Recovery of solvents by vapour permeation
By: António Pedro Eurich Couto Rosado
Hovione  

The Opportunity
The opportunity to introduce an innovative solvent recovery technology in HOVIONE was identified at the beginning of 1994, during the first steps of development of a new production line. An economical method was sought to remove all of the water from alcohols originating from waste steams so that they could be reused.

To reach the required levels of dehydration (< 0 ,5 % in water), the azeotrope with water would have to be overcome. Two options presented themselves:
 - classic technology using distillation with a third solvent to attain
    a new azeotrope.
 - innovative technology - pervaporation;

Evolution of the solution

Since pervaporation allegedly is associated with gains, in terms of energy consumption, and considering the fact that contamination of the recovered solvent with a third solvent could be avoided, we decided to investigate this approach.

After performing laboratory tests, we established the utility not only for the dehydration of alcohols, but also for the dehydration of acetone.

With this data in hand and in collaboration with INETI/ ITA ( Institute of Environmental Technologies ) we submitted an application in the 1st contest to foster the "Support of Productivity Regimen and Industrial Demonstrability" (PEDIP II).

In this way, we were able to order a Pervaporation Unit with a 120 m 2polyvalent membrane surface for continuous operation and with the following capacities for the treatment of residues:

  • Distillates with isopropyl alcohol 8.000 kg / 24 h

  • Distillates with ethyl alcohol 6.000 kg / 24 h

After a committing to the acquisition of the equipment it was discovered that small quantities of solid particles in the distillates could seriously limit the lifetime of the membranes.

Following a series of studies and additional experiments, we decided to select a Vapour Permeation Unit. This is also based on membrane technology, being even more recent, and allows direct recycling of the original waste (the mother liquors being filtered) with energetic and logistic gains.

This unit is composed of two sub-units:

  • An evaporator equipped with a high efficiency "demister", to
    remove non volatil impurities.

  • A group of linked hydrophilic membrane modules, gas permeation
    (sub -units), to remove the water from the solvent by vapour permeation.

In an analogous manner to pervaporation the membrane modules are located in a vacuum chamber that acts as collector and condensation system for the permeate mixture leaving the back side of the membranes as vapour . However, the vapor which is fed to the membranes is dehydrated and not a heated liquid.

After the manufacture of industrial batches using the dehydrated solvents (for validation of reutilization) an evaluation of the gains made was carried out with the assistance of staff from INETI / ITA to check whether the goals set had been achieved.

The results

Comparison between vapour permeation ( based on production values from 1997 ) and azeotropic distillation :

* (Scott, 1995)

Investment Analysis:

The cost of the investment was 95.591.000 PTE

The costs related to labour were added to the operational expenses indicated in the table above, maintenance and waste treatment giving a total of 8,33 PTE/liter of recovered ethanol. It was assumed that the costs would increase linearly with the volume of production up to the maximum installed capacity ( 1.9 million liters/year).

Taking into account that the membranes will have to be replaced every 6 years ( with an estimated cost of 10.000.000 PTE ), we obtain the following returns and updated net costs:

As can be observed, the investment for the installation of a vapour permeation unit, under the conditions described, is economically attractive.
 

With respect to the limitations which appear as the project progressed, it has to be noted that the acquisition value of the solvent recovery unit was substantially higher than the expected cost due to unforseenable factors:

•   - Alteration in composition in one of the wastes to be treated made a more
      polyvalent type of membrane obligatory with a decreased
      permeation capacity;
  
•   - Increase of the capacity required;
  
•   - Alteration from pervaporation to vapour permeation.

Nevertheless, implementation of a vapour permeation unit in HOVIONE lead to results that are encouraging for the study and adoption of innovative solvent recovery technologies.

A combination of indicators were found which demonstrate the advantages of systems that improve the eco-efficiency of processes, revealing that environmental investments turn out to be an advantage and not a constraint.

With the objective of divulging the technology to a wide audience, a session was prepared in cooperation with ITA/INETI.