Exercise 2-1: Basic system data and the particle size distribution of the feed.
The specification of the data will be illustrated using a simple exercise based on simulating the performance of a single hydrocyclone.
Start a new job in MODSIM and draw a flowsheet containing only a single hydrocyclone. Add a stream for the feed and one each for the overflow and the underflow. Add flyouts to each of the streams. The flowsheet should look something like this. You will probably need to move the flyouts to make the flowsheet look a bit neater.
Accept the flowsheet, save the job and start to edit the system data.
The material to be processed will contain only 1 mineral - silica of specific gravity 2.7. The density of the solid material will influence behavior of the particles in most units and this must be specified before any simulation is attempted. Enter this information on the form. Choose to specify specific gravity by mineral.
The basis of the population balance method is the calculation of the behavior of each type of particle in the equipment that is being simulated. To do this, the particle population is conceptually divided into many classes in such a way that the particles in a single class are all similar to each other. The behavior of each class of material is then calculated and the overall behavior is calculate by accumulating the results from each particle class. In general the calculations produce more accurate results the more classes that are used and the narrower the class intervals. But of course the amount of calculational effort will increase as the number of classes increases. MODSIM allows particle populations to be distributed in up to 3 dimensions. The first dimension describes the particle size. The second dimension describes the mineral composition or grade of the particles and the third dimension is available to describe any other particle property that might be useful in any particular simulation. It is necessary therefore to consider carefully what level of detail should be used to describe the solid material so that the results of the simulation are useful and meaningful. In this exercise we will simulate the classification of a homogeneous solid so that only the particle size is relevant. The solid phase should be described by as many size classes as possible to enable MODSIM to model the hydrocyclone in sufficient detail. The student version of MODSIM allows a maximum of 25 size classes so choose this value for this exercise. Because no other properties of the solid are relevant in this exercise, no further distribution of the particle population need be considered. Thus the number of grade classes will be 1 and the number of S-classes will also be 1. MODSIM also requires a specification of the largest particle size that will be relevant in the problem. MODSIM uses this size to set up the size classes that it will use internally to do the modeling and simulation calculations. The classes are contructed using a square root of 2 geometrical sequence starting at the largest size and working downward for the specified number of size classes. In this exercise a largest size of 2mm will be satisfactory. Please note that this size must be specified in m (ie 0.002 in this case). As a general rule MODSIM allows you to specify data in a variety of units but in some cases this would clutter the forms too much and when no alternatives for units are specified you should use the SI system. Internally MODSIM uses only SI units.
This is sufficient data to specify the ore characteristics for this exercise and you should complete the ore characteristics section of the system data form. The system data form looks like this.
The hydrocyclone is primarily a size classification device and obviously the size distribution in the feed material will be of particular importance and must be specified before the operation of the hydrocyclone can be simulated. Other important properties of the feed will be the solids content and the feed rate.
The feed material for this exercise is silica having a size distribution given in the table below.
|Mesh size microns||850||600||425||300||212||150||106||75||53||38|
The feed rate will be set at 50 tonnes/hr of solid and the feed slurry will contain 40% solids by mass.
Since these data items are specific to the material in the feed stream, they must be attached to the appropriate stream in the flowsheet. All the streams are shown in one or other box on the right hand side of the sytem data form. You should now double click on the feed stream number in the feed stream box in order to specify the properties of the feed.
The first thing to do is give the stream a name so that it can be identified easily later.
The next thing to do is to specify the size distribution. Note that the experimental data for the size distribution that is usually determined by screening will include less mesh sizes than the number that you previously specified for internal computations. You can specify just as many mesh sizes as are available and MODSIM will interpolate your data to match the internal size distributions. In the table above there are 11 size intervals (note not 10 as you might think if you count the columns in the table because the oversize on the top screen is not shown explicitly). So specify 11 mesh sizes in the form. Clear the default data and type in the data from the table. Remember to choose the units of size that you are using.
When this is complete, specify the feed rate and the percent solids in the feed in the appropriate boxes on the form.
Note that all other selections on the form are greyed out in this case because they are not required and MODSIM always tries to help the user to specify only the data that is necessary for the problem on hand. The feed data input screen should look like this.
Accept the data when you are satisfied that it is what you want.
Finally you should name the overflow and underflow streams for future reference. To do this double click on the stream number in the "Internal and product streams" box. This will open the "Particle size distribution" form for the chosen stream. Specify the name for the stream and click Accept to return to the previous form.
Save the job. This completes this exercise. Now go to exercise 2-2 which you will find in objective 2 for this module.