Energétique Avancée Process Integration And Exergy Analysis Dr ...
Energétique Avancée – Process integration
5. Improve the MER of a process
Energétique avancée
Process integration and exergy analysis
Dr Francois Marechal
5. Improve the MER of a process
Goals
Different ways of modifying the minimum energy requirement of a process
Short summary
5. Improve the MER of a process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5.1. Distillation columns............................................................................................................ 4
5.1.2. Pressure of the distillation columns................................................................................. 6
5.1.3. Comparing MVR and pressure changes ...........................................................................12
Francois Marechal - v8.12.2002
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Energétique Avancée – Process integration
5. Improve the MER of a process
5. Improve the MER of a process
Before searching for defining a heat exchangers network that satisfies the MER, we will study
the possibilities of improving the MER.
A hierarchical approach can be used. The energy analysis method considers the industrial
system as an onion. The hearth of the process is the reaction. For improving the MER we will
first concentrate on the reactions and the operating conditions of the reactor
Once these are fixed, we are able to try to optimize the separation units (distillations, stripping,
absorptions, membranes,...).
The next layer is the utility system that aims in satisfying the MER at minimum cost.
Once all the previous layers are fixed, the complete list of streams for the heat exchangers is
fixed and the synthesis of the heat exchangers network can be executed.
Improving the MER can be made by analysing the shape of the composite curves.
The analysis of the composite curve shape allows to propose process modifications and to
choose the utilities.
The pinch point identifies the bottleneck of the process where energy has the most difficulties
to flow. The stream involved in the pinch point determination and the one in its
neighbourhood requires more attention.
Which streams defines the pinch point?
The pinch point is always created by the inlet temperature of a stream. It can be the inlet of a
stream or one of the streams created if fluid phase changes occur.
T
T
Cp=A
Cp=(A+B)
Cp=(A+B)
Cp=A
H
H
a) Hot streams
b) Cold streams
Figure 44. The pinch is introduced by the inlet temperature.
The figure 44 illustrates this affirmation. For the hot composite curve figure 44 (a), the pinch
point can occur if the curve features an angular corner pointing to the cold composite
curve. It is only possible if the slope of the curve decreases when the temperature decreases.
It is obtained when a new Cp is taken into account in the lower temperature interval
corresponding to a new hot stream involved. The pinch point will then appear at the upper
temperature of the interval. It corresponds to the inlet temperature of the new hot stream
entering the composite curve.
The same reasoning can be done for the cold composite curve figure 44 (b), where the
angular corner must point to the hot composite curve to have a chance to produce a pinch
point.
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Energétique Avancée – Process integration
5. Improve the MER of a process
The important streams are such that they introduce strong angular corner in the
neighbourhood of the pinch point.
The figure 45 shows how to analyse the composite curves to identify the important streams.
T
3
2
1
Q
Figure 45. The composite curves analysis
1 - The stream which defines the pinch point.
This is the most important because it defines the minimum energy requirement.
2 - The stream with big Cp and specially the condensing or evaporating streams. They
produce steps in the composite curve which prevent the fitting of the two curves. These
streams often define the pinch point. And when their heat load is high, they define most of
the energy requirement.
3 - The stream involved in pseudo pinch point: where the hot and cold composite curves
are close but do not intersect. A slight modification of the process operation can lead this
pinch point to be active.
The same points can be found on the grand composite curve as shown figure 46.
T
3
2
1
Q
Figure 46. Grand composite curve analysis
The process modifications will be suggested by the type of the identified
important streams. The leitmotiv will be "try to fit the hot and cold
composite curves together" or " try to squash the grand composite curve
on the temperature axis".
The process modifications depend on the process context, we can not give an exhaustive list
of the possibilities. They mainly concern operating condition modifications to change the
temperature levels or the flow rates and the structure modifications.
The intrinsic energy efficiency of each unit of the process is important for defining the global
demand of the process.
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Energétique Avancée – Process integration
5. Improve the MER of a process
But, according to their relative position with respect to the pinch point, improving the
energy efficiency of some units might not be useful in the global context.
The units consuming energy above the pinch point have to be minimized.
The same for the units producing energy above the pinch point: their production has to be
maximized.
Caution might be paid for units between the pinch point and a pseudo pinch point, because
the effect of the optimization might be limited by the activation f a new pinch point.
5.1. Distillation columns
In the following, we will analyse two of the more important modifications that can be
proposed from the composite curve analysis in chemical plants. The same philosophy will be
applied for other modifications.
