Role of Simulators in the Process Industry: An Overview and Simulation Case Study of a Chemical Process Plant

We can analyze how challenging the physical and chemical processes are when you are aiming to model them. The manual calculation, even using a digital calculator is too time-consuming. When engineers and technical personnel change variable values repeatedly for some optimization purpose on multiple inter-connected equipment, it becomes tedious and increases the possibility of big mistakes.

Process simulation is used to develop, design, analyze, and optimize technical processes such as chemical processes, complex manufacturing operations, environmental systems, biological processes, power stations, and similar technical functions.

Unit operations are positioned and connected to products or streams in flow diagrams representing process simulation software processes. The program or software solves the mass and energy balance to identify a stable operating point under certain conditions.

Models are used in process simulation to describe properties over a wide range of temperatures and pressures that may not be covered by the actual data that is currently available. These models make assumptions and approximations. Within specific parameters, models also permit extrapolation and interpolation and search possibilities outside the known range of properties.

This article will include details on how the process simulation approach is beneficial for the process industry not in terms of time-saving but also deliver cost advantages in calculating equipment design and optimizing processing parameters. This article will provide an example of important chemical process design and analysis related to energy optimization.

1 Modelling Operations

The basis of the continued development of the simulation software is the development of models for a better representation of actual processes. Models are generated using chemical engineering and control engineering principles to strengthen mathematical simulation techniques.

New and better models for calculating properties are being developed. This could describe, for instance:

• The thermophysical characteristics of pure components and mixtures include caloric data, vapor pressures, viscosities, etc.
• The properties of various equipment, such as separators, reactors, and distillation columns
• Information on the environment and safety

Models can be divided into two main categories:

1. Direct correlations and equations where the parameters are fitted to experimental results.
2. Predictive approaches that use approximated properties.

Usually, equations and correlations are preferred since they almost perfectly represent the condition. Experimental data is often gathered from factual data banks or, in the absence of publicly accessible data, from measurements to generate accurate parameters.

Comparatively speaking to experimental effort and data from data banks, predictive approaches are more cost-effective. These estimating approaches often introduce more mistakes than correlations derived from actual data. Hence, they are typically only employed in the early phases of the process development to locate first approximative solutions and to remove erroneous routes.

The development of mathematical models in the areas of mathematics and complicated problem solving has been assisted by process simulation.

2 Chemical Process Simulators

A process simulator is a software or program used for the modeling of the behavior of a chemical process in a steady state through assessing temperatures, pressures, flow rates, etc. The dynamic behavior of operations and control systems, as well as their reaction to operational disturbances, have all been extensively investigated by the computer programs used in process simulation. The market is now home to software that may be used for equipment size, cost estimation, properties assessment and analysis, operability analysis, and process optimization.

Process simulators enable the following:

• Predict the behavior of a process
• Analyze many cases simultaneously while altering the values of the critical operational variables
• Track a chemical plant during its entire useful life to anticipate expansions or process upgrades
• Change the conditions under which a new or existing plant’s function

The emergence and growth of digital informatics impacted the advancement of several branches of human knowledge. This development included chemical engineering, especially in its application to process modeling. The first attempt at mathematical modeling date back 1950s when the FORTRAN programming language was introduced (FORmula TRANslating). The first process simulator, known as FLOWTRAN, then appeared in the 1970s, marking the start of an ongoing research effort led primarily by academics and occasionally supported by the industry. This research was directed toward increasing the profitability of process operations and providing quick access to alternative evaluations. Process simulators are pre-built subroutine or model libraries that adhere to FORTRAN, C, or Visual Basic procedures for solving equations. The terms “procedures,” “blocks,” or “models” refer to the subroutines or models. Process engineers need to be aware of the rules and presumptions behind each simulator’s models to utilize them effectively. The user guides outline these presumptions. Additionally, it is crucial to consider the models and criteria used to specify phase equilibrium since they impact how accurate the simulations’ results are.

