1. A method for controlling ramp imbalances in a model predictive controller by allowing temporary relaxation of ramp imbalance constraints when within limits to better control a petrochemical process, comprising:determining user specified settings comprising minimum/maximum allowed ramp rates (?inmin??inmax), time to balance (tbal), and set-point recovery rate (?r);

determining a change (?) in current ramp imbalance induced by changes in one or more external disturbance affecting one or more ramp variables;

modifying low (ylo,sp) and high (yhi,sp) set-point limits in accordance with ylo,sp=ylo,sp+min(tbal?,0) and yhi,sp=yhi,sp+max(0,tbal?);

determining soft-landing ramp limit time constants (Tlo and Thi) based on ramp limits (ylo and yhi) and the user-defined minimum/maximum allowed ramp rates in accordance with Tlo=max(Tmin,(ylo?y)/?inmin) and Thi=max(Tmin,(yhi?y)/?inmax) where y is a current ramp and Tmin is a minimum time constant that determines how fast the ramp rate may be driven to zero;

determining soft-land set-point limit time constants (Tlo,sp and Thi,sp) in accordance with Tlo,sp=max(Tmin,(ylo,sp?y)/?inmin) and Thi,sp=max(Tmin,(yhi,sp?y)/?inmax);

determining imbalance ramp rate constraints (?min,ss and ?max,ss) in accordance with ?min,ss=(ylo?y)/max(Tlo,h) and ?max,ss=(yhi?y)/max(Thi,h) where h is the dynamic control horizon;

determining imbalance set-point ramp rate constraints (?min,sp,ss and ?max,sp,ss) in accordance with ?min,sp,ss=(ylo,sp?y)/max(Tlo,sp,h) and ?max,sp,ss=(yhi,sp?y)/max(Thi,sp,h);

resolving static control process problem by imposing the determined imbalance ramp rate constraints (?min,ss and ?max,ss) at a priority higher than the ramp limits (ylo and yhi) and imposing the determined imbalance set-point ramp rate constraints (?min,sp,ss and ?max,sp,ss) at a priority higher than the ramp set point limits (ylo,sp and yhi,sp);

updating constraints for the ramp limits (ylo and yhi) in accordance with ylo?yk+Tlo?k and yk+Thi?k?yhi where k is a prediction time and ?k is a ramp rate at the prediction time;

updating constraints for the set-point limit trajectories in accordance with ylo,sp,k?yk+Tlo,sp?k and yk+Thi,sp?k?yhi,sp,k where set-point limit trajectories (ylo,sp,k and yhi,sp,k) are defined as ylo,sp,k=min(ylo,sp+k?r,ysp) and yhi,sp,k=max(yspyhi,sp?k?r);

resolving a dynamic control process problem using said updates; and

controlling the ramp imbalances in the model predictive controller by said ramp imbalance constraints to better control the petrochemical process.