hi 08 utilities
TRANSCRIPT
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Heat Integration UVa | Synthesis 08. Utilities 1
HEAT INTEGRATION
Synthesis
8. Utilities
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Heat Integration UVa | Synthesis 08. Utilities 2
Outline
Introduction
Heating and Cooling
Designing with composite curves
Grand composite curve
Common utility design
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Heat Integration UVa | Synthesis 08. Utilities 3
Introduction
Utilities are the responsibility of the technical staff designing/retrofittingthe plant. One must:
Select the most suitable type for energy utilities
Select their levels (T, P)
Dimensioning
Each utility has its own characteristics:
Mass, volume and energy flows ability (big turbines or small ORCs)
Level variation pattern (isothermic, changing T...)
Capacity to combine with other utilities
...that make it more suitable for some processes than for other
Typical cases:
Change a spoiled/damaged utility device (steam boiler)
Plant revamping (modernize) or debottlenecking (enlargement)
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Heating
Cooling water (with/without cooling tower) (sea water)
Air Cooled Exchanger / Condenser
Low-temperature ('sub-ambient') refrigeration
Vapor-Compression / Vapor-Absorption
BFW pre-heating / Combustion air pre-heating
Steam generation (heat from process)
Bottoming ORC (organic Rankine cycles) co-generation
Steam (LP, MP, HP, VHP...)
From a steam generator (boiler)
From a heat recovery steam generator (engine/turbine)
Steam generation through co-generation (turbine)
Flue gases form a gas engine/turbine (topping)
Thermal Fluid ('Oil') systems
FurnacesHeat pumps
and Cooling
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Heating and Cooling
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Objective: To select the utility or combination of utilities optimized for theproblem and fix their most suitable operating parameters and quantities
Pinch analysis and H vs. T composite curves can be used:Example: introducing an intermediate level of steam:
How much use from the new level? How do they affect original steam?
Utility is an energy stream like the other (level, thermal inertia, duty)
Increase until the appearance of a utility pinch
Designing with composite curves
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Example: steam generation from BFW using process heat
How much steam at given T can be generated from BFW at ambient T?
This is a complex refrigeration utility:
Until the appearance of a utility pinch (may appear in the preheating)
Designing with composite curves
q = m Cp TVTBFWHVAP
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Example: dimensioning a gas turbine co-generation
What is the minimum mass flow of flue gases/fuel required for a process
Until the appearance of a utility pinch
Summary: the calculation of utilities is possible but not easy
Designing with composite curves
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Grand composite curve
Can be calculated by theproblem table algorithm
Shows the energy profile of theproblem: where heat arises ordies away
Incoming intervals:
Heat flow decreases with T
Enthalpy consumption regions
Outgoing intervals:
Heat flow increases with T
Enthalpy consumption regions
Utilities are well understoodwhen plotted on this diagram
T* vs H diagram of heat cascade -heat flows across boundaries-
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Grand composite curve
Represents the T vs. H characteristics of a problem, enthalpy flows orheat in excess for each temperature level
Lets you easily visualize the pinch/es and quasi-pinch/es, and theheating and cooling requirements
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Grand composite curve
The process heat exchanging with the surroundings utilities takesplace only at hot and cold ends (highest and lowest points H coord.)
In re-entrant areas ('pockets')internal transfer can occursbetween heat-surplus andheat-deficit regions of theprocess (Tmin at least)
If heat is taken from a pocketmust be restored later (colder)
Also 'pseudo-utility' design
Reactors
Separation systems
Turbines/engines
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Grand composite curve
Solving previous examples:
Intermediate steam level
Steam generation from BFW
Co-generation minimumfuel mass flow
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Common utility design Steam systems
High temperatures, but critical point: Less carrying capacity / Too high P
Usually several levels (LP, MP, HP, VHP...) for efficiency and cost
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Common utility design Steam systems
Minimize level temperature
Maximize flows at lowlevels (normally)
Complexity increases withthe number of levels
Different alternatives /setscan be easily explored
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Common utility design Furnaces (simple)
Used in cases of very high temperature or large duties
Very complex utility; here a simplified model
Flue gases loosingheat linearly (mCp)from adiabatic flameT to stack T
Real T < Flame T(radical formation,excess air)
Stack T limited by:
Acid condensationUtility pinch
GCC informs onwaste heat
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Common utility design Thermal oil
Heat transfer from liquids at high temperature (not constant)
Seeks to reduce the flow by reducing the final T
Initial T fixed by oil
or process stabilityfinal T limited by:
Process pinch
Utility pinch
C ili d i S b bi C li
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Common utility design Sub-ambient Cooling
Vapor-Compression / Absorption generation of a liquid refrigerant
(Constant T boiling) or (mixed refrigerant) heat removal
Several P/T levels tomaximize efficiency
Very complex
designs
Heat leaks fromoutside to inside
High cost (shaft
work, mechanical orelectrical):minimizing shaft
work, no heat flow
C tilit d i C li t
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Common utility design Cooling water
Air-cooled exchangers and combustion air pre-heating are similar
Mass flow andtemperatures
Cooling towerdimensioning
Sea water:pumping rates
H t d Th l E i
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Heat pumps and Thermal Engines
Heat pumps:
The smaller the differencein temperature, the better
'Big nose' processHeat output = heat input +shaft work (1st principle)
Difficulty in placing:
Topping cycle (gas turbine): rendering heat output above the pinch
Bottoming cycle (ORC): taking heat input from bellow the pinch
Heat pump:
Taking heat from bellow the pinch
Rendering heat above the pinch