Reducing energy consumption and heat exchanger network

1. May 2, 2015

Maylis

1. The problem statement, all variables and given/known data
If you are allowed to make changes of column pressure and temperature, stream preheating or cooling temperature, please propose a process change that can significantly reduce the energy consumption at DTmin of 10°C. The modified process should maintain the product quality and production rate unchanged.

Utility Information

Low pressure steam (LP, 140°C) = 0.03 $/kWh = 240$/kW-Yr

Medium pressure steam (MP, 170°C) = 0.05 $/kWh = 400$/kW-Yr

High pressure steam (HP, 200°C) = 0.07 $/kWh = 560$/kW-Yr

Hot Oil (HO, supply 450°C – return ≥ 300°C) = 0.15 $/kWh= 1200$/kW-Yr

Electricity = 0.1 $/kWh = 800$/kW-Yr

Cooling water (CW, 30°C) = 0.005 $/kWh = 40$/kW-Yr

Chilled Water (ChW, 5°C) = 0.05 $/kWh = 400$/kW-Yr

Refrigeration

(R-10, -10°C) = 0.1 $/kWh = 800$/kW-Yr

(R-50, -50°C) = 0.3 $/kWh = 2400$/kW-Yr

(R-100, -100°C) = 1 $/kWh = 8000$/kW-Yr

(R-200, -200°C) = 5 $/kWh = 40000$/kW-Yr

2. Relevant equations

3. The attempt at a solution
This is my converged aspen simulation of a process to produce acetone (AC) from isopropyl alcohol (IP). I used a heat exchanger simulation software and here are my streams

Basically just inputting the heat duties of all the units. I just do a heat curve analysis and when the heat capacity changes I add a separate cooler, which is why there are blank rows in "Name", because the specific heat capacity is assumed to be constant only in one phase.

This is my composite curve

This is my heat exchanger network

I'm wondering what I could do to improve the efficiency of this process? Or at least places I could start looking and what I should look for.

Last edited: May 2, 2015
2. May 4, 2015

Maylis

Some new idea, I decided to add a heater before the reactor in order to preheat the IP, since I can use low pressure steam which is cheaper than hot oil for the reactor. However, for some reason the Aspen file will not converge when I add this heater. The amount of IP being fed is changed, even though I specify the amount. In the OP the mass flow rate is 19483 kg/hr, now its 21123 kg/hr. It is quite strange.

The Energy report from SPRINT

This is my original energy report from the previous iteration

3. May 4, 2015

BvU

Wouldn't trust any answer at all from a non-converged A+ simulation.
I have preciously little to go by, but my advice is to check the pressure drop in the feed preheater. T1 has trouble now (offgas flow disproportional) and then T2 crashes.
Good luck !

4. May 4, 2015

Maylis

If you wish, I can send you the Aspen file, but in the mean time I will try your suggestion

5. May 4, 2015

BvU

Either that or increase the feed pressure.
Something else that came to mind: does the reactor do the same thing with a feed of 25 degrees as it does with a feed of 140 degrees ?

6. May 4, 2015

Maylis

As far as I know there is no difference

7. May 4, 2015

BvU

If there are no temperature dependencies in the reactor, then it must be the pressure ...

8. May 4, 2015

Maylis

Ah yes, I forgot that the pressure in the preheater should be 2 atm.

9. May 6, 2015

Maylis

Okay, so I added a preheater before the reactor to IP, and a cooler for the AC.

This is my grand composite curve, and I should somehow use it to improve my energy efficiency. I know the flat blue lines are the utilities that I need, but I am unsure how to translate this curve into my process so that I know where I should put heaters/coolers.

And here is my utilities cost (this one is higher because DTmin is 20 C), but I have improved it with what I have done so far

Last edited: May 6, 2015