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Solvent Cleaning Technology – An Overview

Apart from aqueous cleaning, ‘wet chemical’ processes using solvents are particularly widespread in industrial parts cleaning today.  While water-based cleaners are effective in removing polar contaminants e.g., aqueous coolants and lubricants (oil/water emulsions), polishing pastes, additives and salts, solvents (VOC) have emerged as the optimum cleaning medium for dealing with non-polar substances such as greases, oils and waxes.
 
Overview
VOC’s are defined as an organic compounds that have high vapour pressure under normal atmospheric conditions to significantly vaporize and enter into the atmosphere. Hydrocarbons are obtained in oil refineries. Here crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. Virtually all oil companies extract hydrocarbons in their refineries and market them commercially. For industrial cleaning processes mainly three product categories are used as shown in table 1.
 

 

Basic characteristics

Known as
 

Hydrocarbons
 
(HC)
 

Contain C- and H- atoms
Usually flammable fluids
Hydrocarbons used in CLI: C9-C13 Isoparaffins
Similar to Diesel fuel or Kerosene without lubricants
 

Non-chlorinated Hydrocarbons
Isoparafines
 

Chlorinated Hydrocarbons
 
(CHC)
 

Contain C-, H- and Cl- atoms
Usually not flammable
Toxicological and adverse environmental effects
 

TCE/PCE
Trichloroethylene (“TRI”)
Perchloroethylene (“PER”)

Oxygenated Hydrocarbons
 

Contain C-, H- and O-atoms
Usually inflammable
E.g. alcohols, ethers, esters, ketones
 

Polar solvents
Modified alcohols
 
In advanced manufacturing, concentrated oils are being increasingly used, e.g., in CNC-turning and milling, grinding, punching and hardening processes.  As a result, the products will typically be contaminated with large amounts of such processing oils.  However, sensitive downstream operations – such as coating (CVD and PVD), painting, adhesive bonding, soldering/brazing and welding operations require high levels of surface cleanliness and oil-free parts in the interests of process quality.  Here, solvent-based systems offer benefits over aqueous processes in terms of surface quality (absence of grease) and cleaning fluid longevity / reconditioning needs.  Chips and particles which are insoluble in the solvent are removed along with the oil as they can no longer adhere to the surfaces of the parts.
 

Contamination

 
Water -based cleaners
 

Chlorinated hydrocarbons (CHC)
(e.g. TCE and PCE)

Non-chlorinated
hydrocarbons (HC)

 
Organic, non-polar
e.g. oil, fat, grease
 

moderate

very good

very good

 
Organic, polar
e.g. colophony resin,
water based coolants (emulsions)
 

moderate

moderate – good

moderate – good

 
Inorganic, polar
e.g. salts
 

very good

moderate – good

Moderate
 

 
Solids (e.g. chips, particles, dust
 

 
 
depending on machine configuration
 
 
Innovative polar solvents / modified alcohols combine the benefits of aqueous and solvent-based cleaning techniques. Thanks to their balanced oil and water solubility profile, these cleaning media allow both non-polar contaminants (such as greases, oils, etc.) and polar (such as emulsion, salts, etc.) contaminations to be removed at the same time.  In Europe such solvents are increasingly gaining popularity, while in Asia they have not yet become widespread. Dürr Ecoclean GmbH provides equipment technology for all common media and will choose the optimum medium in terms of specific cleaning requirements and global availability.  The technology embodied in a given system is expressed in the following code:
C          –           (Non chlorinated-) Hydrocarbon technology (HC)
S          –           Chlorinated Hydrocarbons (CHC)
P          –           Polar solvents
W         –           Water based technology
 
The use of solvents – health and environmental aspects
Chlorinated solvents (CHC) are still in use in industrial part cleaning applications today.  They offer well-known advantages such as an almost ‘residue-free’ surface quality of the cleaned product, high degreasing performance, very good drying characteristics, plus worldwide availability of the solvents.  Parts contaminated with oil and grease are frequently cleaned by brushing, often manually, in vats containing TRI and PER.
 
Tables 3 and 4 illustrate the patterns of hydrocarbon (CHC, HC) impact on humans and on the environment. Accordingly, the specifications for environmental management systems such as, e.g., DIN ISO 14001, call for a responsible use of these substances and a high level of awareness in handling them.  In order to protect humans and the environment, the use of chlorinated solvents should be reduced or take place in a controlled way in advanced processing systems.
 

