Introduction to Chemical Engineering

Introduction to Chemical Engineering

September 7, 2013
I have been updating the material on use of Aspen Plus to include version 8.0-8.2. The way you get to the various windows is somewhat different, although most of the operating windows look very similar. Thus, some changes are needed. These will be completed about October 1st and posted on the Wiley website, and perhaps elsewhere.

I also received a pleasant surprise recently – the Korean translation of the book. Shown above are various versions of Introduction to Chemical Engineering Computing, the first edition, Chinese and Korean translations of it, and the 2nd edition.

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July 31, 2013

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Last week I attended part of the Summer Institute run by the Center for Process and Analytical Chemistry (CPAC – but not the political one!) (www.cpac.washington.edu). While I didn’t attend all the sessions (one of the privileges of being retired), I did hear some interesting talks. Kurt VandenBussche, UOP Honeywell, talked about big data in the petroleum industry. Apparently, companies are collecting billions of data points per day. This is then analyzed to decide what are the most profitable products to make from a given source, crude oil or from a bio-refinery. Sometimes this is done using science (Composition Based Modeling, Stephan Jaffe, ExxonMobil) based on properties of molecular segments, but sometimes it is just based on statistics and probability, with no understanding of the underlying reason. So much for chemistry classes! I joked that UOP has become the NSA of refining, since they have so much data available. (Aside: My own suggestion to the NSA to avoid people taking data out of the building with flash drives is to walk around the building while chewing gum, and put gum in all the flash drive sockets.)

The trend worldwide is that crude is becoming, on average, heavier with more sulfur. Just the opposite is happening in North Dakota, where the crude is lighter and has less sulfur. 

Another talk by Ray Chrisman of Atochemis srl (Italy) talked about continuous micro-processing (i.e. where micrdofluidics plays a role), in which he is an expert. He emphasized the need to spend more effort on separation and purification in small, easily-reproducible devices for extraction, crystallization, distillation, membrane separation, and chromatography. 

Apparently when making isobutanol in a biorefinery, the broth must be removed and the isobutanol removed, and the broth recycled, since the isobutanol kills organisms. Thus, attention goes beyond the chemical reactor and includes the separation system as an integral part of the reactor.

There were a few references to some continuous microprocessor systems that have been scaled up by adding identical systems. The idea of microprocessors is that you learn how to make one small one that has all the mixing, reaction, and separation, then make many to run in parallel, taking advantage of the processing techniques developed by the computer chip manufacturers. This is the subject of a book that contains an article written with my undergraduate research students about transport in microfluidic devices.

Bruce A. Finlayson, Pawel W. Drapala, Matt Gebhardt, Michael D. Harrison, Bryan Johnson, Marlina Lukman, Suwimol Kunaridtipol, Trevor Plaisted, Zachary Tyree, Jeremy VanBuren, Albert Witarsa, “Micro-component flow characterization,” Ch. in Micro-Instrumentation, (M. Koch, K.Vanden Bussche, R. Chrisman (ed.), Wiley, 2007).

More info about my microfluidic work is on my University web site: http://faculty.washington.edu/finlayso/ and choose papers. See also the book website, http://www.ChemEComp.com/, since many chapters of the book are devoted to flow and mass transfer in microdevices.

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Chemical Reactor with Mass Transfer Limitations – in Comsol

In chemical reactors with a catalyst, sometimes the rate of reaction is governed by the concentration on/in the catalyst, which can be different from the concentration in the bulk fluid. This happens when the rate of mass transfer is not fast enough to get the material to the catalyst as fast as it is reacting. One cause is the boundary layer around the catalyst due to fluid flow. While that can be a complicated problem, we usually just lump everything together and define a mass transfer coefficient that defines the rate that the chemical passes through the boundary layer, usually depending on the difference between the concentration in the bulk stream and on the catalyst. Then one must solve an algebraic equation to find the concentration on the catalyst. This is done in the book using MATLAB, p. 155. It can be done in Comsol, too, using the feature to solve algebraic equations.  The latest addition to the book website, www.ChemEComp.com, shows how to do this. The example, while simple, shows how to solve ordinary differential equations as initial value problems along with an algebraic equation that must be satisfied at each point along the reactor. In this case, as we found with some of the mass balance problems, the initial guess of the variables defined by algebraic equations is important and some experimentation was necessary, but it was easily done.

Go to the book web site, www. ChemEComp.com and look at Download or chemical reactor with mass transfer.

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System Mass Balances in Excel

I’ve added some more material to the web site – solving mass balances for processes with recycle streams, using Comsol. In process simulators, one usually solves the equations by taking the input to a unit, solving that unit, taking the output and putting it into the next unit. If there are no recycle streams, you are done. If there are recycle streams, though, you do this over and over until the problem converges (it usually does). When using Comsol, however, you are writing algebraic equations like you would in Excel (i.e. like the formulas you’d put in the cells there). Then you must solve those algebraic equations, which are usually nonlinear. Sometimes that type of calculation does not converge, and you need to supply a reasonable guess to get a solution. You can see in the example when that was necessary for the problems solved. Go to the book web site, www. ChemEComp.com and look at Download or system mass balances.

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How to solve CSTRs in Comsol

I’ve added some material to the web site – solving equations for Continuous Stirred Tank Reactors (CSTR) using Comsol. There are some things you have to be careful of when the parameters lead to multiple solutions or to no steady solutions at all. This material repeats, using Comsol, what is in the book on pages 161-168: steady CSTRs, transient CSTRs, and ones with weird oscillations. In the latter case one must adjust tolerances in the numerical integration, and the writeup shows how to do that. Here is one of the graphs that indicates a problem.Image

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For Beginners with Comsol and with Excel

If you are a new user of Comsol Multiphysics, there is a nifty way to work through the first example in the book. A pdf version of the first example in Chapter 9 (heat conduction) is available at www.ChemEComp.com. You can download that and put it on the left side of your screen, and then open Comsol and put it on the right. Then just follow along the steps shown in the book, one by one.

Also, I’ve put up an expanded Appendix A, Excel. The second edition assumes that most users have used Excel before, but all haven’t. Thus the pdf version on the web site is an expanded one, like that in the first edition but redone for more modern versions of Excel. So that is a place to start if you are not a whiz with Excel.

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New Resources Added to Website

I’ve added new resources to the website, chemecomp.com. There you will find Problem 9.26 worked. This is a problem to examine the proposition that if one has a transport problem with a large velocity (Peclet number) it is easier if one converts to gamma= exp(c). You can see there the amount of improvement.

Also available are the meshes for the gradient problem in 11.29. This is used for creating a concentration profile across a width. While the geometry isn’t hard to create, it is tedious, so I’ve done it for you. Also there is the mesh for the hole pressure problem (10.28) and the serpentine mixer (text in Ch. 10 and 11 and problems 10.26 and 11.24.

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