9 Application of Equations of State in Exxon's Production Operations S. W. HOPKE
Downloaded by TROY UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch009
Exxon Production Research Co., Houston, TX 77001
Equations of state are the heart of Exxon's computer simulation of phase behavior in field separation facilities, pipelines, gas plants and condensate or volatile oil reservoirs. These simulation programs are executed scores of times each working day. There are several state equation methods available to our engineers. One of them, the BWRS equation, (1 - 3), is more accurate than the others and is used for almost all of our simulations that employ state equations. The BWRS equation is Starling's modification of the BenedictWebb-Rubin equation of state. It contains eleven adjustable pure component parameters plus a binary interaction parameter for each component pair. Thus, a typical 20 component mixture would be characterized by 220 pure component parameters and 180 different binary interaction parameters--a total of 400 constants. Exxon's set of constants were determined by multi-property regression, a procedure in which parameters are adjusted until available data on density, enthalpy, vapor pressure, K-values, sonic velocity, and specific heats are all matched simultaneously. The large number of constants to be determined requires that these data be accurate and that they cover a wide range of conditions. Nearly 20,000 data points were used to determine our set of constants. The large amount of data required limits the components that can be handled to the relatively few for which such data exist and may also place a practical limit on the number of parameters desirable in an equation of state. To be useful for typical petroleum production applications, a state equation must be able to predict the phase behavior of mixtures containing hydrocarbon fractions. The BWRS equation can do this for mixtures containing fractions with molecular weights as high as 500. The BWRS parameters for these fractions are obtained from correlations of the parameters, made dimensionless by dividing by appropriate powers of the critical temperature and critical volume, with the acentric factor. Accurate predicted K-values for light components in oils result from correlations of binary interaction parameters. Reliable simulations of absorber plant 221
In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
Downloaded by TROY UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch009
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P H A S E EQUILIBRIA A N D F L U I D PROPERTIES IN C H E M I C A L
INDUSTRY
performance are p o s s i b l e even when only the molecular weight and d e n s i t y o f the a b s o r p t i o n o i l s are known. In condensate and v o l a t i l e o i l r e s e r v o i r s , i n h i g h pressure s e p a r a t i o n f a c i l i t i e s , and i n p i p e l i n e s a s s o c i a t e d w i t h p r o d u c t i o n of r e s e r v o i r s i n remote l o c a t i o n s , phase behavior i s dominated by hydrocarbons h e a v i e r than C ^ Q . Since the c o n c e n t r a t i o n s and prop e r t i e s o f these heavy hydrocarbon f r a c t i o n s are not u s u a l l y known, r e l i a b l e phase behavior p r e d i c t i o n s f o r these produced f l u i d s a r e i m p o s s i b l e unless they are based upon experimental phase behavior data f o r the f l u i d i n q u e s t i o n . Consequently, to p r e d i c t the phase behavior o f these systems, we take the f o l l o w i n g approach. First, the composition o f the f l u i d i s measured t o C10+ and the molecular weight and d e n s i t y determined f o r each o f the C5 t o C10+ f r a c t i o n s . The C^o"" f r a c t i o n i s then s p l i t i n t o s e v e r a l c o m p o n e n t s — t y p i c a l l y the C I Q , C15, C 2 0 and C25 f r a c t i o n s . This s p l i t and the b i n a r y i n t e r a c t i o n parameters of these heavy hydrocarbon f r a c t i o n s w i t h other components are adjusted u n t i l the model matches experimental phase behavior data f o r the f l u i d . The minimum phase behavior data r e q u i r e d i s the volume percent l i q u i d a t s e v e r a l pressures and at a constant temperature. Because of the l a r g e number o f constants i n v o l v e d , the BWRS method r e q u i r e s s l i g h t l y longer computation times than s i m p l e r , l e s s accurate methods. Considerable a t t e n t i o n to improving comput a t i o n e f f i c i e n c y has enabled us to use the BWRS equation i n t r a y t o - t r a y column c a l c u l a t i o n s and i n a one-dimensional c o m p o s i t i o n a l r e s e r v o i r s i m u l a t o r , a p p l i c a t i o n s which i n v o l v e l a r g e numbers o f s t a t e equation c a l c u l a t i o n s . While the BWRS equation o f s t a t e i s adequate f o r most o f our a p p l i c a t i o n s , there are problem areas. Some low temperature proc e s s i n g c o n d i t i o n s approach the c r i t i c a l r e g i o n , where c a l c u l a t i o n s l o s e accuracy and are d i f f i c u l t to converge. To date, we have not s y s t e m a t i c a l l y evaluated the accuracy near mixture c r i t i c a l p o i n t s nor s t u d i e d ways t o improve convergence i n t h i s r e g i o n . We have not had great success modeling water w i t h the BWRS equation. Water i s present i n most p r o d u c t i o n systems and we have had t o use separate c o r r e l a t i o n s f o r water behavior. Binary i n t e r a c t i o n parameters, e s p e c i a l l y f o r component p a i r s that g r e a t l y d i f f e r i n molecular s i z e , tend to be temperature dependent. This i n d i c a t e s t h a t the mixing r u l e s can be improved. In the f u t u r e , we p l a n t o continue i n c o r p o r a t i n g new data i n t o our c o r r e l a t i o n s as they become a v a i l a b l e . We a l s o await w i t h i n t e r e s t s t u d i e s c u r r e n t l y underway i n the u n i v e r s i t i e s t h a t might l e a d to f a s t , g e n e r a l i z e d equations o f s t a t e t h a t can handle water and p o l a r compounds w h i l e r e t a i n i n g adequate accuracy f o r e n g i neering a p p l i c a t i o n s . 1
In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
9.
HOPKE
Equations of State in Exxon's Operations
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Literature Cited
Downloaded by TROY UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch009
1. Lin, C. J. and Hopke, S. W., AIChE Symposium Series (1974), 70, No. 140, 37. 2. Hopke, S. W. and Lin C. J. presented at 76th National AIChE meeting, Tulsa, Oklahoma (1974). 3. Hopke, S. W. and C. J. Lin, Proc. 53rd Gas Processors Assn. Annual Conv. (1974) p. 63.
In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