 
Chlorinated Hydrocarbons (CHC`s)
 

 
 
Non-chlorinated Hydrocarbons (HC`S)

 

 
(TCE)
 

(PCE)

Hazard
Symbol

T = toxic
 
 

Xn = harmful
 

Xn =harmful
 

Risk-phrases

R36/38: Irritating to eyes and skin
R45: May cause cancer
R67: Vapors may cause drowsiness and dizziness
R68: Possible risk of irreversible effects

R40: Limited evidence of a carcinogenic effect

R65: Harmful: may cause lung damage if swallowed
R66: Repeated exposure may cause skin dryness or cracking
 

 
Chlorinated hydrocarbons (CHC`s)
 

Non-chlorinated solvent hydrocarbons (HC`S)

 

(TCE)

(PCE)

Hazard symbol

 

N = dangerous for the environment
 
 
 

 

Risk-phrases

 
R52: Harmful to aquatic organisms
R53: May cause long-term adverse effects in the aquatic environment

 
R51: Toxic to aquatic organisms
R53: May cause long-term adverse effects in the aquatic environment

 
(some: R53: May cause long-term adverse effects in the aquatic environment

Water/soil

 
Poisonous for water organisms.
Not easily degradable

 
Not acutely poisonous for water organisms
Moderately degradable

 
WGK
(category according to German water resources law
 

WGK 3

WGK 1

Atmosphere / air

 
Formation of surface ozone (summer smog)
à negative effect on humans and environment
 
Negligible depletion of the stratospheric ozone layer
Negligible contribution to global warming
Negligible contribution to formation of acid rain
 
 
Trichloroethylene is classified as R45, meaning that it may cause cancer. Perchloroethylene shows a limited evidence of carcinogenic effects and is classified as R40. Besides, these chlorinated solvents are poisonous for water organisms.
 
Solvent Emissions – VOC Regulation
It is the stated goal of the global community to reduce emissions of volatile organic compounds (carbon tetrachlorides, CTC) as they are considered important precursor substances in the formation of ozone.  This objective was enshrined on 1 January 1990 in the Montreal Protocol, which has been signed by many countries.  The Indian government, for instance, enacted the Carbon Tetrachloride (CTC) Sector Plan in the aim of reducing CTC by 100% by 2010.  As a substitute for the readily volatile CTC substances commonly employed in industrial cleaning (CFC-113, methyl chloroform, etc.), the Montreal Protocol recommends an increasing use of chlorinated and non-chlorinated solvents and aqueous cleaning media. 
 
Further, the Kyoto Protocol was signed in 1997 in the aim of reducing atmospheric greenhouse gases. Greenhouse gases are believed to be the main reasons for global warming. The list of greenhouse gases controlled by the Protocol includes not only carbon dioxide and methane, but also partially halogenated chlorofluorocarbons and perfluorated hydrocarbons. Chlorinated and non-chlorinated hydrocarbons (CHC, HC) likewise belong to the VOC group.  They are believed to promote the formation of near-ground (surface) ozone.  In Europe and the U.S., special regulations have therefore been imposed to limit these emissions. 
 

 
Chlorinated hydrocarbons (CHC`s)
 

 
Non-chlorinated solvent hydrocarbons (HC`S)
 

 

(TCE)

(PCE)

 

 
Emission limits (C ?)
 

 
2 mg/m³

 
20 mg/m³

 
75 mg/m³

 
TLV-TWA (Threshold Limit Value – Max. Workplace Concentration
 

 
 
5 ppm

 
 
25 ppm

 
 
177 ppm
 
 
These regulations limit exhaust air emissions by imposing limit values. They also recommend the substitution of R45, R46, R49, R60 and R61 classified solvents in view of their toxic properties.  In an industrial cleaning context, this recommendation concerns trichloroethylene (TCE).  Based on these classifications, many countries have defined so-called maximum workplace concentrations (TLV-TWA, Threshold Limit Value – Maximum Workplace Concentration) to control and regulate workplace emissions.
 
The Indian Carbon Tetrachloride (CTC) Sector Plan of 12 February 2004 likewise recommends the use of enclosed systems and an orientation towards European maximum workplace concentrations (TLV-TWA). 
 
Companies in many countries have limited or banned the use of chlorinated solvents because of their impact on humans and the environment, to protect their workers, and to comply with DIN ISO 14001 requirements. In order to meet maximum workplace concentrations and to cut emissions for the reasons outlined above, the use of closed cleaning systems is therefore recommended.  Dürr Ecoclean addresses this requirement with its self-contained equipment designs for both chlorinated and non-chlorinated hydrocarbons.
 
