(58k) Simulation and Optimization of Gas Based Ammonia Plant Using Advanced Tools of MS Excel

Now a days, Chemical engineering practice is highly influenced by advancement in computer hardware and software. Most of tasks are carried out by using commercially available software packages, such as Microsoft Office suite for word processing and spreadsheet, Polymath, MATLAB, etc. for solving numerical problems, ASPEN for computer aided plant design. MS Excel tools speed up productivity, to be more efficient when working with massive amounts of data and calculations. Solver is a powerful analysis tool with Excel used for optimization and simulation of engineering calculations and can be even more powerful if used in conjunction with VBA, to automate solving of multiple models which use different input parameters and constraints.

For any chemical industry, one of key factors for running the plants efficiently is to get material & heat balance across each equipment and performance of critical equipments and machineries on real time basis in DCS. Such online output assists plant operator to run the plant in optimized way.

In this article, efforts have been put to effectively use Microsoft Excel and Visual Basic for Application (VBA) programming and Solver Tool for simulation and optimization of gas-reforming based Ammonia Plant of industrial scale. The program can directly communicate with DCS of Ammonia Plant to obtain the required inputs to run the simulation program.

Indian Farmers Fertilizer Cooperative Limited (IFFCO) runs 3 nos. Ammonia plants based on HTAS technology and 2 nos. Ammonia plants based on KBR technology. Earlier IFFCO Team developed an Advanced level Excel program for HTAS based technology which was later on modified suitably for KBR based Ammonia plants of IFFCO.

The input data required for the program are segregated in one excel sheet. These input data are taken from DCS on real time basis and fetched in the input sheet of the Excel File. From the input sheet, the data transfers to various calculation sheets of the excel file. The calculation sheets involve various multiple implicit equations which require iterative calculations. Computation of such complex equations became possible by using advanced tool of Excel i.e. Solver Tool. The Solver Tool used in various sheets of the Excel File runs automatically once the input data is fetched to the input sheet. The excel sheet uses VBA modules for the determination of steam properties at the given pressure & temperature. There is a limitation with Solver Tool in terms of initial values it has considered for the iterative calculation, if it is not within the acceptable range, the calculation may not converge. To overcome this issue, initial values to the variable of Solver Tool have been provided through VBA module. Codes have been also provided in the VBA module to run Solver Tool automatically.

The program calculates real time performance of Ammonia plant using DCS input data. As the program involves iterative calculation in various sheets of excel file through Solver Tool, the total run time of program is around 15 minutes. So, a scheduler has been provided to get online DCS input data at the interval of 30 minutes. There is an option of changing the time interval. The output report is segregated in one excel sheet. From the output result sheet of Excel file, the output data is sent to HMI machines in Control Room through the Server. The input and output data are shown in form of graphics in these HMI machines.

By incorporating advanced tools of MS Excel, the calculation has become very simple. This approach for the evaluation of complete material and energy balance of Ammonia plant through excel file is unique in fertilizer industry.

The salient features of the in-house program is as under:

• The program can be useful for all Ammonia Plants by suitably modifying the excel program as per the process involved in that plant.
• It can directly communicate with Ammonia Plant DCS system by on-way communication and carry out the entire simulation on separate computer.
• Material balance of entire gas-based Ammonia Plant: Total material balance and component wise material balance for the process stream across each equipment are carried out. For Reactors like Primary Reformer, Secondary Reformer, etc., chemical kinetic and stoichiometric balances of the chemical reactions have been applied for the balances. In case of condensation involved in any equipment, flash calculation is carried out at the outlet of each equipment to determine composition of various components at vapor phase and liquid phase.
• Heat duty of all exchangers: For the calculation of heat duty of the exchanger, enthalpy of inlet and outlet stream is calculated.
• The program has an option of selecting either Peng-Robinson (PR) or Soave-Redlich-Kwong (SRK) Equation of State for the properties of fluids.
• Primary Reformer Furnace performance: Primary Reformer furnace is the heart of the Ammonia Plant and it consumed about 80 % of fuel energy in the Ammonia Plant. Steam- natural gas reforming reactions takes place in Primary Reformer furnace of Ammonia Plant in presence of catalyst. Hence, energy consumption optimization of Primary Reformer furnace is of prime importance for overall productivity of the Plant.

Following performance indicators are evaluated:

• Heat duty of Radiant section is calculated from both flue gas side and feed gas side. This involves enthalpy calculation of inlet and outlet stream across Radiant Section from flue gas side and process gas side.
• For the flue gas side, theoretical air requirement for the complete combustion of NG fuel & tail gas in Primary Reformer furnace is calculated. The excess air to Primary Reformer furnace is calculated using wet O2% at Radiant top. By this way, flow and analysis of flue gas coming out from the radiant section of Primary Reformer is evaluated.
• In Convection Section, heat duties of all coils are calculated from flue gas side as well as process gas side.

Also, above results can be used for checking the accuracy of temperature indicators in the flue gas side as well process side of each coils at each location of the furnace.

• Reactors performance: The program calculates Approach to Equilibrium (ATE) of Primary Reformer, Secondary Reformer, High Temperature Shift Converter and Low Temperature Shift Converter. For exothermic reaction, ATE is defined as the difference of equilibrium temperature and actual outlet temperature at the outlet of Reactor. Whereas, for endothermic reaction, ATE is defined as the difference of actual temperature and equilibrium temperature.
• Compressor performance: The program calculates polytropic efficiencies and shaft power of Synthesis Gas Compressor, Process Air Compressor and Refrigeration Compressor.

Enthalpy Determination Method is adopted for the performance evaluation. It is based on the pressure, temperature and gas analysis of inlet and outlet gas. Combined with gas mixture properties and known mass flow rate, it is a simple process to determine overall compressor power and efficiency.

• Above performance parameters’ can be obtain on real time basis with input data received from DCS.
• The program consists of modules for specific calculation (like material balance, calculation of heat duty, properties calculation of fluids, etc.). For any chemical process such modules can be used to develop similar type of excel based program for complete performance evaluation of that plant.