Reboilers in distillation

All machines have drivers. A distillation column is also a machine, driven by a reboiler. It is the heat duty of the reboiler, supplemented by the heat content (enthalpy) of the feed, that provides the energy to make a split between light and heavy components. A useful example of the importance of the reboiler in distillation comes from the venerable use of sugar cane, in my home state of Louisiana. 

Kayihan, F., Optimum Distribution of Heat Load in Distillation Columns Using Intermediate Condensers and Reboilers, AfC/iS Symp. Ser., 192(76) 1, 1980. 

Reboiler. The case shown in Figure 8 is common for reboilers and condensers on distillation towers. Typically, this AThas a greater impact on excess energy use in distillation than does reflux beyond the minimum. The capital cost of the reboiler and condenser is often equivalent to the cost of the column they serve. 

Chlorobenzene mixtures behave in distillation as ideal solutions. In a continuous distillation train, heat may be conserved by using the condensers from some units as the reboilers for others thereby, saving process energy. 

Bottoms and three side-cut strippers remove light ends from products and may utilize steam or reboilers. In Fig. 13-92 a reboiled stripper is utilized on the light distillate, which is the largest side cut withdrawn. Steam-stripping rates in side-cut strippers and at the bottom of the atmospheric column may vary from 0.45 to 4.5 kg (1 to 10 lb) of steam per barrel of stripped liquid, depending on the fraction of stripper feea hquid that is vaporizea. 

FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

The previous discussion focused on the use of indirect fired heaters as line heaters to provide the necessary heat to avoid hydrate formation at wellstream chokes. Indirect fired heaters have many other potential uses in production facilities. For example, indirect fired heaters can be used to provide heat to emulsions prior to treating, as reboilers on distillation towers, and to heat liquids that are circulated to several heat users. The sizing of indirect fired heaters for these uses relies on the same principles and techniques discussed for wellstream line heaters. 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

Level in Distillation Column Usually About ot Top of Reboiler Tube Sheet. 

Figure 10-96B. Horizontal thermosiphon reboiler on distillation column shell and tube design, not kettle. Boiling in shell.

Pressure balance across reboiler Assume fluid in distillation column at reboiler tubesheet level. 

Reboilers are required for distillation columns to vaporize a fraction of the bottom product, as discussed in Chapter 9. It may also be the case that a liquid needs to be vaporized for other purposes, for example, a liquid feed needs to be vaporized before entering a reactor. The discussion here will focus on reboiling a distillation column, but the same principles apply to other types of vaporizers. 

A Develop the equations describing an inverted batch distillation column. This system has a large reflux drum into which the feed is charged. This material is fed to the top of the distillation column (which acts like a stripper). Vapor is generated in a reboiler in the base. Heavy material is withdrawn from the bottom of the column. 

Another example is sketched in Fig. 8.3i>. A hot oil stream is used to reboil a distillation column. Controlling the flow rate of the hot oil does not guarantee a fixed heat input because the inlet oil temperature can vary and the AT requirements in the reboiler can change. The heat input Q can be computed from the flow rate and the inlet and outlet temperatures, and this Q can then be controlled. 

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]. 

When a vapor condenses to a liquid, we say that the latent heat of condensation of the vapor is liberated. In a steam reboiler, this liberated heat is used to reboil the distillation tower. When a vapor, or more commonly a liquid, cools, we say that its sensible heat is reduced. For a small or slight temperature change, the change in latent heat might be large, while the change in sensible heat will be very small. 

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by... 

Extractive distillation, that is, fractional distillation in the presence of a solvent, is used to recover aromatic hydrocarbons from, say, reformate fractions in the following manner. By means of preliminary distillation in a 65-tray prefractionator, a fraction containing a single aromatic can be separated from the reformate, and this aromatic concentrate is then pumped to an extraction distillation tower near the top, and aromatic concentrate enters near the bottom. A reboiler in the extractive distillation tower induces the aromatic concentrate to ascend the tower, where it contacts the descending solvent. 

Unlike continuous distillation, batch distillation is inherently an unsteady state process. Dynamics in continuous distillation are usually in the form of relatively small upsets from steady state operation, whereas in batch distillation individual species can completely disappear from the column, first from the reboiler (in the case of CBD columns) and then from the entire column. Therefore the model describing a batch column is always dynamic in nature and results in a system of Ordinary Differential Equations (ODEs) or a coupled system of Differential and Algebraic Equations (DAEs) (model types III, IV and V). 

Figure 2.2 Idea] and nonideal stages in distillation systems, (a), (f>) Thermosiphon reboiler arrangements (c). id) fractionation tray arrangements (ej, (j) inter-condenaor arrangements. Arrangements (6), (d), and (/) are ideal stages. Arrangements (a), (c), and (e) an nonideal stages.

Is the reflux ratio in distillation column optimized Increase in reflux ratio increases the product purity. However, this causes a larger pressure drop in the column, and increases the reboiler temperature and the reboiler heat duty. Hence, there is a delicate balance among these factors, and often the reflux ratio has to be continuously adjusted to meet the change in feed composition and other day-to-day variations. 

The flue gas generated in the tubular reformer is used for preheat of reformer and prereformer feed, natural gas preheat, steam superheat and preheat of combustion air. The synthesis gas generated in the tubular reformer is cooled by high-pressure steam generation (4), preheat of boiler feed water and reboiling in the distillation section (5). 

The flue gas heat content preheats reformer feed. Likewise, the heat content of the process gas is used to produce superheated high-pressure steam (5), boiler feedwater preheating, preheating process condensate going to the saturator and reboiling in the distillation section (6). 

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