Heat transfer systems serve as crucial utilities at plants. So, boosts in their performance and efficiency can significantly impact operations.
Consider stereospecific reactions. Temperature usually affects relative yield of enantiomers. This has prompted companies to perform organic synthesis at low temperature using reagents such as n-butyl lithium that produce intermediates that after further processing lead to products with greater regularity and better selectivity. However, n-butyl lithium is very unstable at room temperature and needs excellent cooling control.
Air Products and Chemicals, Allentown, Pa., has been working with manufacturers to help them get better temperature differentials in their exchangers by using liquid nitrogen (LIN) to cool heat transfer fluids.
“In Air Products’ alternative, intermediate heat transfer fluid (HTF) —typically methanol, Syltherm XLT or a similar equivalent — is cooled by LIN in a counter-current flow heat exchanger. The HTF is then pumped into the jacket of the reactor vessel, where it removes heat from the reaction. The warmed fluid returns to the heat exchanger to be re-cooled by the LIN. The temperature of the HTF is controlled by varying the flow of LIN,” says Jon Trembley, lead, cryogenic applications, Basingstoke, U.K.
“Cryogenic cooling also provides rapid responses in cooling that are sometimes necessary to come with reaction kinetics and provides the flexibility to run reactions at lower temperatures should that be required in the future. Recovery of the vaporized nitrogen also means the operational running costs of cryogenic cooling system are controlled,” notes Marna Schmidt, an industry manager based in Allentown, Pa.
Trembley challenges the portrayal of LIN as an expensive option. “When improved reaction yields and selectivity, reduced unwanted byproducts, and the relatively low capital costs involved are taken into consideration, LIN also becomes an economically attractive choice. Because LIN is used in the reaction cooling process merely as a source of refrigeration, it is not affected by the process other than to vaporize and warm up slightly. So, if the evaporated LIN from the cooling process can be recovered and used elsewhere in the plant — such as for purging and blanketing — the costs of the system can be dramatically lessened and are minimal compared to mechanical refrigeration,” he explains.
Vendors of more-conventional heat transfer fluids, of course, also aim to help producers improve operations. For instance, Solutia, St. Louis, promotes its no-additional-cost Total Lifecycle Care (TLC) program that includes system design support, start-up assistance, 24/7 access to technically trained experts and more.
The experience of Mexichem (formerly Grupo Primex), a manufacturer of polyvinyl chloride resins and other materials, in Altamira, Mexico, highlights the importance of such services.
As of 2005, the company was operating two parallel heat-transfer systems — one running for more than 15 years with Therminol 66 fluid, which is suitable for operation up to 650°F and pumpable to 27°F, and the other working for several years with a diaryl-alkyl-based product rated to 660°F.