APPLICATION OF FLOW SHEET SIMULATION

LITERATURE REVIEW

2.6 APPLICATION OF FLOW SHEET SIMULATION

• Simulation of Existing Plants: Simulation of an existing plant may be very useful when there is a need for changing the operating conditions. The simulation is helpful to find the best strategy for raising the production, to enhance the efficiency of operation through better energy integration, to adapt the existing plant to a different raw material, or to a requirement for products with different compositions.

The major benefit of process simulation has been that it produces better process designs with lower capital and operating costs in less time than hand methods alone hand methods in combination with stand alone computer programs. The Monsanto Company (Monsanto, 1974) estimates the saving obtained by using the FLOWTRAN simulation program at 28 man-months in one project, and a potential yearly net gain of $750,000 in increased production in another one. However, there are several other intangible, yet highly important, benefits that are perhaps less well understood. These are the transferability of process simulations and the improved communication via the use of a process simulation. The different engineering groups, which deal with the same plant in different stages, may use the simulator as a basis for faster and improved communication.

Briddell (1974a, b) summarizes the most severe pitfalls in the use of simulation. Among these are carrying out a simulation for its own sake, not having clear objectives, being overly rigorous, not knowing how much to assume, and not working with the people who have the problem. For proper use of simulation, Briddell proposes to judge,the necessity of computer simulation and the degree of sophistication that must be employed against a realistic cost estimate.

The main use of simulation programs is within' the company developing the program since the team, which developed the program can use it most easily. New users may face difficulties in training the people who will use the systems. In addition, problems may

arise in communication with the developers when there are difficulties in the operation of the simulation program or interpretation of results.

The extension of the study of process simulation in the universities in recent years may accelerate the use of process simulation in the chemical industry in succeeding years.

Monsanto Co. has made available its FLOWTRAN system to aid universities in such training (Institute News, 1974). For continuing research in process simulation, it is important to remember the main types of industrial uses and the most critical problems of the industrial user. Despite the fact that industrial process simulators have the capability of handling a large set of interacting units, much use is made of them on comparatively simple processes. Often, simply a material balance (and not an energy balance) has been adequate for the application. Using the physical property capabilities of the system for simple condensation curves has been another frequent use of simulators.

Morris et al[25] discussed the features of a process simulator program that enhances the' conversational abilities of engineering minicomputers and enables doing flowsheet design in a natural manner.

Although considerable attention has been given to the problem academically, recycle calculations, in terms of which stream to tear, have rarely caused much of a problem. The engineer, using his process knowledge, can generally select suitable tear streams.

Convergence acceleration, although always desirable, has not been of major concern since the bounded Wegstein method has proven satisfactory in a broad range of industrial simulators. Major concern in industrial process simulation has been with the area of physical properties (especially at cryogenic temperatures). Developing and using these properties over large ranges of temperature and pressure has often been the major stumbling block to a successful simulation.

This section describes some of the ways that flowsheet simulation has helped companies in process development, in plant design and retrofitting, and in plant operations. [2]

2.6.1 A New Way of Doing Process Development

A leading chemical manufacturer in the United States, interested in diversifying into new products, was developing a process to make a new polymer. At the early conceptual stages, the development team prepared a flowsheet simulation model of the process, including preliminary costing and economics. Pilot-plant experiments were used to determine parameters in the model. The model was then used to optimize the process and to assess its economics.

This represented a new way of doing development in that company. The pilot plant was not used to prove the workability of the process that was done on the computer. Instead, the pilot-plant experimental program was used to get the data needed for the model, which involved experiments specifically designed for this purpose. Thus the flow sheet model drove the development program. Armed with the results of the flowsheet model, the development team was able to get management approval to go ahead with plant construction in record time.

The plant was constructed and successfully brought on stream with much less than the usual number of start-up problems. The leader of the development team was promoted to Director of Process Development and is now a Vice President with the company.

2.6.2 Speed Commercialization

In another example, a specialty chemical manufacturer was interested in bringing a new high-value product to market as soon as possible. They used flow sheet simulation to model the process and determine the preliminary process economics. The project was highly attractive.

Because of the confidence gained with the model, the company decided to bypass pilot- plant work and go directly from bench-scale experiments into commercial production. As a result, the product was brought to market 18 months ahead of schedule.

Because of the company's head start, a major competitor decided not to enter the market.

As a result, the company enjoyed increased revenues measured in the millions of dollars and was able to establish a dominant position in the market.

