Dewatering of 2,2,3,3-tetrafluoropropan-1-ol by hydrophilic pervaporation with poly(vinyl alcohol) based Pervap™ membranes

Highlights

• Pervaporation of water-tetrafluoropropanol (TFP) mixtures was studied.

• Effect of different process conditions on separation efficiency was evaluated.

• Efficient dewatering of TFP using Pervap™ hydrophilic membranes was proven.

• Batch mode pervaporation with Pervap™ 2200 and 2216 membranes was suggested.

Abstract

Pervap™ 2200, 2201, 2216, 2255, and 2510 hydrophilic, PVA membranes were investigated in pervaporation of water-tetrafluoropropanol mixtures. Physicochemical properties of membranes were characterized by determining the contact angle (CA) of water and glycerol and the surface free energy (SFE). The separation and transport properties of membranes were determined during vacuum pervaporation in contact with water-TFP mixtures containing up to 22 wt.% water.

It was found that all membranes were hydrophilic (CA < 90°) and the polar component of SFE was much higher than the dispersive one. The apparent activation energy for water transport was very high (E_app = 81.3 kJ/mol), indicated that water transport is activated thermally. Pervap™ membranes were very selective during the dehydration process of TFP. In contact with the Pervap™ 2200, 2201, and 2216 permeate contained practically pure water, regardless the feed composition. The process separation index (PSI) for Pervap™ 2200 and 2216 was close to 5000 kg m⁻² h⁻¹, suggesting that both membranes can be efficiently applied in the batch pervaporation process for TFP dehydration.

Introduction

Tetrafluoropropanol (TFP) can be obtained in a telomerization reaction (Eq. (1)) of tetrafluoroethylen with methanol [1], [2]. Synthesis proceeds at 120–122 °C, and leads eventually to the product with n = 1 and the yield of ca. 80% [1], [2].

$$n{\text{C}}_2{\text{F}}_4+{\text{CH}}_3\text{OH}\to \text{H}{({\text{CF}}_2{\text{CF}}_2)}_n{\text{CH}}_2\text{OH}$$

Dry tetrafluoropropanol is widely used as cleaning agent in dry cleaning, as a dye solvent in an electronic industry during CD-R(W) and DVD manufacturing, and as an ingredient of the fabric finishing agents [3], [4], [5]. Pure TFP and/or its water azeotrope mixture are also used as a new type of working fluids in an organic Rankine’s Cycle, which is a promising process for the conversion of low temperature heat into electricity [6], [7].

Tetrafluoropropanol is an organic solvent classified by REACH regulation as a hazardous substance. The most important physicochemical properties of TFP are gathered in Table 1. Taking into consideration that TFP is bio-accumulative and harmful to organisms, it is very important to remove and/or recover TFP from wastewaters. Nowadays, various processes such as Fenton, O₃/H₂O₂, O₃/UV defluoridation and/or sorption on activated carbon are used for treating TFP wastewaters [4], [8], [9], [10].

The application of TFP at the industrial scale has yet some limitations. One of the basic requirements for a wider application of TFP is a must of low water content in the solvent, which depends on the final application of TFP [3], [5], [6], [7]. Binary mixture of TFP and water forms an azeotrope at TFP/water composition of 72.5/27.5 (m/m) – Table 1. For that reason, the simple distillation as a method for TFP dewatering is not effective. The promising solution of this limitation would be application of hydrophilic pervaporation [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].

Pervaporation (PV) is a separation process where a binary or multicomponent liquid mixture is separated by a partial vaporization through a membrane [19], [21]. Basics of pervaporation process are described in detail elsewhere [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Compared with the traditional separation method for liquid mixtures, PV has many outstanding advantages, such as high selectivity, low energy consumption and flexible operation condition. This process is particularly appropriate for solvents dehydration [3], [11], [13], [15], [17], [26], [27], separation of organic–organic mixtures [28], [29], [30], [31], and organic recovery or removal from dilute aqueous solution [23], [24], [32], [33], [34], [35], [36], [37], [38], [39]. However, pervaporation performance is often limited due to membrane fouling and aging, concentration polarization, and the presence of non-volatile solutes and electrolytes in feed mixture [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53].

