A Novel Orange Juice Concentration Method Based on C 2 H 4 Clathrate Hydrate Formation

A novel orange juice concentration method by C2H4 hydrate formation was presented. The experiments were carried out in a high-pressure stirred reactor. The changes of temperature and pressure during C2H4 hydrate formation were discussed under different pressure (2.10 and 4.43 MPa). Dehydration ratio was defined in order to evaluate the separation efficiency. It was found that secondary nucleation happened under higher initial pressure. The dehydration ratios were 20.2 and 99.3% with initial pressure at 2.10 and 4.43 MPa, respectively. The results demonstrated that removal of water by formation of C2H4 hydrate is a potential technology for orange juice concentration.


INTRODUCTION
Fruit juice concentration is recognized word-wide as a way to preserve and reduce transport cost.Despite recent advance in fruit juice concentration such as evaporation, membranes (ultrafiltration, reverse osmosis and membrane distillation) and freeze concentration and so on, there remain numerous challenges (Jiao et al., 2009;Jesus et al., 2007;Guignon et al., 2012;Aider and de Halleux, 2008).
In recent years, hydrate separation technology has attracted scientific interest in the field of CO 2 capture (Eslamimanesh et al., 2012;Fan et al., 2009;Li et al., 2009), hydrogen or methane recovery (Liu et al., 2014;Zhong and Englezos, 2012) and desalination (Javanmardi and Moshfeghian, 2003;Park et al., 2011).Hydrates (also called clathrate hydrates) are nonstoichiometric crystalline inclusion compounds form through the combination of water and suitably sized "guest" molecules, typically under low temperature and elevated pressure conditions (Sloan and Koh, 2008).The basic principle of aqueous concentration by gas hydrate is that under suitable conditions of temperature and pressure, the gas with water forms solid clathrate hydrate which can be removed from the concentrated solution by mechanical separation.Since gas hydrate can be formed above 0°C, energy consumption becomes smaller than that in freezing (Eslamimanesh et al., 2012;Fan et al., 2009;Li et al., 2009).Despite recent advances in hydrate separation technology, there is little report on aqueous concentration via hydrate formation, especially in application in juice concentration (Huang et al., 1966;Purwanto et al., 2001;Andersen and Thomsen, 2009).In fact, as early as the 1960s, Huang et al. (1966) utilized CH 3 Br and CCl 3 F hydrate to concentrate apple, orange and tomato juices.No difficulty was encountered in removing approximately 80% of the water from the substrates.However, the concentration process diminished the color and flavor of most substances and frequently imparted a slightly bitter aftertaste.Because the hydrate gas is not environmental friendly, the practical use of juice concentration by CH 3 Br and CCl 3 F hydrate formation is limited.Until to 2001, Purwanto et al. (2001) proposed concentration of coffee solutions by use of xenon hydrate.Andersen and Thomsen (2009) investigated the possibility of using gas hydrates for concentration of sugar juice.It was found that the process is not suitable for sugar production, but could be interesting for concentration of heat sensitive, high value products.
To our knowledge, there is no report on orange juice concentration via formation of C 2 H 4 hydrate and the study of C 2 H 4 hydrate formation processes are not clear.Therefore, the purpose of this study is to develop a novel orange juice concentration method via C 2 H 4 hydrate formation and investigate the processes of pressure and temperature changing with time in different initial pressure.
(China) and orange juice was supplied by Beijing Huiyuan Group (China).The contents of reducing sugars, total acid, vitamin C, soluble solid and water content of orange juice are listed in Table 1.

