Applications of Liquid Carbon Dioxide for Removal of Surface Contamination

Chapter 5

Applications of Liquid Carbon Dioxide for Removal of Surface Contamination Rajiv Kohli The Aerospace Corporation, NASA Johnson Space Center, Houston, TX, USA

Chapter Outline 1 Introduction 2 Surface Contamination and Cleanliness Levels 3 Characteristics of Liquid Carbon Dioxide 3.1 Carbon Dioxide as a Dense Phase Fluid 3.2 Liquid Carbon Dioxide 3.3 Principles of Liquid Co2 Cleaning

1

171 172 173 173 174 182

3.4 Cleaning Systems 3.5 Application Examples 4 Other Considerations 4.1 Costs 4.2 Advantages and Disadvantages of Liquid CO2 Cleaning 5 Summary Acknowledgments References

185 187 191 191

192 194 194 195

INTRODUCTION

Carbon dioxide (CO2) is a highly desirable alternate cleaning medium to chlorinated solvents, hydrochlorofluorocarbons (HCFCs), trichloroethane and other ozone-depleting solvents for precision cleaning [1–27]. It is inexpensive, naturally abundant, relatively inert towards reactive compounds, nontoxic, nonflammable, and can be easily recycled. The CO2 used for cleaning is recycled, that is, gas collected from industrial processes which would otherwise have been emitted through smokestacks to the atmosphere. This gas is purified and used in the cleaning process (as well as in other applications), before it eventually returns to the atmosphere. CO2 in its different states has been used for removal of surface contaminants in a wide variety of industries for many years. CO2 gas can also be used for cleaning by blowing it at high velocity over the contaminated substrate, but this method is only effective for particles larger

Developments in Surface Contamination and Cleaning, Volume 11. https://doi.org/10.1016/B978-0-12-815577-6.00005-0 Copyright © 2019 Elsevier Inc. All rights reserved.

171

172 Developments in Surface Contamination and Cleaning

than about 50 μm [22]. Solid CO2, in the form of dry ice pellets, is well established for cleaning applications such as paint removal on aircraft, nuclear and asbestos decontamination, and cleaning and restoration of historical monuments [21,22,24]. CO2 snow cleaning, which employs a less-dense form of dry ice, is a gentle precision cleaning technique that can remove thin films of surface organic contaminants (< 10 nm thick), as well as particles in the 30 to 40 nanometer range [17–19]. Supercritical CO2 (SCCO2) has both gas-like transport properties and liquid-like solvent properties which makes it more flexible than the other states for contaminant removal, although the process must be operated at very high pressures [25–27]. The liquid state of CO2 exhibits attractive physical properties that are increasingly being exploited for surface contaminant removal. Liquid CO2 (LCO2) cleaning involves immersion in a cleaning vessel with agitation to increase the effectiveness of the removal process. Both SCCO2 and LCO2 are dense-phase states of CO2. However, as compared with SCCO2 cleaning, the operating pressures in LCO2 cleaning are considerably lower. The applications of LCO2 for removal of surface contaminants were reviewed recently as part of a broader review of dense-phase CO2 [25]. The focus of this chapter is to update the previously published information on LCO2 and its cleaning applications. SCCO2 cleaning is reviewed separately in Chapter 6 in this volume.

2 SURFACE CONTAMINATION AND CLEANLINESS LEVELS Surface contamination can be in many forms and may be present in a variety of states on the surface [28]. The most common categories include: particles; organic contaminants which may be present as hydrocarbon films or organic residue such as oil droplets, grease, resin additives, and waxes; molecular contamination that can be organic or inorganic; ionic contamination; metallic contaminants present as discrete particles on the surface or as trace impurities in matrix; and microbial contamination. Common contamination sources can include machining oils and greases, hydraulic and cleaning fluids, adhesives, waxes, human contamination, and particulates, as well as from manufacturing process operations. In addition, a whole host of other chemical contaminants from a variety of sources may soil a surface. Cleaning specifications are typically based on the amount of specific or characteristic contaminant remaining on the surface after it has been cleaned. Cleaning specifications are typically based on the amount of specific or characteristic contaminant remaining on the surface after it has been cleaned. Cleanliness levels for space hardware and for other precision technology applications are specified for particles by size (in the micrometer (μm) size range) and number of particles, as well as for hydrocarbon contamination represented by nonvolatile residue (NVR) in mass per unit area for surfaces or mass per unit volume for liquids [29–31]. The surface cleanliness levels are based on contamination levels established in industry standard IEST-STD-CC1246E

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for particles from Level 5 to Level 1000 and for NVR from Level R1E-5 (10 ng/0.1 m2) to Level R25 (25 mg/0.1 m2) [31]. Many of the products and manufacturing processes are also sensitive to, or they can even be destroyed by, airborne molecular contaminants (AMCs) that are present due to external, process or otherwise generated sources, making it essential to monitor and control AMCs [32]. An AMC is a chemical contaminant in the form of vapors or aerosols that can be organic or inorganic, and it includes everything from acids and bases to organometallic compounds and dopants [33,34]. A new ISO Standard 14644-10, “Cleanrooms and associated controlled environments – Part 10: Classification of surface cleanliness by chemical concentration” [35] is now available as an international standard that defines the classification system for cleanliness of surfaces in cleanrooms with respect to the presence of chemical compounds or elements (including molecules, ions, atoms, and particles). In many other commercial applications, the precision cleanliness level is defined as an organic contaminant level of less than 10 μg of contaminant per cm2, although for many applications the requirement is set at 1 μg/cm2 [31]. This cleanliness level is either very desirable or is required by the functional use of parts such as metal devices, machined parts, electronic assemblies, optical and laser components, precision mechanical parts, and computer parts. Standard ISO 14644-13 gives guidelines for cleaning of surfaces in cleanrooms to achieve defined levels of cleanliness in terms of particles and chemical classifications [36].

