7 replies to this topic

[Jome128]

Dear Professional:

 

I am in trouble to understand, and setup the following rigorous method to get the mixture latent heat of vaporization done by Hysys.

For single component system, the latent heat of vaporization at relieving condition could be determined from a flash calculation as the difference in the specific enthalpies of the vapor and liquid phase in equilibrium with each other.

 

But, for multi-component systems, it will be more complicated, because the vaporization of the liquid initially in the vessel at the start of the fire proceeds in which the temperature, vapor flow rate and physical properties of the vapor and liquid in equilibrium which each other change continuously as fire proceeds. The peak relief load will not happen at the start of the fire. In such system require a time dependent analysis to determine the required relief area at the corresponding relief rate.

 

STEP 1 : Define fluid in the vessel is at maximum Operating pressure (P0) & operating temperature (T0) prior to fire start.

HYSYS : Stream 1 is at P0 & T0. Flash stream 1 in VESSEL V-100 and associate vapor and liquid outlet are stream 2 & 3.

STEP 2a : Determine liquid volume (Vl0) and vapor volume (Vv0) which will be defined by the physical vessel dimension and piping volume. Calculate liquid volume (Vl0) and vapor volume (Vv0) consider liquid level in the vessel is at maximum liquid level (L0) and piping vapor & liquid volume.

STEP 2b : Adjust the inventory (vapor mass and liquid mass) in the vessel until you achieve the vapor
volume (Vv0) and liquid volume (Vl0).

HYSYS : Copy stream 2 to stream 4. Copy stream 3 to stream 5. Mix stream 4 & 5 into stream 6. ADJUST stream 4 Mass Flow to obtain Stream 6 Vapor Actual Volumetric Flow to Vv0. ADJUST stream 5 Mass Flow to obtain Stream 6 Liquid Actual Volumetric Flow to Vl0.

STEP 3 : Bring the the system to Relieving pressure (Pr0=121% of Pset). By maintaining system volume at V0 (=Vv0+Vl0). The system is at relieving condition. The temperature at this point is Tr0.

HYSYS : Define a new stream 7 with a BALANCE unit (mole balance only). Set Stream 7 pressure at Pr0. ADJUST stream 7 temperature until Overall Actual Volumetric Flow to V0 (=Vv0+Vl0).

STEP 4 : Next step is further input heat (Q1) to achieve 1-2 deg C above Tr0 but maintain pressure at Pr0. Normally the total volume (V1) at this point potentially higher than V0.

HYSYS : Flash stream 7 in VESSEL V-101 into Vapor outlet, stream 6 and Liquid Outlet stream 11. Attach a heat stream (Q-100) into V-101. SET stream 8 temperature 1-2 degC above stream 7.

STEP 5 : Part of the vapor (m1) from the system shall be removed in order to bring the system volume (V1) back to V0 as the physical volume maintain.

HYSYS : SPLIT stream 8 into stream 9 and 10. Mix stream 9 & stream 11 form stream 12. ADJUST stream 9 Mass Flow (m1) to obtain Stream 12 Overall Actual Volumetric Flow to V0.

STEP 6 : Above has shown that m1 (stream 9) will be removed with the heat input of Q1 (Q-100) and maintaining at set pressure. Heat input is used to heat the vapor and liquid from Tro to Tr1 (sensible heat) and vaporized m1 of liquid (Latent Heat of Vaporization).

Latent Heat of Vaporization,
 

Hvap = [Q1 - m8 x dT x (Cp7+Cp8)/2 - m11 x dT x (Cp7+Cp8)/2]/m9

where
m8 - Mass of stream 8 (Vapor)
m9 - Mass of stream 9 (Removed Vapor)
m11 - Mass of stream 11 (Liquid)
Cp7 - Stream 7 (Mixed) specific heat capacity
Cp8 - Stream 8 (Vapor) specific heat capacity
Cp11 - Stream 11 (Liquid) specific heat capacity
dT = Tr0 - Tr1

STEP 7 : Determine Fire heat load input (Qc1) from API equation (Q=43.2A^0.82).

STEP 8 : Determine PSV mass flow rate with M1= m1 x (Qc1/ Q1)

STEP 9 : Calculate the Relief area (A1) base on M1 and fluid properties from stream 9.

STEP 10 : Repeat (4) - (9) until you get the maximum Relief area (Amax).

