초임계 유체 세정 (Supercritical Fluid Cleaning)
일반적으로 화합물의 온도와 압력이 임계점 (critical temperature) 이상에서는 액체와 기체의 구분이 없어지게 되고 이러한 영역을 초임계 유체 영역이라고 부른다. 이러한 초임 계 영역에서는 액체와 기체의 특성이 혼합되어 나타난다. 초임계 유체는 기체와 같은 낮은 표면장력을 가지고 있기 때문에 세공구조에도 쉽게 침투한고 용질의 확산계수가 액체에서의 값과 비교할 때 10배 이상이 크기 때문에 물질전달 속도가 커서 용이하게 오염물질을 부품 으로 부터 빼앗아 갈 수 있다. 또한 용매에 대한 용해력 (solvent power)은 일반적으로 용 액의 밀도에 비례를 하는 데 초임계의 밀도가 액체와 비슷하기 때문에 상당한 용해력을 갖 는다. 더우기 초임계 상태에서는 압력이 조금만 변하더라도 연속적으로 기체에서 부터 액 체에 이르는 범위의 밀도조절이 가능하며 따라서 용해력을 압력에 따라 민감하게 조절할 수 있다. 일반적으로 세정에 널리 사용되고 있는 초임계 유체에 대한 물성을 표1에 나타내었 다.
표 1 전형적인 초임계 용매의 임계물성
용매 화학식 분자량
g/gmol
비점
℃
빙점
℃
임계온도
℃
임계압력
atm
밀도
g/cm3
암모니아 NH3 17.03 -33.4 -77.7 132 111.3 0.24 이산화탄소 CO2 44.01 ++-56.6 *-78.5 31 72.8 0.47 에탄 C2H6 30.07 -88.6 -183.3 32 48.2 0.20 프로판 C3H8 44.11 -42.1 -189.7 97 41.9 0.22 CFC-11 CCl3F 16.04 -164.0 -182.5 198 43.5 0.55 육뷸화황 SF6 146.05 -50.5 *-63.8 45 37.1 0.74 물 H2O 18.02 100.0 0.0 374 217.7 0.32
++ @5.2atm, *: 승화온도
초임계 유체 세정제중에서 가장 널리 이용되고 있는 대표적인 세정제는 초임계 이산화 탄소이다. 초임계이산화탄소는 수계 세정제와 견줄만한 장점을 지니고 있다. 우선 환경을 파괴하지 않으며, 비활성이고, 독성이 없을 뿐 아니라 가격도 매우 싸다. 오염물질은 쉽게 농축되고 쉽게 이산화탄소와 분리되어 가치있는 물질을 회수하거나 폐기물을 처리하기가 유 리하다. 더우기 이산화탄소는 계속하여 순환하면서 사용하기 때문에 세정제를 폐기하는 문 제도 없고 활성이 떨어지지 않아서 운전비용도 적게 든다. 이산화탄소는 부품에서 쉽게 제 거되어 잔사나 부식의 문제가 전혀 없는 장점이 있다. 반면 최대의 난점은 세척장치가 고 압으로 제작되어야 한기 때문에 장치의 제작비가 높다. 그러나 초임계 이산화탄소의 세정 능력, 낮은 운전비, life cycle cost 등을 고려하여 분석한 결과 많은 응용분야에서 유리한 것
으로 알려지고 있다.
초임계 이산화탄소는 실리콘이나 불화에테르와 같은 오일에는 우수한 용매이다. 금속 과는 서로 상용할 수 있으며 플라스틱은 응용에 따라서 한가지씩 검토를 하여야 한다. 일 반적으로 고밀도 폴리에틸렌 (HDPE)과 cross-link된 고분자에는 나쁜 영향을 주지 않는다.
초임계 이산화탄소는 독성이 없기 때문에 의료기기의 정밀세정에 사용될 전망이 밝으며, 임 계온도가 낮기 때문에 온도에 민감한 부품의 세정에도 유망하다. 그러나 초임계 이산화탄 소는 비극성물질이기 때문에 오염물질이 친수성물질이거나 극성화합물일 경우에는 효과적이 지 못하다. 따라서 이러한 문제를 극복하기 위하여 cosolvent나 entrainer와 같은 것을 사 용한다. 극성오염물질의 용해도를 선택적으로 개선하기 위하여 분자량이 작은 알코올을 사 용하는 것도 좋은 결과를 얻어내고 있으나 반드시 부품과의 상용성 (compatibility)과 환경 처리 등과 같은 문제를 검토하여야 한다.
초임계 이산화탄소의 세정 장치에 대한 간략도를 그림 1에 나타내었다. 세정할 부품은 세정기용기내에 장착한다. 초임계 이산화탄소 (72.8기압 이상, 31℃이상)를 용기를 통해 보 내면 부품의 표면으로 부터 오염물질이 용해되어 초임계 이산화탄소로 나온다. 오염물질을 실은 초임계 이산화탄소는 용기를 떠나고, 팽창밸브와 열교환기를 거치면서 온도와 압력이 떨어지고 이 과정에서 용해력 (solvent power)를 잃게 된다. 용해력을 잃은 이산화탄소로 부터 오염물질은 침적되어 분리기에서 모여지고, 오염물질이 없어진 이산화탄소는 다시 가 압과 가열 공정을 거쳐 초임계 유체가 되면서 세정용기로 들어간다. 초임계 이산화 탄소를 세정제로서 이용할 수 있는 분야, 부품, 오염물질은 표 2에 나타내었다.