3

CHARACTERISTICS OF LIQUID CARBON DIOXIDE

The key characteristics of LCO2 relevant to cleaning applications are discussed in the following sections.

3.1 Carbon Dioxide as a Dense Phase Fluid1 Critical phenomena and the supercritical phase were first discovered in 1822 [37,38]. Above a critical temperature, the distinction between the liquid phase and the gas phase disappears, resulting in a single supercritical fluid phase. The term critical point was coined in 1869 from experiments performed on CO2 [39]. The attractiveness of near-critical and supercritical fluids as solvents stems from their unique combination of liquid-like and gas-like properties. Table 5.1 compares the diffusivity, viscosity and density of a typical organic fluid in the liquid, gas, and supercritical fluid state. The properties of the supercritical phase

1. Throughout this chapter “dense phase” refers to liquid carbon dioxide and “fluid” refers to the liquid phase.

174 Developments in Surface Contamination and Cleaning

TABLE 5.1 Comparison of Physicochemical Properties of a Typical Organic Fluid in the Liquid, Gas, and Supercritical Fluid States State

Diffusivity (cm2/s)

Viscosity (mNs/m2)

Density (kg/m3)

Liquid

<105

1

1000

2

Supercritical fluid

10

Gas

0.1

3

10

2

10

300

102

1

are intermediate between the gas phase and the liquid phase, with the diffusivity and viscosity similar to the transport properties of gases, but the density is similar to that of a liquid.

3.2 Liquid Carbon Dioxide At room temperature, CO2 can exist as a liquid if kept in a closed system at elevated pressure. When CO2 is compressed, it acquires increasingly liquid-like densities. Depending on the pressure and temperature, CO2 is obtained as a liquid or as a supercritical fluid. Upon heating the liquid CO2 in a closed vessel, it changes into a supercritical fluid above 304.3 K and 7.38 MPa. On the pressure–temperature phase diagram for CO2 (Fig. 5.1), the liquid phase exists above the triple point (0.52 MPa, 216.45 K) and the supercritical phase exists above the critical point (7.38 MPa, 304.2 K). Carbon dioxide has no liquid state at pressures below 0.52 MPa. The surface tension of liquid CO2 is
1.5 mN/m and reaches zero at the critical point and allows LCO2 and LCO2-based solvents to penetrate and wet nearly all features of a part. Also, CO2 has no permanent dipole moment and thus has low polarizability, but it has a strong quadrupole moment which affects its physical properties, including the high critical pressure of CO2 [46–52]. It can act as a hydrogen bond acceptor, so hydrogen bond donors are soluble in LCO2.

3.2.1 Physical and Transport Properties Liquid CO2 has a high density (
950 kg/m3 at 277 K). Small changes in the temperature or pressure near the critical point result in large changes in density, as seen in Fig. 5.2 for CO2 [42, 53–59]. Because the solvating power of a fluid is generally related to its density, this gives LCO2 its strong solvating powers near the critical point. Thus, the tunable density, and therefore the tunability of solvent power, which can be varied from gas to liquid via a simple change of pressure at constant temperature, and the solvation effects at the densities in the vicinity of the

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FIGURE 5.1 Pressure – temperature phase diagram for CO2. Pc is the critical pressure; Ptp is the pressure at the triple point; Tc is the critical temperature; Ttp is the temperature at the triple point [40–45].

FIGURE 5.2 Variation of density of CO2 as a function of temperature at different pressures.

176 Developments in Surface Contamination and Cleaning

critical point provide the most attractive attributes of supercritical fluids for surface contaminant removal. However, as the temperature is raised, the change becomes more pronounced only at higher pressures. This makes it difficult to control the density near the critical temperature, and, consequently, control of processes in the critical region is difficult. The transport properties of LCO2 are also important to its application as a precision cleaning fluid. The gas-like viscosity (
0.11 mPa.s at 277 K) enables it to effectively penetrate fine scale structures such as high aspect ratio vias, through-holes, small pores and cervices, and clean components with complex geometries and tight spaces. The viscosity rises with pressure similarly to the density, however, the effect is less pronounced (Fig. 5.3) [56,60–63]. In general, the viscosity is an order of magnitude lower than the viscosity of typical organic solvents [40]. The self-diffusivity of CO2 in the vicinity of the critical point is approximately 5  104 cm2/s, which is nearly two orders of magnitude larger than the diffusivity of solutes in normal liquids [1]. Many organic solutes too have significantly higher diffusion coefficients in the supercritical region, although the diffusion coefficients drop off in the critical region and are nearly zero at the critical point [64]. This has the advantage of faster transport of the contaminants and shorter process times during removal of contaminants near the critical point. Finally, the very low surface tension of LCO2 (
1.5 mN/m at 293 K) provides for excellent wettability of components with complex geometries that make it very attractive in the dense phase for commercial precision cleaning applications [40,41,43,65–67].

FIGURE 5.3 Variation of viscosity of CO2 as a function of temperature at different pressures.

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3.2.2 Solubility Considerations Liquid CO2 is a very good solvent for nonpolar, low molecular weight organic compounds, such as greases, oils, lubricants, and fingerprints [1,4]. It is this ability to dissolve organic compounds that underlies the commercial applications of LCO2 cleaning. On the other hand, hydrophilic compounds, such as inorganic salts and metal ions, do not dissolve. This has been attributed to the large quadrupole moment and the weak van der Waals forces in CO2 [68,69]. Extensive compilations of the solubility of organic compounds in sub-critical CO2 can be found in [70–74]. The solubility behavior in binary mixtures of CO2 and a cosolvent can be considered in terms of the contributions of the individual forces to the total energy of vaporization, E. Thus, E ¼ E d + Ep + Eh

(5.1)

where Ed, Ep, and Eh are the energy contributions due to dispersion forces, polar interactions (dipole-dipole forces), and hydrogen bonding, respectively [75]. The Hildebrand solubility parameter, δ, is (E/V)½ and Eq. 5.1 becomes δ2 ¼ δ2d + δ2P + δ2h

