Comparing PD-PLUS with Other Simulators

Engineers evaluating PD-PLUS for their use will note the absence of a few features found in other simulators, such as facilities for handling systems with solids or electrolytes. Some engineers need these things, but most do not. Remember, PD-PLUS was not intended to have everything for everyone, but only what most need.

PD-PLUS offers the benefits of speed, robustness, simplicity of use, and cost, and is an ideal tool for many. However, if it lacks a capability that is absolutely required for a certain type of work you do, then another tool may be more appropriate, despite the extra cost. This is a decision you must make. These days, many companies find that one tool alone is not adequate for all needs. As is the case with many licensees of PD-PLUS, it could be used as a cost-effective tool for most of your needs, and another could be used as required. At least one of the high-end simulators now is offered for use over the Internet, on a pay-as-used basis. Others may be leased for short periods.

There is a natural tendency among process engineers investigating PD-PLUS to compare it with the dominant packages. This often is not an appropriate comparison to make, because these products are vastly different and address different classes of users. Still, people do make the comparisons, so some comments on how the products compare and differ are given below.

PD-PLUS and others are alike in that they are used to generate steady-state heat and material balances for chemical processes. Beyond that, it is more useful to summarize differences, in the following categories:

User interface - ease of use

The interface to a process simulator often is a very personal matter. Some engineers will not even consider a simulator if it does not have a Windows-based interface, particularly a menu-based system for entering information describing a model. Most simulators these days use a graphical and menu-based interface exclusively, centered around an on-screen process flow diagram (PFD). New pieces of equipment in the model are placed on the diagram, stream connections are made graphically, and then parameters for the streams and blocks are entered on forms. After calculations are completed, results may be viewed on forms, or a report file may be printed. Some like this paradigm, while others find it very clumsy and clearly prefer something more streamlined.

Although PD-PLUS has a Windows-based menu window for controlling the various aspects of building and running a model, it uses a keyword-based input language to describe input streams, flowsheet blocks, and everything else in the model. The keyword system is very easy to use and is quite readable, even to engineers who have never seen the program before. By comparison, earlier keyword input systems for other simulators were cryptic and difficult to use.

With PD-PLUS, defining a new block is a simple matter of typing in its description. As an example of a simple block describing a flash drum, imagine the time it would take to set up in another product what the following PD-PLUS input section does:


*FLASH D1002 'Cool effluent, separate in Separator Drum'



 ;Piping detail must provide gravity drain to decanter

Inserting parts of other PD-PLUS models is a simple cut/paste operation with whatever file editor the user has selected. Comment fields and paragraphs of bulk remarks may be inserted liberally to make the resulting file self-documenting. In fact, engineers who work frequently with multiple variations on a few basic processes normally build a new one by copying parts of similar models done before. A good archive of previous work becomes very useful. A model built using keyword format is virtually incorruptible, version independent, and easy to review with any text browser.

In most other simulators, the on-screen (PFD) is the only way to introduce new equipment and connect it to other blocks. Merging selected pieces of different models to create a new one often is very awkward and time-consuming. Also, users of such programs learn to save their models frequently in backup files, because the working files are prone to becoming corrupted often as changes are accumulated.

Users of simulation programs normally make repeated changes to process configurations and conditions in the course of developing the process or debugging the model. The speed with which such changes can be made with the PD-PLUS keyword scheme, compared with the time it takes using a graphical/menu system, becomes very important when the simulator is used extensively. Of course, a new user must become familiar with the PD-PLUS input system in order to use it, but this is true for any program. An organized course in using one of the others is a practical necessity for being able to use it, yet new PD-PLUS users routinely pick up the keyword language on their own, with liberal assistance from the example problems provided.

Unit operations

Several of the other simulators really do have a great wealth of unit operation capability. PD-PLUS has all the operations normally encountered by most engineers, but there are some exceptions that could be important to you. For example, some provide three-phase distillation and simultaneous reaction with distillation, whereas PD-PLUS does not. Some have liquid-liquid extraction, but to do this in PD-PLUS requires use of multiple decanter blocks, connected by recycles. The technique does work, but it isn't very elegant. PD-PLUS also lacks the solids-handling capabilities found in some others.

On the other hand, some of the normal operations found in some simulators can be difficult to get to work properly. For distillation columns, for example, the column block found in one of the high-end simulators simply does not converge as well as the column block in PD-PLUS. Both use "inside-out" techniques, but the inner loops work differently. In PD-PLUS the ability to converge columns with specifications on product quality and columns with main and vent condensers is superior. Many engineers find that getting the other product to converge their columns requires good flow and temperature estimates. PD-PLUS is more forgiving and more robust in this regard. It is interesting to note that at least two other simulators have implemented the same method first introduced by Deerhaven back in 1985 (see "Origins of PD-PLUS").

