CHAPTER 3 The Simple Surface Area Measurement Module

Transcription

1 CHAPTER 3 The Simple Surface Area Measuremet Module I chapter 2, the quality of charcoal i each batch might chage due to traditioal operatio. The quality test shall be performed before usig it as a adsorbet. For commercially activated carbo, maufacturers or providers have to verify their products by determiig some properties such as hardess, total ash cotet, desity, moisture cotet, surface area ad volatile matters cotet [Schaeffer, 2002]. Surface area is oe of parameters that relates to our purpose, moreover, it is usually a accepted value for geeral applicatio of commercial porous material. The pricipal method of the measurig surface area of porous materials is by adsorptio of a particular adsorbate oto the surface. The relatio of the adsorbed amout uder equilibrium with partial pressure of adsorbate is kow as the adsorptio isotherm. 3.1 Physical Adsorptio Theoretically, there are two types of adsorptio; physical adsorptio ad chemical adsorptio. Chemical adsorptio is a adsorptio oto a specific surface locatio thus the adsorbed amouts do ot represet all vacacy surface areas. Typically, it is examied at a elevated temperature at which reactios occur, ad strog bodig betwee adsorbet ad adsorbate always relates. For complete overall surface area determiatio, reversible or physical adsorptio is the most suitable as idicated by the followig reasos [Lowell ad Shields, 1984]: 1. Physical adsorptio is accompaied by low heats of adsorptio without chagig the adsorbet structure durig the adsorptio measuremet. 2. Physical adsorptio approaches equilibrium rapidly. 3. Adsorbed molecules are ot restricted to specified sites ad are free to roam the etire surface area. For this reaso surface areas determied by physical adsorptio are more accurate. The physical adsorptio isotherms are classified by Bruauer et. al. [Bruauer et.al.,1940], geerally kow as BDDT classificatio ad illustrated i Figure3.1. Type I isotherms are

2 ecoutered whe adsorptio is limited to oly a few molecular layers. I the case of physical adsorptio, this isotherm represets microporous powder whose pore size does ot exceed a few adsorbate molecular diameters. Type II isotherms are the result of moolayermultilayer adsorptio o oporous or macroporous materials. Type III isotherms are typical of a system i which the forces of adsorptio are relatively weak. Type IV isotherms are similar to Type II isotherm at low value of P/Ps while at high value of P/Ps the pore codesatio occurs ad a hysteresis effect associated with the pore codesatio is observed. Type V isotherms are similar to Type III but at high value of P/Ps the pore codesatio takes place. Figure 3.1 The five types of adsorptio isotherm accordig to BDDT classificatio [Bruauer et.al.,1940] 3.2 Moolayer Capacity Determiatio I may cases, the adsorptio amouts icrease further by multilayer adsorptio. The iflectio poit or kee of the isotherm usually occurs ear the completio of the first adsorbed moolayer. The poit of moolayer adsorptio is very clear i some isotherms, but is was ot clear or ot at all foud i other cases. Various methods have bee proposed to determie the completed moolayer. There are two widely acceptable equatios; the Lagmuir ad BET equatios, to predict the umber of adsorptive molecules required to exactly cover the solid with a sigle molecular layer.

3 A) Lagmuir Equatio This equatio is derived from adsorptio ad desorptio rate o moolayer adsorptio [Satterfield, 1993] thus it is i theoretically suitable for Type I isotherm or adsorptio amouts approached to a certai value. Adsorptio rate relates to ucovered surface ad pressure of adsorptive gas while desorptio depeds o covered surface. At equilibrium, adsorptio ad desorptio rate are equal as show i equatio (3.1). k (1 θ ) P = k θ (3.1) 1 2 Where θ is covered surface; (1-θ ) is ucovered surface, P is pressure of adsorbate gas ad k 1 & k 2 are rate costat of adsorptio ad desorptio, respectively. Thus, θ = KP 1 + KP (equilibrium costat, k K = 1 ) (3.2) k Whe covered surface is referred to a mole fractio of adsorbed molecule at ay P to completed moolayer, θ = m Equatio (3.3) shows the well-kow form of Lagmuir equatio. 2 m KP = 1 + KP (3.3) Rearragemet of equatio (3.3) gives P 1 P = + (3.4) K m m A plot of P/ versus P gives a straight lie of slope 1/ m ad itercept 1/ m K from which moolayer capacity ca be calculated. B) The Bruauer, Emmett ad Teller (BET) Equatio Bruauer, Emmett ad Teller exteded Lagmuir equatio to multilayer adsorptio [Bruauer et al., 1938 ad Bruauer et al., 1940]. Assumptios of this equatio are followed. 1) Adsorptio of the first layer is assumed to take place o a array of surface sites of uiform eergy.