(5.2)

where δd, δp, and δh are the Hansen solubility parameters for the dispersion, polar, and hydrogen bonding interactions, respectively [75,76], and V is the molar volume. This approach considers the solubility in terms of the intermolecular forces of the solvents that have unique molecular structures and exhibit unique solubility behavior. Thus, the Hansen solubility parameters are of significant value in predicting the solubility behavior of cosolvents. For cosolvents such as methanol, ethanol, propanol, acetone, ethylene glycol, and water, the values for δd do not differ significantly from the values for SCCO2. By contrast, the polar and hydrogen bonding Hansen parameters, δp and δh, are considerably higher than the values for LCO2 [75]. This would suggest that there is a lack of miscibility in these binary mixtures. In the liquid state, these cosolvents have high polarities and they form self-associates. Also, these compounds can hydrogen bond with their own molecules in CO2, leading to reduced dipole moment and polarity of the binary mixture. The reduced polarity of the cosolvent component due to self-association leads to better matching of the polarity with LCO2 and high miscibility, as has been observed in the CO2– alcohol binary and ternary systems [77,78]. In fact, it is possible to form and maintain a single phase at high pressures over the entire composition range [77]. The large quadrupole moment of CO2 affects the value of δ, which reduces the nonpolar δd for ionic compounds. For example, at pressures above 20 MPa, the value of δ for CO2 is higher than the value for ethane, yet CO2 is incapable of dissolving an ionic surfactant with a hydrocarbon tail, such as AOT (sodium bis(2-ethylhexyl) sulfosuccinate) [79], which readily dissolves in ethane.

178 Developments in Surface Contamination and Cleaning

Nearly 20% of δ value can be attributed to the quadrupole moment, which reduces δd considerably below the value for ethane. Until recently, the lack of solubility of hydrophilic solutes in CO2 has been a serious limitation to widespread use of LCO2 for removing polar inorganic contaminants and particles in precision cleaning applications. This limitation is increasingly being overcome by new developments to incorporate cosolvents in CO2, such as low molecular weight alcohols and acetone, to increase the solvent strength. For example, the addition of only 2 mol percent of tri-n-butyl phosphate to CO2 increases the solubility of hydroquinone by a factor of 250 [41]. Surfactants and water too have been added to SCCO2 to form microemulsions and dendritic micelles, as well as chelating fluorinated ligands [41,80–90]. These molecules can dissolve inorganic solutes and ionic species.

3.2.3 Thermodynamic Data and Properties To understand the chemical interactions in CO2/cosolvent systems, reliable thermodynamic properties are needed at the high temperatures and pressures for the species under consideration at process operating conditions. As very few direct measurements have been reported on the molar thermodynamic properties of cosolvents in LCO2, it becomes necessary to consider thermodynamic models to predict these properties. Recent advances in high temperature solution chemistry make it possible to accurately predict the properties of aqueous and nonaqueous species by extending the thermodynamic data base into the near-critical and supercritical regions for CO2 as well as for water, because the solution behavior of CO2 in these regions is analogous to nonpolar supercritical water. The presence of highly polar compounds in the binary CO2 systems requires that a rigorous molecular thermodynamic model take into account all of the interactions in the system, including hydrogen bonding, electrostatic attraction, and ion hydration, as well as hard sphere repulsion and dispersion-attraction forces [91,92]. The Helmholtz energy A of the system is composed of the sum of the individual contributions of these interactions: A ¼ ARef + AAssoc + ABorn + ACoul + ADis + ARep

(5.3)

where the superscripts denote, respectively, the reference state for an ideal fluid; the association term due to hydrogen bonding among polar molecules and electrostatic interactions between solvent–solute molecules; Born interaction due to ion formation; Coulombic ion–ion interaction; the dispersion term; and the repulsion among the solute–solvent molecules. In the case of CO2 systems, depending on polarity of the cosolvent and the composition of the system, not all of these interactions will take place. A substance-dependent equation of state (EOS) limits the applicability of the EOS to predict thermodynamic properties.

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A more universal approach for predicting the thermodynamic properties of aqueous species over a wide range of temperatures and pressures is based on the Born equation [93,94] for the absolute standard partial molal Gibbs free energy 0 of solvation ΔGj which can be written for the jth ion as:   N 0 Zj2 e2 1 0 ΔGj ¼ 1 (5.4) 2re, j ε where N0 denotes Avogadro’s number, ε is the relative permittivity of water, e is the electronic charge, re,j is the effective electrostatic radius of the jth ion, and Zj is the formal charge on the jth ion. The bulk relative permittivity can be used in the Born equation if provision is made to include dielectric saturation, which is a function of pressure and temperature, in the definition of the electrostatic radius. A widely used approach is the use of the revised Helgeson–Kirkham– Flowers (HKF) equation for predicting thermodynamic properties [95,96]. Although this approach was developed strictly for ions in aqueous electrolyte solutions, the basic principle is applicable to the CO2 binary systems because of strong solvent clustering about the solute even in nonpolar systems [97]. Regression analysis of the available experimental thermodynamic data for alkali and alkaline earth chlorides showed the following close correlation between the crystal radius rx,j and the electrostatic radius of the ions [96]: re, j ¼ rx, j + Zj ðkz + gÞ (5.5) ˚ for cations and near 0 A ˚ for anions, Here kz is a constant ranging from 0.94 A and g is a generalized temperature-dependent solvent function that accounts for dielectric saturation and the compressibility of the solvent at high temperatures and pressures. In low-density liquids, this g-function has the distinct physical sense of contracting the associated molecule. By using calorimetric and density data at lower temperatures and pressures, it is possible to calculate values of the gfunction and re,j from room temperature and pressure into the supercritical region. The revised HKF model provides correlation algorithms that permit prediction of species-dependent parameters in revised equations-of-state for aqueous ions and electrolytes. The algorithms can be used together with values of the 0 standard partial molar entropies (S0 ), volumes (V 0 ), and heat capacities (Cp ) of the ions at 298 K and 0.1 MPa to calculate the standard partial molal thermodynamic properties at pressures and temperatures to 500 MPa and 1273 K [95,96,98–102]. The HKF model is more generally applicable and more accurate than strictly electrostatic or density models [103–106]. Furthermore, the fact that the EOS parameters can be estimated from the correlation algorithms makes these equations widely applicable, which is largely not true of other proposed models [107–111]. The equations in the other models can be used only if sufficient experimental data are available for regression analysis, and even then only for a given species over the pressure and temperature range represented by the data.

