Distillation with constant distillate quality

Figures 12.110and 12.111 show the same test with the material balance scheme in place. Again the distillate quality would be better controlled with no feedforward, rather than with just one of the ratios kept constant. While keeping either ratio constant marginally improves control of bottoms quality, keeping both constant will result in virtually no change.

A colourless, odourless, neutral liquid at room temperature with a high dielectric constant. The amount of water present can be determined directly by Karl Fischer titration GLC and NMR have been used to detect unreacted propionic acid. Commercial material of high quality is available, probably from the condensation of anhydrous methylamine with 50% excess of propionic acid. Rapid heating to 120-140° with stirring favours the reaction by removing water either directly or as the ternary xylene azeotrope. The quality of the distillate improves during the distn. 

A 50 mole per cent mixture of benzene and toluene is fractionated in a batch still which has the separating power of 8 theoretical plates. It is proposed to obtain a constant quality product with a mole per cent benzene of 95, and to continue the distillation until the still has a content of 10 mole per cent of benzene. What will be the range of reflux ratios used in the process Show graphically the relation between the required reflux ratio and the amount of distillate removed. 

This is a point to consider process improvements. One problem is that the biopolymers we can buy are not of constant quality. Would it be possible to get other materials that are more reliable (This would mean a major redo of the work in the lab.) At this point you should also obtain a good idea of the prices of the ingredients and the energy used in the process. These may force you to go back to the lab to change the process. A fairly obvious improvement is to reduce the amount of water. This will reduce the size of equipment, the energy requirement and the amount of waste. However, you may be limited by a low solubility of the polymers, or by too high a viscosity of the solutions produced. The stream of waste water is a nuisance, especially because it contains formaldehyde. Could we separate the formaldehyde and recycle it This might be possible with a membrane or with distillation. 

Fig. 13.9 presents the flowsheet for the separation section of a HDA process. The first column is a stabiliser. The vapour distillate - a mixture of H2, CH4 with traces of benzene and toluene - can be used to hold constant the pressure. The quality of the bottom product is ensured by controlling a sensitive temperature in the top zone (inferential control) with the reboiler duty. Reflux and bottoms flow rates control the levels in reflux drum and sump, respectively. 

Let s take a look at the control loops. Now the make-up stream of reactant B is fed on level controller LCf of the buffer tank. The flow rate of the exit stream (Recycle) is set at a constant value by a simple specification on the stream s script. There are also two level controllers LCt and LCb for the top and bottom inventories of the distillation column, which manipulate the distillate and bottom products, respectively. Besides, the top column pressure is kept constant by means of the condenser duty. Quality control is implemented only for the bottom product, the reflux being fixed. We considered a composition measure with a first-order lag transmitted to a controller that manipulates the reboiler duty. 

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