3 replies to this topic

[karthiben]

I am preparing an optimised heat exchanger system for a chemical plant (fictional) as part of an assignment by reducing the demand for heating steam and cooling water (utilities) by using process streams to heat and cool one another, rather than relying solely on the utilities.

Unfortunately, every time we try to make connections between the streams to the heat exchanger or to tanks, HYSYS automatically adds a "cold cutter" to the end of the stream as well as new undefined stream out of the "cold cutter" to the heat exchanger. This happens every time we make a connection. I have no idea what a cold cutter is, and I've tried googling the terms HYSYS "cold cutter" and other variations to these search terms to no avail.

What does this term mean? What am I doing wrong and is there anything you think is should keep in mind or consider when doing this assignment?


For more detail, read on.

3 process streams must be cooled, while two must be heated.

Stream 1201 contains a mixture of MIBK (41.0 mol%) and MEK (59.0 mol%) from the bottom of a flash tank. Prior to entering a distillation column in another part of the plant, it must be heated from 15°C to 55°C. Its flowrate is 2.82 T/hr and pressure 220 kPa.

Stream 1208 must be cooled to 35°C before going to a storage tank in the tank farm. It contains a mixture of 17 mol% methanol with the remainder ethanol.
Its flowrate is 45.8 kmol/hr at 65°C and 133 kPa.

Stream 1211 is to be cooled before being recycled to the butanol section of the plant for re-processing. Forty five mole percent of the stream is the 1-butanol isomer with the balance iso-butane. It must be cooled from 150°C to 100°C. The flowrate of this stream is 31.8 kmol/hr and the pressure 120 kPa.

Stream 1204 contains pure allyl alcohol at a volumetric flowrate of 4.1 m3/hr at 90°C and 385 kPa and it must be cooled to 40°C.

Finally, the product stream, 1252 (containing pure MIBK), must be heated from 45°C to 105°C prior to being piped to an adjoining plant at 124.4 kmol/hr and 165 kPa. (All pressures quoted above are absolute.)

Saturated steam is available at 125°C and 232 kPa and costs A$3.50/tonne, while

Cooling water is available at 20°C and costs A$0.05/tonne. It is essential that the heating steam and cooling water leave this section of the plant at no less than 115°C, and no more than 35°C
respectively.

Air-cooled heat exchangers could also be constructed, with the air assumed to enter at 30°C and exit at 40°C. Energy costs associated with these exchangers are around A$0.80 per GJ of energy supplied. Currently the company uses heat exchangers combined with the steam and water to effect the required temperature changes.

I'm supposed to prepare a process flow diagram (PFD) using HYSYS, which shows all the relevant streams and unit information. as well as prepare a brief qualitative description of what would happen to your built system if the flow rate of each stream was to change (say by ±20%) one at a time.

Pressure drops across each of the heat exchangers are assumed to be 30kPa. minimum approach temperature between two streams is 10°C (the pinch temperature).

THATS ALL FOLKS!

[Padmakar Katre]

Dear Karthiben,

Please provide the Cooling Water Supply pressure. Probably I can help you out

Regards,

Padmakar Katre

[joerd]

See if you have two different Fluid Packages in your simulation. Hysys uses stream cutters to transfer stream data from one property package to another, to give you (the user) control over how this should be accomplished (i.e. how to transfer the components, if they are different, and how to transfer stream conditions, such as pressure and temperature, or enthalpy, or vapor fraction).
You might use pinch analysis to come up with the optimum exchanger configuration. Hysys allows that via its Composite Curves utility, but there are many tools around - maybe even on this site.

[smalawi]

hi,

Joerd is right. you are using deferent fluid packages in your simulation. usually this is the case when you cant find all the components in your simuation in one physical model.

you can spot this in hysys by having several component lists in the stream property box. vhanging them will change the components. also from the component enviroment.

the reson hysys does this is very simple. the base or referance properties of each pakcage is deferent and the enthalpy as a result is deferent. also some packages are specific to some group of components. the steam tables for example are for steam only, the same applies for amines. to avoid the cutters try to chose a package which represents the majority of your compononts. if this is not possbile, try to chose the cutters transfer option to represent your condition and it will work.

i hope this helped, heat intergration is very intersting its more of an art.

cheers,