diff --git a/pages/science/reactors/pfr.rst b/pages/science/reactors/pfr.rst
index 38b48f51..b9464828 100644
--- a/pages/science/reactors/pfr.rst
+++ b/pages/science/reactors/pfr.rst
@@ -83,7 +83,7 @@ overall mass conservation equation for the reactor can then be written as:
 
 .. math::
 
-  \rho u \frac{du}{dz} = - u \sum_j \frac{A_{s,j}}{V} \dot{m}_s - \frac{dp}{dz}
+  \rho u \frac{du}{dz} = - u \dot{m}_s - \frac{dp}{dz}
   \tag{2}
 
 *Energy conservation:*