus AT for clean and fouled conditions. (Used by permission Hajek, J. D. Private communication. Deceased.)...

Assume the reboiler -will have a full shell diameter top outlet elbow vapor nozzle. Thus, a flux of (104,000) (0.90) or 93,600 Btu/hr (fP) is possible if steam temperature is adequate. At the latter flux, the fouled AT = 204°F. This -will require a steam temperature of 298 + 204 or 498°F equivalent to 693 psia steam. Because only 200 psig steam is available do-wnstream of the control valve at the chest, a lower flux must be used. 

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. 

CHANTRY, W. A. and Church, D. M. (1958) Chem. Eng. Prog. 54 (Oct.) 64. Design of high velocity forced circulation reboilers for fouling service. 

The amount of liquid vaporized in the reboiler should not be more than 80%, otherwise this will tend to lead to excessive fouling of the reboiler. For kettle reboilers, there is no recirculation. But for thermosyphon reboilers, a recirculation ratio can be defined as ... 

This usually lies between 0.25 and 6. The greater the value of recirculation ratio, the less fouling there is in the reboiler. Lower values tend to be used in horizontal thermosyphons and higher values (greater than 4) used in vertical thermosyphons. The recirculation ratio is a degree of freedom at the discretion of the designer. This should be fixed later when the detailed design is carried out. 

Example 15.4 A reboiler is required to supply 0.1 krnol-s 1 of vapor to a distillation column. The column bottom product is almost pure butane. The column operates with a pressure at the bottom of the column of 19.25 bar. At this pressure, the butane vaporizes at a temperature of 112°C. The vaporization can be assumed to be essentially isothermal and is to be carried out using steam with a condensing temperature of 140°C. The heat of vaporization for butane is 233,000 Jkg, its critical pressure 38 bar, critical temperature 425.2 K and molar mass 58 kg krnol Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The thermal conductivity of the tube wall can be taken to be 45 W-m 1-K 1. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 W m 2-K 1. Estimate the heat transfer area for... 

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