There are other irritating things in some simulators that are not a problem in PD-PLUS. For example, at least two others do not like having a stream enter two different blocks, something occasionally very useful. Consider the case of a feed/bottoms exchanger connected to a column. The column bottoms could be a recycle ("tear") stream, but this creates unnecessary convergence complexity. It is more efficient to do the feed heating first, then the column, and finally the exchanger to cool the bottoms. Similar and more complicated modeling situations are easily imagined, but this simple case makes a point clear. With PD-PLUS, the feed can be passed first through an exchanger block with nothing on the hot side; some would call this a heater block. After the column block, the same feed stream can be passed through a two-sided exchanger to cool the bottoms. To do this with another simulator, one must do a stream-copy operation on the feed stream, feed the first copy to a heater block, and the second copy to the exchanger block. The PFD created for this sequence gives the appearance of there being a real splitting of the feed stream and a second heat exchanger in the process, reducing the usefulness of the drawing for describing the process itself. Also, the artificial copying of the feed stream is seen by many as a clumsy way of getting around an unnecessary program constraint.

The approach most other simulators seem to take is that everything in the model represents some real equipment and that the PFD the user is forced to construct represents the actual process. In reality, the way a process engineer models a process often has calculations that do not represent real equipment. Also, there are times when one real piece of equipment is described by multiple blocks in the model, and times when pieces of real equipment do not appear in the model at all. PD-PLUS takes the approach that the user knows what set of calculations he wants done, and any correspondence to real equipment is not the concern of the program. PD-PLUS does not produce a PFD, or any other graphical output, for that matter.

Physical property options

Some simulators appear to have every physical property option known to man. If the intent is to be everything for everyone, such a collection is necessary, but expensive. Keep in mind that PD-PLUS was developed to satisfy the needs of most process engineers at an affordable price.

The pure compound library that PD-PLUS uses contains information on over 1500 compounds. Provided with the program is a utility that allows addition of proprietary compounds to the library. In any model, there also may be both general non-library compounds and narrow-boiling hydrocarbon pseudo-components, such as may be used to represent refinery systems. A crude breakdown facility is provided for converting a hydrocarbon stream defined by a distillation curve into a set of such pseudo-components for use in the model.

The options for VLE K values include several for dealing with refinery, petrochemical, and cryogenic systems. There also are several models for nonideal liquid activity coefficients, for use with chemical systems. All simulators have such a range of options.

Some simulators insist that the system being modeled be described by one completely rigorous thermodynamic model. In the real world, there are very often chemical systems that defy description by such a model. For such systems, there may be measured data that cannot easily be fit to a model but which nonetheless may be useful in building a working model for design purposes. To accommodate such data, PD-PLUS allows input of tabular K value and enthalpy data, as a function of temperature, for any component. There also can be tables of partial pressure or heat of mixing as a function of both temperature and composition. Such tabular data may be used to supplement equations of state or other methods used for other components in the system. The philosophy used in PD-PLUS is to allow the user to combine rigorous thermodynamic models and empirical data to describe his system in a way not possible by either approach alone.

A complete list of physical property options is provided in "Technical Summary."

Graphical output options

Most other simulators provide various forms of graphical output, and, except perhaps for the PFD, much of this can be useful. As stated earlier, PD-PLUS does not produce a PFD, or any other graphical output. All output is in the form of text files. If graphical output is important for your use of a simulator, then a more expensive product may be a better choice for you.

Speed, size

PD-PLUS is arguably the speed champion among general chemical process simulators. The difference in speed may be only a factor of two or three compared to a few others, but actual measurements have shown it to be more than an order of magnitude faster than at least one of the dominant products. Such a dramatic difference in speed can make a real difference in the way you work with a simulator and let you be far more productive. For more details, giving actual timings for a number of typical process models, see "Why Choose PD-PLUS?"

Perhaps one of the reasons for the remarkable speed advantage of PD-PLUS lies in the compactness of the program. A great deal of effort has gone into writing highly efficient program code, with careful grouping of often-used calculations into a core set of subprograms. The program has been written according to what was thought to be simply good programming practice, yet the total size of the executable code is so small compared to that of other simulators that one has to wonder how well written the others were. Some advocates of PD-PLUS have humorously referred to the larger products as "fatware."

In fairness, other simulators do typically offer more unit operation options and graphical features, and that requires more code. Total disk space required for them typically is hundreds of megabytes, and real memory required for efficient execution of those simulators can be hundreds of megabytes. Fortunately, computers of today have such resources, although overhead associated with just loading programs that need these resources is not trivial.

In contrast, all of PD-PLUS, including the simulator, its utilities, compound databank file, and .pdf form of the user manual currently fit, in compressed format, in a single 3.9 megabyte installation program! Expanded, the files need about seven megabytes of disk space. Besides contributing to rapid loading and execution, the compact size of PD-PLUS is one reason it lends itself so well to imbedding in other applications, such as online data monitoring and process control. Trying to do this with a bulky, inefficient simulator is indeed a formidable task, far more difficult than with PD-PLUS.


Simply put, PD-PLUS is the least costly of any full-flowsheeting, steady-state chemical process simulator. The first copy license fee is only $1,995. That is a one-time cost, not an annual fee to be paid. In contrast, other simulators either have much higher prices or have annual lease costs an order of magnitude higher than the one-time cost of PD-PLUS. For more discussion of cost, see "Why Choose PD-PLUS?"