1 reply to this topic

[theleftcoast]

Hello everyone,

I am working on a heat exchanger network as part of my senior design project. Right now, I have two designs and want to get some feedback regarding which configuration is most feasible. The HX network is supporting a hydrotreating reactor and the stream properties are listed below (temperatures are on the flow diagrams). I included a heat release curve that I marked up with the equipment arrangement shown in Layout 1. My concern with Layout 1 is that the reactor effluent on the tube side of the vaporizing exchanger is much hotter than the boiling liquid on the shell side. The book we have been using for this course (Product and Process Design Principles by Seider) recommends that the maximum overall driving force be limited to 45F to ensure nucleate boiling and avoid undesirable film boiling which could "choke" the vaporizer. Another concern is that the liquid could come in contact with tubes that are around 600-700F in some parts of the exchanger which could create fouling issues. To try and mitigate these problems, I came up with the flow diagram shown in Layout 2. I added a waste heat boiler for steam production and use that saturated steam to vaporize the feed in a separate exchanger. This arrangement would reduce the temperature differences in the vaporizer and possibly help us avoid the film boiling and fouling issues of Layout 1. Is there a better way to lay out the HX network? If my designs (or concerns for that matter) aren't reasonable, I would appreciate some guidance.

Thank You!

Total Feed Rate - 2,700,000 lbs/day
Feed Pressure - 400 psia
Hydrocarbon Composition (volume fractions)
C3 - 0.05
C4 - 0.33
C5 - 0.43
C6 - 0.17
C7 - 0.02
Other Components
130 lb/hr H2S
130 lb/hr CO2
1650 lb/hr

[joerd]

Good work already, and a very clear presentation as well. Even though you are right about the potential boiling issues in the vaporizer, the design that you propose is not how it's done. Take a look at http://en.citizendiu...ki/Hydrotreater which has a clear process flow scheme. First of all, the recycle gas (mostly hydrogen) is mixed in before the heat exchanger, which changes the boiling regime. Also, the feed/effluent exchanger will consist of multiple shells so that you can get closer to countercurrent flow and achieve the "temperature cross": the vaporized feed exit temperature is higher than the cooled reactor effluent temperature. (of course there can be no real temperature cross in the sense that the cooled reactor effluent will always be hotter than the hydrocarbon feed). One other thought that I had about your steam system is that the condensing/boiling curve of steam is a straight line, at one temperature, which doesn't fit the sloping heating and cooling curves of your process very well and must therefore be quite inefficient.

hope this helps a bit,