First Evidence of Solvent Spillage under Subcritical Conditions in

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Materials and Interfaces

First evidence of solvent spillage under subcritical conditions in aerogel production Alberto Bueno, Ilka Selmer, Raman Subrahmanyam P., Pavel Gurikov, Wibke Loelsberg, Dirk Weinrich, Marc Fricke, and Irina Smirnova Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00855 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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Industrial & Engineering Chemistry Research

First evidence of solvent spillage under subcritical conditions in aerogel production Alberto Bueno*a, Ilka Selmer a, Raman S.P. a, Pavel Gurikov a, Wibke Lölsberg b, Dirk Weinrichb, Marc Fricke b, Irina Smirnova a a

Hamburg University of Technology, Institute of Thermal Separation Processes, Eißendorfer Straße 38, 21073 Hamburg, Germany.

b

BASF Polyurethanes GmbH, RAP/LF - E41, Elastogranstr 60, 49448 Lemfoerde, Germany [email protected]

Abstract The first evidence of solvent spillage under subcritical conditions during aerogel production is presented. The main objective was to understand the underlying phenomena controlling the solvent extraction kinetics during autoclave pressurisation. Alginate, silica and polyurethane as gels and ethanol, methyl ethyl ketone and ethanol/water as solvents were investigated. When CO2 diffuses in the gel solvent, there is a relative volume expansion of the liquid solvent. This expanded liquid mixture spills out of the gel and accumulates as a separate liquid phase at the bottom of the autoclave. A lag time was observed between the start of the autoclave pressurisation and the moment in which liquid starts to accumulate in the autoclave. The time needed for the solvent to start accumulating at the bottom of the autoclave is controlled by capillary forces and the saturation of the CO2-gas phase, on which temperature has an important

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effect. Low operating temperature and thereby low solubility of the solvent in the gas phase is suggested as the factor that enhances the kinetics of solvent removal at subcritical conditions.

1

Introduction

Aerogel technology has gathered sufficient momentum in the last decade to garner industry interest for commercialisation 1. The best example is the advances in the silica aerogel industry in the last decade for thermal super-insulation applications. However, aerogel production is a cost intensive process, and the industry is constantly on the lookout for a cost effective solution for the production of these materials. For example, at present, there are only a handful of companies led by Aspen Aerogels, which produce silica aerogels by the low temperature supercritical drying route using CO2. Other silica aerogel producers such as Cabot and JIOS Korea exploit the well-established chemistries of silica to produce super-hydrophobic aerogels through the ambient pressure route. Aerogels are produced if the solvent present inside the gels can be removed without pore collapse by considerably reducing the capillary forces generated during the solvent removal. If a gel can withstand the capillary forces acting on its backbone (due to low surface tension and/or reinforced backbone) then there is an alternative technological solution to produce aerogels, namely the evaporative drying. For almost all hydrophilic gels with a delicate structure, production processes based on supercritical drying are the surest way to preserve the mesoporous structure of the wet gel. The main reason why supercritical drying allows for the preservation of the gel mesoporous structure is the absence of surface tension and thereby capillary forces 2. 2 ACS Paragon Plus Environment

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Systems that have been successfully dried with CO2 include many organic gels derived from synthetic and biological polymers

3,4

. Especially in the case of biopolymer gels that are

intrinsically soft and hydrophilic the preservation of the structure is almost impossible with ambient pressure and freeze drying techniques 5. Biopolymer aerogels present the next generation of aerogel materials and can open up exciting commercialisation opportunities in the field of pharmaceutical 6,7, food 8, packaging and cosmetic industry 9. One of the reasons that make supercritical CO2 drying expensive at an industrial scale is the limitation of batch processing. In the last years, many efforts have been made in the evaluation and optimisation of supercritical CO2 drying procedure. Although, previous works have shown that the mass transport during the supercritical drying can be viewed as dominantly diffusive 10– 13

, they fail however to quantitatively describe the overall kinetics of the drying. We believe that

the pressurisation step may be a key to the optimisation of the overall supercritical drying process helping to reduce compression requirements and CO2 consumption. The general problem that we believe the present supercritical drying processes and aerogel production concepts face, is that they are viewed as a simple extension of the well-established supercritical extraction processes. In case of supercritical extraction, the solubility of the active component in the supercritical CO2 is limited and is heavily reliant on pressure and temperature of CO2 (and thus its solvation power). This is not the case with the supercritical drying, as complete miscibility between the solute (here it is the organic solvent) and CO2 is easily ensured by working above the critical pressure of the mixture, which in turn is determined by the operating temperature. Furthermore in case of supercritical extraction, the solute at the maximum constitutes 5-10 wt.% of the feed whereas, in case of supercritical drying, it constitutes more than 95 wt.% (solvent) of the initial feed material. 3 ACS Paragon Plus Environment


