(* This is written in the ASCEND Language ... ASCEND is an
environment to pose modeling problems and solve the resulting
equations in an object oriented fashion ... ASCEND is
copyrighted by Carnegie Mellon University ... the project
has been directed by Prof. Art Westerberg in the Chemical
Engineering Department ...*)

(* ASCEND is free software, released under GNU ... and you
can get it from http://www.cs.cmu.edu/~ascend ... The
software has been released WITH source code ... it compiles
under Solaris, Linux and IRIX (SGI) ... thanks to the excellent
job in putting the software together by the folks at CMU! ...
and autoconf ... I have no idea of how people compiled things
before ./configure ! *)

(* This example written by Krishnan Chittur (chitturk@uah.edu)
Chemical Engineering Department, University of Alabama in Huntsville,
Huntsville, AL 35899 (205) 890 6850 (V), (205) 890 6839 (FAX) *)

(* This is the NRTL Model for liquid phase activity
coefficients ... contains information pertinent to the
1998 AIChE student design problem *) 

(* obviously the modeling style here is primitive ... write down
the equations all together in one model and solve it .. it can
get quite confusing for real problems!*)

REQUIRE "atoms.a4l";

MODEL nrtl;

	components IS_A set OF symbol_constant;
	components :== ['acetonitrile','toluene','water','pxylene'];
	x[components] IS_A fraction;
	G[components][components] IS_A factor;
	tau[components][components] IS_A factor;
	gamma[components] IS_A factor;
	A[components][components] IS_A molar_energy;
	alpha[components][components] IS_A factor;
	term1[components] IS_A factor;
	term2[components] IS_A factor;
	term3[components] IS_A factor;
	term4[components] IS_A factor;
	term5[components] IS_A factor;
	term6[components] IS_A factor; 	
	RG IS_A gas_constant;
	TA IS_A temperature;

	FOR i IN components CREATE
	term1[i] = SUM[tau[k][i]*G[k][i]*x[k] | k IN components];
	term2[i] = SUM[G[k][i]*x[k] | k IN components];

        FOR j IN components CREATE 
	term3[j] = SUM[G[m][j]*x[m] | m IN components];
	term4[j] = SUM[x[n]*tau[n][j]*G[n][j] | n IN components];
	term5[j] = SUM[G[k][j]*x[k] | k IN components];
	END FOR; 	

	term6[i] = SUM[x[j]*G[i][j]*(tau[i][j] - term4[j]/term5[j])/term3[j] | j IN components];
	ln(gamma[i]) = term1[i]/term2[i] + term6[i];
	END FOR;

	FOR i IN components CREATE
	FOR j IN components CREATE
	tau[j][i] = A[j][i]/(RG*TA);
	END FOR;
	END FOR;

	FOR i IN components CREATE
	FOR j IN components CREATE
	G[j][i] = exp(-alpha[j][i]*tau[j][i]);
	END FOR;
	END FOR;



METHODS
	METHOD clear;
	x[components].fixed := FALSE;
	G[components][components].fixed := FALSE;
	tau[components][components].fixed := FALSE;
	A[components][components].fixed := FALSE;
	alpha[components][components].fixed := FALSE;
	TA.fixed := FALSE;
	END clear;

	METHOD specify;
	x[components].fixed := TRUE;
	G[components][components].fixed := FALSE;
	tau[components][components].fixed := FALSE;
	A[components][components].fixed := TRUE;
	alpha[components][components].fixed := TRUE;
	TA.fixed := TRUE;
	END specify;

	METHOD values;
	x['acetonitrile'] := 0.2;
	x['toluene'] := 0.3;
	x['water'] := 0.3;
	x['pxylene'] := 0.2;
	A['acetonitrile']['acetonitrile'] := 0 {cal/mole};
	A['toluene']['acetonitrile'] := 724.0 {cal/mole};
	A['water']['acetonitrile'] := 1217.0 {cal/mole};
	A['pxylene']['acetonitrile'] := -210 {cal/mole};
	A['acetonitrile']['toluene'] := 790 {cal/mole};
	A['toluene']['toluene'] := 0 {cal/mole};
	A['water']['toluene'] := 4375.0 {cal/mole};
	A['pxylene']['toluene'] := -242 {cal/mole};
	A['acetonitrile']['water'] := 1100 {cal/mole};
	A['toluene']['water'] := 5680.0 {cal/mole};
	A['water']['water'] := 0 {cal/mole};
	A['pxylene']['water'] := 2456 {cal/mole};
	A['acetonitrile']['pxylene'] := 1413 {cal/mole};
	A['toluene']['pxylene'] := 226.0 {cal/mole};
	A['water']['pxylene'] := 1387.2 {cal/mole};
	A['pxylene']['pxylene'] := 0 {cal/mole};
	alpha['acetonitrile']['acetonitrile'] := 0;
	alpha['toluene']['acetonitrile'] := 0.9353;
	alpha['water']['acetonitrile'] := 0.5654;
	alpha['pxylene']['acetonitrile'] := 0.294;
	alpha['acetonitrile']['toluene'] := 0.9353;
	alpha['toluene']['toluene'] := 0;
	alpha['water']['toluene'] := 0.2;
	alpha['pxylene']['toluene'] := 0.287;
	alpha['acetonitrile']['water'] := 0.5654;
	alpha['toluene']['water'] := 0.2;
	alpha['water']['water'] := 0;
	alpha['pxylene']['water'] := 0.2;
	alpha['acetonitrile']['pxylene'] := 0.294;
	alpha['toluene']['pxylene'] := 0.287;
	alpha['water']['pxylene'] := 0.2;
	alpha['pxylene']['pxylene'] := 0;
	TA := 300.0 {K};
	END values;

END nrtl;