Flashing under vacum 2

[zamu2010]

Dear Group members.
I am looking into heat recovery by flashing the stream at high conditions to lower conditions.
I want to recover the heat from the dischrge stream (65% liquids and 35% solids) of the reactor (leaving at 150 C and 10 bar g)by flashing it in flast tank (.8 bar g). Would it be possible to recover some heat by this method?? Is it a standard engineering practice??
Thanks for your help.
Best Regards
ZAMU

 

[sshep]

Hey zamu,

The flash tank will generate clean (solid free) vapor which could then be condensed against a heat sink (process stream). The vapor will be at the pressure of the flash drum which is likely to be significantly below 150C- i.e. it may not be a useful temperature. First do a flash calculation on the carrier liquid to determine the final temperature at 0.8bar. Adjust the amount of vapor predicted in your flash calculation by adding in the heat capacity of the solids going from 150C to the final temperature.

The primary advantage of what you are proposing is that you can "partially dry the solids", and recover vapor which can be used for heating. The disadvantage is that the temperature at which the heat can be recovered will be lower than if the solid-liquid mixture is simply cooled against a heat sink.

best wishes,
sshep

 

[zamu2010]

Thank you very much for your reply sshep,
Would it be possible to operate a flast tank at 0.8 bar and energy economical???
Whats about flashing in two steps??
Mine problem is I can't use the heat recoverd by simply cooling against heat sink due to high heat requirements (steam at high temperature) for heat requirements.
This is my though it could be a good idea both to recover some heat and have the discharge of the tank at lower water contents.
What you suggest??
Thanks in advance for your reply.

 

[ione]

In order to establish whether recovery is not economically affordable but profitable, you have to evaluate the amount of flash steam produced and then you have to consider:

-) The need of a demand for low pressure flash steam that either equals or exceeds the flash steam being produced. If the amount of flash steam exceeds its demand, surplus must be vented to atmosphere.
-) The need to have a continual supply of condensate to ensure that enough flash steam can be released for economic recovery.
-) A possible application for low pressure flash steam is in space heating equipment.

 

[sshep]

Hey Zamu,

Perhaps you can find a heat sink (BFW or tower preheat, side heaters, etc) other than where you are currently putting heat. Barring that the 150C came from a heat of reaction, there must have been some heating of feeds from ambient to process temperature, and so the waste heat from cooling of products should have a home, at least in part.

The quantitative process of minimum heat use is the well known "pinch analysis" of which you may be familiar. The usual training failure for pinch studies is that engineers are not taught to consider changes (mixing schemes, pressure profiles, temp vs flow trade-offs, etc) to the process being studied. EXAMPLE: if two towers each require 10MW heat input at 150C in the reboiler, we look for waste heat available at 150C. Instead, if we raise one tower pressure and lower the other, the temperature profiles are shifted such that we save 10MW of utility by using the hotter condenser waste heat to drive the other tower. This can be done by making steam off the condenser if direct exchange is an impractical retrofit.

If you are like most older plants, the waste heat from many parts of the process goes straight into cooling water. Given your enthusiasm to save energy, you may do even better to look at other parts of the process that might give more energy savings "bang for the buck" than what you are considering.

best wishes,
sshep