Membranes Reboilers

Purification of the reboiled caustic soda is important to remove sizes (raw-mercerization), dyes (mercerization of dyed materials), fibers, and impurities released from the fibers. Important techniques are filtration, centrifugation, flotation processes, and oxidative processes [29-31]. The application of membrane processes for reconcentration is limited to low concentrations of NaOH because of the insufficient chemical stability of the membranes. 

Here, a batch membrane separator is considered, as depicted in Fig. 4.26(a). The difference between this process and the reactive reboiler which was considered in Section 4.2 is that a membrane is introduced above the vapor phase. For the further analysis, the following assumptions are made ... 

Distillation to 90-volZ ethanol requires multistage operation. The process calculations were performed according to the Ponchon-Savarit method.(12) Assuming the concentration of the ethanol bottoms stream to be 1 volZ, and a reflux ratio of 1.5 times the minimum, the heat input to the reboiler corresponds to an energy requirement of 3.2 kWh/gal of 90-volZ product as distillate, or about 2.7 times that of a CCRO process employing the hypothetical, improved membrane. 

In a given application, 100 mol/s of a saturated liquid containing 37 mol% ethanol and 63 mol% water must be separated to yield a product which is 99 mol% ethanol, and a residue containing 99 mol% water. The solution will be fed to a distillation column operating at atmospheric pressure, with a partial reboiler and a total condenser. The reflux ratio will be 1.5 times the minimum. The distillate will enter a membrane with parameters am = 70 and 0 = 0.6. The membrane boosts the concentration so that the permeate stream is the ethanol-rich product (xp = 0.99). The retentate stream is returned as a saturated liquid to the column to the tray at the nearest liquid concentration. 

Many studies on systems in the current literature did not consider the Joule-Thompson effect caused by the expansion of permeate gas due to the pressure difference between the high retentate pressure and the low permeate pressure, also known as transmembrane pressure. This expansion leads to a decrease in the permeate temperature, which in turn decreases the membrane permeance. So, ignoring the Joule-Thomson effect may result in a wrong estimation of membrane separation performance and consequently of the reboiler/condenser duties and utility savings obtained from an HMD system. The membrane model employed in the present study takes into account the Joule-Thompson effect by including the following energy balance [Equation (10.2)] ... 

There are two major costs that play an important role in the feasibility of retrofitting a distillation column to HMD. One of them is the capital cost of the membrane module along with compressor, exchangers and associated installation, piping and labor costs (that is, cost of retrofitting). The other is the utility cost, which includes condenser and reboiler... 

In order to minimize utility costs, design variables of the membrane unit can be chosen to give better separation. For example, a larger membrane area will lead to more separation for a given feed flowrate and permeate pressure. This may lead to a reduction in condenser and reboiler duties of the associated column. While there is a decrease in utility cost of the column, there is a corresponding increase in the capital cost of the membrane. This results... 

In sum, distillation is a countercurrent vapor-liquid operation with the external reflux of a condensed liquid phase at the top and the external recycle or reflux of reboiled or vaporized vapor at the bottom. This reflux or recycle feature produces a sharp separation between the two key components of the feed mixture, and the same applies to membrane operations. The key components are the two components of a mixture between which the separation is to be made. 

Figure 4.4 Schematic representation of a membrane operation corresponding to the bottom section or stripping section of a distillation column (a) without external recycle or reflux (b) with external recycle or reflux (or reboil).

The layout is thus similar to a distillation column but without introducing an external reflux or recycle at the top and without reboil at the bottom. Alternately, the top cell can be viewed as inidating the reflux stream L, and the bottom cell as initiating the reboil stream V. In distillation column parlance, the top membrane cell then corresponds to the reflux condenser, and the bottom membrane cell corresponds to the reboiler. That is, in distillation operations, a partial reflux condenser and accumulator can be perceived as an extra stage (the distillate product is recovered as a vapor), and the reboiler (called a partial reboiler) as an extra stage if the bottoms product is recovered as a liquid. 

This is the relationship between internal and external reboil ratios as they pertain to distillation, which also applies here to the stripping section in multistage membrane separations. 

The calculations, in this respect, become similar to those employed for single-stage flash vaporization and multistage distillation with reflux and reboil, or for absorption or stripping. All the latter utilize the concept of an equilibrium stage. It should be emphasized, however, that the adaptations are constituted to apply to the nonequilibrium rate phenomena associated with membrane permeation. The calculations are similar in form but not in content. For one thing, the permeate flow rate per unit of membrane area (that is, the permeate flux) becomes part of the distribution coefficients or K-values for each component. An extra element of trial and error is therefore introduced. 

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