I use scanimage rather than simple-scan but there the hpaio phrase must be in quotes.
In ~/.kshrc I have a function which works fine
scan() {
if [[ "$1" = "" ]] ; \
then echo "Please enter a file number" ; return 1; \
fi ; \
/usr/local/bin/scanimage -v -p \
--device='hpaio:/net/Deskjet_2540_series?ip=192.168.1.132' \
--format jpeg -x 210 -y 297 --resolution=300 --mode=color \
>$1.jpg
}
Best