표 2 초임계 이산화탄소를 세정제로 사용하는 분야 및 오염물질의 종류
관련 분야 재 료 및 부 품 오 염 물 질
우 주 선 고압케이블, 베어링, 리베트 실리콘 오일, 윤활유 레 이 다 Connectors, transformer, 케이블 Flux residues, dielectric oils 레 이 저 Optical benches, o-ring 기계유, Plasticizers
가스 시스템 Seals Plasticizer, monomers
Cleaning Aid Cotten ball/wipers Cotten-tipped application
Organic extractables, Triglycerides Adhesive residues
온도 압력 강하
오염된 부품 초임계 CO2/오염물 초임계 CO2/오염물
세 정 조 분 리 조
온도 압력 상승
세척된 부품 초임계 CO2 CO2 오염물질
(배출/회수)
그림 1 초임계CO2를 이용한 세정장치의 개념도
년도 연구비(천원) 조달방법
과기부 기업
1차년도 100,000 70,000 30,000
2차년도 200,000 140,000 60,000
3차년도 200,000 140,000 60,000
계 500,000 350,000 150,000
2. 분야별 세부과제 및 연구목표
3. 소요연구비(규모, 조달방법)
4. 과제 관리방법
5. 참고문헌
Supercritical Fluid Cleaning as a Solvent Alternative 1,1,2-trichloro-1,2,2-trifluoroethane, CCl
2FCClF
2;
1,1,1-trichloroethane, CH
3CCl
3Revision: 1/99
Process Code Navy: ID-03-99; Air Force: CL01; Army: LOP Usage List: Navy: Low; Army: Low; Air Force: Low
Substitute for: Solvent cleaning operations such as vapor degreasing
Applicable EPCRA Targeted Constituents: Trichlorotrifluoroethane (CFC-113) (CAS: 76-13-1) and Methyl Chloroform (MCF) (CAS: 71-55-6)
Overview:
Supercritical fluid is a high pressure cleaning process that takes advantage of the fact that the
fluid chosen becomes an extremely effective solvent for many organic materials, once in its supercritical state. It is a cleaning process that penetrates small openings and is especially useful for precision or intricate components like gyroscopes, accelerometers, nuclear valve seals, laser optic components, special camera lenses, electromechanical assemblies, and porous ceramics. The process works well removing liquid contaminants, including silicone, petroleum and dielectric oils, flux residues, lubricants, adhesive residues, and fats and waxes.
However, it is not very effective on heavy soils, nor for removal of particles or salts, except in circumstances where it is used in conjunction with agitation or ultrasonic cleaning.
The supercritical point is the pressure and temperature condition above which a chemical can no longer be vaporized, but, at the same time, the fluid does not retain its liquid-phase characteristics. Supercritical fluids have qualities unique to their fluid state; that is, unlike the characteristics and properties of either the vapor or the liquid phases. Small changes in temperature and pressure produce significant changes in density and solvent power. This combination of characteristics allows for greater mass transfer rates, effectively decreasing the time required to move the contaminants into the bulk supercritical fluid stream, thus providing rapid cleaning.
Carbon dioxide is probably the most widely used fluid in supercritical cleaning applications.
CO2 is especially useful, since it is non-toxic, non-flammable, and non-ozone depleting; has a supercritical temperature near ambient temperatures (good for temperature sensitive parts);
and exhibits excellent solvent properties in its supercritical state. Carbon dioxide supercritical cleaning does require high operating pressures in the range of 1,500 to 2,000 psig, but operating temperatures of only 35 to 65℃. As a result, most supercritical cleaning equipment has been designed for high pressure operation and is relatively small. High pressure cylindrical chambers of supercritical cleaning equipment are intended to hold primarily small, intricate parts or parts with deep crevices, tiny holes, or very tight tolerances that normal alternative precision cleaning processes, specifically aqueous or semi-aqueous processes, have difficulty cleaning.
A basic CO2 supercritical cleaning system has two primary cleaning vessels: the extraction vessel, in which the component to be cleaned is placed and flooded with supercritical carbon dioxide and, as the CO2 dissolves the contaminants, it flows to a separator vessel where the fluid is subjected to a pressure and temperature change (pressure is reduced and the carbon dioxide vaporizes). As that occurs, the solubility of the contaminant in the carbon dioxide decreases, causing the contaminant to separate from the bulk fluid. Once all the CO2 is evacuated from the separator, the concentrated contaminant is usually in residue form, often as an oily or tar-like liquid that is simply drained from the separator. The residue can then be recovered, recycled, or reused, if suitable; otherwise, the residue is disposed as the sole component; no solvents, wastewater, or other contaminants are present to increase the volume of waste disposed.