The first modification is the columns pressure modifications.
The second will be the mechanical vapour recompression in distillation column train.
When starting from simulation or validation results, data collection is an important step of the
energy integration. The boiler and condenser case requires a few comments.
A boiler is a cold stream to be heated up.
A condenser is a hot stream to be cooled down. Its flow rate includes the distillate and the
reflux.
In a "short-cut" distillation column simulation, the condenser and boiler temperatures are
considered as constant, equal respectively to the boiling or the dew point. As no more
precise information is available, this choice is a conservative case.
Figure 47 gives the detail about the streams entering the integration.
Condenser : hot stream
C
T=Tc
T=Tc
Qc
Q=Qc
Q=0
A
Qb
COLUMN
T=Tb
T=Tb
Q=0
Q=Qb
B
Boiler : cold stream
Qc:
Condenser heat load (simulation results).
Tc:
Condenser temperature (Boiling point of the distillate C).
Qb:
Boiler heat load.
Tb:
Boiler temperature (Dew point of the residue B).
Figure 47. Distillation column: the streams entering the integration.
A single distillation column includes one boiler and one condenser. For reason of simplicity,
the fluid phase changes will be considered isothermal. The corresponding grand composite
curve is given figure 48. The boiler will exchange with the hot utility (steam for example) and
the condenser will be cooled down by cold utility (cooling water, for example).
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Energétique Avancée – Process integration
5. Improve the MER of a process
CW
T
Steam
Boiler
Condenser
Cooling Water
Steam
H
Figure 48. Single distillation column: grand composite curve.
When simulating a distillation column with a more precise model (plates simulation), the
composition and flow rates of the condenser and boiler stream on the distillation column side
are calculated. In this case, the streams entering the integration can be rigorously taken into
account: the H-T diagram can be considered.
Figure 49a gives the case of the hot stream in a total condenser.
Tc1
Tc2
Pc1
d1
Pc2
a=1
a=0
Tc1
Tc2
Pc1
Pc2
Plate 1
d1
d1
R
D
d1:
condenser flow rate = distillate + reflux (R+D).
Tc1: Saturation temperature at Pc1 of the vapour leaving the first plate of the column.
Pc1:
Condenser inlet pressure.
Tc2: Saturation temperature of the liquid leaving the condenser at Pc2. In certain cases,
the stream will be undercooled.
Pc2:
Condenser outlet pressure.
Figure 49a. Condenser
The same analysis can be done for the cold stream in a boiler of the figure 49b.
Plate n-1
Tb2
a=0
a=1
Plate n
Pb2
Tb1
Tb2
Qb
Pb1
Pb2
d1
d1
d1
Tb1
Pb1
d:
Flow rate in the boiler. It is calculated from the inlet and outlet temperatures and the
heat load of the boiler.
Tb1: Saturated liquid temperature at Pb1.
Tb2: Saturated vapour temperature at Pb2.
The composition is the residue composition.
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5. Improve the MER of a process
Figure 49b. The boiler.
Other configurations can be studied according to the specific technology used in the boilers
and condensers.
5.1.2. Pressure of the distillation columns
When integrating columns together, pressure column changes are also possible.
The analysis of the composite curves can be done in the same way. In figure 50, the
objective of the process improvement is to put the stage CA (condenser of column A) above
the stage BB (boiler of column B). The MVR is a way to realize this objective, an other way is
to change the columns pressure in order to change the temperature levels of both the boiler
and the condenser.
For example figure 50, if the column B pressure can be decreased in order to have a boiler
BB temperature below the condenser CA temperature, the resulting curves will be the case
(b). The hot and cold composite curves fit better and the grand composite curve is closer of
the temperature axis. In this case, the energy recovery is the boiler BB heat load for both hot
and cold utilities.
T
T
Q1
Q1new
BA
BA
BB
CA
CA
C
BB
B
Q2
CB
H
H
T
T
Q1
Q1new
BA
BB
C
BA
A
CA
CB
CB
BB
Q2
H
H
(a)
(b)
Figure 50. Pressure changes in integrated distillation columns.
If pressure decrease was not possible in column B, the pressure of column A can be
increased. The mechanical power will be smaller than in the case of the MVR as the feed of
column A is a liquid.
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5. Improve the MER of a process
An important remark is that the operation of the column is modified. The new operating
conditions have to be calculated by simulation. In case of retrofit or revamping study, the
results will be carefully analysed to verify the technological feasibility of the new running
conditions.
