Benefits Beckon in Heat Transfer

Developments provide operational and economic improvements.

By Seán Ottewell, Editor at Large

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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.

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.

The system passes condensate returning from the steam distribution system through a flash steam separation vessel. The separate flash-steam and condensate streams travel through a dedicated plate exchanger to heat pressurized feed water before it enters the boiler. The two streams then are combined and go to the boiler feed tank. Because that stream is sub-cooled, it’s sufficiently warm to begin heating cold feed but not hot enough to overheat the tank. Heat and water previously lost from the system can be recovered, reducing utility bills, water treatment chemical costs and carbon dioxide emissions.

The FREME system is available as a pre-engineered skid-mounted unit (Figure 1), taking the stress out of designing, specifying, building and installing steam, hot water and other systems, says the company. And with less work to do on-site, the installation process is simpler, safer and speedier, it adds.

Such a system recently was commissioned by Abbey Corrugated, Blunham, U.K. Before the project, water entered the boiler at 154°F–158°F. It now arrives at 280°F–288°F. “…It’s fair to say that the savings from this project were in the region of 25% of the gas used by the boiler,” notes Paul Gale, facilities manager.

In early November FREME won the energy category in the annual awards for chemical engineering innovation and excellence presented by the Institution of Chemical Engineers, Rugby, U.K.

Model Results
 “Studies around heat exchange equipment show that about 90% of energy consumption on a typical process is associated with some sort of heat exchange. So companies want to get the most return on investment per BTU,” says Tom Ralston, Reading, U.K.-based product manager, exchanger design and rating, for AspenTech, Burlington, Mass. The total installed cost of heat transfer equipment today typically accounts for about 30% of overall plant investment, he notes. “So it’s central to exploring the cost benefits of almost any energy saving proposal.”

A key issue today is fouling. “Rigorous modeling is very important here if, for example, a stream contains materials that polymerize at a certain temperature and which would lead to a significant fouling deposit then good predictions can be vital. The rigorous exchanger model can predict local film temperatures within an operating process and allow the operator to ensure he is outside the limits where polymerization can take place. The process can be maintained with minimum downtime and maximum efficiency.”

Perstorp, Perstorp, Sweden, has used Aspen’s Tasc+ to optimize heat exchanger performance and eliminate downtime. “A most important factor in apparatus design is the fact that non-working equipment is often very expensive. Cost of downtime is $90,000 to $150,000/day for a typical large-scale polyol factory. For that reason a reliable design tool such as Tasc+ is of highest importance,” says Oleg Pajalic, process engineer.

The full impact of new technologies such as twisted tube exchangers from Koch Heat Transfer, Houston, Texas, and hiTRAN wire matrix inserts from Cal Gavin, Alcester, U.K., only can be gauged by a simulation including rigorous models of each exchanger, Ralston also points out. While modeling novel proprietary exchangers isn’t a trivial task, the biggest challenge is the conceptual design of the interface between the software tools, which relies on a strong collaboration between the partner companies, he says.

However, the rewards can be impressive: optimization of a large feed/effluent heat exchanger revamp on a major European plant by use of Tasc+ and hiTRAN inserts has resulted in a 20% increase in overall film coefficient, re-use of the existing shell, annual fired heater savings of $75,000 and a 1,700-m.t./yr decrease in carbon dioxide emissions.


 

Seán Ottewell is Chemical Processing's Editor at Large. You can e-mail him at sottewell@putman.net.

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