An apparatus capable of mixing a biopharmaceutical composition including a liquid, comprising: - a bag , made of flexible plastic material, provided with a bottom wall , a top wall and a tubular sidewall extending from the bottom wall to the top wall so that the bag delimits an interior volume (V); - a tank (T) provided with a base that is rectangular, the tank (T) comprising a side wall that is rigid and provided with four panels extending upwardly from the base , in order to delimit a receiving compartment; - a stirring device located in the interior volume (V) and adapted to be driven from below, at the opposite from the top wall ; wherein the bag is arranged above the base and between the four panels , in the receiving compartment, wherein the bag is configured such that, in an unfolded configuration of the bag with the liquid present inside the bag, has a parallelepiped configuration, wherein the bag is configured such that, once placed inside the tank (T) and unfolded according to the unfolded configuration, is: - extending vertically, parallel to a central axis (X) of the tank (T) that intersects an upper opening (O) of the tank (T), - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels , and - satisfying the following relationship: V2 ≥ 1500 L where V2 is the capacity of the bag , characterized in that the bag includes four rounded corners (RC1, RC2, RC3, RC4) that are each spaced from the side wall of the tank (T), and satisfies the following relationship: 0.1 < 4 r / p < 0.45 where p is an inner perimeter at said base to delimit the receiving compartment and r is a curvature radius as measured at any one of the four rounded corners (RC1, RC2, RC3, RC4) - a bag , made of flexible plastic material, provided with a bottom wall , a top wall and a tubular sidewall extending from the bottom wall to the top wall so that the bag delimits an interior volume (V); - a tank (T) provided with a base that is rectangular, the tank (T) comprising a side wall that is rigid and provided with four panels extending upwardly from the base , in order to delimit a receiving compartment; - a stirring device located in the interior volume (V) and adapted to be driven from below, at the opposite from the top wall ; wherein the bag is arranged above the base and between the four panels , in the receiving compartment, wherein the bag is configured such that, in an unfolded configuration of the bag with the liquid present inside the bag, has a parallelepiped configuration, wherein the bag is configured such that, once placed inside the tank (T) and unfolded according to the unfolded configuration, is: - extending vertically, parallel to a central axis (X) of the tank (T) that intersects an upper opening (O) of the tank (T), - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels , and - satisfying the following relationship: V2 ≥ 1500 L where V2 is the capacity of the bag , - extending vertically, parallel to a central axis (X) of the tank (T) that intersects an upper opening (O) of the tank (T), - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels , and - satisfying the following relationship: V2 ≥ 1500 L where V2 is the capacity of the bag , characterized in that the bag includes four rounded corners (RC1, RC2, RC3, RC4) that are each spaced from the side wall of the tank (T), and satisfies the following relationship: 0.1 < 4 r / p < 0.45 where p is an inner perimeter at said base to delimit the receiving compartment and r is a curvature radius as measured at any one of the four rounded corners (RC1, RC2, RC3, RC4) <EOS>