Optimized Case

Thus, besides the three streams requiring heat exchange, there are also the reboiler duties to consider as well. For the base case from the previous two projects, the first tower’s reboiler had a temperature of 102.1 ° C and the second tower’s was 58.4 ° C. So, trying to use all of the streams and eliminate as much of the utility load as possible, the following configuration was developed:

 

The basic approach was to use the largest heat stream to provide most of the heating (for the reboilers especially) and allow the cold stream to cool the wastewater outlet (C1DN) and the reactor outlet. Finally, the remainder of the exchange to the proper system temperatures is achieved from utilities.

Utilizing Hysys’ Pinch Utility, the final pinch characteristic can is determined and plotted below:

From this Figure, it is obvious that the system of exchangers is valid and the pinch temperature involved is greater than the agreed upon minimum of 10° C.

Somewhat of a concern when using a process stream to heat the reboilers is that of control and modulization. It is desirable to have the capability to separate the different units within a process and heat integration works against this in some instances. This is of special concern during start up and shut down. Nevertheless, our process allows a quick fix for this as the process stream in use contains water, so there is no problem with allowing a back-up piping system and valves leading to the low pressure steam boiler. This would only involve bypassing the process stream until the unit was online and then allowing the reactor outlet to provide the heating duty for the reboiler.

Now, in comparison to the original system, this network has an added heat exchanger with a cost of about $46,000 – this is above the capital cost of the base case. However, the utilities will more than make up for this. Besides using heat integration, increasing the cooling water duty on exchanger E-402 in order to force the outlet of stream 11 to 35° C saves a significant amount in the refrigerated water utilities. In the end, the savings due to the reduced amount of utilities is staggering, as will be seen in the Cost Analysis report section.