180 Developments in Surface Contamination and Cleaning

More recently, a group contribution model has been proposed for improved prediction of the thermodynamic properties of organic aqueous solutions based on the contributions of the functional groups of the compounds [112–118]. In this model, the thermodynamic functions are determined as the sum of the structural contributions of individual species based on the assumption of functional group additivity. An alternate approach has been to adjust the correlation algorithms of the revised HKF model based on new parameters generated from experimental high temperature data and a correlation developed for the Gibbs energy of hydration [8]. Both these approaches have improved the prediction accuracy and have extended the applicability of the revised HKF model to aqueous systems, nonelectrolytes, ionic and neutral species, and organic and inorganic species over the entire pressure and temperature range from ambient into the supercritical region. A new unified theoretical model has been proposed based on correction to the Born equation for non-Born hydration effects [119]. This model has the advantage that the effective ionic radius is very accurately defined by the standard state Gibbs free energy of hydration. Only two parameters in the model are required that can be derived from the experimental standard state partial Gibbs free energies of the species. The constants are independent of temperature and pressure, which makes the model very useful for predicting the high temperature thermodynamic properties of the electrolyte species. The correlation algorithms in the revised HKF model can be modified for application to CO2 binary systems [9]. Regression analysis of available thermodynamic data for 150 polar and nonpolar fluids at 298 K and 0.1 MPa has been performed to obtain the revised EOS parameters. This information has been used to calculate the thermodynamic properties in several binary systems with LCO2 and SCCO2. One example of the calculated values for molar volume is shown in Table 5.2 for CO2 + 1-propanol system along with recent experimental data [73,74,121]. In general, the agreement is within 1–3% between the calculated values of the thermodynamic functions and the limited available experimental data for CO2 binary mixtures of the species considered here. Based on these results, the correlation algorithms of the revised HKF model with modified EOS parameters can be applied to predict the thermodynamic properties of CO2 binary mixtures. The revised HKF model has also been used to estimate the molar volume of saturated alcohol/CO2 mixtures at different temperatures and pressures (Table 5.3). The estimated values are in good agreement (within
5%) of the experimental data [120,122], thereby suggesting that the assumption of CO2 as a nonpolar solvent analogous to supercritical water is generally valid [6].2 2. An independent check of the application of the updated revised HKF model was made by estimating the standard molar heat capacity of NaCl (aq) solution in the temperature range 573 to 673 K. The estimated values were in excellent agreement with recent experimentally measured values [123,124].

8.31 8.75 –

2.25

4.37

6.48

8.26

–

–

0.2

0.4

0.6

0.8

0.9

0.95

6.61

4.53

2.31

1.87

1.88

0.1

Calculated

Experimental

8.0

Mol fraction

Pressure (MPa)

2.44

2.89

2.91

2.43

1.84

0.87

0.45

Experimental

9.0

2.51

2.97

3.01

2.48

1.91

0.89

0.46

Calculated

1.08

1.32

1.46

1.32

1.01

0.53

0.25

Experimental

9.8

1.09

1.39

1.52

1.36

1.21

0.59

0.27

Calculated

TABLE 5.2 Comparison of Molar Volumes (1022 × m3.kmol21) for CO2 + 1-Propanol Mixtures at 313 K for Different Mol Fractions of CO2 Applications of Liquid Carbon Dioxide Chapter 5

181

182 Developments in Surface Contamination and Cleaning

TABLE 5.3 Calculated Values of the Molar Volume of 2% alcohol/CO2 Mixtures as a Function of Pressure and Temperature Molar Volume, cm3.mol21 Alcohol

250 K

350 K

450 K

100 MPa

36.75

41.12

43.04

200 MPa

35.68

38.03

40.59

250 MPa

34.97

37.61

39.27

100 MPa

53.08

59.35

62.36

200 MPa

51.60

55.74

58.59

250 MPa

50.69

54.93

56.66

Methanol

Ethanol

3.2.4 Vapor–Liquid Equilibria The EOS approach using a cubic perturbed hard-body model of the solute can be employed to calculate the vapor–liquid equilibria in CO2 binary systems [125,126]. This model is applicable to mixtures of polar and nonpolar fluids [127]. With this EOS, the conventional linear mixing rule is replaced with a volume parameter for the mixing rule, as the linear mixing rule is not adequate for supercritical mixtures [128]. The calculated values for the vapor–liquid equilibria are shown in Table 5.4 for several CO2 binary short chain alcohol systems together with available experimental data [81,82,122,129,130]. The agreement is very good over the entire composition range. The present method can be employed to predict phase equilibria in other binary CO2 systems for which there are no experimental data. 3.3 Principles of Liquid CO2 Cleaning Liquid CO2 exhibits strong solvent power for dissolving nonpolar organic compounds. Organic contaminants that are most often encountered in industrial applications consist of nonpolar compounds as films or as particles. Thin films are removed easily by dissolution either in pure CO2 or in a mixture with a cosolvent. Organic particles too can be removed because CO2 will help loosen the particles on the surface. For highly polar contaminants, the solubility can be enhanced by adding a suitable cosolvent in an amount sufficient to maintain the near-critical liquid phase at the system operating conditions (temperature and

