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PD-PLUS : Chemical Process Simulator
price : 1990 $

What it is

PD-Plus is a PC-based program for simulating chemical processes, including refinery systems and non-ideal chemicals. It handles multiple-operation flowsheets with recycle streams. An interactive mode provides complete user control of the simulation and rapid evaluation of different process conditions. PD-Plus may be interfaced with or driven by other software packages easily.

Example of PD-PLUS running in Windows 2000 (DOS window)

PD-PLUS

Technical Summary

PD-PLUS is a very fast, large-capacity, exceedingly robust, PC- based program for simulating chemical processes, including refinery systems and non-ideal chemicals. It handles multiple-operation flowsheets with recycle streams. An interactive mode provides complete user control of the simulation and rapid evaluation of different process conditions.
All operations allow multiple feed streams and in general can produce multiple products, except where noted. Complete heat balancing is generally done, but can be suppressed where not needed for calculation efficiency.

  • Column, rigorous simulation of all types of columns
  • Flash, vapor-liquid equilibrium flash, can separate a pure liquid water phase, heat may be added or removed, pressure drop may be given
  • Mixer, multiple streams are mixed to form a single product
  • Split, multiple product streams are split from the total feed
  • Separator, composition of each product is specified by fractions of the feed components to be split off
  • Heat Exchanger, a single stream is heated or cooled, or two streams exchange heat
  • LNG, Plate Fin Heat Exchanger
  • Reactor, multiple equations with extents of reaction are handled
  • Compressor, calculates power to compress a gas, permits multiple stages operations with interstage cooling
  • Expander, calculates work available from expansion of a gas
  • Pump, calculates power and temperature rise for pumping a liquid stream
  • Feedback Controller, varies a stream or block parameter to obtain a result
  • Calculator, in-line Fortran blocks
  • Decanter, separates a feed into two immiscible phases, using fixed K-values or activity coefficents.

Distillation - Column Block, one great strength of PD-PLUS is the column block, which permits simulation of a wide range of column types, including fractionators, absorbers, strippers, crude and vacuum columns. Convergence is exceedingly reliable and rapid. Initial estimates very close to the final answer normally are not required.
The configuration may be a single column or a main column with one or more sidestrippers. There may be multiple feeds to the column, and more than one feed to any stage. Feed stream flows may be varied to meet performance specifications. There may be a single vapor sidedraw, a single liquid sidedraw, and heat added or removed at any stage. There may be a stripping gas stream or a reboiler at the bottom stage of a sidestripper. The main column may have pumparound coolers, returning one or more stages above or below the draw stage. Heat duties may be fixed or varied to meet specifications. Product and internal flow rates may be varied to meet specifications; or, any flow may be fixed in absolute mole, weight, or liquid volume units or as a fraction of a particular feed or the total feed, on mole, weight, or volume basis, wet or dry. Flows estimated as initial values may be in any units. Pressures may be specified on individual stages; the program will interpolate for other stage pressures. Temperature may be specified or estimated at any stage. Subcooled reflux temperature may be specified. TBP, ASTM D86, or ASTM D1160 distillation temperature for any volume fraction distilled may be specified for a product stream. The definitions of initial and end points may be adjusted. Tables of TBP, D86, and D1160 distillation curves may be printed for all streams. Free water may be allowed to form and be decanted at the reflux drum when dealing with hydrocarbon separations. Internal or external reflux ratio may be specified. Liquid/vapor mole flow ratio may be specified at any stage. The absolute mole, weight, or volume fraction of a single component or group of components in a product stream may be specified. The mole, weight, or volume fraction recovery (loss) one component or group of components into a product stream may be specified. API gravity or Reid or true vapor pressure of a product stream may be specified. An extremely rapid "superfractionator" convergence option is provided for solving columns having up to 350 stages.

