Reboiler charge

Table 8.6. Benefits of Recycling Based on Fixed Reboiler Charge. [Mujtaba, 1989]...

Dls is the solution of Equations 8.1-8.4 given Bs0, x Du and x B2 tnr is the batch time without recycle using 5 kmol (fixed reboiler charge) of fresh feed for the given separation x DI and x B2 (this time is same as those reported in Table 8.1 as tnr)... 

In addition to the above-mentioned problem, numerical difficulties may arise. The system (model equations) describing the multicomponent off-cut recycle operation needs to be reinitialised at the end of each main-cut and off-cut to accommodate the next off-cut to the reboiler. To optimise these initial conditions (new mixed reboiler charge and its composition) it is essential to obtain the objective function gradients with respect to these initial conditions. 

The column compositions are initialised to the composition of the mixed reboiler charge and a total of 2% of the fresh feed is used as column holdup. Half of the column holdup is assumed to be in the condenser and the rest is distributed equally over the plates. Piecewise constant reflux ratio was used, with 3 time intervals for the main-cut separation and 1 interval for the off-cut. 

The amount of solvent (F, kmol) charged in the reboiler is varied and the amount of feed (Bo, kmol) is adjusted to have the reboiler charged to its maximum... 

The process is approximated as a number or relatively small steps, each of which corresponds to a defined time interval. In each step the shortcut distillation model is applied based on the initial reboiler charge for that step and the desired constant distillate composition. The resulting product is used as the reboiler charge for the next step. The computations are continued until the available number of stages cannot produce the required distillate composition. 

Given the initial reboiler and the distillate compositions, calculate the minimum reflux ratio and the minimum number of stages by the Fenske and Underwood methods. With a reflux ratio that is twice the minimum, calculate the number of theoretical stages by the Gilliland method. The outcome is the initial reboiler charge and composition for the next step. 

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... 

In batch distillation the mixture to be distilled is charged as a batch to the still and the distillation carried out till a satisfactory top or bottom product is achieved. The still usually consists of a vessel surmounted by a packed or plate column. The heater may be incorporated in the vessel or a separate reboiler used. Batch distillation should be considered under the following circumstances ... 

In the arrangement discussed, the feed is introduced continuously to the column and two product streams are obtained, one at the top much richer than the feed in the MVC and the second from the base of the column weaker in the MVC. For the separation of small quantities of mixtures, a batch still may be used. Here the column rises directly from a large drum which acts as the still and reboiler and holds the charge of feed. The trays in the column form a rectifying column and distillation is continued until it is no longer possible to obtain the desired product quality from the column. The concentration of the MVC steadily falls in the liquid remaining in the still so that enrichment to the desired level of the MVC is not possible. This problem is discussed in more detail in Section 11.6. 

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. 

Not only do the types of boiling look different, they sound different as well. Technical operators in charge of evaporators, reboilers, and other boiling equipment sometimes judge the operation of their equipment from the noise emitted. 

Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280psig and —75°F. Freon refrigerant at — 90°F is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification. 

The crude C4 mixture is charged to a 70 tray extractive distillation column T-l that employs acetonitrile as solvent. Trays are numbered from the bottom. Feed enters on tray 20, solvent enters on tray 60, and reflux is returned to the top tray. Net overhead product goes beyond the battery limits. Butadiene dissolved in acetonitrile leaves at the bottom. This stream is pumped to a 25-tray solvent recovery column T-2 which it enters on tray 20. Butadiene is recovered overhead as liquid and proceeds to the BDS reactor. Acetonitrile is the bottom product which is cooled to 100°F and returned to T-l. Both columns have the usual condensing and reboiling provisions. 

Tall oil is a byproduct obtained from the manufacture of paper pulp from pine trees. It is separated by vacuum distillation (50 mm Hg) in the presence of steam into four primary products. In the order of decreasing volatility these are unsaponifiables (US), fatty acid (FA), rosin acids (RA), and pitch (P). Heat exchangers and rcboilers are heated with Dowtherm condensing vapors. Some coolers operate with water and others generate steam. Live steam is charged to the inlet of every reboiler along with the process material. Trays are numbered from the bottom of each tower. 

A hydrocarbon stream containing 60 mol % isoprene is charged at the rate of 10,000 pph to the main fractionator D-l at tray 40 from the top. The solvent is acetonitrile with 10wt% water it is charged at the rate of 70,000 pph on tray 11 of D-l. This column has a total of 70 trays, operates at lOpsig and 100°F at the top and about 220°F at the bottom. It has the usual provisions for reboiling and top reflux. 