The commercial and academic process simulators more divulged are, among others: ASPEN PLUS®, CHEMCAD®, ASPEN HYSYS®, DESIGN II®, SPEED UP®, HYSYM®, and PRO II®.

Let’s move toward an example and understand how a chemical process simulation is carried out through software.

3 Optimizing the Energy Requirements for Styrene Production using a Retrofit Approach: An Example

Styrene monomer is a valuable chemical, and the main route for producing styrene is dehydrogenating ethyl benzene. This process consumes a large amount of heat energy due to the high-temperature steam. This work uses the existing process flow diagram (PFD) retrofit approach to optimize energy consumption. Before moving to the simulation software, the importance of styrene is described. Further, different important commercial processes have been mentioned here to select only a processing route that has worth and then optimize it based on energy utilization.

3.1 Styrene Importance

Styrene is extensively used to make rubber and plastic products. It is used in packaging materials, drinking cups, insulation for electrical uses, carpet backing, plastic pipes, automobile parts, and insulation for homes. Styrene can be used to make strong lightweight and flexible products such as Polystyrene (PS). The styrene-based chemicals include expanded polystyrene, styrene-butadiene rubber, styrene-acrylonitrile, and acrylonitrile butadiene styrene. The maximum quantity of styrene is used in the manufacture of polystyrene. Styrene is applied in food containers, medical applications, consumer electronics, automobile parts, aircraft, wind energy, armor vehicles, fuel cells, and building insulation.

It is crucial to analyze the possible route for styrene production and find the most efficient commercial process before proposing any retrofit approach.

3.1.1 Most Prominent Industrial Processes for Producing Styrene

Method 1: Ethylbenzene Oxidation

In 1977, Stoler introduced this process. In this process, ethylbenzene reacts with oxygen to form ethylbenzene hydroperoxide and propylene to form propylene oxide (PO). The co-product phenylmethyl carbinol is dehydrated to styrene. This process’s main disadvantage is difficulty in balancing the markets for PO and styrene because of the coproduction of 2.25 tons of styrene for every ton of PO. The mass ratio of styrene and polypropylene produced together of 2:1, but the market demand is different [1].

Method 2 & 3: Ethylbenzene side-chain Chlorination by Dichlorination and Ethylbenzene side-chain Chlorination by Dehydration

The process uses ethylbenzene chlorination at high temperatures to produce ethylbenzene chloride, which is then treated with water. This method includes chlorine and demands high-cost raw materials. It is a costly method for styrene production, and another disadvantage of this method is the monomer’s chlorine contamination.

Method 4: Pyrolysis from Petrochemical Products

In this process, styrene is produced by the thermal cracking of petroleum. Thermally cracked petroleum is firstly distilled at 120C to 160C, and this fraction is sent to the extractive distillation column. The extractive distillation is performed with an organic solvent in the presence of a nitric polymerization inhibitor in which styrene is dissolved. The dissolved fraction of styrene is treated with nitric acid and scrubbed with alkali to recover styrene. This method involves petroleum processes which involve high cost due to carbon being catalyst poison making more cost needed. The unavailability of raw materials and carbon in petroleum poisoned the catalyst making this method unfavorable for styrene production [2].

Method 5: Ethylbenzene Catalytic Dehydrogenation

This process involves an ethylbenzene catalytic reaction and it is a commercialized process for the production of styrene that involves ethylbenzene and iron oxide catalyst.

In this method, fresh ethylbenzene is mixed with recycled steam, then steam is added before sending it into the reactor. Crude styrene from the reactor is sent into the distillation column for further treatment [3].

Advantages of ethylbenzene catalytic dehydrogenation:

• Minimize the partial pressure of ethylbenzene
• Minimize the loss to thermal cracking by shifting the equilibrium in the direction of higher styrene production
• Avoid carbon formation and extreme coking
• Steam cleans the catalyst of any carbon that forms

In Figure 1, the term EB, R-1, R-2, HE, C-1, C-2, and EBR are ethylbenzene, reactor-1, reactor-2, heat-exchanger, distillation column 1, distillation column 2, and ethylbenzene respectively.