The reasoning is already valid for the integration of more than two columns.
The problem has to be analysed globally as it can be seen in figure 51. Pressure increase of
the column A has no effect on the energy recovery as well as the pressure decrease of
column B. But if the pressure of column C is decreased, the pinch point BB CA can become
active, and in this case pressure modifications of A or B pressure can become attractive.
Q1
T
T
Q1
BA
B
B
B
A
C
CA
A
B B
B C
C
CB
B
B C
C
C
C
C
Q2
Q2
H
H
Figure 51. Integration of three distillation columns.
The rules for pressure modifications in integrated distillation columns are:
Decrease the pressure of the column which boiler is above and the closest of the pinch point.
Increase the pressure of the column which condenser is below and the closest of the pinch
point
5.1.2.2. Mechanical Vapour Recompression (MVR)
The mechanical vapour recompression (MVR) consists in compressing the vapour entering the
condenser before the exchange (figure 53), so as the condensation temperature is increased
and the heat can be used to heat up the boiler. The figure 52 gives the evolution of the
temperature and the heat load of the condenser when the compression ratio increases.
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5. Improve the MER of a process
T
t
Q
Figure 52. Temperature and heat load of the condenser as a function of compression ratio.
The heat load of the condenser increases since the mechanical power is added in the process
and must be evacuated to not perturb the column operation. Additional cooling will be
required to perform the sub cooling necessary to obtain the saturated liquid after the
expansion valve.
The expected energy saving is related to the use of the energy of the condenser in the
boiler. This is obtained by changing the temperature level of the condenser using the
mechanical power. Since mechanical power is more expensive than thermal energy, this
mechanical vapour recompression is not always interesting. Cost study has to be performed.
The MVR structure and the composite curves after mechanical vapour recompression in a
single column is described figure 53.
W
T
Qc+W
Qi
Qb
Qi
Q
Qb
Figure 53. MVR in a single column.
The condenser line is now above the boiler and it can heat up the boiler. Additional cooling
Qi = Qc + W - Qb is required.
The energy recovery is very interesting: W of mechanical power gives an energy recovery
of Qb for the hot utility and Qc-Qi for the cold utility.
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Energétique Avancée – Process integration
5. Improve the MER of a process
The economy depends on the different energy costs. A common value can be used as an
idea: 1 mechanical kW by electricity = 3 kW of heat. The mechanical vapour recompression
will be attractive if the energy recovery is three times greater than the mechanical power
required for the recompression. Indeed, it depends on the availability of the energy on the
plant: utility network, with excess or not of steam for the condensation or for expansion in
steam engines; or the electricity cost.
When the column is integrated in a process, the recompression is not always interesting. The
pinch point is once again the key for the energy recovery:
In figure 54, the column is above the pinch point, in the heat sink. In this case, the heat of
the condenser can be used to heat up other cold streams than the boiler. The mechanical
vapour recompression adds W of mechanical power which is directly converted into thermal
power with no other significant effect: the economy of hot utility is the mechanical power
introduced in the process.
T
T
Qc+W
Qb
Qb
Qc
Q1-W
Qo
Q1
Qo
H
H
Figure 54. MVR above the pinch point
In figure 55, the column is below the pinch point in the heat source. The boiler takes its heat
load on the hot process streams above him. The MVR produces an extra hot stream heat load
of W which is evacuated by cooling water. This case is surely not interesting.
Q1
Q1
T
T
Qc+W
Qb
Qb
Qc
Q2
Q2+W
H
H
Figure55. MVR below the pinch point.
In figure 56, the boiler of the column is above the pinch point and the condenser below.
When recompressing the condenser vapour, a hot stream is added in the heat sink
decreasing the total hot utility demand. Moreover, a hot stream is taken out of the heat
source decreasing the cold utility requirement. In this case, the MVR has similar effect as
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Energétique Avancée – Process integration
5. Improve the MER of a process
when the column is not integrated. This effect can be better because the condenser can
leave the heat load to the boiler but also to other cold streams above the pinch point. The
hot and cold composite curves fit one to the other.
T
T
Q1
Q1-(Qc+W)
Qb
Qc+W
W
Qb
Qc
Qb
Qc
Qb
Q2
Q1
Q2
Q2-(Qc+W)
H
H
Figure 56. MVR around the pinch point.
