In Chemical Process Industries (CPI), distillation columns are the main steam consumers. And, as a process engineer, we are continuously keep on working to minimize the steam consumption norm. So that we can reduce overall variable cost of the product. Therefore, to achieve this objective we can work upon various improvement initiatives. These various techniques can be like, parameter optimization, waste heat recovery using preheating, heat integration of columns using pinch technology, etc. Apart from this we can look into the feasibility of one more technology to reduce steam consumption, which is use of heat pump in distillation column.
So, in this article we will try to understand about heat pump and its possible use in a distillation column. First, let us understand, what is a Heat Pump?
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What is a Heat Pump?
Fundamentally, as we know heat flows spontaneously from high temperature to low temperature. So, using a heat pump we can transfer heat from low temperature to higher temperature. Or in other words, we can convert energy at lower temperature into higher temperature energy.
You can see the utility of a heat pumps in our surroundings, which includes heating of houses and offices during winters and cooling in summers. The examples of heat pumps in our homes are air conditioners and refrigerators. In these appliances refrigerant is used, which evaporated in outside air and reject heat. This is the heat which refrigerant absorbs at lower temperature (i.e., temperature which is maintained inside the AC room and refrigerator) than atmosphere.
During winter heat pump absorbs heat from atmosphere and rejects into the office or building to maintain the temperature higher than outside atmosphere. In this process the flow of refrigerant is in reverse direction than a refrigerator or air conditioner system.
Apart from this we can see another example of a heat pump in our industry is a thermo-compressor. Where we compress the low pressure & temperature steam to convert into high pressure & temperature steam. This objective we can achieve using a thermo vapour compressors (TVR) or a mechanical vapour compressor (MVR).
Heat Engine, Refrigerator & Heat Pump
All three are cyclically operating devices. Heat engine absorbs energy from high temperature reservoir, generates work and rejects balance energy into a low temperature reservoir. While refrigerator and heat pump are the reverse cycle of a heat engine. Here, it absorbs heat from low temperature and work is applied to reject heat into high temperature reservoir. Performance or efficiency of these cyclic devices, we can estimate using below equations:
- Heat Engine Efficiency: η = |W|/|QH|
- COP of Refrigerator: COPR = |QL|/|W|
- Heat Pump COP: COPHP = |QH|/|W|
Below is the schematically representation of the heat engine, refrigerator and heat pump.
A thermal power plant is the example of a heat engine. While refrigerator and heat pump are the reverse of a heat engine. Both these devices, refrigerator and heat pump absorb heat from low temperature reservoir and use work, which is applied by a compressor and reject heat at higher temperature reservoir. Please clear in your mind refrigerator is also a heat pump.
Various Possible Ways to Use a Heat Pump in Distillation Column
So, we discussed about the heat pump in above section. Now, we will look into the application of heat pumps in a distillation column. In a distillation column this heat pump is a compressor. In this compressor, vapour from distillation column goes into the compressor and adiabatic compression takes place. Because, we are note removing any heat of compression temperature of the outlet vapour rises. Subsequently, this high temperature vapour we can use into column reboiler to supply the heat in place of steam.
So, thermodynamically we can see in a distillation column, we absorb heat from column top/condenser (i.e., a low temperature reservoir) and work upon it using compressor and reject this heat in column bottom/reboiler (i.e., a high temperature reservoir).
There can be below three type of configurations which we can use to install a heat pump in a distillation column.
Direct Vapour Compression
In this setup, column vapour directly enters into a compressor and after adiabatic or polytropic compression superheated vapour reject heat in column reboiler. Subsequently condensed vapour goes for column reflux and product draw.
External Vapour Compression
Here one buffer fluid is used in column condenser which evaporates and converts into the vapour. Subsequently, this vapour enters into the compressor and after adiabatic or polytropic compression gets superheated. High temperature vapour enters into the column reboiler and reduce equivalent steam consumption. This type of installation is recommended where compression of direct vapour is hazardous or fluid is very corrosive in nature.
Bottom Flashing
Third possible way for heat pump installation can be for the column where we are using direct steam. In other words, there is no reboiler, this bottom goes into the condenser and converts into the vapour. Subsequently, this vapour from condenser enters into the compressor. After adiabatic compression vapour temperature increases and we can use this heat in column bottom.
Now, to understand the heat pump use in a distillation column we will consider an example in subsequent section.