Components, Pure Component Library, components may be retrieved by number from a library. Non-library components may be defined by giving their necessary properties. Hydrocarbon fraction pseudocomponents may defined, and the program will estimate their properties from boiling point and gravity. There is a choice of API Technical Data Book (1980, 1987), Kesler-Lee (1976), and Winn (1957) methods for predicting fraction properties. Water need not be first or last. A separate crude breakdown program is provided to convert a laboratory TBP or ASTM D86 or D1160 distillation into a set of defined light ends plus narrow-boiling pseudocomponents. Blending of multiple assays is allowed.
The large pure compound library provided with the full PD-PLUS contains properties for 1,508 compounds of all types. This library includes information from the AIChE/DIPPR Data Compilation Project, the API Technical Data Book, Petroleum Refining, and other sources. The file maintenance program allows changing existing data and adding new compounds to customize the databank. The program always allows non-databank compounds to be defined for any problem.

K values may be calculated by "data generator," by ideal K values plus liquid activity coefficients, or by user-supplied tabular data. Tabular data may be used to override data generators for selected components. Such data may be in the form of simple pseudo-vapor pressure vs. temperature, or there may be curves of partial pressure vs. composition at multiple temperatures, for double-interpolation. Methods for vapor and liquid fugacity and liquid activity may be specified by general system name or separately.
Data generators for hydrocarbon systems include Maxwell-Bonnell vapor pressures, Soave-Redlich-Kwong (API), and Graysoneed. Vapor fugacity options include Soave, Redlich-Kwong, and ideal. For liquid fugacity there are Soave, Graysoneed, and ideal. For nonideal liquid activity, there are Wilson, NRTL, UNIQUAC, and regular solution. There is also a sour water VLE option.
The K value of water out of hydrocarbons may be estimated using vapor pressure and the solubility in kerosene, if desired. User- provided K data may be in the form of actual data points (two to six values) or equation coefficients. Coefficients may be for the Antoine equation or the DIPPR (AIChE) vapor pressure equation (five coefficients).

Enthalpy options, enthalpies may be generated automatically for hydrocarbon systems using API Soave-Redlich-Kwong, Lee-Kesler, API (similar to older Johnson-Grayson), or ideal gas enthalpy plus latent heat. Tabular data points (two to six points) may be given for one or more components, even when a method such as SRK is used for the others. Tabular heat of mixing data may be given for highly unusual systems.

PRODE and PD-PLUS

Prode distributes a broad range of software products to process industries in Italy, we found PD-PLUS a low cost, very competitive product and began to propose this simulator in 1996, since then we have had quite good results and according with the author (Richard Russel) we began to promote this software via internet, at present we do not market this product outside Europe.

Compare PD-PLUS with other simulators

Engineers evaluating PD-PLUS for their use will note the absence of a few features found in other simulators. Included are 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:

  • The 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-style interface. 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.
    PD-PLUS uses a keyword-based input language to describe input streams, flowsheet blocks, and everything else. 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'
    FEED IS STREAM 12A, VAPOR IS STREAM 13,
    LIQUID IS STREAM 14, TEMP= 40 C, PRESSURE= 150 KPAG,
    ;Piping detail must provide gravity drain to decanter

    Inserting parts of other PD-PLUS models is a simple cut/paste operation with a file editor. 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 in keyword format is virtually incorruptible, version independent, and easy to review with any text browser.
    In the typical Windows-based simulator, 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 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 what some deem the dominant simulator 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.
    There are other irritating things in some simulators that are not a problem in PD-PLUS. For example, at least one other does 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 program 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.
  • 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.
    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 calculation options and graphical features, and that requires more code. Still, total disk space required for them typically is hundreds of megabytes, and real memory required for efficient execution of those simulators can be 64 or 128K 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, and compound databank file, currently fit, in compressed format, on a single 1.44 megabyte diskette! Expanded, the files need just over three 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.
  • Cost, simply put, PD-PLUS is the least costly of any full-flowsheeting, steady-state chemical process simulator. There 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.
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