1.662

1.976

2.300

2.634

3.009

3.415

3.810

4.215

303

313

323

333

343

353

363

373

0.89

4.215

3.790

3.415

2.979

2.614

2.320

2.016

1.679

1.558

Calculated

Measured

5.55

6.274

7.605

T (K)

314.5

325.2

337.2

9.011

7.632

6.571

Ethanol

14.996

14.128

12.899

11.429

9.878

8.580

7.247

5.920

5.370

Calculated

9.189

7.762

6.522

Calculated

0.54–0.55 Measured

15.026

13.942

12.838

11.389

9.883

8.573

7.240

5.887

5.360

Measured

0.51

10.363

8.825

7.453

15.666

15.497

14.198

12.800

11.239

9.548

7.952

6.293

5.681

Calculated

10.735

8.927

7.511

Calculated

0.39–0.40 Measured

15.553

15.401

14.044

12.737

11.186

9.490

7.968

6.343

5.674

Measured

0.29

– –

– –

10.845

9.349

7.894

Measured

Continued

10.908

9.516

7.792

Calculated

0.15–0.22

12.545

11.858

10.719

9.639

8.209

6.820

6.282

Calculated

12.453

11.794

10.639

9.585

8.284

6.896

6.313

Measured

0.09

5

7.712

6.301

5.834

Calculated

0.64–0.68

1.540

298

Mol fraction

Measured

T (K)

Mol fraction

Methanol

TABLE 5.4 Comparison of the Measured and Calculated Pressures (MPa) for Vapor–Liquid Equilibria for Selected CO2 Binary Systems. The Mol Fraction is for the Binary Component

Applications of Liquid Carbon Dioxide Chapter

183

0.65–0.66

Measured

5.433

6.226

6.902

Mol fraction

T (K)

314.5

325.2

337.2

7.823

6.519

5.948

Calculated

8.708

7.605

6.895

Measured

0.52–0.55

9.427

7.811

6.602

Calculated

Butanol

11.025

9.466

7.984

Measured

0.31–0.35

11.038

9.791

8.223

Calculated

11.776

11.976

10.130

8.618

– 9.873

Calculated

Measured

0.20–0.22

TABLE 5.4 Comparison of the Measured and Calculated Pressures (MPa) for Vapor–Liquid Equilibria for Selected CO2 Binary Systems. The Mol Fraction is for the Binary Component—cont’d

184 Developments in Surface Contamination and Cleaning

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185

pressure). The discussion and calculations presented in the previous section can help define the solubility and phase equilibria and the process operating conditions for removal of contaminants. In many precision cleaning applications, the contaminants are present on the surface as nonorganic particles. These particles adhere to the surface by strong van der Waals, electrostatic or capillary forces [9]. Mechanical agitation using ultrasonic or megasonic transducers or magnetic stirring have been found to be effective in removing particles adhered to the surface [131–137].

3.4 Cleaning Systems The cleaning mechanism for LCO2 is based on dissolving the contaminants in the solvent. The operating pressure and temperature are generally higher for supercritical CO2 than for LCO2 cleaning. The LCO2 cleaning process is primarily a degreasing process. The principle of operation of an LCO2 cleaning system is similar to that of closed vapor degreasing and immersion cleaning systems in which the part is immersed in the solvent for cleaning [5,89,131–155]. A representative process schematic is shown in Fig. 5.4 [137]. The parts to be cleaned are placed into the cleaning chamber. Clean LCO2 is automatically transferred from a supply vessel within the recovery system into the cleaning chamber. The cleaning chamber can be configured with different process options such as a turntable for spinning the part, a means for ultrasonic or megasonic agitation of the bath, or sprays for supplemental rinsing of the

FIGURE 5.4 Schematic of a representative LCO2 cleaning process [137].

186 Developments in Surface Contamination and Cleaning

FIGURE 5.5 Typical mechanisms for LCO2 cleaning of surfaces: (a) spraying; (b) gas bubbling; (c) rinsing [151].

surface. Fig. 5.5 shows some of these cleaning mechanisms schematically [151]. These mechanisms enhance the effectiveness of the removal process for particulates. A novel technique is bidirectional centrifugal agitation that produces a shearing action across the surface, as well as changes the properties of the fluid while it traverses from the edge to the center of a planar substrate [150]. Following the cleaning cycle, contaminated LCO2 is transferred from the cleaning chamber into the recycling system for separation and recovery operations. Clean dry parts are removed from the chamber. A typical cleaning cycle takes approximately 15 to 30 minutes, including loading and unloading parts and depends also on the quantity of parts. The process throughput can be 1–2 batches per hour with a single 10-liter cleaning chamber and up to 6 batches per hour in a system with three 10-liter working chambers. The CO2 recovery system separates contaminants from the LCO2. The contaminants are captured and filtered before disposal. The recovered CO2 is transferred into the supply tank for reuse. The recycling system is capable of recovering better than 95% of the CO2 for reuse. The reclamation system may have several features, including additive (cosolvent) injection and the capability of recovering CO2 off-line or on-line. State-of-the-art integrated cleaning systems include cleaning, rinsing, and distillation contained in a single unit with a programmable logic controller (PLC) for automated processing. Process temperatures generally range from 280 to 300 K, while pressures range between 5 and 8 MPa for LCO2 cleaning operations. Depending on the chemical conditions, the cleaning system can be tuned to operate under subcritical (liquid) or supercritical conditions, and in a vertical (centrifugal agitation) or horizontal (centrifugal/tumbling agitation) orientation. CO2 flow rates range between 10 kg/hr and 80 kg/hr, depending on the capacity of the system. Commercial systems with large cleaning vessels up to 760-liter capacity are available [134–137].

