One means of generating such a low-grade heating medium is by I
applying the hot liquid belt technique. The equipment that is j usually needed is heat exchangers, a drum, a pump, and a heat j transfer liquid suitable for the temperature range of up to about ' j
200°F. Ideally, this liquid should be non-fouling, non-corrosive, I
not excessively viscous and non-toxic. Cooling water is rarely |
suitable, because it tends to cause accelerated fouling at i
¡temperatures greater than about 150°F, but steam condensate, ' |low-viscosity hydrocarbon oil or glycol are often suitable.
jFigure 4.5 shows a typical hot-liquid belt. Exchangers in the bottom group extract heat from various process streams at temperatures j .
greater than approximately T (commonly set at about 180°F). These I streams typically include column overheads, bottoms, and product !
streams, and often non-distillation streams such as compressor !
discharges. If the plant vents low-pressure or flash steam to atmosphere, it cam be re-routed to a condenser and used to heat the ; liquid. If the belt liquid is steam condensate, this steam can be directly injected into the drum liquid. The heated liquid is now 1 .
available to perform column preheating, interreboiling, and sometimes even reboiling duties in the top group of exchangers (Figure 4.5). ¡While preheating column feeds, these exchangers cool the liquid back j I to T, (commonly set at about 130°F), and make it suitable to return | to tne bottom group of exchangers (Figure 4.5). |
A number of application guidelines apply to hot-liquid belts: j -
| (i) It is important to keep the cold temperature T_ within a j desirable range. If this temperature rises, some of the , , cooling functions may not be performed. Often, a trim j cooling water exchanger is installed downstream of or in !
parallel with the top group of exchangers to control this temperature.
(ii) The hot temperature can be tightly controlled, but is often allowed to float. The hotter this temperature is, the more heat is available in the top group of exchangers, which may enhance the quantity of energy saved. Should T. drop below its normal temperature, the top group of exchangers may be unable to provide the normal amount of heating. If r the hot liquid belt is installed as part of a new plant design, some spare reboiler capacity or means of heating the liquid should be provided in case temperature T. drops; in a r revamp, spare reboiler capacity is usually available.
A TyplCfiL HOT-LIQUID
(iii) The hot-liquid belt system is particularly suitable for revamps, because most of the exchangers need not be tightly designed. The objective of the preheaters is essentially to preheat all that they can rather than to perform a fixed duty. This makes it very convenient for reusing available exchangers that have been withdrawn from other services.
(iv) The hot-liquid belt does not only save energy in column reboilers, but it also reduces cooling water heat loads, condensate losses (eg low-pressure steam vented to atmosphere), fouling of cooling water condensers in hot-distillate service, and problems with noisy steam vents.
(v) A hot-liquid belt is suitable for transmitting heat over considerable distances, and can thus simultaneously serve several different units. Successful application of this technique between different units half a mile apart was reported (30).
Pumparounds have been used in refinery fractionators for several decades as a means of conserving energy and reducing the internal vapor leads in the upper sections of the fractionators. A typical pumparound (Figure 4.6) takes liquid from an intermediate tray in the column, pumps it through a heat exchange system, and returns the cooled liquid to the column 2-4 trays above the withdrawal point. Pumparounds are commonly used in columns with side draws (such columns are referred to as "fractionators" in refinery jargon). Pumparounds are not unique to refineries; similar arrangements are often used in other manufacturing processes when large quantities of heat are to be removed by direct-contact heat exchange.
In principle, the pumparound is a special form of the hot liquid belt (4.2.3), in which heat is extracted by direct contact instead of by-indirect heat exchange. The direct heat exchange area serves as an internal intercondenser in the column. The pumparound can in principle be regarded as a hot liquid belt which extracts heat from an intercondenser and transmits it into services such as preheating, reboiling, steam generation and others.
As with an intercondenser, the further down the column a pumparound is located, the higher the temperature of the pumparound liquid and the higher is the grade of energy it extracts. Therefore, it is desirable to remove the heat as far down the column as possible. A common practice is to use a pumparound near the point of withdrawal of each side stream.
To maximize heat removal as low down as possible, columns are operated to generate as much liquid as possible for each section of column by the pumparound directly above it, and to minimize the amount of liquid "spillover" from the section above. "Spillover" from the section above is usually minimized to that required for control (roughly about 10 percent of the sidedraw flow (27). Reducing the "spillover", however, may be limited by fractionation.
As with intercondenser?, an increase in pumparound (or intercondenser) duty is followed by a reduction in the liquid rate (and therefore the L/V ratio) in the section above the pumparound. Such a reduction requires a greater number of trays in the section above the pumparound in order to maintain the separation. While the additional trays are usually available with energy-saving designs, they may not be available in some of the older units, and a greater amount of "spillover" needs to be tolerated.
A pumparound in a refinery fractionator can be taken either from the same tray as the sidedraw product, with a return a few trays above, or it may be taken a few trays below the sidedraw product, with a return just below the draw tray. The latter arrangement (Figure 4.6) gives a slightly hotter pumparound, which enhances the grade of heat of the pumparound, and also enables spillover measurement; for these reasons, it is considered better (27).
Pumparound circulation rates are usually maximized in order to maximize the temperature along the ptunparotrnd route. Since the heat duty stays essentially constant, an increase in pumparound circulation increases both the pumparound return temperature and the temperatures along the route. This maximizes heat recovery at the j hottest possible service, and therefore, at the highest thermal | grade. Further, if the pumparound circulation falls too low, the temperature in one of the exchangers may approach a temperature pinch, which may in turn restrict the pumparound duty. Typically, temperature drops about 50-100°F along the pumparound route.
¡This technique has been pioneered in refinery crude distillation I columns (27), but is applicable for most distillation columns whose I top product is mostly liquid with a wide boiling range, and have a j column overhead temperature sufficiently high to enable heat
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