A bottom receiver/reboiler which is charged with the feed to be processed and which provides the heat transfer surface. 

The reboiler is charged with the material to be processed and heat is applied to it to bring the material to its boiling point temperature. 

The column initialisation is only required for the first inner loop optimisation problem (described in section 6.2). The liquid composition on the plates, condenser holdup tank and in the distillate accumulator (differential variables) at time t = 0 are set equal to the fresh charge composition (xB0) to the reboiler. The DAE model equations are solved at time t = 0 to provide a consistent initialisation of all the remaining variables. The final values of all these variables at the end of the distillation task in each inner loop problem are stored and used for column initialisation for the subsequent inner loop optimisation problems. At the beginning of each task, the distillate accumulator holdup is set/reset to zero. 

Dynamic optimisation of this type of periodic operation was first attempted and reported in the literature by Mayur et al. (1970), who considered the initial charge to the reboiler as a fresh feed stock mixed with the recycled off-cut material from the previous distillation task. Each batch cycle is then operated in two distillation tasks. During the Task 1, a quantity of overhead distillate meeting the light product specification is collected. The residue is further distilled off in Task 2 until it meets the bottom product specification. The overhead during Task 2 meets neither specifications (but the composition is usually kept close to the that of the initial charge for thermodynamic reasons) and is recycled as part of the charge for the next batch. As the batch cycle is repeated a quasi-steady state mode of operation is attained which is characterised by the identical amount and composition of the recycle (from the previous batch) and the off-cut (from the current batch). Luyben (1988) indicates that the quasi-steady state mode is achieved after three or four such cycles. 

The necessity of the above discussion will now be realised (also see Christensen and Jorgensen, 1987). Figure 8.2 shows a quasi-steady state mode of operation, with off-cut recycle. A fresh charge BO, of composition xB0 is mixed with the off cut (Rl, xRi) from the previous batch to produce a mixed charge to the reboiler (BC, xBc) The main cut (Dl, x D]) is produced over the time // (Task 1), leaving a residue (B1, xbj). At this time the distillate is simply diverted to a second receiver, and further distillation in Task 2 for time t2 produces the off cut and the final bottom product (B2. x B2) where, B2 is the solution of Equations (8.1-8.4) as mentioned before. 

Liquid compositions of plates, condenser holdup tank and accumulator (differential variables) at time t=0 are set equal to the fresh charge composition (xB0) to the reboiler. It is also possible to set these values to mixed charge composition (xBC). Reboiler holdup and compositions were initialised to the mixed charge (BC, xBC) at each iteration of PO. Mujtaba and Macchietto (1988) and Mujtaba (1989) considered Type IV-CMH model for the process and the model was solved at time t=0 to initialise all other variables. The first product (D1, xD/) (see Figure 8.2) was drawn off starting from t = 0. For the second distillation task no re-initialisation was required. The distillate was simply diverted to a different product accumulator and integration was continued. 

Referring to Figure 8.2 and given a batch charge (BO, xB0)> a desired amount of distillate DI of specified purity x D1 and final bottom product B2 of specified purity x b2 Mujtaba (1989) determined the amount and composition of the off-cut (Rl, x R1) and the reflux rate policy r(t) which minimised the overall distillation time. In this formulation instead of optimising Rl, xR1) the mixed charge to the reboiler (Bc, xBC) was optimised and at the end of the solution the optimal (Rl, x RI) was evaluated from the overall balance around the mixer in Figure 8.2. The dynamic optimisation problem is formulated as ... 

The batch distillation column consisted of 3 internal plates, reboiler and a total condenser. The reboiler was charged with a fresh feed of 5 kmol with Benzene molefraction 0.6. The total column holdup was 4 % of the charge. Half the holdup was in the condenser and the rest was distributed over the plates. The vapour load to the condenser was 3 kmol/hr. The required product purities were x oi = 0.90 and x B2 = 0.15. The solution of Equations 8.1-8.4 therefore gives DJ = 3.0 kmol and B2 = 2 kmol. This problem is same as case 3 shown in Table 8.1. Three reflux ratio (control) intervals were used to achieve (Dl, x Di) and one control interval to achieve (B2, x B2). 

In all the case studies presented in this chapter it was assumed that the amount of fresh feed to be processed in the long production campaign is fixed for every batch cycle. And the reboiler was oversized to some extent so that it could accommodate the extra charge from the off cut recycle. The optimal amount of recycle was obtained within that bound (40% oversized) so that maximum benefit could be achieved out of the given column. 

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