Heat reboilers

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to the reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown... 

Figure 21.2 shows the entire low-temperature process in block flow format. Following the reactor and cooler, the liquid EDC is washed, removing unreacted chlorine and catalyst. Product EDC is obtained in two distillation steps drying, where water and a small amount of light by-products are removed and purification, which removes a small amount of heavy by-products. Both of these distillation steps use steam-heated reboilers. 

In most jacketed reactors or steam-heated reboilers the volume occupied by the steam is quite small compared to the volumetric flow rate of the steam vapor. Therefore the dymamic response of the jacket is usually very fast, and simple algebraic mass and energy balances can often be used. Steam flow rate is set equal to condensate flow rate, which is calculated by iteratively solving the heat-transfer relationship (Q = UA AT) and the valve flow equation for the pressure in the jacket and the condensate flow rate. 

When considering the steam side of steam heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 8.1. The steam is on the tube side, because the shell side was selected for the process fluid. If the reboiler is a thermosyphon, or natural-circulation reboiler, then low-process-side pressure drop is important. For a horizontal reboiler, it is easiest to obtain a low pressure drop for the fluid being vaporized by placing it on the shell side. 

The depropanizer bottoms are further processed in the debutanizer for separation of C4 product from light pyrolysis gasoline. The debutanizer operates at a moderate pressure of 0.4 to 0.5 MPa, and is a conventional fractionator with steam heated reboilers and water cooled condensers. 

When steam-heated reboilers are used, only one degree of freedom is available therefore, only one controller can be installed without overdefining... 

The low exergetic efficiency is typical for distillation systems with close boiling mixtures and with high energy requirements in the reboiler. An alternative is to use reboiler-liquid flashing. A compressor is used to return the reboiled vapor to the bottom of the column. The required reboiler duty is somewhat larger than the required condenser duty, and so an auxiliary steam-heated reboiler is needed. Thus, a trade-off is made between the power used in the compressor and the large reduction in reboiler steam. 

Armed with the thermodynamic fundamentals of heat management, we now take a closer look at the unit operation control loops for heat exchangers. We start with utility exchangers. These are used when heat is supplied to, or removed from, the process. Examples are steam-heated reboilers, electric heaters, fuel-fired furnaces, water-cooled condensers, and refrigerated coolers. 

Liquid reaching the bottom of the column is partially vaporized in a heated reboiler to provide boil-up, which is sent back up the column. The remainder of the bottom liquid is withdrawn as bottoms, or bottom product. Vapor reaching the top of the column is cooled and condensed to liquid in the overhead condenser. Part of this liquid is returned to the column as reflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product. In some cases only part of the vapor is condensed so that a vapor distillate can be withdrawn. 

The vapor effluent from the top of the column enters a condenser at 329 K and emerges as a liquid at 303 K. Half of the condensate is withdrawn as the overhead product, and the remainder is refluxed back to the column. The liquid leaving the bottom of the column goes into a steam-heated reboiler, in which it is partially vaporized. The vapor leaving the reboiler is returned to the column at a temperature of 398 K, and the residual liquid, also at 398 K, constitutes the bottoms product. A flowchart of the process and thermodynamic data for the process materials follow. 

Steam heating Condensing Low-to-high pressure gas heating and cooling finned tubes are often used) Reboilers Low-pressure (<50 psig) steam heating Reboilers... 

Vapor produced in the condenser is compressed back to column bottom pressure before entering the column. Figure 17.13 shows the application of this scheme to the propylene-propane separation problem. The bottoms liquid is flashed to 72psia to remove the required heat in the condenser. Additional heat added during isentropic compression is insufficient to make up the difference between reboiler and condenser duties. Therefore, the auxiliary steam-heated reboiler is needed. 

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). 

Sensible-heat reboilers are most commonly controlled by the b rpass scheme in Fig. 17.2a. This scheme is inexpensive and can almost always be implemented, but it suffers from sluggishness. An increase in reboiler duty closes the bypass valve, which initially raises the reboiler flow. This is accompanied by a rise in reboiler pressure drop, which routes some of the added flow back through the bypass. Eventually, the system will reach equilibrium, but only after some back-and-fbrth flow fluctuations. This flow sluggishness may compound the temperature sluggishness described earlier. 

A common problem with sensible heat reboilers is variation in heating medium inlet temperature. A heat input (Btu) controller (Fig. 7.2e) can alleviate this problem. Heat input control can be implemented either with a direct (Fig. 7.2e) or a bypass control system. The Btu controller compensates for changes in heating medium temperature. Heat input controllers are psuticularly useful in services where the temperature difference across the exchanger is small (AT effects dominate), and small variations in heating medium temperature significantly affect boilup. 

Figures 5.30 and 5.31 give temperature and composition profiles in the two columns. Notice that the temperature in the condenser of the high-pressure column is 407 K and the condenser heat duty is 5.86MW (see Fig. 5.28). The temperature in the base of the low-pressure column is 345 K and the reboiler duty is 14.77 MW. This 62 K temperature differential indicates that these two columns could be heat-integrated the condenser of the high-pressure column serving as a reboUer in the low-pressure column. Since the heat duties are not equal, an additional steam-heated reboiler would be needed in the low-pressure column. An example of heat integration is presented in Section 5.4.

There is only one reboiler duty to manipulate, and only one variable can be controlled by this input. As discussed later, we will use a control structure in which a tray temperature in the high-pressure column is controlled by manipulating the heat input to the reboiler in the high-pressure column (the only steam-heated reboiler). Then, we will vary the feed split (the fraction of the total feed fed to the low-pressure column) to control a temperature on a tray in the low-pressure column. 

Addition of heat Reboiler Removal of heat Condenser... 

Heizner, A. E., "Operating Performance of Steam-Heated Reboiler", Chemical Engi-... 

With the use of the RadFrac unit operation block in Aspen Plus, the heating (reboiler) and the cooling (condenser) equipment of a distillation eolumn are built in with the column... 

When considering the steam side of steam-heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 12.1. The steam is on the tube side. 

---