Design of a continuous process for the production of highly substituted granular carboxymethyl starch

A process design for the continuous production of highly substituted granular carboxymethyl starch (CMS) from potato starch is presented. Experiments in a batch reactor showed that aqueous isopropyl alcohol(IPA) is the optimal reaction medium to prevent the substituted starch fr om swelling and to keep it in a granular form. The degree of substitution (DS) that can be obtained in a single reaction step is limited to about 1.3. It is possible to increase the DS in a consecutive batch reaction where a lower water concentration is used. The continuous process consists of two continuous stirred tank reactors in series, each with a specific H2O/IPA ratio. For a complete recycling of the organic reaction medium a hybrid dehydration section is evaluated. Based on this design, granular carboxymethyl potato starch with a DS of 1.5 or higher can be produced continuously. For an annual production of 3600 tons of CMS, the first CSTR should have: a starch content of 20 % (w/w); a H2O/IPA ratio of 10/90 w/w; a temperature of 40 degreesC; a volume of 11 m(3); and a residence time of 4.5 h. To increase the DS to 1.5, the intermediate CMS, with a DS of 0.8, is fed to the second CSTR, having a starch content of 20% (w/w); a H2O/IPA ratio of 5/95 w/w; a temperature of 40 degreesC; a volume of 7.4 m(3). and a residence time of 3.6 h. (C) 2001 Elsevier Science Ltd. All rights reserved.