TCE Process Improvement

Q: We are using trichloroethylene (TCE) in a three-tank immersion degreasing machine. Tank 1 = coarse cleaning (operating at 70-80°C), Tank 2 = coarse cleaning (operating at 70-80°C) (this stage has ultrasonic capability but has been disabled). Tank 3 = Vapor degreasing (90-100°C).

Immersion times are 2-3 minutes in each stage. We have a serious issue with high TCE usage and poor cleaning. Chiller temperature is maintained between 10-15°C. We also see condensation issues leading to water contaminating the solvent.

Filtration is done on manual basis and percent purity or concentration of the solvent is never checked. Could you please suggest how to optimize with respect to substitute solvent, how to determine concentration and purity, installing a continuous distillation system (would it be better?) and how to check residual oil? —A.P.

A. You will first need to implement some preventative maintenance and quality control.  There are many common sense practices that can be implemented to minimize solvent evaporation.  You will likely find that once implemented, you will see a significant improvement in the cleaning process and a decrease in solvent consumption. The solvent alternatives do not always perform as well as your current solvent and are usually significantly more expensive.  In your case, you would want to tighten up the equipment and controls to minimize exposure to your employees.

The first thing you would want to check is the compatibility of the cleaning solvent (TCE) and the lubricant you are attempting to remove. Since you are seeing a significant amount of water contamination of solvent, you will want to make sure that your lubricant is not water based.  If it is, you will introduce a significant quantity of water that the solvent system will not be able to handle. It would be necessary to move to a mineral oil lubricant prior to continuing with the solvent degreaser. This would be a prerequisite in order to obtain reasonable levels of cleanliness.

Also related to the water issue is the use of a water separator in the system. I am not sure if your system is equipped with one, but this is critical to remove the water introduced into the system from the parts and high ambient humidity. Water will float on top of solvent since it is less dense, so a water separator uses this principle to introduce solvent from the system into a covered, baffled weir on the side of the degreaser. This point is cooler than the interior of the degreaser and will allow natural separation to occur more quickly. 

The next thing to check is the temperature settings of your first two stages (70-80°C).  The TCE boiling point is 87°C, so if you are operating these two stages in the range of 70-80°C, you are getting very close to the boiling point of this solvent. With these being used for coarse cleaning, you do not need to run these temperatures this high. I would recommend a much lower temperature (30-50°C) to conserve on solvent. 

To be clear, you will need chillers on all heated stages of this cleaning system to contain the solvent and minimize evaporative losses.  If stages 1 and 2 do not have a chiller, it is recommended that you discontinue heating these stages.

Additionally, the chillers will need to be sized adequately to minimize vapor losses.  The cooling temperature you are using is very good, but if the freeboard distance (distance from the top of the solvent to the top of the degreaser) is too short, the result is excessive solvent loss.  It is often recommended to design a degreaser with a minimum freeboard ratio of 0.75 for open top degreasers.  This means that the height of the freeboard had to be at least 75 percent the length of the smaller of the two opening dimensions of the degreaser.

Additional measures to mitigate fugitive solvent emissions from the degreaser would be to install a limit on the hoist to 11 feet per minute (3.3 meters per minute), keep fans and air ventilation away from the degreaser, insure parts are up to temperature in the vapor degreaser to minimize dragout, orient parts to minimize dragout and install sliding covers on each entry tank.  It is important that these be sliding to avoid a piston effect of a hinged cover that will pull solvent out of the vapor zone when opened.

Regarding quality of solvent, the oil cannot be filtered from the solvent/oil mixture. The solvent dissolves and solubilizes the oil. The only practical and effective means of purifying the solvent is to have a distillation unit that is connected to the entire system. In your case, you would want to counterflow the solvent from stage 3 to 2 to 1 and then into a distillation unit.  With this equipment you will concentrate the oil based on the difference in boiling points between the solvent and the oil. The solvent vapor can be condensed and is then returned to the cleanest part of the system, stage 3. As more oil accumulates in the sump of the distillation unit, the boiling point of the mixture will continue to rise.

Most solvent manufacturers publish boiling temperatures that equate to an approximate oil percentage in the solvent.  For instance old information I had from a TCE manufacturer in the U.S. suggested a 90°C boiling sump temperature equated to approximately 30 percent oil while a 95°C boiling temperature was approximately 50 percent solvent. A general rule of thumb would be to close off the distillation unit at approximately 30 percent oil and then continue to boil until the sump temperature would reach about 50 percent oil.  In this case the sump bottoms would be drained and removed by the solvent supplier for further processing.