Solvent machine concepts
In general terms, closed solvent-based cleaning systems are distinguished by the following advantages:
•      Very good de-greasing performance, even with high oil or grease input levels
•      Residue-free product surfaces
•      Ability to clean different materials (e.g., stainless steel, steel, aluminium) with the same solvent
•      Effective and cost-efficient reconditioning of the cleaning fluid by distillation, resulting in low fluid consumption.
•      High energy efficiency
•      Rapid drying (vacuum drying), even of complex part geometries
•      No cleaner or waste water disposal costs
•      No need for sophisticated, costly bath and media control
•      Reduced corrosion problems.
 
Closed chlorinated solvents (CHC) technology
Despite innovative technologies and new cleaning media, many problems in industrial part cleaning and degreasing can only be solved optimally with cleaning processes relying on CHCs. Typical industries include optical instruments, medical equipment, etc. The use of CHCs is, however, restricted by the requirement that the concentration of solvent be reduced to a maximum of one gram per cubic meter of air in the working area, and by the closed design specified for CHC cleaning systems.
 
Conventional semi-closed system technology achieves this by saturating the process air via condensers and a downstream adsorption cycle in which the process air is finally de-saturated completely via an activated charcoal module. This module, however, has to be exude at regular intervals depending on the application – a step which takes several hours to complete. During that time the cleaning system is shut down, unless exude can take place via a parallel treatment module. Unfortunately this procedure calls for a high capital investment, on the one hand, and the heating and cooling energy required for exude drives up operating costs on the other. Another drawback of this type of treatment is the heavy burden in the solvent and the resulting high consumption of stabiliser.
 
The situation is completely different if the innovative system of process air volume displacement (PAVD) is implemented. The system developed by Dürr Ecoclean permits a fully enclosed, waste-air free operation of CHC systems. A costly de-saturation of the process air by absorption and an application-dependent exude is no longer necessary. The parallel treatment module necessary for continuous cleaning with conventional systems, and naturally, the capital investment it involves, are no longer required, either. The patented system from Dürr Ecoclean is based on the process principles of vacuum de-saturation and volume displacement. Since the work chamber is evacuated, the concentration of solvent in the contaminant removal area is reduced to below the legal limit of one gram per cubic meter of air. Owing to volume displacement the de-saturated but still solvent-containing air passes into a vapour lock before the work chamber is opened. Once the work chamber has been reloaded and evacuated to the level required for the process, the solvent-containing air is automatically recycled. On account of the much lower level of solvent contamination obtained with this forward-looking process, the consumption of stabiliser is also reduced accordingly. The PAVD system has already been implemented in numerous customer-specific facilities. Many different optional features guarantee that these systems are suitable for virtually any application in industrial part cleaning and ensure an optimum cleaning performance without solvent-containing air in the contamination removal area. Thus, such advanced CHC system technology makes it possible to achieve high savings on capital investment and operating costs as well as a continuous availability of the cleaning system with a view to protecting humans and the environment.
 
Dürr Ecoclean’s S-machine technology is designed for a multi-level cleaning process including ultrasonic, immersion cleaning, steam de-greasing and vacuum drying stages. This approach supports a huge variety of applications in all fields of industrial cleaning.
 
Non-chlorinated hydrocarbon (HC) technology
Cleaning systems operated with non-chlorinated hydrocarbons are considered particularly powerful and environmentally friendly in degreasing and cleaning metal parts when used in closed machine concepts. This technique was initially developed to eliminate CHC technology. With more than 800 machines now in place worldwide, this well known technology has evolved into the leading oil removal and precision cleaning solution from Dürr Ecoclean. Especially in fuel injection and aircraft applications, many such machines have been installed. All of Dürr Ecoclean’s hydrocarbon-based installations operate with virtually zero emissions under vacuum to optimize the cleaning effect. To ensure maximum safety, the complete process runs in an atmospheric vacuum of 100mbar. These cleaning systems are designed for a multi-level cleaning process including ultrasonic, immersion cleaning, steam de-greasing and vacuum drying stages. The underlying design concept, with a closed work chamber for cleaning and drying as well as integrated heat recovery minimizes the use of energy and various media. It adapts easily to specific tasks due to its modular construction. Effective distillation technology is integrated as standard to ensure a long service life and consistent quality of the cleaning medium. Stainless steel piping is standard, as on other Dürr Ecoclean products.
 
One key advantage is the possibility of carrying out a HC-based conservation treatment, which can be realized via combined process. After cleaning with HC, the products are conserved using a mixture of hydrocarbon and conservation oil. This very effective technology offers new opportunities to Export orientated customers who need to protect their parts for overseas shipment and for the customers willing to store the cleaned parts for longer duration.
 
Author: Dipl. Ing. Rainer Straub
 
For more details, contact:
Durr Ecoclean India
Info.india@ecoclean.durr.com

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