 

 

 

 LH_calc.PNG   10.56KB   138 downloads

 

 

 

Could anyone please give me a kindly help to share some light about the step 2b, step4, step6 or share the Hysys code of this method if you'd like?

 

 

I appreciate your help greatly.

 

Thank you.

[PaoloPemi]

by definition latent heat = (dew point enthalpy - bubble point enthalpy),

 

for pure components (isothermal) and (in some cases) narrow boling mixtures you can consider the amount of heat required  to vaporize a small fraction of liquid as constant,

 

however for wide boling mixtures there can be large differences in the heat required to vaporize a small fraction of liquid going from bubble point (light components start to vaporize)  to dew point (heavy components end to vaporize).

 

so at given pressure and phase fraction you need to estimate the amount of heat required to vaporize a small fraction of liquid,

 

with a different software (PRODE PROPERTIES) I solve a flash operation and

the result gives the requred value.

the procedure is similar to the zone by zone simulation required when solving a heat exchanger,

In Excel it takes just a series of macros for each flash operation.

 

if that may help with PRODE PROPERTIES you solve the initial V-P flash with a single macro to calculate temperature when PSV opens,

then, given a time step, PSV area and outlet pressure calculate the discharged mass,

adjust (mass and heat balance) vessel conditions solving a flash operation and go on.

 

From your post it seems to me that you are solving basic flash operations as V-P with complex iterating procedures (see step 3),

I do not know if that is a limit of your software or just a wrong procedure (someone more expert may clarify)

 

also it is not clear how you solve mass + heat balance at each time step,

 

Pardon my  comments (which may depend from being not familiar with your software) but on the whole the procedure you described looks very complex,

I hope some expert will suggest alternatives..

Edited by PaoloPemi, 25 August 2013 - 05:35 AM.

[Bobby Strain]

It's quite simple. Set up liquid flashes in series with about 5% vaporization each. Size the PSV for each vapor property. Pick the largest required orifice. Adjust vapor rate for liquid heating if you wish. Be careful that you purchase the valve with the proper materials for a reasonable temperature.

 

Bobby

[gegio1960]

Jome,

I'd like to kindly ask you the following

1) how many components are present in your mixture?

2) which kind of components?

thanks and regards

[AlertO]

Jome

 

i try to answer you step by step.

 

step 2b: it wants to fix the vessel volume (also the nearby piping), but we will input this value as a volume flow rate in HYSYS stream.

step 4: it creats the liquid flashing in series e.g. 5% vaporization as Bobby said by increasing the fluid temperature. 

step 5: after that, the constant volume is required in next combination; hence, a spilted vapor flow is presented ( total volume at this state is higher than the previous one). This split vapor is one of our relief flows (generated in the further states).

step 6: it calculates the latent heat of that splited vapor; anyway, if you've done it in hysys, the calculation is a lot easier than done by that formular.

 

Please note that this only 1 state and not be finished yet.

 

 

Hope this may help you.

[Jome128]

Can anyone kindly help me out to explain to me the following equation for latent heat calculation ?

 

Hvap = [Q1 - m8 x dT x (Cp7+Cp8)/2 - m11 x dT x (Cp7+Cp8)/2]/m9

 

Thank you very much.

[PaoloPemi]

at a given step (i.e. solving H-P , PF-P etc. flash operation)

you know composition and properties (density, enthalpy)

for actual and previous point,

 

for calculating the heat (J/Kg) required to vaporize a Kg of liquid

in that interval  you can solve H vapor - H liquid
by setting the same composition

or with a equivalent formulation keeping in account average

cp for liquid and vapor.

 

I do that with PRODE PROPERTIES

and the resulting procedure is very simple.

 

If you are unable to solve that

possibly you can utilize the depressuring procedure

which is available in these tools.

[ChemEng01]

 

Can anyone kindly help me out to explain to me the following equation for latent heat calculation ?

 

Hvap = [Q1 - m8 x dT x (Cp7+Cp8)/2 - m11 x dT x (Cp7+Cp8)/2]/m9

 

Thank you very much.

 

I think the equation should actually be ;

 

 

Hvap = [Q1 - m8 x dT x (Cp7+Cp8)/2 - m11 x dT x (Cp7+Cp11)/2]/m9

 

Please correct me if I'm wrong

 

 

cheers