Benefits Beckon in Heat Transfer

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.

Energy Recovery Unit

Figure 1. FREME system is available as a pre-engineered, skid-mounted unit.
Source: Spirax Sarco.

Cradle-To-Grave Care
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.

The system with Therminol 66 has performed without incident, says Mexichem. However, after just three years, performance of the other system began to decline.

“First we experienced pluggage in our instrument tubing,” says Francisco Nava, production manager. “Soon after, we observed damage to the mechanical seals and problems began occurring in the heat transfer process. As a result, we were experiencing losses in distillation efficiency, increases in system vapor pressures, increased unplanned downtime, and impacts to our finished product quality.”

Mexichem turned to Solutia for a complimentary fluid analysis, part of the TLC program. It showed the non-Solutia fluid was degrading rapidly, reducing its ability to operate efficiently. Degradation products accounted for more than half of the fluid composition and the fluid was precipitating crystalline solids under certain conditions.

“Because the alternate heat transfer fluid wasn’t doing a good job, our pump seals were failing and we were losing yield. At the same time, our system utilizing Therminol 66 heat transfer fluid was operating smoothly. The decision to switch to Therminol 66 heat transfer fluid in the other system was clear,” says Nava.

Steam Savings
Many plants rely on steam as a heat transfer fluid but condensate recovery systems are so effective that often too much energy is captured. So, steam specialist Spirax Sarco, Blythewood, S.C. and Cheltenham, U.K., has launched a flash recovery energy management equipment (FREME) system that’s designed to overcome that problem.

FREME is a completely closed steam system under constant pressure that can recover energy from returned condensate and flash steam without wastefully dumping or venting surplus energy from the system. Instead it feeds energy from the returned condensate into the high-pressure side of boiler feed pumps.