On-line version ISSN 0104-6632
Braz. J. Chem. Eng. vol.20 no.1 São Paulo Jan./Mar. 2003
A study of paint sludge deactivation by pyrolysis reactions
L.A.R.Muniz; A.R.Costa; E.Steffani; A.J.Zattera; K.Hofsetz; K.Bossardi; L.Valentini
Departamento de Engenharia Química, Centro de Ciências Exatas e Tecnologia, Universidade de Caxias do Sul, Rua Francisco Getulio Vargas 1130, CEP 95001-970, Caxias do Sul, RS, Brazil
The production of large quantities of paint sludge is a serious environmental problem. This work evaluates the use of pyrolysis reaction as a process for deactivating paint sludge that generates a combustible gas phase, a solvent liquid phase and an inert solid phase. These wastes were classified into three types: water-based solvent (latex resin) and solvents based on their resins (alkyd and polyurethane). An electrically heated stainless steel batch reactor with a capacity of 579 mL and a maximum pressure of 30 atm was used. Following the reactor, a flash separator, which was operated at atmospheric pressure, partially condensed and separated liquid and gas products. Pressure and temperature were monitored on-line by a control and data acquisition system, which adjusted the heating power supplied to the pyrolysis reactor. Reactions followed an experimental design with two factors (reaction time and temperature) and three levels (10, 50 and 90 minutes; 450, 550 and 650°C). The response variables were liquid and solid masses and net heat of combustion. The optimal operational range for the pyrolysis process was obtained for each response variable. A significant reduction in total mass of solid waste was obtained.
Keywords: pyrolysis, paint sludge and batch reactor.
One of the most serious problems in industries that make use of the paint process is the generation of waste known as paint sludge. This is the paint that collects in the washed water from the paint cabin. Paint sludge was analyzed and classified into hazardous and active waste. Many studies on the characterization, disposal and, mainly, applications of paint sludge have been published. Erdman and Johnson (1998) extended the use of the hollow screw indirect heat exchange processor, used by the food, chemical and minerals industries, and developed the large particle recycler. They tested it for paint waste sludge volume reduction and low-temperature volatile organic compound vapor exclusion or recovery. Mitchell and Schweers (1989) used organic polymers to reduce generation of hazardous waste and discuss costs associated with paint sludge disposal.
Ford Research Laboratory has intensively studied applications for paint sludge. Kim et al. (1996) addressed the technical feasibility of converting paint sludge into activated char and reusing the char in paint spray booth water to capture paint solvents from spray booth air. They used pyrolysis reactions to activate paint sludge and tested it as an adsorbent. Kim et al. (1997) developed a process by which water-soluble paint solvents are biologically degraded in an activated sludge reactor. Nakouzi et al. (1998) described an alternative to landfill disposal whereby paint sludge is converted into ceramic composites that can be used as reinforcing materials.
Pyrolysis is a possible way to make paint sludge become chemically inert. It can be generically defined as thermal degradation in the absence of or with a minimum amount of oxygen, with a simultaneous production of oils and gases that can be used by the chemical industry and for the production of energy (Sodero et al., 1996).
In this work, pyrolysis of paint sludge in a fixed-bed reactor was studied.
Initially, the reactor was purged with nitrogen and fed with 20 g of paint sludge. The reactor operated in batch mode and was fed and emptied at the top. A transducer, connected to a coil to disperse heat, was used to measure pressure. Temperature was measured by a J thermocouple, inserted in a well in the center of the reactor. The thermocouple was plugged into an Ultra SlimPak signal conditioner (G428-0001 model) and a data acquisition card computerboard (CIO-DAS-Jr 08 model). Elipse Windows Software® was used for data acquisition, and temperature control and power supplied to the reactor were used as manipulable variables. Reactions were conducted in the absence of oxygen in order to eliminate toxic oxygenated gases.
Pressure in the reactor rises only due to gases produced during the reaction. At the end of the reaction the products were removed from the reactor at a side exit and passed through an expansion valve and the liquid phase was collected at the bottom of the flash separator. The dark oil in liquid phase obtained was distilled to remove the heavy oil. The light oil and gas phases were analyzed by gas phase chromatography. A schematic diagram of the process is shown in Figure 1.