The other important aspect when dealing with organic solvent/CO2 systems is that one typically encounters gas-expanded liquids. Dissolution of several gasses (CO2, C2H6) under elevated pressures in organic solvents such as alcohols and ketones results in a considerable increases of the liquid phase volume

14

. At the critical pressure of the mixture (which is temperature-

dependent), the gas solubility becomes infinity and the system is in a single phase. Even though gas expanded liquids have been extensively studied such as enhanced oil recovery

16

15

and have applications in oil extraction

, their applications to the aerogel processing are not well

understood. In the scarce literature

10,17

available regarding solvent expansion in aerogel

production, its role is only explained from a theoretical perspective, and there is no concrete experimental evidence to support the concept. Consequently, there is a lack of understanding on how the phenomenon influences the drying kinetics and whether it can be utilised for the optimization of the aerogel production. Our intention in this work is to prove the concept of solvent removal from wet gels inside the two-phase region of the CO2-solvent system using gas expanded liquids. The kinetics of the solvent removal in the CO2-solvent two phase region was measured using a high pressure view cell. Factors that control the kinetics of the solvent removal are also discussed.

2

Materials and Methods


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2.1

Reagents

Sodium alginate from brown algae (Sigma Aldrich: 71238) composed of approximately 70% guluronic acid

18,19

was purchased from Sigma-Aldrich. Calcium carbonate (light, precipitated

powder, particle size ca. 1 µm) was provided by Magnesia GmbH (Germany). D-(+)-Gluconic

acid -lactone (GDL) (Sigma Aldrich: G4750) with a purity above 99.0% was used. Carbon

dioxide with a purity of 99.9% was supplied by Praxair GmbH. Denatured ethanol (from Roth, K928.2) with a concentration greater than 99.8% and a methyl ethyl ketone (MEK) content of ca. 1% was used for solvent exchange.

2.2

Apparatus

All the experiments were done using an autoclave equipped with two glass windows (Figure 1). The capacity of the autoclave is 500 mL and it was positioned horizontally. Carbon dioxide was liquefied using a refrigeration unit and supplied using an air operated pump, followed by heating using a heat exchanger. The temperature of the autoclave was controlled with an accuracy of ± 3 K using an external heating jacket connected to an on/off controller. The pressure inside the autoclave was controlled manually using a needle valve (± 5 bar). Due to several sampling ports present at the bottom of the autoclave, a volume calibration was done to determine the total volume of the autoclave including the sampling ports. This initial volume calibration experiments were performed with ethanol at a constant temperature. To correlate the position of the gas/liquid interface inside the autoclave with the volume of the liquid phase, a known volume of ethanol was injected into the high pressure view cell using an ISCO 500D syringe pump (at ambient pressure). At each known volume, a picture was taken 5 ACS Paragon Plus Environment


using a digital camera (Logitech C920 HD Pro) positioned at one of the windows; meanwhile, at the opposite window, a white led light together with a diffuser were fixed. Each picture was analysed with a script developed in MATLAB®. The script identifies the position of the liquid/gas interface. The position of the interface was then correlated with the known volume of the liquid. A fourth degree polynomial (n >1500 R2 = 0.9994) was used to correlate the volume with the interface position. The same script was used to determine the position of the interface during the actual experiments. During the experiments below the critical point of the mixture, the position of the liquid accumulated at the bottom of the autoclave was tracked using the camera and converted into the volume using the position-volume calibration. For tracking the ethanol removal above the critical pressure of the mixture, samples from the autoclave outlet were taken every 120 s and analysed using a gas chromatograph (CG). Prior to injecting in the GC the samples were depressurised to ambient pressure and heated to 393 K in a heated line.

