Alcohol is stored in conventional storage tanks at site, which are non-insulated & low pressure fixed roof tanks. In this kind of storage system, there are three different types of losses of stored liquid.
1. WORKING LOSS
Losses due to displacement of inner air space during filling & evacuation. The material is lost each time during filling while fresh air intake results in additional evaporation inside the tank to maintain its partial pressure of liquid in vapor space.
2. BREATHING LOSS
Losses due to contraction and expansion of vapor space due to variation in day – night temperature. This also causes similar effect as explained above in item 1.
3. LOSSES DUE TO HEAT GAIN
Due to difference in ambient temperature & bulk liquid temperature, the material stored gets energy input through radiative & convective heat transfer from atmosphere. This is significant energy gain by the system compared to other two losses explained above.
We have assumed following parameters during this study.
Breathing losses and working losses were estimated based on detailed EPA method described in AP-42 compilation of air pollutant emission factor Vol-1 by US Environmental Protection Agency.
Heat Losses were estimated based on Eq-6, 7 & 4 page-235 from ‘Process Energy Conservation’ by Richard Greene.
1. This is the first step to calculate storage tank losses i.e. list out all the tanks with dimensions and average liquid level in each tank separately.
2. Now find out the average temperatures for Day & Night for each month separately as listed below in our case.
3. Now assume a sun temperature & calculate vapor composition, Specific heat & latent heat of vapor mix. If it is pure solvent its easier. In my case it was Ethanol & water mix.
4. Assume interface temperature at tank walls & calculate heat transfer coefficient. I can provide worksheet & formula reference if anybody needs it.
5. Calculate losses & summarize them as below.
6. This will give you loss table for day & night separately so you need no of tanks x 2 worksheets for calculating these losses.
7. Now simply calculate possible recovery by condensation. I did it @ 12°C chilled water.
Based on detailed calculation for each month, it was identified that the total alcohol losses from all storage tanks are in the range of ~715 TPA. The breakup of these losses is given below.
- Breathing Losses - 34 TPA
- Working Losses - 24 TPA
- Heat Losses - 654 TPA
- Total Losses - 712 TPA
- Recovery - 568 TPA
We can economically recover ~568 TPA (~80%) of ethanol by condensing vent vapors using chilled water of 12 °C.
The cost of this operation was ~ Rs. 1500 / Te of alcohol with a net saving of Rs. 105 Lac /Year.
Comparison with absorption system.
When using absorber I could plan to recover >98% alcohol compared to 80-81% using vent condenser saving additional Rs. 40 Lac / year.
The cost of recovery was also lower due to higher power consumption for chillers. The absorber can be designed to target ~higher alcohol concentration at the bottom of column. We finalized it at 21% due to total economics of recovering it later.
This is not only important from economics point of view but is very critical from stringent Environmental norms.
So I prefer this solution when you are having a pure solvent the only judgment needed is to minimize “after processing” cost which depends on different setups in different industries.