Process improvements in selective ethylene oligomerizations

A reaction system comprising:

• a reaction zone configured to form an oligomer product, the reaction zone comprising ethylene, an organic reaction medium, a catalyst system, the oligomer product, and optionally hydrogen;
• a first reaction zone inlet connected to the reaction zone and configured to introduce the catalyst system comprising i) a chromium component comprising an N2-phosphinyl amidine chromium compound complex, an N2-phosphinyl formamidine chromium compound complex, an N2-phosphinyl guanidine chromium compound complex, or any combination thereof, and ii) an aluminoxane to the reaction zone;
• a second reaction zone inlet connected to the reaction zone and configured to introduce an ethylene feedstock mixture comprising ethylene, at least a portion of the organic reaction medium, and optionally hydrogen to the reaction zone;
• one or more reaction zone outlets connected to the reaction zone and configured to discharge a reaction zone effluent comprising at least a portion of the oligomer product from the reaction zone;
• an ethylene feed line comprising ethylene;
• and
• an organic reaction medium feed line comprising the organic reaction medium, wherein the ethylene feed line and the organic reaction medium feed line are configured to combine to form an ethylene feedstock mixture line, wherein the ethylene feedstock mixture line is connected to or coupled to the second reaction zone inlet;
• wherein the catalyst system is fed into the reaction zone separately of the ethylene feedstock mixture.