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Manually changing the ESP DBS File

 


 

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How to change the ESP DBS File?

On rare occasions a user will need to change the contents of the DBS file to quickly adjust an equilibrium constant for a reaction without having to regenerate the chemistry model. Typically this occurs with DBS files used with OLI's alliance partner products who do not have the ability to generate models.

For our example, we are going to adjust the equilibrium constant for the dissociation of the bisulfide ion (HS-1)

In principle modifying just this dissociation to get a better Sulfide ion concentration is a bit dicey. The dissociation is important for both the VLE of H2S as well as any solubility of any sulfide solids. Changing a single equilibrium constant is like knocking out the middle floor on a three story house..

The user can modify the DBS file to change the coefficients but it is tricky and OLI cannot support the effort beyond what we tell you here. The first step is to locate the following record in the dbs file:

THE SECTION BELOW PERTAINS TO:AKNAM=K-VALUE NAMES LIST

Which has the following contents



Each equilibrium constant begins with the letter K. Notice that the dissociation constant for HSION=HION+SION is KHSION and appears in the 15th position.

Now location the section:

THE SECTION BELOW PERTAINS TO:AKVAL=K COEFFICIENTS

Each equilibrium constant has a total of 8 parameters. The first is a record type value and the next 7 follow this formula if the first record is type 1 or 3

TYPES 1 AND 3: K = 10**(A + B/TK + C*TK + D*TK**2) * 10**(E + F*P + G*P**2)


If type 2 then this formula is used:

TYPE 2: K = EXP(AX + BX + CX)

WHERE AX = -A / (298.15 * 1.9872)
BX = -B * ( 1/TK - 1/298.15) / 1.9872
CX = C * ( 298.15/TK - 1.0 - LN(298.15/TK)) / 1.9872

You can simply count out to the 15th occurrence and start changing things. In your sample file it is:


So the dissociation for HSion is a type 2 and loosely related to the Gibbs energy of reaction.

Small changes here are big changes in the equilibrium constants and you should expect non converged cases. Also please use a text editor and not a word processor

Do not regenerate the model after doing this or the dbs file will be rebuilt.

A brief statement about activity coefficients: our activity coefficients were developed simultaneously with the equilibrium data. Changing the equilibrium data except in the most dilute of cases will invalidate the activity model which means that the calculation of the activity of water will be suspect. OLI cannot effectively support this model if these changes are made.

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