Steam Expansion Under Vacuum

A simplistic and approximate way to this would be to look at steam tables (Kenan & Keyes). In that, saturated vapor at 150 psia has specific volume of 3.015 cu ft/lb or density about 5.3 kg/cu M. which is close to your number 5.6497. Then going to the table at 0.25" hg (about 6.3 mm) the specific volume is 2423.7 cu ft/lb or about 151 cuM/Kg. At a stripping steam rate of 300 Kg/hr with those figures, there will be 45,300 cu M of stripping steam available which at 8 cu M/ton of oil would let you strip more than 5600 tons per hour of oil if all the steam was used strictly and only to provide vapor to strip. Some steam is condensed to provide sensible heat to the oil and latent heat to vaporize the odor bodies, etc. and I'm not sure whether that is all accounted for in your factor of 8 cu M/ton

You might want to do more rigorous calclulations of course to analyse this, taking into account that the steam is superheated, which gives it more volume per Kg and if the steam is actually at lower absolute pressure in the deodorizer under the new system it will have a higher enthalpy and make the heat duty of the vacuum system greater. You need to look at the overall heat balance as well as the stripping gas ratio.

I'm curious about what you mean by "ice condensers". Are you converting from barametric condensers between stages of your jets to some kind of surface condenser? This is done sometimes to eliminate the aqueos waste stream from barametric condenser seal legs. But I don't know what you mean by "ice" condensers.

hello,

sorry forgot to mention the steam is superheated, thus the volumes and densities change, but litrature on superheated steam especially under a 1mBar vacuum is impossible to find, it not even in Perry handbook. from what i know the condition that will effect the volume will be the Vacuum, the temp, the amount of steam per ton of oil and effectiveness of strip steam.

Ice condensers are condensers used to super cool the water vapour, air and impurities at around -33C with the use of ammonia on tube side. this crytalises on the tubes, they are then super heated into a melting vessel. the vacuum is pulled using liquid ring pums and small 2m 2 stage jet ejectors. its long o explain so have a look at the diagram above sure you can work it out.

Thanks for the diagram. I can see the operating principles for that kind of system.

On the question of steam properties at very low absolute pressure I think it should be valid to treat your gas phase as ideal gas using Raoult equation for phase calculations. I checked the steam table data in the range 1" Hg to 0.25" Hg for saturated steam and it checks ideal gas law within less than 1% error. It should be valid to extrapolate that to even lower absolute pressure at reasonable superheat temperature, i.e. reasonable removed from critical temperature. So if you know the pressure and temperature you can calculate the volume.

Your calculation analysis would then be to estimate the stripping steam volume rate at your current known deodorizer operation, say 3mb, 600 kg/h to get your current steam to oil requirement in terms of steam volume at internal deodorizer condition. (your present figure of 8:1 is based on 10 bar steam) Then calculate the steam volume at the new conditions say, 1.5 mb and 300 kg/h and estimate the oil rate possible at that steam oil ratio.

Again, I suggest you have to also look at the heat balance. How much of your current 600 kg/h steam supply is used to heat the oil to operating temperature and vaporize the odor bodies and is actually not applied as carrier for gas stripping. You will still need that steam in additon to stripping steam.

My previous comments focused on the steam properties since that was the thrust of your question and also the question of whether the ice condenser system was adequate to handle the necessary steam load. However I have wondered about whether you have analysed the equilibrium stripping situation in terms of the mol ratio of the odor bodies to the steam. Reducing the steam rate from 600 kg/h to 300 kg/h for the same oil rate will make a big change in that ratio. The stripper will be following a different operating line. Will that make a change in quality of your deodorized oil on equilibrium considerations given your existing deodorizer geometry?

I do not believe it will have an effect on the deodorised oil, as the 8 m3 figure is based on 10mbar vacuum, thus increasin the vacuum will allow expansion of the steam at a better vacuum hence possibly requiring less m3 to the ton of oil, i guesstimate the equilibrium will be the same.

Using the Ideal gas law equations and superheated steam volume data that i canot find for 1mbar,The relationship between superheated steam and higher vacuum showed that under a better vacuum the amount of strip steam is decreased, taking some assumptions into account such as pressure drops (looking into other sites, the Scrubber at present and condition, size and physical refining basis reducing the amount of distillate due to more FFA being removed in Chemical refining), volume of steam required per ton of oil (CCP figures at 10mBar), deodoriser temperature and final vacuum at the deodourisers the amount of tonnage could allow for the same tonnage of oil to be processed.

But i need help to work out wha calcs i need to actually do, and extrapolation of the data does not sound easy, will give it a go on excel

Estimating the specific volumes of steam at low absolute pressures from 10mb down to 1 mb is straight forward if you accept the assumption of ideal gas properties. The steam tables give specific volume values down to about 8mb which you can read directly. Take the 4 lowest pressure values given in table for saturated pressures; converted to mb these are: 8.35, 16.71, 25.06, 33.42; then plot these against their given specific volume values and extrapolate the straight line down to 1 mb. Adjust the results upward to superheated temperature using the incremental changes in specific volume given in the superheat table. (that one only goes down to 33.4 mb, but that probably will not introduce much error) It should give you results that are reliable enough for an engineering analysis of the deodorizer condenser question.

An overall rigorous assessment of the system would of course be much more complex as you mentioned in your last comment.

Where did you find these steam values the ones i have looked at all only go down to 100mBar or 0.125 Bar, but seems you have found some on the lower threshold, what book are you using.

My old college book: Keenan, J.H. and Keyes, G.K., Thermodynamic Properties of Steam, John Wiley & Sons, New York 1st edition 1936

The table for saturated pressures start at 0.25 psia.

I'm surprised, did I not perform the units conversion correctly?

to convert 0.25 psia to mbar:

0.25/14.7 = 0.017 Atms

0.017 x 1.0132 = .0172 bar == 17.2 mb

hno calulation was correct but thanks for breaking it down anyway. need to find that book you have. thanks