Methods for operating polyethylene reactor systems

A method of operating a polyethylene reactor system, the method comprising:

• feeding ethylene, a diluent, and a chromium-based catalyst to a first polymerization reactor;
• contacting ethylene and the first optional comonomer with the catalyst in the first polymerization reactor to form a first product comprising a first polyethylene, wherein the ethylene concentration in the first polymerization reactor is from 7.5 mol % to about 8 mol %;
• feeding the first product from the first polymerization reactor to a second polymerization reactor;
• contacting ethylene and an optional comonomer with catalyst from the first reactor in the second polymerization reactor to form a second product comprising the first polyethylene and a second polyethylene, wherein the ethylene concentration in the second polymerization reactor is from about 7.5 mol % to about 8 mol %;
• discharging the second product from the second polymerization reactor;
• and
• controlling one or both of a molecular weight or a breadth of molecular weight distribution of the second product by adjusting a rate of hydrogen fed to the second polymerization reactor,
• wherein no hydrogen is fed to the first polymerization reactor, and
• wherein the first polyethylene and the second polyethylene in the second product define a combined monomodal molecular weight distribution.