In the previous sections two solutions have been proposed to the need to produce thermal energy to power the stripper reboiler. We will now compare these two alternatives starting step by step.
As a first point of reflection, we start from efficiency. Although it is not logical to compare the efficiency of a heat pump with a traditional boiler, only hints will be provided. The industrial boilers on the market are a solid and well-known technology and have a high efficiency, up to 92%. In practice, only 8% of the heat produced by the natural gas entering the boiler is lost. As regards the heat pumps, their efficiency reaches 300-400% because two different energy quantities are used to calculate the efficiency. Electricity is known to be a more precious form of energy than thermal energy. The COP is nothing more than an efficiency index that quantifies how much heat is produced compared to the electricity that enters the system. A heat pump is competitive when it produces at least double the electricity fed into the system. In this case, the COP is 4.6.
Another important issue is the comparison on the lifetime of the two proposed alternatives.
A boiler is competitive and efficient for 10-15 years after which it becomes obsolete and must be renewed. A heat pump has a longer life span ranging from 15 to 25 years. This is also because the moving parts of the heat pump are only in the compressor and with a careful maintenance, heat pump can last many more years than traditional boilers.
The space required by the two solutions must also be considered. As for the boiler, it is more compact and requires less space. The only problem is that you need a chimney for the flue gases and therefore you have to have an area available for the construction of a stack or provide a pipeline capable of transporting the flue gases up to an existing stack. The chimney, if it already exists, must be sized correctly to ensure a certain speed of the flue gases and a certain flow rate. A heat pump requires more space because it requires a compressor and two heat exchangers which take up a lot of space. Consequently, the space required by this technology is greater than the space required by a traditional boiler.
Now the costs are considered. Within these, installation costs and operating costs must be distinguished. The investment costs are those incurred for the installation of the two systems.
In the case of the traditional natural gas boiler these costs are low because it is a widely used technology. The costs incurred for the investment of the purchase of a boiler are recovered within 1-2 years. As far as the heat pump is concerned, the initial costs are higher since it is a technology not yet widespread and it is necessary to consider the costs for the purchase of multiple instruments such as the compressor, two heat exchangers and the working fluid.
From the literature it emerges that the costs for the investment of a heat pump fall within 3-4 years.
As far as operating costs are concerned, the hourly cost of the two technologies has been seen above. The Table 5.13 shows the running costs of the two solutions.
Table 5.13: Operating costs incurred in the case of the use of a boiler and a heat pump.
Operating costs Boiler
Natural gas cost per day 7602,07 €/day
Heat pump
Electricity cost per day 6374.73 €/day
It is clear that by using a heat pump you can save up to 1000 € per day. Furthermore, the price of natural gas is much more variable than the price of electricity.
Another parameter to compare is maintenance. A boiler needs annual maintenance to maintain high efficiency and allow safe operation; a heat pump needs less maintenance.
The last point, which is also the most important for the work done in this thesis, is the environmental impact of the two solutions. The Table 5.14 shows the emissions of the two proposed alternatives.
Table 5.14: Carbon dioxide emissions per hour of the two solutions proposed solutions.
CO2 emissions per hour Boiler
CO2 emissions per hour 3566,26 kg/h
Heat pump
CO2 emissions per hour 1133,3 kg/h
It is evident that in the case of a traditional boiler the emissions are three times as using a heat pump. Furthermore, the CO2 produced by the boiler is considered in the global balance of CO2 emitted by the refinery. As for the heat pump, on the other hand, the CO2 produced to produce electricity is not considered as it comes from the electricity grid. The advantage is that no more carbon dioxide is produced in the refinery area, which is often a maritime
Chapter 5
area, but the emissions are in the power plant that produces the energy. If the electricity comes from non-fossil sources, this amount of carbon dioxide appears to be zero.
Furthermore, in the event that a carbon tax is envisaged by environmental laws, the costs associated with the production of carbon dioxide from the boiler must be added to the operating costs; as for the heat pump, these costs are zero. However, the heat pump has an environmental disadvantage which is linked to the working fluid used. This fluid, if dispersed in the atmosphere, degrades only after 7 years.
Summing up the main points developed, the heat pump wins over the use of a traditional boiler, especially in the environmental field. However, if there are constraints related to the space available or to the availability of money, the use of a traditional boiler can be an alternative. In the case of the adoption of a boiler, the carbon dioxide emissions of the refinery will increase. The two proposed solutions are compared in the Figure 5.8. It is clear that the adoption of a boiler nullifies more than half of the emissions avoided with the use of carbon capture.
0 5000 10000 15000 20000 25000
Without CCS With CCS (Heat
Pump) With CCS
(Boiler)
Figure 5.8: CO2 emissions in the three possible scenarios: without the use of CCS, use of CCS with heat pump and use of CCS with natural gas boiler.
Acronyms
ARU Amine regeneration unit
BFW Boiler feed water
BP British Petroleum
CCR Continuous catalyst regeneration
CCS Carbon Capture and Storage
CCU Carbon capture and utilization
CDU Crude distillation unit
CHP Combined heat and power
COP Coefficient of performance
CRF Catalytic reformer
DEA Diethanolamine
DRS Delayed recovery scenario
ELECNRTL ElectrolyteNRTL, a method in ASPEN Plus
ENRTL-RK Unsymmetric standard state Electrolyte NRTL with Redlich-Kwong equations
EOR Enhanced oil recovery
EP End point
GDP Gross Domestic Product
GWP Global warming potential
HDS Hydrodesulfurization
HDT Hydrotreater
HP High pressure
HPS High pressure steam
IBT Initial boiling point
ISO Isomerization unit
KHT Kerosene hydrotreater
KSW Kerosene sweetening
LP Low pressure
LPG Liquefied Petroleum Gas
LPS Low pressure steam
LSW LPG sweetening
MDEA Methyl diethanolamine
MEA Monoethanolamine
MP Medium pressure
MPS Medium pressure steam
NHT Naphtha hydrotreater
NSU Naphtha splitter
NZE2050 Net zero emissions by 2050
ODP Near-zero ozone depletion potential
PCC Post combustion capture
POW Power Plant
PSA Pressure swing absorber
PZ Piperazine
RSU Reformate splitter
SDS Sustainable development scenario
SGP Saturated gas plant
SR Steam reformer
SRU Sulphur recovery unit
SRU Sulfur recovery unit
STEPS Stated policies scenario
SWS Sour water stripper unit
TEA Triethanolamine
VBU Visbreaker unit
VDU Vacuum distillation unit
VGO Vacuum gas oil
VHT Vacuum gasoil hydrotreater
WHSG Waste heat steam generator
WWT Waste water treatment
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Chapter 5
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