An experimental design for pyrolysis reaction was developed considering reaction time and temperature as control variables. Liquid and solid mass and net heat of combustion were chosen as response variables. First-order central composite design with two factors and three levels was the method used for experimental planning.
The experiments were divided into three categories of paint sludge resin: alkyd, latex and polyurethane. Results of the three experiments are summarized in Tables 1, 2 and 3. Net heat of combustion is the energy obtained by combustion of the overall gas phase produced after passing through the flash separator. This was calculated by simulation with known gas phase mass and composition. Chromatographic analysis indicated the presence of methane, ethane, ethylene, propane, air, and carbon dioxide in gas phase. The liquid phase is composed mainly of benzene, toluene, ethylbenzene, ethyltoluene, xylene, naphthalene and hydrocarbons from C6 to C13. The liquid and gas phases are used commercially as solvent and combustible gas, and the remainder of the solid mass can be thought of as a residual waste. The costs involved in this process are mainly due to labor and energy, therefore, maximizing net heat of combustion and minimizing batch time is the way to minimize costs. This research focused on operational conditions that improve net heat of combustion and liquid mass is a secondary response. A statistical analysis showed that the solid mass obtained at the end of the reaction is little influenced by reaction time and temperature.
Reactions were carried out according to the experimental conditions shown in Table 1. Reductions in solid waste mass were around 70%. Figure 2 shows the net heat of combustion as a function of reaction temperature and time for paint sludge with alkyd resin.
As can be seen at high temperature a gas with high energetic potential was produced, and there was a little influence of time.
Reactions were carried out according to the experimental conditions shown in Table 2. Reductions in initial waste were 75% on average. Figure 3 shows the variation in net heat of combustion as a function of temperature and reaction time.
In this case, longer reaction times resulted in higher net heats of combustion. Higher temperatures produced a gas with improved energetic potential.
Reactions were carried out according to the experimental conditions shown in Table 3. In this case, the solid waste mass was reduced 96% on average. Figure 4 shows the net heat of combustion as a function of temperature and reaction time. As in the first case, at high temperatures a gas with a high energy potential was produced, and there was little influence of time.
Pyrolysis appears to be a good alternative for the treatment of paint sludge wastes. The solid phase had a weight reduction with no significant variation. The alkyd resin had an average reduction in the weight of initial solid mass of 70% latex resin, 75%, and polyurethane resin, 96%.
The liquid phase had a variation in yield that was not significant. The paint sludge alkyd resin showed an average yield of 34%, the latex resin, 56%, and the polyurethane resin, 63%.
As the costs involved in the pyrolysis process are due to labor and energy, the gas phase obtained in the process can be burned and the net heat of combustion can be used as part of the energy supplied to the process. Reaction time is not a significant variable, so we must operate the process in a short time regime. The best conditions for carrying out the process were a high temperature (650°C) and a short reaction time (10 minutes).
An economical analysis, quantifying operational costs and profits with products (gas and liquid), is in progress and will allow definition of the optimal operational point.
Erdman, A. Jr., Johnson, J. (1989). Low Temperature Thermal Stripping and Volume Reduction of Waste Sludge with a Hollow Screw Indirect Heat Exchange Processor. Environmental Conference, Proceedings of the Technical Association of the Pulp and Paper Industry, 71-73. [ Links ]
Kim, B.R., Kalis, E.M., Salmeen, I.T., Kruge, C.W., Demir, I., Carlson, S.L., Rostam Abadi, M. (1996). Evaluating Paint Sludges Chars for Adsorption of Selected Paint Solvents. Journal of Environmental Engineering, 122(6), 532-539. [ Links ]
Kim, B.R., Podsiadlik, D.H., Yeh, D.H., Salmeen, I.T., Briggs, L.M. (1997). Evaluating Conversion of an Automotive Paint Spray Booth Scrubber to an Activated Sludge System for Removing Paint Volatile Organic Compound from Air. Water Environment Research, 69 (7), 1211-1221. [ Links ]
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Address to correspondence
Received: March 5, 2002
Accepted: August 21, 2002