Process for the controlled production of polyethylene and its copolymers

A process for the controlled production of polyethylene and its copolymers in gas phase fluidized bed reactor(s), isolated or combined, under polymerization conditions, in the presence of hydrogen, oxygen, inert diluents and a Ziegler-Natta catalyst, wherein said process comprises the steps of:

• a) In the laboratory, periodically testing the MFR2 and density of a resin;
• b) Establishing desired values or limits for the set of CVs and CCVs which comprise production rate, the resin MFR2 and density, catalyst productivity, opening of a temperature control valve, monomer partial pressure, superficial velocity of the gas in the reactor, bubble point of a recycle stream and difference between the inlet and outlet temperature of the water in a cooling system of the reactor;
• c) Determining in real time the value of the variables of item (b), the resin properties having their value continuously inferred with the aid of mathematical models;
• d) Establishing limits for the set of MVs which comprises flow rate of catalyst, ratio between the concentrations of one or more comonomers and ethylene, ratio between the flow rates of one or more comonomers and ethylene, temperature of the fluidized bed, ratio between the flow rates of oxygen and ethylene, ratio between the concentrations of hydrogen and ethylene, ratio between the flow rates of hydrogen and ethylene, flow rate of a recycle stream, flow rate of inert diluent(s) and opening of a purge valve;
• reactor pressure, level of the fluidized bed, density of the gas in a recycle stream and superficial velocity;
• e) Establishing limits for the variation rate of the variables cited in item (d);
• f) By using mathematical models of the process, simultaneously calculating the values which should be taken in a stationary condition by the variables cited in item (d), considering that the level of a fluidized bed and the reactor pressure will remain constant, so that the desired values established in (b) will be reached without violating the limits established in items (b) and (d);
• g) Determining the sequence of adjustments to be made along a predetermined period of time, so that the variables cited in item (d) reach the desired values calculated in item (f) considering the dynamics and the relevance of each of the variables cited in (b) and the constraints established in (e);
• h) Adjusting the flow rates of catalyst, comonomer(s), cooling water, oxygen, hydrogen, inert diluent(s), the flow rate of the recycle stream and the opening of the purge valve so as to satisfy the previous items;
• i) based on the laboratory tests, periodically correcting the value inferred for the resin properties.