The three cases here above can be represented in the thermal cascade (figure 57). The
mechanical power is used to change the temperature level of the stream recompressed. The
mechanical power raises the heat along the thermal cascade, it is a heat pump.
F10-W
F10
F10-(Q+W)
T
Q+W
F9+Q
F9
F9-(Q+W)
W
F8+Q
F8
F8-(Q+W)
F7
F7-(Q+W)
Q
F7
Q+W
F6
F6
F6
0
W
F5
F5
F5-Q
Q+W
Q
F4
F4+Q+W
F4-Q
W
F3
F3+Q+W
F3-Q
F2
F2+W
Q
F2-Q
F1
F1+W
F1-Q
(a)
(b)
(c)
Figure 57. Mechanical power and thermal cascade.
In case a, the heat load Q is taken in the interval 7 and risen to interval 9. The problem table
shows that the F10 (hot utility) is reduced of W, the thermal power to obtain zero flow at the
pinch point. The mechanical power is used as hot utility.
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Energétique Avancée – Process integration
5. Improve the MER of a process
In case b, the heat load Q is taken in the interval 2 and risen to interval 4. In this case, the
zero flow at the pinch point imposes an increase of W for F1, the cold utility. The mechanical
energy W is directly sent to the cold utility.
In case c, the heat load Q is taken in the interval 5 below the pinch point and is risen to
interval 6 above the pinch point. This result to a net hot utility recovery of Q+W to find zero
flow at the pinch point, and a cold utility recovery of Q.
The use of mechanical power to rise heat in the thermal cascade is energetically
interesting when it rises heat from below to above the pinch point.
In process integration, mechanical vapour recompression does not necessary involve the
matching of a condenser and the boiler of the same column. Distillation columns can be
integrated together and the condenser of one column can heat up the boiler of another. This
feature allows to reduce the compression ratio as shown figure 58. The compression ratio
and the mechanical power required (W2) for the MVR in the integrated columns (condenser
of A and boiler of B) is less than the one (W1) for the MVR of column A.
T
T
Q1-QcA - W1
T
Q1-QcA - W2
Q1
CA
B
B
A
BA
A
BB
BB
CA
B
C
B
A
C
C
C
B
B
B
Q2
Q2-QcA
Q2-QcA
H
H
H
W1
W2 B
Qb
A
W1> W2
Qb
Figure 58. Integrated distillation column wit MVR.
The mechanical vapour recompression is economically effective when the compression ratio
is small enough. The shape of the composite curves and specially the grand composite curve
helps to detect the opportunity for vapour recompression.
A vapour recompression can be envisaged when grand composite features two stage (figure
59). One below the pinch point (A) which correspond to the recompressed stream and one
above (B), where the recompressed stream will discharge its heat load. The choice of the
compression ration will be done so as to be DTmin/2 above the stage B.
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Energétique Avancée – Process integration
5. Improve the MER of a process
T
B
A
H
Figure 59. MVR and grand composite curve.
The condition necessary to detect the opportunity of mechanical recompression is thus the
presence of two stages surrounding the pinch point. The rest of the grand composite curve
must also be considered to analyse if no other pinch point will occur.
The easy way to make this analysis is to consider the recompressed stream as a
hot utility for the heat sink.
In the same way, to determine the amount of heat which can enter the mechanical
recompression, the grand composite curve below the pinch is to be analysed.
The easy way to make this analysis is to consider that the recompressed stream is
fictively cooled down by a cold utility in the heat source.
The figure 60 illustrates these rules. Even if the condenser Qc is recompressed the heat
recovery will be of Q2 and the pinch point changes.
T
T
Q1
Qb
Qc
Qc
Qb
Q2
Q2
H
H
Figure 60. Detection of MVR opportunity.
5.1.3. Comparing MVR and pressure changes
The following table displays the differences between MVR and pressure changes.
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Energétique Avancée – Process integration
5. Improve the MER of a process
Energy Analysis and Synthesis in Industiral Processes
Comparing MVR and pressure changes
MVR
pressure modification
Pressure
increase
increase or
decrease
Mechanical power
compressor
compressor, pump
Temperature
condenser
boiler and condenser
Loads
condenser
boiler and condenser
Operation
no change
new set point by
simulation
Integration
can be self
only with other
integrated
columns
Technological
new compressor new pressure
constraint
F. Maréchal - ## - //
Form flexibility point of view, the pressure modification leads to more integrated process
because it involves two columns or more. It has to be analysed by simulation to verify the
operability of the modified process.
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