Process for controlling gas phase fluidized bed polymerization reactor

A process for controlling a gas phase polymerization reaction in a reactor when changing from a first product made at a first set of conditions to a second product made at a second set of conditions, said process comprising the steps off:

• (a) comparing the first product reaction temperature and the second product reaction temperature, change the product reaction temperature setpoint to the second product reaction temperature if said second product reaction temperature is lower than said first product reaction temperature,
• (b) setting a melt index setpoint that is either 0-150% higher or 0-70% lower than the desired second product melt index value,
• (c) setting a reaction temperature setpoint that is:
• 1.degree.-15.degree. C. above the desired second product reaction temperature if the second product melt index value is higher than the first product melt index value, or 1.degree.-15.degree. C. below the actual second product reaction temperature if the second product melt index is lower than the first product melt index,
• (d) setting a product rate-limiting reactant partial pressure setpoint that is:
• 1-25 psig either below the first product rate-limiting reactant partial pressure if the second product melt index value is higher than the first product melt index value, or above the first product rate-limiting reactant partial pressure if the second product melt index value is lower than the first product melt index value;
• (e) maintaining said melt index setpoint, temperature setpoint, and rate-limiting reactant partial pressure setpoint values until said polymerization product exhibits an average melt index and average product density with an acceptable range from the desired second product melt index value and second product density value;
• (f) changing said melt index setpoint to the desired second product melt index value;
• (g) changing said product reaction temperature setpoint to a value that is:
• (i) 0.degree.-15.degree. C. above said desired second product reaction temperature if the second product melt index value is higher than the first product melt index value, or (ii) 0.degree.-15.degree. C. below said desired second product reaction temperature if the second product melt index value is lower than the first product melt index value;
• (h) changing said rate-limiting partial pressure setpoint to a value that is:
• (i) 0-25 psig either below the desired second product rate-limiting partial pressure if the second melt index value is higher than the first melt index value, or (ii) 0-25 psig above the second product rate-limiting partial pressure if the second melt index value is lower than the first melt index value;
• and
• (i) changing the reaction temperature setpoint and the rate-limiting reactant partial pressure setpoint values to the desired second product reaction temperature and second rate-limiting reactant partial pressure value when the reaction product exhibits an average melt index value within acceptable limits of the second product melt index value.