Example of Heat Pump in Distillation
Let us consider a distillation column to distil the ethanol-water mixture. In this column feed contains Ethanol: 50 wt.% and Water: 50 wt.%. The feed flow rate is 5000 kg/h and feed temperature is 40 0C. This is an atmospheric distillation column, top product is the azeotropic composition of ethanol & water, contains 95 wt.% ethanol and remaining is water. The bottom product from this column is aqueous waste contains traces of ethanol (around <100 ppm). You can refer below figure to understand the distillation setup for our case.
Distillate rate, D = 5000*(50/100)*(100/95) = 2632 kg/h (considering ethanol in bottom negligible)
Reflux rate, R = D*3 = 7896 kg/h (Reflux ratio is 1:3)
Vapour rate from column top, V = R + D = 10528 kg/h
Latent heat of top vapour, LH1 = 221 kcal/kg
Feed specific heat, Cp1 = 0.81 kcal/kg-C
Column bottom temp, T2 = 105 0C
Feed temperature, T1 = 40 0C
Steam Enthalpy, H1 = 517 kcal/kg (@ 3.0 bar saturated steam)
Reboiler duty, Q1 = 5000*0.81*(105 – 40) + 10528*221 = 2,326,688 kcal/h
Steam requirement, MS = Q1/H1 = 5003 kg/h
Cooling water requirement, MC = 10528*221/6 = 387,781 kg/h (@ 6 0C ∆T)
Equivalent power for cooling water, 40m discharge pressure, PE = MC*9.81*40/3600/0.8/0.9 + 17500 (Watt for cooling tower fan) = 76.21 kWh
Therefore, total estimated utility cost for above distillation will be, C1 = MS*1.1 + PE*7.5 = 6075 Rs/h. (@ Steam Cost 1100 Rs/MT & Power Cost 7.50 Rs/kWh). Which is 6075*8000/100000 = Rs. 486 Lacs.
Distillation with Heat Pump
Now, let us consider the above example with using heat pump. To understand this, you can refer below figure.
So, in above figure we can see, this a direct vapour compression heat pump system. Here, column vapour directly enters into the compressor and after polytropic/adiabatic compression heats up (i.e., at outlet pressure is 3.0 bar the adiabatic temperature rise will be 144 0C). This hot vapour goes into the reboiler and provide heat to the column and condense @ 108 0C. Subsequently, this hot condensed liquid goes into a feed preheater and cools down to around 75 0C. The cold liquid from feed preheater partly goes for reflux into the column and partly we recover as a distillate or product.
Moreover, there is one more reboiler with steam, which supplies balance heat to the column and ensures stable column operation.
We have vapour rate from the column is, V = 10528 kg/h (this enters into the compressor).
After compression at 3.0 bar pressure temperature increases from 78 to 144 0C (Here, we are considering adiabatic compression). This vapour condenses at 108 0C and 3.0 bar, to supply heat to the column through reboiler. Total power consumption in compressor with 75% efficiency is, PE1 = 306 kWh. (Please note theses data are based on CHEMCAD simulation).
Additional steam consumption is zero here. So, distillation column operating cost with heat pump will be as below:
C2 = PE1*7.50*8000/100000 = Rs. 183.6 Lacs/annum
Therefore, savings in operating cost by using heat pump will be = C1 – C2 = 486.0 – 183.6 = Rs. 302.4 Lacs/annum.
Apart from above saving there will be no requirements for the cooling water and condenser also. However, we need to provide a condenser as a standby provision, this is in case of any problems in heat pump operation. This will ensure the stable column operation.
Conclusion
Finally, when we plan for a heat pump in a distillation column, first we need to evaluate its technical feasibility. If, it is technically feasible then second question comes to work out the financial feasibility. Any economic advantage of a heat pump depends on the cost of electricity in comparison with the cost of steam. As a thumb rule if difference between distillation column top and bottom temperature is around 20 – 250C or less, using heat pump will be a feasible option. Because for higher temperature differences you need to compress column vapour more and require more electric power.
Also, there should be around 30 0C temperature difference between compressed hot vapour and column bottom temperature. Otherwise, required surface area for reboiler will be higher side and increases initial capital investment.
As a process engineer you need to see, where you can use this heat pump in your plant. Because this is a huge opportunity to save energy and our blue planet as well.
Thanks for reading.