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Ind. Eng. Chem. Res. 1999, 38, 3738-3745
Adsorption of Glycols, Sugars, and Related Multiple -OH Compounds onto Activated Carbons. 1. Adsorption Mechanisms Daniel Chinn† and C. Judson King* Department of Chemical Engineering and Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720
Several different experimental methods were used to understand better the mechanisms of adsorption of glycerol, glycols, and sugars from dilute aqueous solution onto activated carbons. From batch equilibrium experiments, the affinity and capacity (25 °C) of carbons for single and multiple -OH compounds were greatest for solutes having larger positive deviations from solution ideality. Carbons with progressively oxidized surfaces exhibited reduced uptakes for all multiple -OH solutes. Larger isosteric heats of adsorption on carbons were measured for compounds with fewer -OH groups or higher molecular weight. Thermogravimetric analysis showed that the volatility of the adsorbed multiple -OH solute was reduced by higher molecular weight and solute hydrophobicity. Collectively, these findings show that the adsorption mechanism is characterized by: (a) attractive dispersion interactions between surface and solute, (b) solutionphase nonidealities, and (c) secondary, competitive interactions of the solute -OH groups(s) or surface oxygen groups with water. Introduction Multiple -OH compounds (e.g., glycerol, glycols, and sugars), by virtue of their nonvolatility and hydrophilicity, are extremely difficult to separate from water. Current recovery methods involve evaporation of all the water, which is energy intensive, nonselective among nonvolatile solutes, and prohibitively expensive for process and waste streams of high dilution (1-5 wt. %). A more energy-efficient and economical separation process would find use in many industrial applications, among them ethylene glycol (EG) recovery from spent aircraft deicer and automobile coolant;1 product recovery from the manufacturing stage, where ethylene and propylene oxide are hydrated with an excess of water to form the corresponding glycols; sugar recovery from waste streams leaving canneries, wineries, distilleries, and breweries; and recovery of multiple -OH solutes from the dilute product streams in corn wet-milling and fermentation processes.2 To address these challenges, researchers have investigated several novel methods of separating glycols from water. A glycol recovery process combining water evaporation, reverse osmosis (RO), and pervaporation (PV) was developed at the bench scale.1 An aqueous stream of 25 wt. % glycol was fed into a vacuum evaporator, where the overhead and bottoms streams were 0.5 and 70 wt. % glycol, respectively. The PV process used a poly(vinyl alcohol)/polyacrylonitrile membrane to concentrate the bottoms stream up to 95 wt. % glycol, which was suitable for reuse. The dilute overhead stream from the * To whom correspondence should be addressed: Provost and Senior Vice President, Academic Affairs, Office of the President, University of California, 1111 Franklin St., 12th Floor, Oakland, CA 94607-5200. Phone: (510) 987-9020. Fax: (510) 987-9209. E-mail:
[email protected]. † Current address: Zeneca Ag Products, Western Research Center, 1200 South 47th St., P.O. Box 4023, Richmond, CA 94804. Phone: (510) 231-1129. E-mail: daniel.chinn@ agna.zeneca.com.
evaporator was fed into the RO process, where the permeate product (0.05 wt. %) was sufficiently dilute for conventional waste treatment, and the retentate stream (7 wt. %) was recycled back to the evaporator. Researchers from our laboratory explored chemical complexation and reversible reaction as means of separating cis-vicinal glycols from water. The degree of liquid-liquid extraction was enhanced by using an organoboronate complexing agent.3,4 The organoboronate achieves its selectivity by a steric effect; the cisvicinal -OH groups in the aqueous phase form a planar, five-member-ring intermediate with the agent in the organic phase. To recover the original glycol product, the organoboronate is protonated to its original form by acidification, causing release of glycol back into the aqueous phase. In another scheme, cis-vicinal glycols were reacted with aldehydes under acidic conditions to form a more volatile and hydrophobic compound, a dioxolane, which can be more readily extracted or stripped.5 The glycol product is recovered by hydrolysis of the dioxolane, with concurrent recycle of the aldehyde reagent. Although data showing that carbons have some affinity for sugars have been available since the pioneering days of liquid chromatography,6 actual equilibrium isotherms reported for sugars and other multiple -OH solutes adsorbing onto active carbons are rather scarce. Measured isotherms were reported for adsorption of 13 saccharides and four polyhydric alcohols onto a single type of carbon, with parameters in the isotherm equations correlated to physical properties of the solutess molar refraction, number of carbon atoms, and number of oxygen atoms.7 Schlieker et al.8 compared isotherms of 1,3-propanediol on an active carbon with those on silica zeolite. All of these studies suggest that activated carbons are promising adsorbents for many types of multiple -OH compounds. In addition to having good sorption capacity, carbons are inexpensive and appear to be adequately regenerable.
