In 8051 9 bit uart mode is used when communicating with multiple devices what is the provision for the same in lpc2148
Again, that is a feature of the 8051 hardware - it has nothing specifically to do with Keil!
The 9-bit "multiprocessor" facilities can be handled equally well in any other development tools from any other manufacturer.
The interesting thing here, is of course that the user manual UM10120 which claims to be for LPC213x don't mention any 9th bit. What does the user manual for LPC2148 say? Or do you expect people to download user manuals for processors they don't use just to do your work?
If the processor don't have 9-bit transfer mode (which lots of processors don't), then it's extra interesting trying to use 9-bit transfers - basically playing with parity forced high or low on transmit and checking for parity error or not on receive... Anyway - how to implement 9-bit transfers isn't a Keil question but a processor question.