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Similar to vapor degreasing, LCO2 immersion cleaning has only limited capability to remove particulate matter from a surface without additional mechanical enhancements such as ultrasonics and megasonics, or sprays [131–137]. The process will remove most light and medium molecular weight hydrocarbon oils, gross particulate contamination, drawing compounds, and other machining fluids. The best applications are those where organic vapor degreasing solvents will work. LCO2 will not remove rust, paint, coatings, or most adhesives. These are typically removed by other surface preparation techniques such as abrasive blasting or surface stripping. As opposed to aqueous systems, once the pressure is released, LCO2 rapidly returns to a gas without leaving a liquid within the surface porosity. This is particularly advantageous on parts with complex geometries (such as capillaries) or porous structures (such as sintered parts or implants). The mechanical properties of medical polymers with higher crystallinity are not affected by LCO2 exposure, indicating good tolerance to LCO2, although more amorphous polymers show swelling and distortion [156]. These results are relevant to evaluating the potential of LCO2 for sterilization technology.

3.5 Application Examples Dense-phase CO2 has been employed for a wide range of precision and commercial cleaning applications including metal surfaces, sintered porous metal substrates, glass, optical elements, Si wafers, polymers, parts with complex geometries and tight spaces, sterilization and disinfection of medical equipment, cleaning microbially-contaminated surfaces, garment cleaning and pesticide mitigation in museum collections. LCO2 has been successfully used with additives (cosolvents, surfactants, dispersants, and chelating agents) to remove a wide variety of contaminants [3,5,12,15,20,89,134–137,142,157–174]. The types of contaminants removed include greases, lubricants, silicone oils, machining oils, flux residue, trace metals, photoresists, outgassing compounds, printing ink, adhesives, and small particles. Addition of a small amount of a polar cosolvent such as ethyl alcohol has been found to decrease the critical diameter for nucleation of a contaminant particle by more than an order of magnitude. The cosolvent tends to adsorb on the surface to increase the particle– surface distance and decrease the attractive adhesion force. Even nanoparticles may be dislodged from the surface by use of appropriate surfactants.

3.5.1 Textile and Garment Cleaning Textile and garment cleaning has been the widest commercial application for LCO2 as a viable alternative to conventional dry cleaning technologies utilizing perchloroethylene and petroleum-based solvents. Since the first patent was issued in 1977 [175], there have been a large number of developments in the cleaning technology and in the equipment and process for dry cleaning,

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including development of surfactants and co-solvents [138,176–199]. These developments have enabled the LCO2 dry cleaning process to reach a level of technical maturity to effectively remove nonpolar, water-soluble, and polar soils simultaneously from garments, as well as mechanical and sonic agitation to assist in removing particles. In addition, heat-free drying with LCO2 prevents damage to the garment fibers and extends garment life to improve financial and environmental performance.

3.5.2 Sterilization and Disinfection In the health and food sectors, the risk of transmission of diseases due to contamination is a subject of growing concern. As an example, viral and bacterial contamination of implants and allograft tissue, widely used by transplant surgeons for orthopedic (joint replacement), trauma, and cancer (surgical reconstruction) procedures, can have disastrous consequences for the patient and are a major concern [200]. Similarly, safe contamination-free liquid foods and beverages are critical to human health [201]. Efforts to minimize disease transmission have generally included donor screening, bioburden assessment, and aseptic handling, as well as sterilization before, during and after processing [202–218]. Terminal sterilization refers to a sterility assurance level (SAL) of 106 (SAL6 is considered the standard for medical devices [219]) and describes the process that ensures that the medical devices and implants are sterile at the point of use. Common sterilization methods include: steam autoclaving; dry heat; sterilizing gaseous (for example, ethylene oxide, chlorine dioxide, hydrogen peroxide) or liquid (such as glutaraldehyde, hydrogen peroxide, formaldehyde) chemicals; gamma, X-ray or electron beam irradiation; and UV-ozone treatment. All of these methods have certain drawbacks and limitations and cannot be applied to many materials and substances that are temperature sensitive or reactive with other forms of sterilization. For example, achieving terminal sterilization with these methods frequently compromises the osteogenic and biomechanical properties of the allograft. A novel approach to sterilization, with emphases on reducing the process temperature and minimizing contamination, is based on dense-phase CO2 technology [202–218]. The process involves exposure of the item to LCO2 or SCCO2 with additives such as hydrogen peroxide or peracetic acid, at temperatures in the range 313 K to 333 K and pressures from 5 to 30 MPa. Treatment times can range from 5 minutes to 6 hours to achieve SAL6, depending on the microorganism. Clinically relevant gram-positive and gram-negative vegetative bacteria can be deactivated using LCO2 in single step, but the treatment efficacy increases rapidly with temperature. Rapid pressurization/depressurization cycles also have a very positive effect on sterilization efficacy due to membrane disruption and cell lysis. Bacterial spores can also be sterilized with CO2-based processes.

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Liquid CO2 can penetrate and sterilize delicate products and materials, such as sintered metal porous implants, allograft tissue and engineered tissues including skin, ligament, tendon and bone, without damage to the structural or chemical integrity of the products. The very low surface tension of LCO2 facilitates penetration into the interior of the tissue, thereby allowing inactivation of embedded pathogens. Other biological sterilization applications include: inactivation of viruses; elimination of endotoxins and pyrogens; production of sterile immunogenic preparations; surgical implants and medical devices; sterilization of active and inactive pharmaceutical ingredients; and pest control (killing of insect eggs and larva).