2.3

Ca-alginate slabs preparation for solvent expansion measurements

Suspensions of CaCO3 in Na-alginate aqueous solution were prepared for two different Naalginate compositions: 1.5 wt. % and 3.0 wt. %, with a CaCO3 content of 0.36 g CaCO3 for each gram of Na-alginate. The suspension was homogenised using an Ultra Turrax T45 and was left to rest for 15 min to allow the air trapped in the suspension to escape. A fresh solution of GDL was prepared in water with a GDL amount of 0.64 g per gram of Naalginate. The GDL solution was poured in the respective suspension and homogenised using the Ultra Turrax T45 mixer for 10 seconds. 500 g of the resulting suspension was poured into 6 ACS Paragon Plus Environment

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polypropylene moulds of 20 cm by 20 cm and a height of 3.5 cm. The amount of water used for dissolving the GDL was chosen such as the final Na-alginate composition was kept at 1.5 wt.% or 3.0 wt.%. The gelation time was around 40 seconds, and the gels were allowed to age for 24 hours in the closed container. After ageing the gels, they were cut in three equal slabs. Since the solubility gap between CO2 and water is large, it is necessary to exchange this water with a solvent in which CO2 is completely soluble. The solvent exchange was done by adding water-ethanol mixtures in increments of 20 wt.% ethanol after every 2 hours until a composition of at least 99.0 wt.% (measured using Anton Paar DMA 4500 density meter) was obtained at equilibrium 5. The solvent exchange was done in the same moulds used for gelation, and the slabs were turned over each solvent exchange cycle. The thickness of the slabs after the solvent exchange was approximately 0.5 cm with a length of 15 cm and a width of 5 cm.

2.4

Solvent Spillage Kinetics

The slabs with an ethanol composition of at least 99.0 wt. % were preheated in an ethanol bath to 308.15 K or 333.15 K. After an equilibration time of one hour, the slabs were weighed (Wslab) and immediately positioned inside the preheated autoclave. The slabs were positioned over a mesh located in the middle of the autoclave to leave the bottom half of the autoclave free for the accumulation of the spilt liquid from the gel (Figure 1). The autoclave was pressurised at a rate of 1 bar/s and, the desired final pressure was kept constant. The volume of the liquid phase accumulating at the bottom of the autoclave was



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measured as described in Section 2.2. A video of the solvent removal at subcritical conditions is provided in the supporting information. Pictures were also taken every 5 seconds and analysed afterwards to determine the position of the liquid/gas interphase. The position of the interphase was used to calculate the volume of the spilt liquid (Vspilled) using prior calibration data. Equation (1) was used to determine the normalised amount of the solvent (ethanol free of CO2) that was spilt out of the gel and accumulated in the liquid phase at the bottom of the autoclave. The real extracted amount of the solvent is slightly larger since there is a small amount of the solvent dissolved (evaporated) in the gas phase. To ensure that the equilibrium was reached (Vspilled constant), the autoclave was left pressurised for 6 hours. After this time the autoclave was depressurised, and the gel was dried in an oven at 383 K for 24 hours to obtain the weight of the polymer (Wpolymer). The difference between the weight of the preheated gel (Wslab) and the oven

ℎ % =

∆ + 1

!"#$,&

'()* − ',-.

× 100

(1)

dried gel (Wpolymer) gives the initial amount of the solvent that was introduced into the autoclave.

To analyse the effect of the time required to saturate the gas phase on the solvent spillage kinetics, experiments were performed using CO2 pre-saturated with ethanol by positioning the gel slab in the autoclave as described before and injecting a known amount of ethanol into the autoclave. The autoclave was kept closed and was heated to the desired temperature and left to


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equilibrate for 2 hours under a CO2 atmosphere. After this time the same pressurisation regime described above was carried out. Since our intention was to analyse the rate of the solvent spilling out of the gel and not the rate of the expansion of the solvent that was previously injected into the autoclave to saturate the gas phase, a baseline experiment was performed by following the same procedure as described above but without a gel. The solvent left after the saturation of the gas phase starts expanding as the pressure in the autoclave increases. The rate at which the liquid volume increases (baseline) was recorded and subtracted from the results of the experiments with gels. For describing the ethanol expansion phenomena using CO2, phase equilibria and volume properties for the system carbon dioxide-ethanol are required. With the density of the mixture and mole fraction of carbon dioxide in the liquid phase, it is possible to determine the relative volume expansion of the liquid phase 1 3 using equation (2) 2 ∆2

20

, where is the volume of the

is the mass density (g/cm3), is mole fraction

solvent at temperature T and pressure Po (cm3),

in the liquid phase, Po is the reference pressure (1 bar) and MW is molecular weight (g/mol). Experimental data for the system CO2-ethanol from 19 sources

21–40

and a total of 146

experimental data points were collected. The data were used to determine the relative volume expansion of the system CO2-ethanol at different temperatures (291.15 K to 343.15 K) and pressures (0 to 250 bar) using equation (2) and are plotted in Figure 2.