The greatest concern when using supercritical cleaning processes is the safety risk of high operating pressures. Equipment must be properly maintained to prevent over pressure or failure of high-pressure components. Although carbon dioxide is non-toxic and non-flammable, it can displace oxygen and cause asphyxiation if leakage occurs in closed, occupied spaces. A CO2 monitor may be useful for closed areas, despite the fact that there are early warning symptoms, primarily difficulty in breathing (unlike nitrogen, which can quickly cause asphyxiation without warning).
Materials Compatibility:
Carbon dioxide, in its supercritical state, is compatible with virtually all metals; however, non-metallic components, such as plastics, gaskets, and o-rings must be checked for compatibility. In general, cross-linked polymers and high density polyethylene are not affected by CO2 supercritical cleaning. Cellulose acetate butyrate is one plastic that is not compatible
with supercritical carbon dioxide. Other plastics that are susceptible to damage from supercritical cleaning are generally affected because the carbon dioxide solvates the plasticizers within the plastic and once removed, the absence of plasticizer tends to make the cleaned plastics more brittle. This is usually an undesirable result for plastic components. Compatibility should always be checked and tested, if necessary. The extremely high pressures at which supercritical cleaning takes place make it unsuitable for components containing gas or evacuated spaces because they could implode or deform during the cleaning cycle.
Safety and Health:
The primary safety concern when using supercritical fluids is the high pressure operating range of the equipment. Proper design, operation, and maintenance are critical to safe use of the equipment. In addition, the hydrocarbon gases are flammable; thus, their use requires excellent maintenance measures to safeguard against leaks. Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.
Benefits:
Some of the benefits are an equally high degree of cleanliness; relatively short cleaning times, typically 15 to 30 minutes; completely dry parts at room temperature (no supplemental drying is needed); low operating costs; contaminants are the sole waste; and systems are typically closed-loop, designed to maximize recycling of the carbon dioxide. For difficult applications, the addition of agitation will usually provide a significant improvement in a supercritical fluid system cleaning ability, as well as reduce the time required for cleaning.
Disadvantages:
The disadvantages of supercritical carbon dioxide cleaning are high capital costs, poor removal of hydrophilic (polar molecules) contaminants, high-pressure operation, and, as a result, limited component size, due to equipment design pressure constraints. Development work using co-solvents to aid cleaning of hydrophilic contaminants is in progress.
Economic Analysis:
Supercritical cleaning systems are expensive, but operating and waste disposal costs are usually low. Installed cost of a supercritical carbon dioxide system can range from $60K to
$300K for once-through CO2 use, depending on the complexity of the controls and other components. Recovery and recycling of the CO2 will add $25K to $50K. (Sometimes liquid CO2 at 800 to 900 psig can be used as an alternative to supercritical CO2 if the contaminant is readily soluble in liquid CO2. This can reduce the equipment cost by 10 to 15%.) Operating costs are low; power costs are minimal because cycles are short and no heat is input into the process. Furthermore, liquid CO2 is approximately $0.10/lb (in bottles).
Maintenance costs under contract can run $15K per year, according to one manufacturer.
Waste disposal costs are lower than competing cleaning technologies that require disposal of spent solvent, wastewater, or blasting media, since the waste residue is 100% contaminant. In some cases, there is no disposal of waste, since the contaminant can be recovered, recycled, or reclaimed.
Approval Authority:
Navy: Approval is controlled locally and should be implemented only after engineering approval has been granted. Major claimant approval is not required.
Points of Contact:
Mr. Dan Smudski, Mechanical Engineer SM-ALC/LI(1)
5441 Bailey Loop
McClellan AFB, CA 95652-1133 DSN 633-3787, (916) 643-3787
National Defense Center for Environmental Excellence (800) 282-4392
Vendors:
The following is a list of companies that deal with supercritical equipment. This is not meant to be a complete list, as there are other manufacturers of this type of equipment.
CF TECHnologies, Inc.
Hyde Park, MA (617) 364-2500, Fax (617) 364-2550 Mr. Bill McGovern
EnviroPro Technologies
P.O. Box 5051, 2930 West 22nd Street, Erie, PA, 16512-5051 (814) 838-5888, Fax (814) 838-5755
Sources:
1. EPA Solvent Alternatives Guide, SAGE 2.0, EPA and ICOLP guides for Eliminating CFC-113 and Methyl Chloroform in Aircraft Maintenance Procedures, Oct 93, and Eliminating CFC-113 and Methyl Chloroform in Precision Cleaning Operations, revised Oct 94.
2. Pirrotta, R. and T. Pava, Replacement of CFCs with Supercritical Carbon Dioxide for Precision Parts Cleaning, Proceedings of the International Conference on CFC and Halon Alternatives ?4, p. 532-539, October 94.
3. Mr. Bill McGovern, CF TECHnologies, Inc., Hyde Park, Massachusetts.