5 replies to this topic

[T'ien]

Hello,

 

I was reading through some old forums about the waste heat boiler located immediately downstream of a methane reformer unit. From what I read, most people seemed to agree that the very hot effluent from the reformer should be in the tubes and the boiler feed water can be turned to steam in the shell.

 

However, I have been using HTRI and I am thinking that perhaps the hot syngas should be in the shell and the BFW in the tubes. The reason is because the syngas has a very high volumetric flow rate and I am getting some ridiculous fluid velocities and pressure drop in the tube side if I put the hot syngas there. Here are the details:

 

Hot fluid: reformer effluent (syngas)
Flow rate: 13.5 kg/s, 5.3 m3/s

Temperature change: 850C down to 420C

Inlet Pressure: 2000 kPa

 

cold fluid: boiler feed water

Flow rate: 8.2kg/s, 0.0093 m3/s

Temperature change: 186C up to 233C (saturated steam)

Inlet Pressure: 2900 kPa

 

Could some one please explain to me how you would decide where to put each fluid? As I said, the high flow rate of syngas seems to result in an excessive pressure drop in the tubes and that is why I would lean towards putting it in the shell, but if someone can explain to me why the hot syngas should go though the tubes, please explain to me the reasons why. Thank you very much.

 

T'ien

[Said Salim]

Hello Tien,

 

One vital reason that will encourage me to put the reformer effluent into the tube side is that I don't want to lose the energy to the atmosphere. Instead, I would prefer that  the heat released to be absorbed by the BFW.

 

Thanks

[PingPong]

Syngas in put on tubeside for metallurgical reasons.

[T'ien]

Thank you for the replies. In terms of metallurgical reasons, I am guessing the material needed for the very hot gases is very expensive and we don't want to build the entire shell with this material?

 

I forgot to mention that in my HTRI simulation, I have a pressure drop of around 700 kPa and fluid velocities of 230 m/s in 1 inch tubes. Would you recommend using larger diameter tubes although the overall dimensions of the heat exchanger might increase significantly?

[PingPong]

Why not use more tubes in parallel, so velocity is lower and required tube length is shorter, both resulting in less pressure drop.

[Pilesar]

  The operational and maintenance problems for exchangers like this in severe service often influence the design. It is critical to keep water supplied to the exchanger at all times. Your indicated water side stream does not seem typical. If the exchanger is designed for 100% vaporization, the heat transfer surfaces will dry out and get too hot. Also, boiler feed water is not pure H2O! Minerals concentrate in the liquid phase as steam is generated. These minerals will plate out on heat transfer surfaces, resulting in corrosion. I would expect the water to gravity-feed to this exchanger from an elevated steam drum. The additional static head from the drum results in subcooled water to the exchanger. With a much lower vaporization percentage and higher flow rate, the steam and water mix returns through a thermosiphon action to the steam drum where the steam is separated. The steam drum will also have a continuous blowdown to remove concentrated minerals. 

  The methane reformer waste heat boilers I designed and operated many years ago used a bayonet-style tube-within-a-tube where the water supply fed downwards through the inner tube and returned back to the steam drum through the outer tube. The gas flow on those exchangers was vertically downward. Another style of vertical waste heat boiler that I currently work with has water on the shell side but can have reliability problems with local internal hot spots if the distribution on either side of the exchanger is inadequate. Gas flow in these is vertically upward. I do not know what style is currently considered the best practice.