- October 01, 2000, Arthur J. Legros, SRS Engineering Corp.
In the converting industry, it is normally desirable to recycle wash solvents and waste inks on-site because of the high cost of disposal of hazardous waste and replacement solvent. Although off-site recycling is an option, many converters prefer to recycle their own product for quality control and to reduce their cradle-to-grave liability in having this waste transported off their property.
Facilities often use inks and coatings containing nitrocellulose (NC), which require special considerations because of the potential of an exothermic reaction under certain conditions. In this article we will discuss a safe method that allows you to take advantage of the cost savings of distillation.
The Chemical Background
Chemically, nitrocellulose is an ester of cellulose with nitric acid, and it is encountered commonly in a variety of inks and coatings used throughout the converting industry. The amount of nitrocellulose used in a typical ink system ranges from 2%-30%, based on total solids content.
Nitrocellulose is a flammable material that becomes unstable at elevated temperatures above 320 deg F (160 deg C), and under certain conditions it may react and decompose exothermically, resulting in toxic fumes including oxides of nitrogen and carbon monoxide.
As long as atmospheric oxygen is not introduced onto the material, decomposition will take place without ignition and will continue until the process is complete. If an oxygen-initiated reaction occurs, it is imperative that no one attempts to open the inspection hatch of the distillation chamber under any circumstances, and that the room be evacuated and vented until the reaction is over and the room is safe to re-enter.
The conditions needed to initiate a nitrocellulose decomposition reaction include:
- An adequate amount of NC in the waste stream, which is normally the case in the converting industry.
- The lack of liquid solvent in the dry waste stream.
- Temperature. At no time should dry NC be overheated or be in contact with any surface at unsafe elevated temperatures. Under actual operating conditions, a reaction could be started at temperatures as low as 230 deg F (110 deg C) under the right set of conditions. With operating temperatures under this limit, NC decomposition will not take place.
Selecting a Distillation System
In extracting solvent from NC-based inks and waste solvent, the objective would be to distill under a relatively low, controllable temperature range, while ensuring that the conditions of temperature and dryness needed to start a reaction are not present at the same time.
Operating temperatures are a variable in the converting industry, as different inks and coatings are used and cleaned with a variety of solvents with a wide range of boiling points. Low- boiling flammable solvents such as alcohols, ethyl acetate, and methyl ethyl ketone (MEK) commonly are used. These have a boiling point of less than 200 deg F (93 deg C), while new, high-boiling, low volatile organic compound alternative solvents, such as some N-Methyl Pyrrolidone (NMP)-based products, can have boiling points in excess of 400 deg F (200 deg C). This wide range of operating temperatures requires a high degree of versatility in a distillation system design.
A distillation system should incorporate several interlocked features to provide a safe, economical, and efficient system, ideally suited to the specific requirements of nitrocellulose. Any system used for this type of application should be controlled by a Programmable Logic Controller (PLC) for optimal operation and safety. It also should include the features discussed in the remainder of this report.
Automatic Vacuum System
A vacuum system to reduce operating temperatures is recommended for most solvents, but it is particularly necessary with high-boiling solvents of above 230 deg F (110 deg C).
A good system design, such as a high-performance liquid ring vacuum pump, can be used effectively to reduce the boiling point of the solvent to operate within a safe zone. For high-boiling solvents, the operating temperature often can be reduced by up to 100 deg F (38 deg C) or more. Not only does this allow for a safer operating environment, but it also increases the overall efficiency of the system by reducing the warm-up and cooling cycles, and it is less prone to other phenomena such as foaming and fouling.
Steep Wall Cone Design
A steep cone design for the waste receptacle is highly desirable for optimal efficiency, safety, and ease of use. As the rate of distillation is directly related to the heat transfer surface area between the heating media and the solvent, a steep cone, either immersed in a large oil bath or surrounded by a recirculating hot oil jacket, provides an ideal configuration for this type of application.
Under normal operating conditions, the distillation chamber should be kept full of solvent automatically through an auto fill feature, and as the distillation process goes on, solids naturally tend to fall to the bottom of the cone and remain saturated with solvent.
As the largest heat transfer surface area is toward the top of the cone, where most of the heat is required for distillation, the sludge concentrates at the bottom, where the small surface area provides less heat transfer and, consequently, cooler temperatures than in conventional flat or dished bottom designs.
Steep cones are less subject to hot spots, where a possible accumulation of dried NC residue may exceed safe temperatures and cause a decomposition reaction.
As an added bonus, a steep cone can be fitted with a large, free-flowing discharge gate valve at the bottom, making the removal of sludge a very routine, safe, and easy procedure.
The benefits of a steep cone design are enhanced by the use of self-adjusting, low-maintenance scrapers, which continually sweep the inside surfaces to help the solids fall to the bottom and prevent buildup of material that possibly could result in an exothermic reaction.
A scraper system should have a hub design, which would not restrict a thickened sludge from flowing freely during the discharging operation.
The principal area of concern in a safe distillation process is controlling temperatures within the critical zone limit below 230 deg F with precision and consistency. An automatic solvent distillation system should monitor and control the temperatures in the sludge, vapor space, and hot oil.
By using Resistance Thermal Devices (RTDs) accurate to within 1 deg, you can control the process automatically with accurate and consistent results.
For nitrocellulose applications, the most critical parameter lies in the sludge temperature. By monitoring and controlling this temperature precisely, the system automatically will prevent the sludge from reaching the reaction temperature. The system's PLC program should allow only the distillation process to operate within a specified range. This range should be preset and protected by password security to ensure against accidental changes by unauthorized personnel.
Since a nitrocellulose reaction could ignite in the presence of atmospheric oxygen, an auto cool system should be installed as part of the system. When the sludge temperature has reached the preset level, indicating that the system is ready to discharge, the PLC automatically should start an auto cool cycle to cool the sludge to a safe discharge temperature rapidly, typically around 125 deg F (50 deg C). The sludge then can be discharged safely and handled in accordance with local safety practices and environmental regulations.
Some systems offer a water-quench option in the distillation system in the event of a reaction. Although most distillation systems are equipped with a pressure release valve on the distillation chamber, the introduction of water on an exothermic reaction may result in steam and pressure, and it should be considered a last resort.
In certain cases, where a distillation system is not equipped with the safety features required for this application, adding alkali to the NC material may reduce the risk of decomposition. By adding caustic soda (NaOH) or soda ash (Na2CO3), a saponification process of the nitrocellulose is started, which reduces the potential of decomposition and a reaction.
The best approach to safety, however, lies in selecting a system designed to prevent an accidental reaction in the first place.
Arthur J. Legros is a founder of SRS Engineering Corp., Fallbrook, CA, and has been involved in the industrial market for more than 25 years. Founded in 1985, SRS designs and manufactures a family of automatic solvent-distillation systems for a wide range of applications. Mr. Legros holds a degree in electrical engineering. He can be reached at 760/728-1790..
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