3.5.3 Conservation of Historical Art Objects and Structures Several applications of dense-phase CO2 have been developed for cleaning of historical art objects and structures [220–227]. Museum collections often use organic and inorganic pesticides containing toxic metals such as arsenic and mercury for conservation and protection of the art objects. Unfortunately, this often resulted in contamination of the indoor air from the dust settling on the treated surfaces of the objects. In other cases, fragile textiles of historic value that may have also been damaged must be cleaned to preserve their fiber structure and value. Other materials needing remediation include waterlogged paper and wood, fluoropolymer coated stone, and various fabrics such as silk, wool, and leather. Surface cleaning will not remove the hazardous chemicals embedded in the matrix. The use of LCO2 has been successfully demonstrated for cleaning various objects. Treatment times are generally just a few minutes to extract the contaminants from artifacts without damaging fragile materials and without leaving a residue. Not surprisingly, removal efficiencies of 80– 95% have been achieved for organic pesticides, but inorganic or polar compounds are not removed to a sufficient extent. In the case of a gilded leather tapestry, the surface was not physically damaged or degraded and there was no loss of material by LCO2 cleaning (Fig. 5.6). Extraction effectiveness is improved with a cosolvent such as water, isopropanol, or acetone. Detailed knowledge about the properties of the different materials (contaminants and objects) is necessary in order to prevent any possible damage to objects that are sensitive to LCO2. 3.5.4 Wellbore Cleaning An innovative industrial application of LCO2 is for cleaning of wellbores and near-wellbore areas of gas or hydrocarbon-bearing formations [228,229]. Nearwellbore formation refers to the volume of the formation which is adjacent to the production zone. In many methane-producing coal beds, water introduced into the microcleat system of the coal formation from natural sources or from drilling operations interferes with the production of methane since water combined with debris acts to block the natural flow paths through which the methane

190 Developments in Surface Contamination and Cleaning

FIGURE 5.6 Removal of grease from a gilded leather tapestry (a) before and (b) after cleaning with LCO2 [225].

is produced. The formation pressure in high pressure wells may be sufficient to overcome the presence of the water. However, when the formation pressure drops, methane gas flow is reduced or even stopped. In other wells with a liner or a casing, the presence of hydrocarbon oils and other debris can reduce production by similar blocking mechanisms. The proposed method involves injecting a treatment liquid containing 85 to 100% LCO2, up to 15% of an alcohol, and 0.5% of a surfactant into the well in a predetermined quantity. The oils will dissolve in LCO2 which is soluble in water. The pressure of the treatment liquid in the near-wellbore formation is maintained below the fracturing pressure of the formation, releasing the pressure on the wellbore and allowing the LCO2 to vaporize. The treatment liquid with the contaminants can be blown out of the well or pumped to the surface. The proposed method can be applied to clean various types of wells producing methane or some other gas, liquid petroleum, water, or other desirable fluid or gas.

3.5.5 Well Fracturing with LCO2 The search for cleaner fracturing fluids has led to the development of fracturing using LCO2 as the proppant carrier fluid [230–237]. Proppant slurry is pumped into the induced fracture to keep it open so that the hydrocarbon production from the well can be maintained and significantly enhanced. CO2 is handled at the surface as a liquid. Special blending equipment is used to inject proppant directly into LCO2. At reservoir temperature and pressure, LCO2 vaporizes in the gas well or it vaporizes and is partially dissolved in the reservoir oil in an oil well. The main benefits of using CO2 are its solubility in water and oil, which reduces the viscosity of oil during fracturing treatment. In addition, the low pH of LCO2 (4.5 to 5.0) reduces gel formation and minimizes the swelling of clay formations, allowing the proppant slurry to penetrate further into the fractures. Also, the use of LCO2 reduces the amount of water used in fracturing treatment

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and there is significantly less liquid to clean up. Vaporization of LCO2 to gas provides sufficient energy to remove the fracture fluids and excess residual solids. Using LCO2 can also result in a better network of fractures, making it easier to extract the fuel.

3.5.6 Machining and Cutting with LCO2 Jets A novel application is the use of LCO2 jet for cutting and machining as an alternative to water-jet cutting [238–245]. With waterjet cutting, the workpiece is wetted during the process and dust and debris generated must be collected by extensive filtration. In addition, the used water after machining must be treated for recycling or disposal. The workpieces must also be cleaned and dried after processing. These disadvantages can be overcome by employing a thermodynamically stable coherent LCO2 jet with high specific energy. Such a jet can be formed by compressing CO2 to high pressures (100 to 350 MPa), cooled and expanded through a sharp-edged sapphire nozzle to form a jet suitable for machining. At sufficiently low injection temperatures, the phase change from liquid to gas is delayed and a coherent liquid jet is generated at the nozzle even at atmospheric pressure. However, breakup of the jet is closer to the nozzle which results in lower kerf depth but smaller kerf width. This makes precise machining possible. Different materials, such as fabrics, wood, aluminum, polycarbonate, heat-sensitive carbon fiber-reinforced composites, hygroscopic materials, hygienically-sensitive materials in food and healthcare sectors, and hazardous materials can be cut or machined with LCO2. This is a residue-free machining process without the generation of secondary waste.

4

OTHER CONSIDERATIONS

4.1 Costs Commercial dense-phase CO2 cleaning systems are expensive, but operating and waste disposal costs are usually low [13,134–137,140,246–254]. The installed costs can range from less than US$100,000 for small-capacity (1-liter volume) equipment to several hundred thousand dollars for large-capacity (30-liter volume) equipment. The cost of the equipment increases considerably with size, and also depends on the complexity of the controls and other components, and the degree of automation required. For large parts it may be more cost effective to install lower-pressure-rated equipment and operate the cleaning system in supercritical mode for longer times at lower pressure. The systems are operated in batch mode. Interlocks for continuous operation between high and ambient pressures are too expensive to be justified. Recovery and recycling of the LCO2 will add 15–25% in costs. If the contaminant is readily soluble in LCO2, the system can be rated for lower pressure LCO2 (at 5–6 MPa) which can reduce the equipment cost by 10–5%.

