Indirect Heating of Thermal Fluids
General
Many operations, requiring energy delivery to a liquid-phase (and/or
fluid) stream, require a physical barrier between the fossil burn
(energy release) and the process stream. The barrier reduces heat
transfer efficiency, but is often necessary. Traditional systems depend
upon heat delivery via heat exchangers, fire-tube schemes (heating
coils, or multi-pass fluid heaters) and other methods. Many of these
systems use flame induced, radiant based heating to rapidly deliver
well over 50% of the required energy.
Process Uses
Situations involving high (and/or variable) pressure
systems, separation/purification operations, multi-phase operations,
systems impeded by oxidation (or other possibly reactive/degrading
components of combustion), and/or strictly maintained closed-loop
systems are common boundaries to direct heating of process streams.
More specific operations and their manufacturing
environments include:
- Purification, recovery, and separations; Chemicals/refinery
distillation (reboilers, etc.), and flash evaporators (polymer processing,
slurry separations/purification, brine treatment, etc.).
- Pressurized process streams (chemical reactors,
etc.)
- Processes/products sensitive to oxidation,
other reaction-driven degradation, and/or general fouling (chemical,
food, pharmaceutical processing, etc.)
- Vat or batch systems maintaining a heated
fluid (paint/dye blenders, food deep fryers, refinery-bottoms storage
and subsequent processing, reactor/fermentation vessels, crystallizers,
etc.)
- Thermal fluid, closed-loop-heating systems
for processes, often including those already mentioned, requiring
especially high and smoothly controlled temperature profiles
- Systems requiring high temperatures over large
areas such as calcium chloride crystallizers; Pipe line tracing
- Distillation and reactor feed lines whereby
preheating feed components simplifies the energy delivery and/or
chemistry complexity of that downstream operation
- Heat tracing viscous material (crude, confectionery,
polymer melts, etc.) pipelines to reduce electric driven pumping
- Tool heating, including plastics/rubber extruders,
molds, etc., paper mill platens and rollers, metal fabrication equipment,
laminate setting, and others
- General polymer processing. Polymer processing
plants may require high temperature (> 400F) energy delivery
to several unit operations because of high "pure" polymer
melting points (maintained for extrusion, molding, etc.), and endothermic
and/or equilibrium limited reactions (whereby light byproducts,
often water, must be continuously evaporated and removed for effective/efficient
reactor output). Polyester and Nylon 6,6 are good examples of major
international commodities often utilizing thermal fluids systems
throughout their production cycle
Integrating for Cogeneration
The wide variety of thermal "fluid" heating
applications mentioned above reflects the broad scope in unit operations,
engineering techniques, and process chemistries involved in this concept
category.
For this section, three general interconnection (with
cogeneration) systems will be discussed.
1. Systems not relying on radiant energy delivery.
Systems currently delivering heat to a process fluid via combustion
exhaust energy only (or other forced convection media), either through
a series of tubes, vessel/pipe jacket, or compact heat exchanger (shell
and tube, plate unit, etc.) can be easily adapted to receive cogeneration
based thermal energy. Because the majority of a DG unit's thermal
output is in the form of hot exhaust, the key concerns would be matching
the temperature, gas volume, and pressure parameters to those experienced
prior to cogeneration integration. This may require little or no rebuilding
of the process heat exchange equipment, but will need to consider
the operating tolerance of the DG unit.
2. Systems relying on radiant (flame induced) energy
transfer. Unless there is little radiant energy transfer contribution
(relative to the entire quantity delivered by the process operation)
and/or the flame temperatures are low (< 1,500F), even an unrecuperated
turbine cannot match the heat transfer characteristics expected in
the existing process heat transfer unit. Several combinations may
then compete on a cost benefit and space based analysis. Many systems
delivering a majority of the energy via high temperature, flame induced,
radiation leave a significant amount of the unit volume for flame
(radiant rays) "space" only. If this space were utilized
to generate more passes (fluid tubing), thereby increasing heat transfer
area, the operation could be more readily fit by a cogeneration scheme.
It may be the case that the original heat transfer unit cannot be
properly modified. However, if the feed line to the heater unit is
relatively low temperature (70-300F), a heat exchanger extracting
cogeneration energy prior to entering the main heater could result
in a sizable turn down of fuel delivery to that unit. Another option
would be the use of duct burners to increase the fuel gas temperature
to the required levels.
3. Closed loop, thermal fluid heating systems.
The first two interconnection categories represent traditional methods
of heat transfer to process fluid streams/systems. The second is more
common, but also requires a great deal more case by case analysis,
because of the variety of techniques and principles incorporated in
radiant heat transfer different from those available from cogeneration
exhaust (not considering "reburn" technologies). Thermal
fluid heating systems however, represent a stronger possibility for
a more heterogeneous cogeneration-based heat delivery, retrofit and/or
interface system. From a cogeneration standpoint the only concern
is maintaining total heat transfer characteristics to the heat transfer
fluid on return from the process unit(s). In other words, a 400,000
Btu/hr Dowtherm® based operation can use the same heater design
regardless of whether the system is heating/controlling a polymer
reactor or a paper laminate machine. This would allow for more repetitious
cogeneration designs across broad categories of process operations
and manufacturing sectors.
For more information on each application: