 |
||||
        |
||||
 |
||||
|
Solvent Recovery South of the Border
By Vivek Nayak, Sergio Medrano and Jaime Fermin Reprinted from Environmental Technology, January/February 1999
Volatile organic compounds (VOC's) find wide spread use in the gravure printing and flexible package printing industries. Though water based inks and coatings have made significant inroads into this industry, organic solvent based printing remains the method of choice for high quality and high volume printing. Most solvent based printing facilities in the USA have a device of some form to control emission of the VOC's to the atmosphere.
With the advent of NAFTA, the package printing industry of Mexico by virtue of being a major source of VOC emission is coming under increasing pressure to comply with environmental regulations. One of the major players in the package printing industry in Mexico, Grafo Regia SA de CV located in Monterrey, decided to proactively pursue the emission control issue. Grafo Regia found that most pollution control processes such as thermal/catalytic oxidation, etc. while adequately addressing the pollution control problem, are destructive processes that have no economic payback and often require large inputs of power and fuel. Grafo Regia preferred to pursue a process that would have an economic payback. Adsorption on activated carbon has been utilized for vapor phase solvent recovery for several decades and has proven to be a relatively simple, reliable and economical method of recovering solvents for reuse and as a means of preventing air pollution. It is one of the few air pollution control technologies which can provide an economic payback. Adsorption is a physical phenomenon in which the solvent molecule adheres to the surface of activated carbon by virtue of weak forces (akin to surface tension) called the Van der Waal's forces. The surface of activated carbon is highly developed and is typically in the range of 1100 m2/gm. Most industrial solvents can be recovered using activated carbon. While several methodologies may be utilized for regenerating activated carbon, the predominant method is steam regeneration. In steam regeneration, dry, low pressure steam is injected into the carbon bed usually in a counter current fashion to that of the solvent laden air flow. The steam, due to its high heat content "desorbs" the solvent from the carbon and carries it out of the adsorber vessel to a heat exchanger where the mixture is condensed and subcooled. Steam desorption has been utilized very successfully over the years and is especially effective for solvents which are insoluble in water, in which case, the condensed/subcooled mixture is decanted to separate the recovered solvent and water. The solvent in most cases is returned to the solvent storage tank for reuse. When the recovered solvents are water soluble, as is the case of Grafo Regia, additional processes such as steam stripping, distillation or pervaporation are used to separate the solvent and water, and to dry the recovered solvent. Grafo Regia, a unit of the packaging division of the Femsa Group, produces premier quality packaging, for the beer, carbonated soft drinks, and consumer goods industries. Capabilities include printing on paper, film and foil. Solvent based lamination is also done. While the solvents used in their printing process were primarily ethyl acetate and ethyl alcohol, several other solvents were used in formulations. Grafo Regia selected AMCEC Inc. of Lisle, Illinois to provide the Solvent Recovery System. AMCEC has been a proven force in the solvent recovery system marketplace for many decades and has considerable experience with systems for gravure printing facilities. In order to simplify the printing and recovery processes and to maximize the economic payback, Grafo Regia decided to restrict their inks and coatings to the use of only two solvents (ethyl acetate and ethyl alcohol). Toluene, a hazardous air pollutant (HAP), which was used in some formulations had to be eliminated if the recovered solvents were to be reused for printing on food related packaging. This streamlining of the ink and coating solvent formulations to a two solvent system was done over a period of several months before the solvent recovery system went into operation. Grafo Regia is located in a fully built up area of Monterrey, close to downtown and had no open land available for location of the solvent recovery system. Therefore, it was decided to locate the bulk of the equipment on a reinforced section of the roof. The solvent recovery system comprises four adsorbers of which three adsorbers are on line at any given time while the other is being regenerated or on standby. Present Solvent Laden Air (SLA) capacity is 115,000 NM3/hr (72,500 SCFM) containing up to 350 kg/hr (775 lbs/hr) of solvent vapors. The available roof top space is sufficient to accommodate one additional adsorber so that system capacity may be increased when more presses are added.