Over the last decade only few papers were published focusing on application of pervaporation process for the dehydration of TFP [3], [54], [55], [56], [57], [58].

Huang et al. [3] presented research on the preparation of two types of thin-film polyamide composite membranes: Type A (pristine composite membrane) and Type B (annealed composite membrane). Authors found that Type B membrane shows better transport and separation properties than Type A membrane; in contact with 70 wt.% TFP solution separation factor for Type B membrane was equal to 75 [3].

Wang et al. [54] underlined that the membranes used for the dehydration of aqueous TFP solutions should be resistant to that aggressive organic solvent. Moreover, it was suggested that the hollow fiber configuration can show higher performance in practical applications of pervaporation. Authors reported pervaporation properties of novel PBI/P84 dual layer hollow fiber membranes applied to the TFP dehydration. PBI and P84 are both aromatic polymeric materials known for their robustness in the aggressive chemical environment, as well as mechanical strength and thermal stability [54]. Authors reported also that during pervaporation of 85 wt.% TFP solution, the novel PBI/P84 membrane displays the permeate flux of 0.33 kg m⁻² h⁻¹ and the separation factor equal to 1990 [54].

Lo et al. [55] presented results on the plasma deposition of tetraethoxysilane on polycarbonate membrane and subsequent utilization of SiO_xC_yH_z/PC membranes in the dehydration of TFP aqueous mixtures. The SiO_xC_yH_z/PC membrane prepared using the deposition time of 30 min and applied power of 150 W showed the best properties. In contact with 80 wt.% TFP and operating at 25 °C, the total pervaporation permeate flux was equal to 0.35 kg m⁻² h⁻¹ with the separation factor of 570 [55].

Tu et al. [56] and Lin et al. [57] reported on the preparation of the hydrophilic surface grafted on poly(tetrafluoroethylene) membranes. Authors obtained membranes grafted with polyacrylamide (denoted as PTFE-g-PAAm membrane) and polystyrene sulfonic acid (denoted as PTFE-g-PSSA membrane) [56]. Subsequently a Chistosan-5/PTFE composite membrane was created by casting a chitosan solution on the surface of PTFE-g-PSSA membrane. Authors reported that for 90% solution of TFP in water, the both PTFE-g-PSSA and Chistosan-5/PTFE membranes showed practically identical transport properties, with permeate fluxes of 0.317–0.319 kg m⁻² h⁻¹. It was also found that the permeate contains practically pure water [56], [57].

Chen et al. [58] reported on the pervaporation performances of various commercially available PVA/PAN based membranes, denoted as S1, B1, B2, and B3, nevertheless the commercial names nor membrane manufacturers were not disclosed. The preliminary screening test of the pervaporation properties of these membranes in contact with water-TFP mixture containing 72.5–99 wt.% TFP allowed authors to choose the S1 membrane as the best one. Authors reported that the water flux was linearly decreasing with the decreasing water content in feed mixture. The flux was also strongly dependent of the operating temperature. In the examined experimental conditions of feed compositions (72.5–99.0 wt.% TFP) and feed temperatures (40–70 °C), the separation factor changed in the range 4–370, whereas permeate fluxes of water changed within the range 0.13–4.73 kg m⁻² h⁻¹. Authors observed also a typical trade-off between separation factor and permeate flux [11], [25], [50], [58].

The aim of this research was to assess the efficiency of several hydrophilic Pervap™ membranes based on polyvinyl alcohol (PVA) in the separation of water-TFP mixtures. The influence of the feed composition and process parameters on separation performances of membranes was investigated. The obtained results allowed to suggest the batch pervaporation dehydration process. Moreover, the obtained results were compared with the available literature data [3], [54], [55], [56], [57], [58].