To demonstrate the viability of a novel carbon adsorption process for the recovery of these materials.
A programme has been initiated to demonstrate the viability of a novel carbon adsorption process for the recovery of water soluble organic component from effluent process waters. Environmental benefits include less organic pollution, lower carbon dioxide emission and lower water demands. The programme considered:
development of a novel immiscible solvent regeneration process to overcome the limitations of conventional techniques;
development of improved carbon adsorbents to maximise the adsorption capacity and the regeneration efficiency;
development of a detailed understanding of the factors influencing the adsorption and regeneration of active carbons, and the generation of a database of adsorption isotherms to allow the prediction of multicomponent adsorption for use in the design of the full scale process. A viable process had to produce an essentially anhydrous stream which could then be returned to the product stream. An improved process for ethanol production had been developed which involved the following steps:
alcohol adsorption on the first bed yielding clean exit water;
bed flushing with anhydrous alcohol;
alcohol displacement, using for instance acetone;
displacement of the acetone using clean water.
Preliminary studies within the programme have now shown that this can be simplified considerably, and extended to lower feed concentrations, for the treatment of aqueous effluents. The replacement of a miscible displacer by an immiscible one removes the requirement for step 2 and eliminates the need for distillation in the final step. In addition to the use of the immiscible solvent, it has also been shown that the overall efficiency of the adsorption plus regeneration steps is strongly dependent on the carbon used in the process. The best results to date have been achieved using a polymer derived carbon.
The process can be split into three steps :
In this step the water containing around 1 % volume of mixed aqueous acids is passed up the first of the three columns. For a feed containing acetic and formic acids two adsorption fronts will pass through the column with formic acid arriving at the column exist first. When a certain acid breakthrough level is reached the adsorption stage will be stopped. The key parameters in this stage of process will be the carbon adsorption capacity, which will be a function of both the micropore volume and mean pore size, and the adsorption temperature. Small scale laboratory studies using both equilibrium adsorption and small scale column breakthrough tests will be used to optimise the main carbon preparation parameters (extent of activation) and the adsorption temperature.
2. Solvent Displacement
The key to the process is the immiscible solvent displacement step. Normal carbon adsorption processes omit this step and proceed directly to steam regeneration. Where the concentration of the adsorbed species on the carbon is high this can be quite efficient and will give a higher organics content in the condensed steam effluent than in the original contaminated water feed to the adsorption step. In the case of the water soluble organics the concentration on the carbon is relatively low and steam regeneration would regenerate an aqueous phase at a very similar concentration to the original aqueous effluent feed to the adsorption step.
3. Steam Regeneration
At this stage of the process the carbon bed is saturated with di-isopropyl ether. This must now be removed and the bed returned to a water saturated state ready for the next acid adsorption stage. Preliminary studies have shown that this stage can be achieved with steam although under laboratory test conditions this is not very efficient due to poor heat transfer in the small beds. Hot dry gas regeneration may also be an alternative although this has not been investigated and may be worthy of further consideration if steam proves impractical. This stage of the process can only be realistically tested in the pilot scale. The work will therefore be carried out using commercial active carbons during the pilot unit commissioning phase.
It will be necessary to show that the bed can be regenerated sufficiently for continuous recycle operation to be viable whilst using low or medium pressure steam with a total volumetric flow of not more than ca.4 bed volumes of water as steam.
Funding SchemeCSC - Cost-sharing contracts
WN7 2HE Leigh