∆ =

;#< >';#< + 1? − 1 89":. 6, 7, ;#< 1 − ;#< >'!"4)5 !"4)5 6, 7

=


(2)


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The relative solvent expansion depends only on the mole fraction of the solubilised carbon dioxide at a given pressure and temperature. To study the effect of temperature on the process, the relative volume expansion was fixed and temperature was varied at two levels. The pressure and temperature settings used for the experiments are summarised in Table 1. To study the effect of the porous structure on the solvent spillage, gels of two different alginate compositions were used at each temperature level yielding in total four experimental sets. The experiments of each set were done in duplicate, and the experiments that were the closest to the values shown in Table 1 at equilibrium are presented in this paper. Even though the relative volume expansion can be theoretically determined using Figure 2 for the operating pressure and temperature the experimental relative volume expansion was calculated using equation (3). This equation assumes the following: (i) the volume of the gel remains constant through the process; (ii) the amount of ethanol which is vaporised into the gas phase is negligible (< 7 wt. % of the initial solvent content of the gel), and (iii) the liquid that is

,!@ ∆ = '()* − ',-. A B !"#$,&

accumulated at the bottom of the autoclave is at equilibrium with the gas phase.

3

Results and Discussion


(3)

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During the pressurisation of the autoclave, carbon dioxide starts diffusing into the ethanol contained in the pores of the gel. This process generates a CO2-ethanol mixture whose relative volume is larger than the volume of the initial ethanol contained in the gel. Since the volume of the gel remains constant during the experiment (data not shown), the volume difference between the expanded mixture and the gel spills out and is accumulated at the bottom of the autoclave (Figure 3). Once the preheated gel is placed inside the hot autoclave, ethanol also starts diffusing into the unsaturated gas phase of the autoclave. Accumulation of the expanded mixture (liquid) at the bottom of the autoclave will be only observed once the gas phase is saturated. The system was equilibrated at constant pressure and temperature (see Table 2). The final volume of the spilt phase was measured and the relative volume expansion of the system was determined using equation (3), see Table 2. A good agreement between the theoretical equilibrium values (Table 1) and the experimental equilibrium values (Table 2) was achieved. The observed deviation is most likely due to the inaccuracy of the pressure, temperature control and assumptions considered. Even though the relative volume expansion is not exactly equal between each of the sets, it is close enough for analysing the kinetic behaviour of the process. Keeping the relative volume expansion of all the experimental sets close to each other it is now possible to study the effect that alginate concentration and temperature has on the ethanol removal kinetics. The solvent removal kinetics below the critical pressure of the mixture is shown in Figure 4. An ethanol extraction of 60 wt. % was achieved with virtually no CO2 consumption compared to amounts for overall drying since the only CO2 consumed was used for pressurising the autoclave and compensate for the CO2 dissolving in the liquid phase. For removing of the remaining 40 wt.



% of ethanol it is possible to flush out the accumulated liquid and then increase the pressure above the critical point of the mixture to perform a supercritical drying. The rate at which ethanol is spilt decreases as the alginate concentration and temperature increases. The permeability of a gel increases with decreasing alginate concentration which is in accordance with the results obtained in Figure 4. However, the effect of temperature is somewhat counterintuitive. To understand this phenomenon, the results are split into two stages, which are shown in Figure 5 and Figure 6. In Figure 5 the first stage of the solvent removal is shown. At the beginning of the process, there is a period were no liquid (CO2-ethanol) is accumulated at the bottom of the autoclave (lag time). In these first minutes of the experiment, after reaching the desired final pressure, carbon dioxide is continuously injected into the autoclave to compensate the CO2 that is dissolving into the ethanol contained in the gel. At the end of the lag time, the additional carbon dioxide requirement is reduced considerably. At the same instant, liquid starts accumulating at the bottom of the autoclave (Figure 5). This lag time becomes larger as the temperature and alginate concentration increases. Since the amount of solvent liquid mixture (CO2-ethanol) accumulated in the first minutes is relatively small (

First Evidence of Solvent Spillage under Subcritical Conditions in

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