已知 $F_1、F_2$是椭圆 $C:\frac{x^2}{a^2}+\frac{y^2}{b^2}=1\left(a>b>0\right)$的两个焦点, $P$为椭圆 $C$上一点,且 $\stackrel{\rightharpoonup }{P{F}_1}\perp \stackrel{\rightharpoonup }{P{F}_2}$.若 $∆P{F}_1{F}_2$的面积为9,则 $b=$     .