Ethylbenzene catalytic dehydrogenation main stages are described here:

Pre-heating of ethylbenzene, mixing, and vaporization

In this method, ethylbenzene is pre-heated at ambient temperature in a shell and tube heat exchanger. Recycle steam has an extreme concentration of ethylbenzene, water, and drops of styrene monomer. Another heat tube exchanger is used to vaporize the exit stream entirely, and the temperature reaches 250C.

Dehydrogenation

A catalytic reactor is used for dehydrogenation reaction in adiabatic mode. The reaction temperature should not exceed 610C; otherwise, thermal decomposition of styrene and ethylbenzene occurs.

Cooling

Shell and tube heat exchangers are used here to cool the mixture to 50-70C, and this stream is sent to the separation section.

Separation

The exit stream passes into a three-phase reactor composed of hydrogen and methane. Water, traces of styrene, and benzene are obtained at the bottom of the LLV separator and gaseous stream from the top of LLV. The middle stream contains the maximum amount of styrene sent into the distillation column to recover the styrene [4].

4 Process Selection for Simulation

Catalytic dehydrogenation of ethylbenzene is a commercially utilized route in the production of styrene, a large-scale method for styrene production, and will be used here for simulation. Iron Oxide (Fe₃O₄) is used as the primary catalyst for this method.

4.1 Reaction Kinetics

Styrene production by dehydrogenating ethylbenzene involves a high-temperature low-pressure endothermic gas phase reversible reaction.

Styrene reaction:

Some other side reactions that occur during styrene production have been given by Vasudevan et al. [5]. Table 1 shows the energy taken by these reactions.

Ethene and benzene synthesis:

Methane and toluene synthesis:

Carbon monoxide synthesis:

Carbon dioxide synthesis:

The kinetics of the reaction tells us that the high temperature favors the forward reaction. However, higher temperature also enhances the side reactions. This conflicts between the conversion and selectivity (favored by low temperature).

The kinetics of the reaction tells us that the high temperature favors the forward reaction. However, higher temperature also enhances the side reactions. This conflicts between the conversion and selectivity (favored by low temperature).

5 Simulating a Process Flow Diagram

Now we have chosen the process flow diagram (PFD) that will be followed for simulation and analysis afterward. Further, we have gathered the relevant reaction kinetics for this process.

Here ASPEN PLUS software will be used for simulation and calculation purposes. Aspen Plus Software is a process modeling tool for optimization and conceptual design, including empirical correlations and mathematical models. It is a computer-aided process simulation tool in which we are given the flowsheet. This software’s main advantages are quickly testing the performance of the synthesized process flow sheets, developing optimum integrated design, and minimizing experimental efforts.

First, we will simulate Fig 1 with the input of EB at a flow rate of 10,000L/hr and water at 8,500L/hr. It is important to mention here that we can consider any input stream flow rate because it will not disturb the energy required to produce per unit styrene. Here, the primary purpose is to optimize the energy requirements in Figure 1.

In a running simulation, some calculations, e.g., reactor size and flash column size, either need to calculate manually (theoretical formulas) or require the assistance of experimental or literature review data. But here, we will not go into the details of it.

Table 2: Aspen Plus generated dimensional requirements of flash columns

So, you will need to structure a PFD (here Fig 1) in an ASPEN environment and add the values of temperature, pressure, kinetics flowrates, sizes, etc., where known. Thus, the result obtained is described as follows:

Table 2: Aspen Plus generated dimensional requirements of flash columns

Two distillation columns have been used in series—the first column separates benzene and toluene from ethyl benzene and styrene. In the second column, the separation of styrene and ethylbenzene takes place. Aspen Plus DSTWU design model has been used to calculate the reflux ratio and the total number of stages.

Two distillation columns have been used in series—the first column separates benzene and toluene from ethyl benzene and styrene. In the second column, the separation of styrene and ethylbenzene takes place. Aspen Plus DSTWU design model has been used to calculate the reflux ratio and the total number of stages.