2.6.3 Retrofitting of a Liquid-Extraction Process

A chemicals division of FMC Corporation (Gruber, 1985) was designing a process, shown in Figure 2.6.3, to extract an organic compound (compound A) from an aqueous

Water

A (mildly polar) B (highly polar)

Stage Stage Stage ~ Back

I 2

-

³ Extraction

Xylene"

.

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Figure 2.6.3: Flowsheet of liquid-extraction process

Xylene

Rest of Process SolutIOnof A

in xylene

solution. The solution also contained an organic impurity (compound B); compound A was mildly polar, and compound B was highly polar. Xylene was chosen as the extracting agent, because A is much more soluble in xylene than B and the goal was to extract A from the water without removing B.

The conventional wisdom was to use as little xylene in the extraction step as possible so that the highly soluble A would dissolve in the xylene and not B. This solution of A in xylene was used downstream in the process and since quite a lot of xylene was required, more would be added later in the process. Since B was undesired, any significant amount ofB in the xylene would have to be removed.

To determine the optimum amount of solvent to use, the engineers developed a flowsheet model of the extraction process. They found that, contrary to expectations, the more xylene they used, and the less B was extracted. Since xylene was added later in the process anyway, adding this xylene in the extraction to decrease the undesired B was not a problem.

The reason for the change in extraction strategy was that the simulation showed that the more concentrated solution of A in xylene was more polar; hence it was able to dissolve more B than the dilute solution could, which was more like xylene and therefore less attractive to B.

This finding had serious implications for the process. By using a large amount of xylene in the extraction, the engineers were able to keep the amount of B dissolved low, therefore eliminating a whole back-extraction process and the associated train of equipment.

The engineers verified the results of the flowsheet simulation with a quick pilot-plant study. Without the flowsheet simulation, the engineers would not even have thought to try this approach. The whole study required about 2 days of work and resulted in a saving of

$250,000 in capital investment.

2.6.4 Ammonia Plant Operations

The Chevron Chemicals Company (Moore et aI., 1985) reported their experience of using a flowsheet simulation model of a 1500-ton-per-day ammonia plant located on the Gulf Coast of the United States. Plant engineers run the model on a regular basis to determine the effect of changing key operating parameters and to find the optimum operating conditions. These conditions change with variations in feedstock and ambient conditions.

Chevron engineers report savings in excess of $1,000,000 per year from reduced operating costs as a result of using the model.

2.6.5 Design of a Feed-Forward Control System

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Another chemical company used a flowsheet simulation model to develop a feed-forward process control model of an energy-intensive distillation column. The goal of the process control system was to measure disturbances, such as changes in feed-stream composition, upstream of the process and then to adjust operating variables, such as the reflux rate, to correct for the effect of the disturbances before they had a chance to affect the process.

The control engineers made a series of flowsheet simulation runs to determine the effect of changes in input variables to the process on the output variables. With the information, they were able to implement a feed-forward control scheme that resulted in a 27%

reduction in steam usage and improved product quality and consistency. The company

had several columns of this type around the world and total savings were measured in the hundreds of thousands of dollars per year. The flowsheet model was also used to

"debottleneck" the overall process.

2.6.6 Competitive Evaluation

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An U.S. producer of commodity chemicals, who is the most efficient in the industry, developed. a flowsheet model of their competitor's process. They use the model to estimate the competitor's manufacturing cost. This in formation enables that the U.S.

producer to set the price of their product at a level just low enough that the competitor's profit margin is unsatisfactory, but which results in good business for the U.S. company.

2.6.7 Reverse Engineering

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A chemical manufacturer located in the Far East licenses a process from an engineering firm on a turnkey basis. The engineering firm designs the process, but does not provide technical details. They license the process on a "black box" or performance basis.

The company developed a flowsheet model of the process. They were able to "back- calculate" the important process parameters. With this model, the company was able to explore the effect of process modifications and changes in operating conditions.

Armed with this information, the company was better able to negotiate with the process licensor and was able to obtain a license for a plant expansion under much more favorable terms than they had before.

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2.6.8 Raw-Material Bidding

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An aluminum manufacturer purchases raw materials (bauxite) on the spot market. They have developed a flowsheet simulation model of the alumina refining process. They use their flowsheet model to determine what their manufacturing costs would be using raw materials with the specifications available. This tells the company how much they can afford to bid for the raw material.

2.6.9 General Comments

These examples are intended to provide a feeling for how models of processes are used on an everyday basis in the process industries to make better engineering and business decisions. Rigorous process models produce a powerful economic benefit for the companies that use them. These benefits are in terms of reduced manufacturing costs, improved plant throughput and efficiency, and reduced engineering risks.