Seems to work, now go for testing it

ode.pl

@@ -1381,22 +1381,14 @@ normalize_expression(-Expr, ListVar, NExpr) :- !,
   normalize_expression(TExpr, ListVar, NExpr).
 
 normalize_expression(Term, ListVar, NTerm) :-
-  normalize_term(Term, ListVar, NTerm).
+  normalize_term(Term, ListVar, TermTempo),
+  simplify(TermTempo, NTerm).
 
 
-normalize_term(TermRaw, ListVar, NewTerm) :-
-  simplify(TermRaw, Term),
-  (
-    Term =.. [-, TermP]
-  ->
-    filter_denominator(TermP, Numerator, Denominator),
-    filter_other(Numerator, ListVar, Numbers, Parameters, Variables),
-    rewrite_term([-1|Numbers], Parameters, Variables, Denominator, NewTerm)
-  ;
-    filter_denominator(Term, Numerator, Denominator),
-    filter_other(Numerator, ListVar, Numbers, Parameters, Variables),
-    rewrite_term(Numbers, Parameters, Variables, Denominator, NewTerm)
-  ).
+normalize_term(Term, ListVar, NewTerm) :-
+  filter_denominator(Term, Numerator, Denominator),
+  filter_other(Numerator, ListVar, Numbers, Parameters, Variables),
+  rewrite_term(Numbers, Parameters, Variables, Denominator, NewTerm).
 
 %! filter_denominator(Term, Numerator, Denominator)
 
@@ -1414,10 +1406,17 @@ filter_denominator(Term*X^N, Num, [NTerm|Den]) :-
 filter_denominator(Term*X^N, [X^N|Num], Den) :- !,
   number(N), N>0, !,
   filter_denominator(Term, Num, Den).
-filter_denominator(Term*X, [X|Num], Den) :- !,
-  filter_denominator(Term, Num, Den).
+
+filter_denominator(T1*T2, Num12, Den12) :- !,
+  filter_denominator(T1, Num1, Den1),
+  filter_denominator(T2, Num2, Den2),
+  append(Num1, Num2, Num12),
+  append(Den1, Den2, Den12).
+
 filter_denominator(Term/X, Num, [X|Den]) :- !,
   filter_denominator(Term, Num, Den).
+
+
 filter_denominator(X^N, [], [NTerm]) :- !,
   number(N), N<0, !,
   Nm is -N,

ode.plt

@@ -109,7 +109,7 @@ test(
   clear_model,
   with_current_ode_system((
     add_ode(d(x)/dt = (1*(k9*k1*k2) + (-1)*(k9*k3*x1) + (-1)*(k9*k8*x*x/(x+k7)))/k9),
-    normalize_ode([d(x)/dt= k2*k1+ -1*x1*k3+ -1*x^2*k8/(x+k7)])
+    normalize_ode([d(x)/dt=k1*k2+ - (k3*x1)+ -1*k8*x^2/(x+k7)])
   )).
 
 test(
@@ -117,7 +117,7 @@ test(
   clear_model,
   with_current_ode_system((
     add_ode(d(x)/dt = k7+ -1*k8*x + -1*k9*x/(x+k1)),
-    normalize_ode([d(x)/dt = k7+ -1*k8*x+ -1*k9*x/(x+k1)])
+    normalize_ode([d(x)/dt=k7+ - (k8*x)+ -1*k9*x/(x+k1)])
   )).
 
 :- end_tests(ode).