10.1021/ie990286k CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3739
Objective and Approach With the exception of phenol and its derivatives, the mechanism of adsorption for multiple -OH compounds from solution onto carbons remains a largely unexplored area. The main objective of the present work was therefore to determine the underlying rationales for the degrees of uptake. Our approach is to measure and interpret the adsorption properties (isotherms and isosteric heats of adsorption) for a variety of solutes in terms of adsorbent or adsorbate properties. Thermogravimetric analysis (TGA) was used to measure the volatilities of a variety of adsorbed compounds on carbons. A sufficient understanding of the underlying chemical, thermodynamic, and transport phenomena during adsorption is needed for rational selection of both adsorbents and methods of process implementation. In addition, research must also address the important engineering issues of carbon regenerability and reuse, along with economical recovery of the adsorbed compounds. These matters are covered in Part II of this work. Materials and Methods Materials. Chemical Reagents. Purity and vendor information for all reagents are tabulated elsewhere.9 All solutions were prepared from distilled water that had been passed through a Milli-Q purification system (Millipore Corp.). Adsorbents. Most experimental work used Filtrasorb F400 (Calgon), a steam-activated carbon derived from bituminous coal, as the adsorbent. Heat treatment and controlled acid oxidation were used to modify the bulk and surface properties of F400 carbon systematically. Another carbon used in this work was WVB (Westvaco), a chemically activated carbon derived from wood. Asreceived and modified carbons were characterized by bulk elemental analysis, BET (Brunauer, Emmett, Teller) surface area, water vapor uptake, and surface acidity and basicity as measured by site titration. Methods. Heat Treatment of Adsorbents. Carbons were heat treated in a 55360/55600 series lab tube furnace (Lundberg/Blue M) equipped with a model 808/ 847 digital controller (Eurotherm Corp.). A nitrogen purge stream, flowing through a carrier gas purifier (Supelco), was passed through the furnace at 20-30 mL/ min, with a back pressure of 4-5 psig. The heating schedule was from ambient temperature to 100 °C at 10 °C/min, then an isothermal soak period of 24 h, then a 10 °C/min ramp to 1000 °C, then a second soak period of 24 h, and finally cooling back to room temperature. The carbons were then stored in desiccators sealed under house vacuum. Surface Oxidation of Adsorbents. In a fume hood, heat-treated F400 carbon was slowly added to concentrated nitric acid (69.4 wt. %) in a 500-mL Pyrex roundbottomed flask. About 25 g of carbon were used per 250-300 mL of acid. The suspension was stirred magnetically and maintained at 50 °C or 70 °C for 2 h using a 270W heater (Glas-Col Apparatus Co.) with Powerstat controller (The Superior Electric Co.). After oxidation, the carbons were rinsed exhaustively with boiling water until the extract showed a neutral pH according to PHydrion paper (Micro Essential Laboratory, Inc.). The carbons were then stored in desiccators sealed under house vacuum.
Table 1. Bulk Elemental Analysis of Adsorbentsa (ref 11) carbon
C (wt. %)
H (wt. %)
N (wt. %)
O+S (wt. %)b
ash (wt. %)
F400 F400/HT F400/OX50 F400/OX70 WVB WVB/HT
92.85 92.76 89.32 88.03 87.57 93.42