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Operating costs are generally low. Power costs are minimal (
2.5 kWh for a 120-liter system) because cleaning cycles are short and there is no heat input into the process. The cost of supplying beverage-grade liquid CO2 to the system is quite small (
US$1.30–$1.50 per kg), although high-purity supercritical grade CO2 is considerably more expensive [253,254]. Typically, CO2 consumption per cleaning cycle is approximately 0.8 kg for a 120-liter system. Maintenance costs for the equipment range between 5–7% per year. Waste disposal costs for LCO2 cleaning are lower than competing cleaning technologies as the waste residue is 100% contaminant. If the contaminant can be recovered, recycled, or reclaimed, there is no cost associated with disposal of waste. For contaminants that are readily soluble in LCO2, the costs for cleaning are competitive with aqueous or solvent cleaning technologies. In one example, LCO2 immersion cleaning reduced machined ball point pen tip cleaning costs by more than US$130,000 annually (fully-burdened), with a return on investment in less than 24 months [250]. For sterilization of heat-sensitive materials and devices, dense-phase CO2 has been shown to have significantly lower cost per unit (cubic foot or load) than conventional sterilization with ethylene oxide, and comparable costs with hydrogen peroxide gas plasma sterilization [249]. In one other full-scale demonstration project, the overall costs of LCO2 textile cleaning were found to be 20% lower than percholorethylene dry cleaning, primarily due to the shorter cycle times for each load of garments (no drying step is required for LCO2 cleaning) [255].

4.2 Advantages and Disadvantages of Liquid CO2 Cleaning LCO2 has become an established process for removal of surface contaminants. The advantages and disadvantages of the process are listed in the following sections.

4.2.1 Advantages 1. LCO2 has very low surface tension with excellent wettability, which makes dense-phase CO2 very attractive for cleaning intricate parts, or parts with deep crevices, tiny holes, or very tight tolerances. 2. CO2 achieves the liquid phase near ambient temperature which is a significant advantage for cleaning temperature-sensitive parts. 3. The low viscosity of LCO2 results in high Reynolds numbers for flowing CO2. Such turbulent flow is an advantage in particle and solid contaminant removal applications. 4. The liquid-like high density of CO2 in the liquid phase gives it a very high solvation power for many low molecular weight organic compounds and many common fluorinated solvents. High purity cosolvents, surfactants

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6. 7.

8. 9.

10.

11. 12.

13.

14. 15. 16.

17.

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(anionic, cationic, and nonionic), dispersants, and chelating compounds are readily available to broaden the range of cleaning applications for LCO2. CO2 is nontoxic with a high threshold limit value (TLV) of 5000 ppm as compared with common organic solvents such as acetone (750 ppm) and chloroform (10 ppm). CO2 is nonflammable, which is a significant safety advantage in cleaning. LCO2 is compatible with virtually all metals. High-density cross-linked polymers are not affected by dense-phase LCO2, but low crystallinity amorphous polymers are susceptible to plasticization and resultant brittleness of the component. For most cleaning applications, the degree of cleanliness is equal to or better than conventional aqueous or solvent cleaning processes. A further advantage of LCO2 cleaning in the medical industry is the ability to remediate bacterial contamination by sterilization. Sterilization by LCO2 is a technologically and economically viable alternative to conventional processes. LCO2 cleaning is typically performed in closed-loop systems, designed to maximize recycling of the carbon dioxide. For challenging applications such as particle removal, the addition of agitation can significantly enhance the cleaning effectiveness, as well as reduce the time required for cleaning. The cleaning process times are relatively short, typically 15 to 30 minutes per batch, which leads to reduced process operating costs. Completely dry and clean parts are obtained at room temperature. No supplemental drying is needed which reduces the amount of energy, water, and time required for processing. CO2 is a byproduct of industrial processes. It is plentiful, inexpensive, and recyclable, making the solvent consumption cost an insignificant contributor to the overall cleaning process costs. The process operating costs are low. Energy consumption is generally low because there is no heat input to the process. Energy is required to operate the pumps to perform cleaning. Contaminants are the sole waste. Hence, the waste disposal costs are low. In fact, waste disposal costs may be eliminated if the contaminants can be recovered, reclaimed, or recycled. This is a noncorrosive, environmentally-friendly process. No hazardous wastes and emissions are generated.

4.2.2 Disadvantages 1. The low dielectric constant of CO2 makes it difficult to dissolve polar compounds. 2. The process is ineffective in removing hydrophilic (polar molecules) contaminants, inorganic contaminants, and large particles and other debris.

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3.

4.

5. 6.

Particle removal can be enhanced with mechanical or sonic (ultrasonic or megasonic) agitation, but that increases the capital costs. Cosolvents and other additives are available to remove these kinds of contaminants; however, the process costs are correspondingly high. LCO2 cleaning is a batch process. The high costs of interlocks between high pressure and ambient pressure required for continuous operation are not justified. The high operating pressures of LCO2 cleaning require robust heavy cleaning chambers that scale with the capacity. In addition, peripheral equipment for storage, distillation, and recovery of the CO2 necessitates equipment with a large footprint. Process complexity is high especially if the chemistry has to be tailored for unknown contaminants. This also requires high level of technical skill. Although CO2 is nontoxic and nonflammable, it can displace oxygen and cause asphyxiation if leakage occurs in closed, occupied spaces. CO2 monitoring may be required.

5 SUMMARY Dense-phase CO2 in its liquid state is an established precision cleaning technique with application in many different industries. The gas-like viscosity and the liquid-like density of LCO2 are key characteristics that allow the process to be tuned to the application. In addition, the very low surface tension of LCO2 ensures high wettability and makes it very attractive for precision cleaning applications, particularly for intricate parts with complex geometries. The cleaning process is operated at near-ambient temperatures and lower pressure than supercritical CO2. LCO2 cleaning is a batch process. Applications include textile and garment cleaning; degreasing of metal surfaces, glass, optical elements, silicon wafers, and polymers; sterilization and disinfection of medical equipment implants and biomaterials; inactivation of microbial organisms; wellbore cleaning; pesticide mitigation and cleaning of museum collections; residue-free dry machining; and well fracturing.

ACKNOWLEDGMENT The author would like to thank the staff of the STI Library at the Johnson Space Center for help with locating obscure reference articles.

DISCLAIMER Mention of commercial products in this chapter is for information only and does not imply recommendation or endorsement by The Aerospace Corporation. All trademarks, service marks, and trade names are the property of their respective owners.

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