Anal. Chem. 2003, 75, 3924-3928
An Energy-Efficient Self-Regulating Heater for Flow-Through Applications Purnendu K. Dasgupta,*,† Ellis L. Loree,‡ Jianzhong Li,† and Zhang Genfa†
Department of Chemistry, Texas Tech University, Lubbock, Texas 79409-1061, and Loree Engineering, 7116 Dodge Trail NW, Albuquerque, New Mexico 87120
In many experiments, a flow-through heating arrangement is needed to reduce reaction time. Often the reaction conditions require inertness of the wetted material. Heated reactors based on polymeric tubing, notably PTFE, are the most common, and such reactors are typically used in a manner in which they are put in a heated bath or an otherwise thermally conductive potting in which a heater and a temperature sensor are embedded for heating and temperature control. Polymeric tubes are poor conductors of heat; as such, most reactors of this type have very poor energy utilization. We describe here heated flow-through reactors where a wire runs through the entire length of the tubular reactor and where the wire is directly electrically heated. The wire may or may not be electrically insulated. If the exterior of the tube is well insulated, the energy efficiency of such a reactor in heating the fluid of interest is nearly unity. This makes it most suitable for battery-powered applications. If an appropriate wire with a significant temperature coefficient of resistance is chosen, monitoring the current through the device at constant applied voltage indicates the effective mean temperature of the device and thus allows effective temperature control without an additional sensor/controller with essentially instantaneous response. Temperature control within (0.4 °C at a mean temperature of 65 °C and within (0.9 °C at 87 °C have been achieved. In the practice of flow analysis, flow-through heaters are frequently necessary to accomplish a desired reaction within a reasonable period of time. At other times, it is necessary to maintain a constant temperature, and since heating is easier than cooling, the system may be heated to some constant temperature above the ambient temperature. The list is far too long to pick examples from the literature to illustrate the wide need for flow-through heated reactors. The use of in-line microwave heating/digestion1 has gained popularity in recent years because of the ability to perform contactless energyefficient heating. However, this approach is not practical in field instruments. Regarding the use of conventional heated reactors, from our laboratory we have described in this journal alone the use of such heated reactors within the past decade for the measurement of HCHO,2 carbonyl compounds in general,3 am* Corresponding author. E-mail:
[email protected]. † Texas Tech University. ‡ Loree Engineering. (1) Caballo-Lopez, A.; de Castro, L. Talanta 2003, 59, 837-843.
3924 Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
monia,4 and peroxides.5 Other important applications we recently published elsewhere include the use of heated reactors for the field measurement of atmospheric formaldehyde6 and waterborne arsenic.7 Within this milieu, in the earlier years, we used knotted or woven PTFE reactors put in a water or silicone oil bath. In the more recent examples, to improve heat transfer and thus energy efficiency, we have adopted a scheme where a PTFE tube, woven on a stainless steel screen in a serpentine II design,8 is potted in a low-melting bismuth alloy along with a platinum RTD (resistance temperature detector) sensor and a pair of silicone-embedded heaters.6 Despite its improved energy efficiency, it is still one of the most power consuming components in our fieldable analyzers and metal potting increases the weight. Similar heated enclosures machined from aluminum and capable of holding 1-1.3-m lengths of PTFE tubular reactors are available commercially.9 Usually Nichrome is the alloy that is used in heating elements because it has a very low temperature coefficient of resistance (R) and there is only a small change of current at constant applied voltage as the element heats. In contrast, platinum exhibits a high value of R and is the most commonly used sensing element in RTDs. If heating element wires are chosen such that they have significant temperature dependence of resistance, the heater and the sensor can be one and the same and very fast response becomes possible.10 In the past, Hail and Yost described heating an Al-clad capillary column for gas chromatography using this strategy.11 For a flow-through heater application, the heating element can be in direct contact with the fluid to be heated and this would result in the most energy-efficient heat-transfer system. The heating wire can be bare and be composed of a noble metal such as Pt or be of any suitable metal and have a chemically inert thermally conducting electrical insulation. This makes possible (2) Fan, Q.; Dasgupta, P. K. Anal. Chem. 1994, 66, 551-556. (3) Dasgupta, P. K.; Zhang, G.; Schulze, S.; Marx, J. N. Anal. Chem. 1994, 66, 1965-1970. (4) Genfa, Z.; Uehara, T.; Dasgupta, P. K.; Clarke, T.; Winiwarter, W. Anal. Chem. 1998, 70, 3656-3666. (5) Li, J.; Dasgupta, P. K. Anal. Chem. 2000, 72, 5338-5347. (6) Li, J.; Dasgupta, P. K.; Genfa, Z.; Hutterli, M. A. Field Anal. Chem. Technol. 2001, 5, 2-11. (7) Dasgupta, P. K.; Huang, H.; Zhang, G.; Cobb, G. P. Talanta 2002, 58, 153-164. (8) Waiz, S.; Cedillo, B. M.; Jambunathan, S.; Hohnholt, S. G.; Dasgupta, P. K.; Wolcott, D. K. Anal. Chim. Acta 2001, 428, 163-171. (9) www.globalfia.com. (10) http://www.minco.com/heaters.php; http://www.avatarinstruments.com/ tssman.pdf. (11) Hail, M.; Yost, R. A. Anal. Chem. 1989, 61, 2410-2416. 10.1021/ac0343174 CCC: $25.00
© 2003 American Chemical Society Published on Web 06/12/2003
Figure 1. (a) Sealing arrangement at the end of a heated reactor, (b) Photograph of directly heated reactor woven in a Serpentine-2 pattern. An external temperature sensor (AD-590) is seen in the right center.
very simple, efficient, and stable flow-through heated reactor systems that we describe here. EXPERIMENTAL SECTION We have used a variety of wires for heating and sensing in this application. Bare platinum (Alfa-Aesar), polyimide (trade name ML)-coated bifilar nickel (www.pdwirecable.com), ML-coated Balco (see Table 1, www.pelicanwire.com), and ML-coated tungsten (www.wiretron.com) were all successfully used, and reactor construction is similar. Except for the bifilar wire (two strands separately insulated, as in household two-conductor cable, except in miniature, the fabrication of the other heaters is the same. The reactor has a T- or Y-fitting at each end. One arm of each fitting is connected to the reactor proper. Since very fine (