Method:
Apparatus: The basic experimental setup was adopted from Li et al. (2010) with modifications made to facilitate the higher pressure applications (Fig. 1).It mainly consisted of a stainless-steel reactor (volume 300 mL) equipped with a magnetic stirrer (Nantong Feiyu Science and Technology exploitation CO., China).The reactor was designed to be operated at pressure up to 25 MPa.The reactor was submerged into a thermostat (Tianheng THCD-306) with a stability of ±0.01 K to control the temperature.Two Pt100 resistance thermometers (Westzh WZ-PT100) within 0.1 K accuracy which are placed in the middle and bottom of the reactor, respectively, were used to monitor the temperature of the reactor.A pressure transducer (Senex DG-1300) within 0.01 MPa in accuracy measured the pressure inside the reactor.The pressures and temperatures of the reactor were recorded by data logger (Agilent 34972A).
Procedure: After 80 mL orange juice solution was introduced into the evacuated reactor, the reactor was cooled to the desired value (typical at 275.2 K).When the cell temperature was stabilized, the reactor was vacuumed to ensure the absence of air and then C 2 H 4 was charged into cell until the given pressure.Then the stirrer was started to initiate hydrate formation (500 rpm).During experiment the temperature and pressure were recorded.After hydrate formation finished (the system pressure was stable), the stirrer was stopped.
The molar number of gas consumed: The molar number of gas that has been consumed during hydrate formation can be calculated as follows Eq. ( 1) (Li et al., 2009): where, z is the compressibility factor calculated by SRK equation of state, subscripts f, e refer to the feed gas and equilibrium gas.The volume of gas was assumed constant throughout the hydrate formation process Dehydration ratio: The content of water in hydrate phase is important for evaluating the dehydrate ratio of C 2 H 4 (defined as ratio of water in the hydrate phase and in the initial orange juice).Because it is difficult to directly determine the water cut in hydrate phase in the presence of residual concentrate orange juice, the following assumption C 2 H 4 "reacts" with water to form C 2 H 4 hydrate based on following reaction Eq. ( 2) were adapted: where, n is the hydrate number of C 2 H 4 hydrate, according to experimental result of Falabella (1975), the hydrate number of C 2 H 4 hydrate (n) under this study is approximately equal to 6.76.Therefore, the Dehydrate ratio (D) can be calculated as following Eq.( 3): where, ܹ ு మ ை = Molar mass of water m f = The mass of water in initial orange juice

RESULTS AND DISCUSSION
Figure 2 shows the typical processes of pressure and temperature changing with time in initial pressure of 2.10 MPa.During the C 2 H 4 hydrate formation, the pressure decreased gradually and achieved constant after about 1.5 h.However there were not obvious temperature changes during orange juice concentration process.These results demonstrated that the C 2 H 4 hydrate formation process was gent under lower initial pressure.Based on the formula of (3), the theoretical dehydration ratio was 20.2%.The trends of the pressure and temperature during orange juice concentration process with initial pressure at 4.43 MPa are shown in Fig. 3.As we can see that the during C 2 H 4 hydrate formation, there are three obvious temperature rises, due to intense exotherm of hydrate formation.And the pressure decreased gradually in the whole process except in the first hydrate formation step.Upon comparison of Fig. 2 and 3, we found that secondary nucleation happened under higher initial pressure.The reason was why that with higher initial C 2 H 4 pressure, the concentration of C 2 H 4 dissolved in orange juice increased.Therefore, the concentration of water clusters around dissolved C 2 H 4 molecules could be kept a higher level based on the labile cluster nucleation hypothesis (Sloan and Koh, 2008), which caused the phenomenon of secondary nucleation.In addition, the theoretical dehydration ratio calculated by formula (3) was 99.3%.
Orange juice concentration by C 2 H 4 hydrate formation was presented.During the C 2 H 4 hydrate formation, the pressure decreased gradually and then achieved constant in both initial pressures.It was found that secondary nucleation happened in higher initial pressure.The dehydration ratios were 20.2 and 99.3% with initial pressure at 2.10 and 4.43 MPa, respectively.The results demonstrated that removal of water by formation of C 2 H 4 hydrate is a potential technology for orange juice concentration.

Fig. 1 :
Fig. 1: Schematic of the experimental apparatus (volume changes due to the phase transitions were neglected).

Fig. 2 :
Fig. 2: Temperature and pressure as a function of time with initial pressure at 2.10 MPa

Table 1 :
The contents of reducing sugars, total acid, vitamin C, soluble solid and water content of orange juice used in this study