ADSORPTION The solvent laden air (SLA) from the printing and laminating processes is filtered and cooled prior to entering the carbon adsorbers. The stationary beds of activated carbon "adsorb" the solvent vapors and cleansed air is exhausted to atmosphere. The sequencing is controlled by a programmable logic controller (PLC). The duration of the adsorption cycle is governed by a flame ionization detector (FID) which continuously monitors the exhaust of the adsorber vessels on line. When a predetermined exhaust concentration is detected, signaling "breakthrough" (saturation) of the carbon, the analyzer initiates desorption of the next adsorber, while the adsorber which is idling is advanced to the adsorption mode. The analyzer is backed by a preset timer in the PLC that will initiate the desorption cycle if breakthrough is not detected within the preset time due to analyzer malfunction or extremely low solvent load condition. All valves on the adsorber vessels are operated by pneumatic actuators. All automatic valves are equipped with limit switches which are monitored by the PLC that triggers alarms when valves are not in the proper position. DESORPTION The adsorber which has just completed its adsorption period, is now "desorbed" using saturated steam. The steam is injected into the adsorber vessel through specially designed sparge pipes in a direction which is counter current to the SLA flow. The heat energy in the steam desorbs (releases) the solvent vapors form the activated carbon. The steam and solvent mixture is conveyed through a vapor duct to a series of shell and tube heat exchangers for condensation and cooling. The condensed steam/solvent mixture is decanted in a continuous decanter with the lighter solvent phase (essentially ethyl acetate with about 3.5% water) going to a "wet solvent tank". The aqueous phase which is water containing some ethyl alcohol and ethyl acetate is collected in a 5000 gallon weak liquor tank. POST TREATMENT The steam solvent mixture in the weak liquor tank is treated with a caustic solution to neutralize any acetic acid formed during the desorption process. The neutralized mixture is fed to a steam heated distillation column. The column comprises both a "stripping" section and a "rectifying" section. In the "stripping" section, the solvent is stripped from the solvent/water mixture while in the "rectifying" section the vapor is enriched by a reflux stream of the overhead product. Heat energy for the separation is provided by live steam injected at the bottom of the column. The "bottoms" of the column is clean water that is discharged to sewer while the "overhead product" which is the solvent mixture with a small percentage of water is condensed and collected in an "overhead product" receiver. A portion of this overhead product is used for "refluxing" the distillation column with the balance going to the "wet solvent" tank. The "wet" solvent containing 7 - 8% water is fed to a vapor permeation unit where the wet solvent is passed along a membrane. The water contained in the solvent permeates through the membrane while dried solvent is collected in a storage tank. The water permeate containing a small percentage of solvent is returned to the weak liquor tank. The "dry" solvent blend consistently contains less than 1.0% of water which is suitable for reuse by the presses. GENERAL Materials and Construction: The client required a system with a life cycle exceeding 25 years so all wetted parts of the system are constructed of stainless steel which is resistant to the trace amounts of acetic acid formed. The system was designed by AMCEC Inc. who had all components manufactured in the USA: installation in Mexico was by Grafo Regia using local contractors. In addition to the solvent recovery equipment, AMCEC also designed the suction ducting and provided all the automatic valves and controls for the SLA fume gathering system which brings solvent laden air from the various presses and coaters. The complete system is controlled by a PLC which is integrated with a PC based HMI package. AMCEC also provided comprehensive training for the Grafo Regia personnel. The solvent recovery system was commissioned in mid 1997 and is providing Grafo Regia with high quality recovered solvent that is re-used in their printing process and the recovered solvent value will return the capital investment within five years: and at the same time is meeting their stringent pollution control requirement. Authors: Vivek Nayak is Engineering Manager at AMCEC Inc.. Vivek has a Bachelors Degree in Chemical Engineering and is a registered Professional Engineer in the State of Illinois. Sergio Medrano is Plant Engineering Manager of Grafo Regia, S.A. de CV and has a Bachelors Degree in Mechanical Engineering with a Major in Electrical Engineering. Jaime Fermin is the Environmental Engineer of Grafo Regia, SA, de CV. Jaime has a Bachelors Degree in Chemical Engineering and over 7 years industrial experience covering Environmental Control, Compliance, Wastewater Treatment, Environmental Audits and Soil Remediation.
|
||||
|
||||
|
| Home | Processes | Equipment | Applications | Services | Case Studies | World Offices | Contact Us | |
