Cooling, of course, figures prominently in many processes. Unit operations such as crystallization and various types of freeze-concentration often rely on refrigeration. Water-glycol mixtures or ammonia-based refrigerants frequently are used for cooling to the freezing point of water or slightly lower, while silicone oils or an HTF based on d-limonene handle some specialized applications and lower temperatures. Liquefied atmospheric gases commonly are chosen when even lower temperatures are needed, for jobs such as cryogenic grinding or combined cooling/inerting. What is new is the development of a variety of HTFs for a range in the region of -58°F (-50°C) to -184°F (-120°C) that is well below refrigeration, but not as cold as cryogenics.
"Overall, many of the higher-temperature processes where our HTFs have conventionally been used are mature," says James Oetinger, sales manager for supplier Paratherm, Conshohocken, Pa. "We think that this region of low-temperature processing is a new frontier for us." Conrad Gamble, technical services manager at another HTF supplier, Solutia, St. Louis, adds, "Our market research showed that there was a portion of the low-temperature processing market that was poorly served by technologies based on liquid nitrogen or existing HTFs."
New options
Paratherm’s new product, called CR, is rated to -184°F (-120°C), and has a viscosity of 10 cP at -126°F (-88°C), rising to 20 cP at -141°F (-96°C). Oetinger says that the patent-pending material is a proprietary all-synthetic hydrocarbon and contains no halogens. It has already been put to use in a pharmaceutical application, and has demonstrated the ability to raise throughput because cooling occurs faster (at a lower temperature) than the material it replaced. Paratherm expects lyophilization to be a typical application in pharmaceutical and fine chemicals processes. "At lower temperatures, existing HTFs like silicone oils turn slushy, which makes pumping them more difficult," he says. Compared to d-limonene, which has a very low surface tension and can leak through fittings, the material is easier to contain.
Bedoukian Research Inc., Danbury, Conn., a producer of fragrances, food ingredients and specialty chemicals, already has had success with Paratherm CR. The company has used the HTF in a jacketed reactor to cool an exothermic chemical-synthesis step. Robert Bedoukian, company president, says the fluid "helped us to maintain the desired reaction temperature more efficiently," and, in some instances, to significantly improve throughput of the process. "We’re happy with the results," he says, adding that the company is looking at other exothermic reactions that could be improved with the new product.
Other low-temperature HTFs that have recently appeared on the market include Dynalene HTFs, which are aliphatic hydrocarbons, from Advanced Fluid Technologies, Whitehall, Pa., and Xceltherm 500 HTF, a polyalphaolefin HTF rated to -80°F, from Radco Industries, La Fox, Ill. (Radco’s Xceltherm product line was a 2003 winner of Chemical Processing’s Vaaler award). 3M Co., St. Paul, Minn., offers HTFs based on hydrofluoroether technology; they, like d-limonene, have low surface tensions and, thus, require special considerations in seals and gasketing. The company’s HFE 7000 product, which consists of
1-methoxyheptafluoropropane, has been used in low-temperature pharmaceutical processing. Dow Chemical Co., Midland, Mich., has marketed for several years Syltherm XLT, a formulated product based on dimethyl polysiloxane. It has a recommended use range of -150°F (-100°C) to 500°F (260°C), and boasts an operating life of as long as 10 years.
D-limonene is a naturally occurring terpene derived from orange peels that first appeared in the early 1980s as an HTF to replace trichloroethylene, a toxic material. Florida Chemical Co., Winter Haven, Fla., is a supplier of the terpene, and because the HTF is an all-natural product, the FDA categorizes the fluid as "generally recognized as safe" (GRAS), which is a requirement for food constituents. The company says it can be used successfully in applications near its freezing point of -142.1°F (-96.7°C). However, it is degradable and reactive and, therefore, needs to be used with both a preservative (BHT, a common food preservative) and a neutralizer (sodium carbonate is recommended) for dealing with residual citric acid.
New fluid's low freeze point and viscosity lead to higher efficiencies at lower temperatures.
Source: Paratherm
Process design factors
"There are two basic ways to effect heat transfer in these applications — sensible heat transfer, using fluids like these new HTFs, or latent heat transfer, using liquid nitrogen or other cryogenic gas," he says. The advantage of nitrogen is that temperature can be very tightly controlled because the heat transfer occurs during the constant temperature vaporization of the liquid. Conversely, the advantage of using an intermediate HTF (even if cryogenic nitrogen is the primary source of cold) is that the process operator avoids the risk of an upset condition where product could be frozen at cryogenic nitrogen’s temperature (around
-319°F/-195°C) or where a range of cooling temperatures is desirable.
Arencibia chose liquid nitrogen for the Pfizer Lipitor process, where his technology won out over the alternative of a halocarbon-based mechanical refrigeration system. The patented technology is used to control an exothermic reaction during the manufacturing process, cooling the reactants to -150°F (-100°C). Two units have been in operation since 1998. Overall, Arencibia says, "There is a trend toward lower-temperature, more selective chemical reaction processes, and the new HTFs offer some advantages."
Arencibia adds that material and equipment considerations also play a factor in low-temperature heat-exchange processes. "Carbon steel is out for these processes, because that grade of steel becomes too brittle and glass-lined reactors are also not acceptable, because of differing coefficients of thermal expansion," he says. A conventional shell-and-tube design, either a tube-in-tube arrangement or a spiral-in-spiral one, usually is specified.
One of the heat exchangers that several engineering companies in this field mention is the Heliflow from Graham Corp., Batavia, N.Y. The design, which has been available for many years, uses a spiral configuration of tubing within the heat exchanger shell and has no baffles or gasketing between tubes, thus obviating thermal expansion problems. According to the company, its designs can handle temperature gradients as large as 500°F; it also has experience with exchangers that have cryogenic nitrogen on one side and an HTF on the other. "We ran a series of tests on HTFs a couple years ago," says Eric Klotzbach, supervisor of the heat transfer group at Graham (see related article in CP, January 2003, p. 35). "At that time, most fluids couldn’t perform reliably at temperatures around -100°C [-150°F], mostly because of freeze-up problems. These new fluids look like they could increase interest in the technology."
By Nick Basta, editor at large
