Chemical Engineering

Chemical Engineering

# Mass and Energy Balances in Plant Design

Though early chapters have a lot to say about the shortcomings of engineering education, most of the book explains in detail how to go about designing process plants. MS Excel is the workhorse of engineering design, for reasons I explain within the book. I explain precisely how it is used by Process Plant designers in professional practice (and why), as exemplified by this excerpt from Chapter 8:

“Here is how I set it out, which is pretty similar to the way most other professionals do. You don’t have to do it the way I do, but you do need to address the problems I have done at least as robustly as I have. Much of what professional engineers do habitually is intended to systematically avoid error, and make it easy to spot any residual errors.

Mass and energy balances like these are pretty complicated process models, which are very hard to hold in your head as a completely unified whole. Even the best examples will need rigorous double-checking by a second competent engineer, and you should assume that there will be mistakes before such checking.

I like to make my Excel spreadsheet look like the engineer’s calculation pads we used to use back when we did hand calcs, and I teach my students to do this as well. If I do not do this, I get given spreadsheets which are hard to follow, with bits of calculation over in some obscure corner of the spreadsheet which no one notices.

Forcing the calculations into a succession of virtual sequential A4 sheets makes it easy to set out an annotated logical argument, and to follow the argument being made. So I recommend a vertical stack of these virtual pages be set out on each Excel tab.

I start with a header tab which sets out the given and assumed design parameters and gives an overview of the whole spreadsheet. All uses of these design parameters throughout the spreadsheet should be linked directly back to this header page. This makes it easy to vary the parameters to generate different scenarios.

If the cells containing the parameters have been labelled with descriptive names, the designer and the checker will not need to keep flicking back to the header to see what a parameter is when it is encountered on other sheets. I would in fact recommend that all cells whose values are copied across into calculations are so labelled to reduce errors and facilitate checking.

Each subsequent tab has a stack of pages which represent the design of a unit operation. I like to follow the order of the process flow on the P&ID in my virtual stack of Excel tabs. The mass and energy flows out of each unit operation should be (with the exception of those requiring the breaking of a recycle to avoid circular calculation errors- see later) directly linked to the inputs to the next operation.

This makes the spreadsheet capable of self-modifying to handle the various scenarios which it will be used to balance, removing the possibility that the designer will forget to change cells.

Each of these tabs will have what I call checksums, calculations arranged so that they will be zero if the unit operation’s mass balance is correct. These checksums will be carried across the header tab, where they will appear as a single table, along with checksums for mass and energy balance checks at scales above the single unit operation.

Behind these mass and energy balance/ unit operation sizing tabs I would usually have a few sheets of hydraulic calculations, so that I can include dynamic pump calcs based on the flows from the mass and energy balance in the spreadsheet for convenience. I have standard single-tab Excel spreadsheets (verified by an independent third party and then locked) which I insert for this purpose.”

The important thing about any model of a process is that both you and your fellow engineers understand it. A simpler but more well understood model is superior to a complex one which no-one really understands, as I say elsewhere in the book.

Checkers also need to be able to verify any assumptions made, and see where any data input to the model comes from, which is why so much attention is paid to making the spreadsheet checkable. There should be a lot more words than calculations in a good excel spreadsheet.

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