Modelling and experimental validation of osmotic driven energy efficient process for tea solution concentration

Highlights

• Energy efficient osmotic driven process for Tea concentration.

• Modelling and simulation of osmotic driven membrane separation process.

• FO can be considered as a suitable and affordable alternate to conventionally used thermal process.

• Water and aroma recovery from tea industry effluent by forward osmosis.

Abstract

In the present study, freshly brewed tea (Camellia sinensis var. assamica) extract was concentrated using an aquaporin embedded hollow fibre membrane module (active area, 2.3 m²). The feed and draw (50g.L⁻¹, NaCl) solutions were circulated via tube and shell side of the membrane module with flow rate of 60 and 20 L.h⁻¹ respectively in counter-current mode. For dewatering, thermal and pressure-driven membrane processes are widely used in the food and beverage industries. These processes may not be economical beneficial compare to the forwarded osmosis process coupled with efficient draw solute regeneration process. The change in DS and FS concentrations were analysed using high performance liquid chromatography and ion chromatography. During FO experiment, the reverse and water flux decreased from (8.32 to 7.63) mg.m${}^{\mathrm{-2}}$h⁻¹ and (10.33 to 8.49) L.m${}_{\mathrm{.}}^{-2}$h⁻¹ respectively. The one-dimensional model of the Forward osmosis process was developed. The given model was validated using experimental data to predict the change in volume and concentration of feed and draw tank. The model predicted the experimental performance for aquaporin FO membrane within allowable error limits. The model developed can be successfully used in food processing industries for designing and optimisation of large scale FO process.

Introduction

The concentration process is an essential unit operation in food and beverage processing industries. The concentration of aqueous food is widely practised in order to reduce storage, packaging, transport, and handling cost (Petrotos and Lazarides, 2001). The concentration process prevents spoilage and improves the shelf-life of the product by reducing the water activity (${\mathit{a}}_w$). Evaporation is extensively used process for concentration (or dewatering) of aqueous food. The evaporation process involves the partial removal of water from liquid food by boiling (Hernandez, 0000). In the evaporation process, the volatile phase is separated based on the vapour pressure difference between two or more components. The essential components of liquid food are thermo-labile. In food processing industries, nutritional and organoleptic properties plays an essential role in consumer acceptability. The high operating temperature employed during the evaporation process can result in 90% loss of volatile aroma compound. This results in altered flavours and aroma of the final concentrated product, which eventually reduces the acceptability of the final concentrated product (Olsson and Trägårdh, 1999). Vacuum evaporator, scrapped surface evaporators, and freeze concentrators are suitable for producing superior quality of juice without appreciable loss of flavour, colour, aroma, and nutritive value. Due to the high energy requirement, the liquid concentrator process cannot be considered as an economical process for large scale industrial production (Raghavarao et al., 2014).

In food processing industries, the pressure driven membrane process such as reverse osmosis (RO) has been reportedly acknowledged as a suitable alternative to the thermal based dewatering process. The low operating temperature leads to the accomplishment of high-quality products having higher retention of essential nutritional (polyphenol and antioxidant Gunathilake et al., 2014), aroma, and flavour compounds (Rastogi, 2018). However, high hydraulic pressure, irreversible membrane fouling, and limited maximum obtainable concentration factor have constrained them from the practical application. On the other hand, Nanofiltration (NF), can be operated at lower pressure and delivers high permeate flux with a low operating cost. Also, NF expected pass nutrients through the membrane due to high molecular weight cut-off than the RO membrane. Vincze et al. (2004), compared economic efficiency of NF and RO process for preparation of concentrated coffee extract. The author claimed that the investment and operational cost of the equipment can be reduced using NF process (Vincze and Vatai, 2004) .

Forward osmosis (FO), is an energy efficient low cost emerging membrane technology for the concentration of aqueous food. Unlike pressure driven membrane processes, FO is low (or no) pressure membrane process where water permeate from feed (low osmotic pressure) to draw (high osmotic pressure) solution due to osmotic difference generated by two aqueous solution separated by a hydrophilic membrane. Low hydraulic pressure, processing temperature, irreversible fouling are few advantages offered by FO process. Compared to conventional pressure driven membrane separation processes such as RO, the FO process can be considered as an energy efficient technique provided the diluted high osmotic (draw) solution can be efficiently recovered using less or lower quality energy (Rastogi, 2016). The FO process has been widely studied for concentrating variety of aqueous food products, and Table 2 summarises its application food processing industries. However, suitable membrane, and draw solution are few important parameters that need to be further examined to make application of this process suitable for large scale industrial application.

Tea can be defined as one of the most extensively consumed non-alcoholic beverage (Jain and De, 2019). India is the fourth largest tea producer (7.5% of global production) producing two most popular black tea varieties, namely Assam and Darjeeling tea (Achinto, 2013). The Assam tea (Camellia sinensis var. assamica) are mainly consumed as black tea, due to its distinct earthy flavour and colour (Soni et al., 2015). Tea leaves contains various polyphenols, flavonoids and antioxidants. The composition of black tea is summarised in Table 1 (Sharangi, 2009). The black tea extract consists of antioxidants and polyphenols responsible prevention of diseases such as diabetes, cancer and cardiovascular disease (Khan and Mukhtar, 2013, Greyling et al., 2014). Owing to health benefits of tea consumption, the ready-to-drink (RTD) tea extract is expected to become progressively popular. At commercial scale, the tea beverages are subjected to thermal treatment, which alters the nutritional value and organoleptic properties of tea. To modify this altered organoleptic properties, manufacturers uses additives such as sugar, citric acid, colourant and flavouring agents. Due to the incorporation of additives, such variety of flavoured tea extract does not meet consumer demand. Membrane separation techniques such as MF (Evans and Bird, 2006) and UF (Todisco et al., 2002) can be considered as an efficient technology of clarification. Mild operating condition ensures the preservation of the nutritional components and organoleptic properties of tea extracts. The concentration of tea extract using Osmotic Evaporation (OE) process was investigated using a Hollow fibre (HF) membrane (774 m² m⁻³, surface area to volume ratio). Reportedly, 40% (w/w) of tea concentration was achieved in a 5 h operation with constant flux and no significant loss of tea properties. For liquid food applications the application of CaCl₂ is not advisable due to the presence of scaling ions (Ca^2＋) (Marques et al., 2017). For the preparation of food concentrate, NaCl is the best choice due to high solubility, high water flux, low cost and low reverse solute flux (Achilli et al., 2010).

This study is aimed to evaluate the FO process for concentration of black tea extract in batch mode. To estimate the practicability of this process for large scale application, an object-oriented dynamic one-dimensional modelling approach for the hollow fibre forward osmosis (HFFO) process was proposed using Modelica language and Dymola software tool. The developed dynamic HFFO model was validated with the experimental data by tuning the membrane transport parameter of the HFFO membrane module. The validated dynamic model is used for simulation of HFFO module performance concerning flow configuration such as counter-current and co-current.