Eutectic Freeze Crystallisation

Technology

EFC is a promising technique for the processing of concentrated saline solutions. Ideally, the salt concentration is close to the saturation limit; at lower levels, a concentration step is needed to achieve the ideal level. The technique has been proven on a small scale. The anticipated cost level is attractive when compared to the alternatives, such as evaporation followed by crystallisation. The next steps for the further development of the EFC technology are research under practical conditions, research into further scaling-up in a continuous process, and definition of the implementation possibilities from the end-user’s perspective.

This TKI projects is connected to the ongoing project aimed at EFC modelling via TTIW / Wetsus (participants: Vewin, Evides Industriewater (EIW) and EFC Separations).

Challenge

This TKI project delivers four results:

1. Inventory of water-related (industrial) applications of IEX, and a determination of the possible operating window to carry out the semi-industrial pilot. The possible reuse and valorisation of residuals is also examined.
2. Realisation of a semi-industrial pilot installation.
3. Experimental assessment on-site, based on the pilot, of the operation of EFC under practical conditions.
4. On the basis of the results, establishment of the feasibility of EFC for specified applications compared to alternative methods.

Solution

The inventory of water-related (industrial) applications of IEX as well as the determination of design conditions of the semi-industrial pilot have been concluded. The pilot was realised by EFC Separations and used for batch and continuous research at the EIW location DWP Botlek, using the spent regenerant for the recovery of the excess salt regenerant (NaCl). The salt crystallises at the eutectic point of -21°C and 23.3%. The pilot is equipped with an improved crystalliser with a capacity of 50-300 kg of ice per hour, and a new settler which enables a more efficient separation of the ice and salt crystals.

The pilot was in continuous operation for several weeks, leading one to the following conclusions and recommendations:

• The cold loss was the main challenge during the pilot tests. For example, the melting of ice on the belt filter, melting of ice during transport to the belt filter, and melting of filtrate of the belt filters caused by the heat of the vacuum pumps. There are various options available to prevent this cold loss when the technology is applied in practice.
• The efficiency of the installation can be improved by using newer, higher energy-efficiency cooling machines. In addition, various energy-saving measures can be taken to increase efficiency.
• By washing the ice and salt crystals, on a small scale, one obtains a good quality product (both ice and salt).

The economic feasibility of EFC to process and reuse ion exchange regenerant can be determined by comparing an EFC treatment scenario with one using state-of-the-art processes (evaporation and crystallisation). The economic analysis shows that the costs of the two scenarios are approximately the same, with the EFC scenario having a slight advantage with regard to operational costs. This is because EFC involves lower energy and residual stream disposal costs, and because of the cost advantage resulting from the salt reuse.

In summary, one can conclude that EFC’s potential (both technical and economic) can be realised in the processing of ion exchange regenerant. It should be noted, however, that the next step to an automatic and continuous process still needs to be taken.

Besides the processing of ion exchange regenerant streams, there are many other aqueous saline waste streams that can potentially be processed using EFC. The growing interest in environmentally-friendly processing and/or sustainable reuse of waste streams holds great promise for the application of EFC.