4 2) Uder equilibrium, the rate of codesatio ad evaporatio are equated for each adsorbed layer. The heat of adsorptio for all layers except the first layer is assumed to be equal to the heat of liquefactio of the adsorbed gas. Summatio of the amouts adsorbed i all layers the gave the BET equatio C( P / Ps ) = P P C P P [(1 ( / ))(1 + ( 1)( / )] m s s (3.5) Liear form of equatio (3.5) is ( P / Ps ) 1 ( C 1)( P / Ps ) = + (1 ( P / P )) C C s m m (3.6) Where m is the moolayer capacity ad C is a costat. Plottig of left side of equatio (3.6) with (P/Ps) gives straight lie, kow as BET plot, of which moolayer capacity is determied from slope ad itercept. 3.3 Coditio for Adsorptio Measuremet Theoretically, adsorbate may be ay codesable iert vapors whose molecules should be small ad approximately spherical [Satterfield, 1993]. However its vapor should be hadled at the required temperature (coolat temperature) ad coveietly attaied i the rage of iterested P/Ps, for example P/Ps = whe usig BET equatio for determiig moolayer capacity. The other importat factors are o-toxic, o-explosive, cheap ad easily available. Criteria to choose the appropriate coolat are similar to that for adsorbate s selectio. Usually, coolats i adsorptio measuremet are similar to adsorbate except at their boilig poit. However, chose coditio for adsorptio affects the accuracy of the measuremet. For example, if you select CO 2 adsorptio at room temperature, which vapor pressure of CO 2 is very high, oly adsorbed amout at low relative pressure is performed with ordiary adsorptio apparatus. Or the high-pressurized equipmet is required. The covetioal coditio for surface area determiatio is the itroge adsorptio at liquid itroge temperature (77 K) or its boilig poit [Sig et al., 1985 ad Rouquerol et al., 1994].

5 3.4 Covetioal Surface Area Apparatus Two methods of measurig adsorptio isotherm are volumetric ad gravimetric techiques. Volumetric techique measures the volume of gases adsorbed usig the gas laws while gravimetric techique measures weight chage by microbalace. Both methods have to degas prior to the adsorptio test. The pricipal parts are a adsorptio vessel, vacuum pump, pressure cotroller ad detector, coolig system ad vacuum-tight coectors. Figure 3.2 is a schematic diagram of volumetric BET apparatus accordig to the British Stadard. The covetioal equipmet is maually operated ad complicated to operate ad i additio some parts are very fragile. However, recetly, there is much commercial surface area measuremet equipmet available. Examples of two commercial surface area measuremet apparatus are show i Figure 3.3. Both apparatus are fully automatic coected with computer for aalyzig data. Commercial apparatus are coveiet so that they widely selected for use i researched ad idustrial operatios. Figure 3.2 Volumetric BET apparatus accordig to British Stadard 4359 [Mikhail ad Robes, 1983]

6 Figure 3.3 Pictures of two commercial surface area apparatus [ ad As metioed previously, our aim is to check the quality of charcoal produced by traditioal method thus usig the covetioal or commercial surface area apparatus is too complicated ad too expesive. The people i rural areas or regioal health office caot afford the high operatio ad maiteace cost for such a device. Moreover, i case of covetioal coditio, liquid itroge is used as coolat so that it is difficult to provide i the rural areas. To accomplish our ultimate purpose, the ew module must be easy to use, affordable ad have readily available replacemet compoets i domestic areas. A simple surface area measuremet was desiged ad assembled. After that its performace was ivestigated icludig frictio test, leak test ad correctio values were also suggested. I additioal, error aalysis of this module was calculated ad adsorptio isotherm of simple module was compared with that of covetioal apparatus. I the secod part, ot oly adsorptio test with several kids of adsorbets were coducted ad aalyzed but surface areas of sample were also calculated.

7 3.5 Equipmet Set-up The simple surface area measuremet module is composed of had-made syrige for pressure detector, water-drive aspirator for evacuatio system, dry ice-methaol as the coolat of this system istead of liquid itroge, sample tube, rubber tube ad plastic valve. Cookig oil is used for leakage protectio ad frictio reductio. All compoets are available i a domestic retail-shop. The price of this module is about 100 US dollar. Compoets ad picture of the simple surface area measuremet module are illustrated i Figure 3.4 ad 3.5. The schematic drawig of the simple module is also show i Figure 3.6. Coditio ad device used i the covetioal adsorptio apparatus compared with that i the simple surface area measuremet module is depicted i Table 3.1. Table 3.1 Coditio ad device used i covetioal apparatus ad the simple surface area measuremet module Covetioal Apparatus Simple Surface Area Measuremet Module Pressure Detector Digital Pressure Detector Had-made Syrige or maometer Evacuatio System Vacuum Pump Water Aspirator Adsorbate Gas Nitroge gas Carbo Dioxide Coolat Liquid Nitroge (77 K) Dry ice-methaol (195 K) Tube-system Vacuum glass type or Rubber tube with strig Stailess Steel ad plastic valve

8 Figure 3.4 Compoets of simple surface area measuremet module Figure 3.5 Picture of simple surface area measuremet module

9 Syrige Three-way valve 1 Cylider Three-way valve 2 Sample tube Sample Dry Ice Methaol Coolat Aspirator Rubber tube Figure 3.6 Schematic drawig of simple surface area measuremet module