Hi 08 Utilities

7/28/2019 Hi 08 Utilities

1/19

Heat Integration UVa | Synthesis 08. Utilities 1

HEAT INTEGRATION

Synthesis

8. Utilities


2/19


Outline

Introduction

Heating and Cooling

Designing with composite curves

Grand composite curve

Common utility design


3/19


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)


4/19Heat Integration UVa | Synthesis 08. Utilities 4

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



Heating and Cooling



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




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)


q = m Cp TVTBFWHVAP



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





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-




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




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


12/19



Solving previous examples:

Intermediate steam level

Steam generation from BFW

Co-generation minimumfuel mass flow


13/19


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


14/19


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


15/19


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


16/19


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


17/19


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


18/19


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


19/19


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

Date post:	03-Apr-2018
Category:	Documents
Upload:	geo-bumbac
View:	217 times
Download:	0 times

Hi 08 Utilities

Documents