Available online at www.sciencedirect.com Chemical Engineering and Processing 47 (2008) 2076–2083 Influence of ultrasound on diffusion through skin/leather matrix Venkatasubramanian Sivakumar a,∗ , Gopalaraman Swaminathan a , Paruchuri Gangadhar Rao b , Thirumalachari Ramasami c,d a Chemical Engineering Division, Central Leather Research Institute, Adyar, Chennai 600020, India b North East Institute of Science and Technology, Jorhat, Assam 785006, India c Department of Science and Technology, New Delhi, India d Council of Scientific and Industrial Research, New Delhi, India Received 12 April 2007; received in revised form 23 October 2007; accepted 23 October 2007 Available online 30 October 2007 Abstract Leather is a unique porous material, which is composed of a three-dimensional weave of tanned collagen fibre bundles. Collagen is a fibrous protein well organized in the formation of skin as building block. This paper studies the use of ultrasound in improving the diffusion process through porous skin/leather matrix. A diffusion model for leather processing has been proposed taking into account of pore characteristics in leather. Dye diffusion experiments have been carried out with leather and powdered leather to show the influence of ultrasound under two different diffusion conditions. Apparent diffusion coefficient (D) of dye through leather matrix has been calculated from the experimental dye uptake data. The results indicate that the use of ultrasound could achieve, 16.2 and 8.56 times improvement in D value for leather at 50 ◦ C and 30 ◦ C, respectively as compared to 2.99 and 1.55 times for powdered leather. Scanning electron microscopy (SEM) analysis of leather has also been performed which shows that fibre structure and morphology are not affected by the use of ultrasound. This study throws some light on enhanced transport through porous materials of complex nature such as skin/leather using ultrasound. © 2007 Elsevier B.V. All rights reserved. Keywords: Skin/Leather; Porous material; Diffusion; Ultrasound; Leather processing; Advanced processing techniques; Sonochemistry 1. Introduction Skin/hide, the outer covering of animals acts as thermo- static control for maintaining the body temperature. Skin/hide after its removal from the animal is degradable by biologi- cal action. Therefore, skin/hide is converted into biologically resistant useful material called ‘leather’ with improved func- tional properties by the process called ‘tanning’. Collagen is a fibrous protein involved in leather making. Leather process- ing can be broadly classified as pre-tanning for the removal of non-collagenous materials as a cleaning up process and prepa- ration of the skin/hide for the subsequent tanning, tanning for the stabilization of the collagen from biological action and to impart hydrothermal resistance and post-tanning to impart required functional properties to the final leather. The process- ing involves diffusion of various chemicals through the pores of ∗ Corresponding author. Tel.: +91 44 24916706; fax: +91 44 2411589. E-mail address: vsiva1@rediffmail.com (V. Sivakumar). the hide/skin/leather matrix of pore-size ranging from 3 × 10 -10 to 15 × 10 -5 m. Diffusion plays a vital role in most of the unit operations in leather making. Ultrasound is a sound wave with a frequency above the human audible range (16 Hz–16 kHz). Ultrasound having fre- quency range of 20–100 kHz is termed as power ultrasound [1] and commonly employed for enhancing physical processes such as cleaning, emulsification, crystallisation, extraction etc., and for accelerating or performing chemical reactions [2]. The uses of ultrasound in the formation of nanomaterials [3] and advanced zeolite materials [4] have also been reported. In recent times, the potential use of ultrasound in process industries such as leather, textiles and chemical with the aim to enhance the process effi- ciency, reduce process time and to get improvement in product quality are being studied. The application of this approach in leather processing has been reviewed earlier [5,6]. Our earlier studies demonstrated that ultrasound helps to improve the dye- ing process [7–9], enhancement of fatliquoring process [10] and efficiency of the soaking process [11]. Ultrasound assisted dye- ing and fatliquoring process has also been studied by Xie et 0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.10.020