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; - a tank provided with a base that is rectangular, the tank comprising a side wall that is rigid and provided with four panels (15a, 15b, 15c, 15d) extending upwardly from the base, in order to delimit a receiving compartment; - a stirring device located in the interior volume 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 (15a, 15b, 15c, 15d), 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 and unfolded according to the unfolded configuration, is: - extending vertically, parallel to a central axis of the tank that intersects an upper opening of the tank, - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels (15a, 15b, 15c, 15d), 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, 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; - a tank provided with a base that is rectangular, the tank comprising a side wall that is rigid and provided with four panels (15a, 15b, 15c, 15d) extending upwardly from the base, in order to delimit a receiving compartment; - a stirring device located in the interior volume 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 (15a, 15b, 15c, 15d), 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 and unfolded according to the unfolded configuration, is: - extending vertically, parallel to a central axis of the tank that intersects an upper opening of the tank, - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels (15a, 15b, 15c, 15d), and - satisfying the following relationship: V2 ≥ 1500 L where V2 is the capacity of the bag, - extending vertically, parallel to a central axis of the tank that intersects an upper opening of the tank, - provided with a rectangular cross section, the rectangular cross-section being in correspondence with rectangular distribution of the four panels (15a, 15b, 15c, 15d), 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, 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)