Table 4: Heat Exchanger and heaters requirements and operating specifications

6 Proposed PFD and Energy Analysis

When the output stream from the second reactor (R-2) is discharged with a temperature of 530-550C, it is required to cool down to 650C to put into a flash drum to separate H2 and CH4 gases. Most of the available PFDs of styrene production from ethylbenzene by dehydrogenation reaction involves one or two consecutive heat exchangers for cooling. An intense amount of energy is required for this purpose also with the mechanical work for pumping purposes.

A proposed PFD has been shown in Fig 3. In this Fig 3, the product of the second reactor (R-2) has been used to pre-heat the inlet ethylbenzene (10000 kg/hr). After that, this stream (S8) fulfills the second distillation column’s re-boiling section (C-2) energy requirement. Afterward, stream (S9) can be utilized to pre-heat the inlet water stream and minimize the energy requirements in the furnace.

There are three heat exchangers, HE-1, HE-2, and HE-3, in this retrofit approach for energy recovery.

The outlet of R-2 at 530C enters HE-1 for pre-heating the inlet ethylbenzene. The hot stream exits from the heat exchanger at the temperature of 358C and enters the heat exchanger HE-3 as a hot inlet stream.

The total Energy Required for running the process is shown in Table 6.

= 8690000 cal/s = 0.03446 MMBtu/s

= 124.062 MMBtu/hr

= 2977.34 MMBtu/day

Total Energy recovered = 580.28 MMBtu/day (Table 7)

% age energy saved = (580.28/2977) *100 » 20%

This analysis has shown that this retrofit approach can recover a significant amount of energy. The calculated amount of recovered energy is about 20%. This energy has been recovered by pre-heating or fulfilling the distillation column’s reboiler duties using the second reactor’s outlet. The heat recovery in this process by heat exchanger HE-1, HE-3, and HE-4 comes out to 580 MMBtu/day. The total energy requirement for this plant at this production capacity is about 2977 MMBtu/day. In this work, the kinetics of the reaction of both PFD has been kept the same for comparison purposes.

Further, there is also flexibility in changing the steam/ethyl benzene ratio. For increase or decrease in steam/ethyl benzene ratio, the furnace will operate at low high or low temperature, respectively, but duty will remain the same. However, suppose the heat loss due to insulations and change in the heat exchanger’s dirt factor can encounter. In that case, it can be predicted that 15% of the overall plant energy requirements can be reduced, which is still a good percentage. However, this retrofit model requires a comparatively longer piping system resulting in increased operational cost.

So, we can see how beneficial the simulators are for the process industry and its different operations. The vast library of chemicals, compounds data, and equipment models make an idea easy to implement in a virtual environment and analyze the outcomes before practically implementing it.

7 References

1. Tamsilian, Y., Ebrahimi, A. N., SA, A. R., & Abdollahzadeh, H. (2012). Modeling and sensitivity analysis of styrene monomer production process and investigation of catalyst behavior. Computers & chemical engineering, 40, 1-11.
2. Sethi, G., Myers, N. S., & German, R. M. (2008). An overview of dynamic compaction in powder metallurgy. International Materials Reviews, 53(4), 219-234.
3. Dautzenberg, F. M., & Angevine, P. J. (2004). Encouraging innovation in catalysis. Catalysis today, 93, 3-16.
4. Pérez-Sánchez, A., Sánchez, E. J. P., & Segura Silva, R. M. (2017). Simulation of the styrene production process via catalytic dehydrogenation of ethylbenzene using CHEMCAD® process simulator. Tecnura, 21(53), 15-31.
5. Vasudevan, S., Rangaiah, G. P., Konda, N. M., & Tay, W. H. (2009). Application and evaluation of three methodologies for plantwide control of the styrene monomer plant. Industrial & engineering chemistry research, 48(24), 10941-10961.
6. Luyben, W. L. (2011). Design and control of the styrene process. Industrial & engineering chemistry research, 50(3), 1231-1246.