3 replies to this topic

[Kakashi-01]

I am trying to understand why the optimizer tool converged to a lower objective function value. The simulation yielded lower feed flow rates for natural gas and high pressure steam while producing the same methanol production rate. As a result, the compression, heating, and cooling requirements were lowered. In the base case,natural gas contributed to  78 % the total operating cost.
 

The steam/methane ratio in the steam reformer feed, temperature of steam methane reforming, process operating pressure of the mixed stream, methane conversion rate in the reformer, and hydrogen conversion rate in the methanol reactor are set as independents. 
 

Steam reforming is an endothermic reaction, so we would expect reforming achieves higher conversion rates at elevated temperatures as heat favours production formation. However, at higher pressure the equilibrium shift towards the reactants reducing methane conversion. The optimizer converged to a solution with a higher methane conversion (0.79) compared to the base case (0.72) for a higher temperature (870 C vs 850 C) and higher pressure (34.256 bar vs 29.5 bar).
 

Methanol synthesis reaction is favored at low temperature and high pressures. However, the optimized case resulted in both higher pressure and higher temperature with a higher hydrogen conversion (0.60 vs 0.52). Why has the amount of NG and steam reduced while maintaining the same methanol production rate?

 

My though process is that we want a composition of hydrogen, carbon dioxide, carbon monoxide in the feed to the methanol reactor that increases the single pass conversion for hydrogen producing the same methanol production rate while decreasing natural gas consumption.

 

 

The steam-to-methane ratio in the steam reformer feed, the steam methane reforming temperature, the process operating pressure of the mixed stream, the methane conversion rate in the reformer, and the hydrogen conversion rate in the methanol reactor are set as independent variables.

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Edited by Dumpmeadrenaline, 17 February 2025 - 11:36 AM.

[Pilesar]

Optimization calculations can provide results that are either unconstrained or constrained. If the result is in a region the plant should not operate, then add constraints to form additional boundaries. It is usual for the best optimization solution to be located at one or more constraints. If the answer is just 'wrong' then discuss that with the author of the optimizer.

[Kakashi-01]

The optimizer only allows the use of variables (either specified or calculated), and when specified variables are flagged, they are freed and treated as independent variables. Parameters cannot be entered into the optimizer. The number of stages and the feed stage locations are treated as parameters and must be specified by the user.
 

In the distillation column, the specified variables include the column condenser vapor fraction, column top stage pressure, column stage pressure drop, reboiler pressure drop in the return line, and pressure drop in the inlet condenser piping. Since the objective is to minimize the operating costs of the process we would have to reduce reboiler duty, do you have any ideas on how to work around the simulation limitations, given that the number of stages and the feed stage location cannot be specified?
 

Considering that the adjustable variables are the specified ones mentioned above, and if the feed flow rate and the liquid from the reflux drum are fixed, then the vapor from the reflux drum and the bottom flow rate remain unknown. To minimize reboiler duty, should we operate at a lower pressure to achieve higher vapor volumes? However, we must also ensure that vapor sent to the flare does not backflow into the column.

Additionally, I am uncertain whether I am interpreting the optimizer's results correctly. When I input the temperature of the cooler before the separator to remove the light components, the optimizer converged to a much lower temperature. Wouldn’t a cooler feed to the distillation column require more reboiler duty?

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Edited by Dumpmeadrenaline, 17 February 2025 - 12:35 PM.

[Pilesar]

I do not recognize the software shown in the thumbnail. The method that has often worked for me is to first get a converged steady state result. Then make parameter adjustments in the software (not using the optimizer) and review the new converged solutions. I want to understand the process and which changes might be important. What if the feed were at its boiling point or its dew point? What if the feed tray were changed? What if the column pressure were higher or lower? What if there were more stages? What if the product specification was changed? In this way, I can get a sense of which direction an optimum might be located. Then I can allow the software optimizer to do its fancy matrix math and provide what it thinks is the precise solution. The constraints are important. Reducing column pressure that lowers reboiler temperature also lowers the condensing temperature and may make the column more prone to hydraulic flooding. Separating components upstream of the column reduces the feed to the column. Alcohol/water systems can have azeotropes which must be dealt with during separation. Try to understand the system using case studies of changing parameters manually before throwing it all to the optimizer. Then you will be better able to judge whether the optimizer is giving a practical result.