How to turn a book chapter into a publishable paper? I'm a PhD student in the field of operations management. I've just finished writing a chapter in a handbook and am thinking about rewriting it into a publishable paper. 

How much of the content can be the same and what else do I need to consider - such as empirical data, new conceptual development, etc.?
There is absolutely nothing wrong with using research you've already published, whether in a book chapter or any other format, as a basis for further research that could lead to a new publication. However, it sounds to me like the way you are approaching this is a classic example of putting the cart before the horse. 

Your goal should be to do new research that contributes something to your area, not to artificially boost your publication list by rehashing old material, which as I said in the comments is the definition of self-plagiarism. Once you have found something new to say beyond what you already wrote about in your book chapter, it will become naturally apparent to you when it makes sense to publish your findings, so the question you are asking here will likely never need to be asked. The point is that publishing is merely the last step of the process of doing research, and in some sense the least important step. It is the means to the end of communicating your research results, not the end unto itself.

As for the practical question of how much material from the book chapter you can include in a new publication, there are different schools of thought on this, with some people viewing any inclusion of substantially similar text from earlier publications, especially without an explicit citation and mention of the similarities, quite negatively. I advise you to consult your advisor or a senior colleague who is familiar with the norms in your area about such things.