Sale of part of assets and treatment in books

As per your information, we can get the aquisition cost of Palnt through that Invoice and Calculate the depreciation as if that the only plant by applying the same rate what you used for entire Block and arrive the figure after providing for 10 years

To account for the same, write off the WDV of plant Replaced (Show Net, If realised any value on scrap) 

Capitalize the new Plant Purchased

For Calculating Depreciation as per Income Tax act it will be little different

You can get Cost of that machinery through invoice and then can work out profit or loss