0.52.15 release; correct Cp estimate

Author: yoelcortes

setup.py

@@ -11,7 +11,7 @@
     name='thermosteam',
     packages=['thermosteam'],
     license='MIT',
-    version='0.52.14',
+    version='0.52.15',
     description="BioSTEAM's Premier Thermodynamic Engine",
     long_description=open('README.rst', encoding='utf-8').read(),
     author='Yoel Cortes-Pena',

thermosteam/__init__.py

@@ -31,7 +31,7 @@
 #     update_module(chemicals, numba)
 # use_numba_chemicals()
 # del use_numba_chemicals
-__version__ = "0.52.14"
+__version__ = "0.52.15"
 
 from . import thermo
 del thermo

thermosteam/_thermo.py

@@ -141,7 +141,7 @@ def __init__(self, chemicals, mixture=None,
         if not isinstance(chemicals, Chemicals): 
             chemicals = Chemicals(chemicals, cache, db=db)
         chemicals.compile(skip_checks=skip_checks)
-        if mixture is not None and isinstance(mixture, Mixture):
+        if mixture is not None and not isinstance(mixture, Mixture):
             raise ValueError(f"mixture must be a '{Mixture.__name__}' object")
         if PCF is None: PCF = eq.MockPoyintingCorrectionFactors
         match pkg:

thermosteam/mixture/__init__.py

@@ -689,12 +689,20 @@ def Cn(self, phase, mol, T, P=101325):
         Cn = self.Cn_ideal(phase, mol, T, P)
         if phase == 's': return Cn
         if phase in self.active_eos:
-            eos, _, _, eos_mol = self.active_eos[phase]
+            eos, index, eos_index, eos_mol = self.active_eos[phase]
         else:
-            eos, _, _, eos_mol = self.eos_args(
+            eos, index, eos_index, eos_mol = self.eos_args(
                 phase, mol, T, P
             )
-        Cn += get_excess_property(eos, 'Cp', phase) * eos_mol
+        excess_ref = sum([
+            mol[eos_index[i]] * get_excess_property(
+                eos.to_TPV_pure(T=T, P=101325, V=None, i=i),
+                'Cp', 
+                phase
+            )
+            for i in range(eos.N)
+        ])
+        Cn += get_excess_property(eos, 'Cp', phase) * eos_mol - excess_ref
         return Cn
 
     def H(self, phase, mol, T, P):