Process Dynamic Simulation
The methodology employed in most of our engineering study projects study includes using first principle calculations and process simulation software called Aspen Hysys. Hysys is an abbreviation for Hyprotech Systems, is a process-modelling software, capable of steady-state and dynamic simulations.
It was developed by AspenTech and usually used by chemical process engineers to mathematically model chemical processes, from unit operations to full chemical plants and refineries. HYSYS is able to perform many of the core calculations of chemical engineering, including those concerned with mass balance, energy balance, vapour-liquid equilibrium, heat transfer, mass transfer, chemical kinetics, fractionation and pressure drop. The diagram on the right shows a process, units operations and stream numbers.
The table shows the properties of each stream.
What is Process Engineering Design?
Chemical and Process Engineering
The chemical manufacturing processes have involved from modest to complex production of chemicals converting raw materials such as fossil fuels, water, minerals, metals, and others to become thousands of products central to our everyday modern living. Chemical and process engineering deals with the development, design, operation, and management of converting these raw materials safely and cost- effectively. It is founded on the principles of chemistry, physics, and mathematics. The laws of physical chemistry and physics govern the practicability and efficiency of chemical engineering operations, whilst mathematics aids the essential tool in optimisation and modelling.
Chemical Process Engineering in Design
The design of a process plant starts with an idea to produce a new product or improve an existing one. It is usually a result of one or more chemical reactions and mainly the physical processes such as the separation of the components or a group of components from a natural mixture, are the subject of the design. An excellent example of such a process is crude oil primary separation.
The engineering design of a chemical manufacturing process can be categorised into two broad phases.
The conceptual or process design is the first phase that includes the selection of the chemical process, technology, process conditions, collection of required data, issuing of process flowsheets, selection, specification, and chemical engineering calculations of equipment, and preliminary cost estimation. Typically, in a project organisation, the process engineering design team is heavily involved in this phase also known as the front-end engineering design (FEED).
The second phase, basic plant design, includes the detailed mechanical design of equipment, detailed design of electrical systems, civil structures, piping and ancillary services. The support design group is responsible for these engineering activities as they have the expertise in other engineering disciplines required to complete the plant design.
A typical chemical process plant design can be illustrated in these phases, as shown below.
The different phases of the project as shown above illustrates that the goal of plant design is to complete them within the framework of projects. The process engineers are heavily involved in FEED within this project phase and constantly interact and communicate with other engineering disciplines.
Process Engineering Key Role
Establishing the steps in a chemical plant design is essential and recognises the involvement of the multidiscipline engineering. Since process engineering design is conducted at the front-end of plant engineering design, therefore, outputs or deliverables from this stage have a significant impact on the subsequent design stages. The non-exhaustive list of process engineering workflow is illustrated below.
Seamless collaboration among different engineering disciplines is key to a successful chemical plant design execution. With this tight collaboration with other engineering disciplines, process engineers set work priorities, choose equipment, select instrument from alternatives, identify key hydraulic and elevation constraints. Aligning process engineering activities as well as deliverables sets the groundwork for the whole design execution and strategy.
The process information flow is communicated through the activities and deliverables that the process engineering owns or shares with another engineering discipline. One deliverable example is the piping and instrumentation diagram (P&ID). The P&ID is a schematic representation used to facilitate the design process, convey intent, or construct and communicate information to the client and all involved disciplines.
Process Plant Consultants
Large-scale organisations may utilise their own design teams to conduct the entire project design and even execute the plant construction, all within their organisation. However, most commercial-scale process plants are very complex, consisted of various unit operations linked together by piping and automatically controlled via basic process control system (BPCS). Many process engineers working in a process plant are operations engineers specifically trained to troubleshoot an underperforming- or optimise a chemical plant but may lack the skills and experience to design a chemical plant.
To minimise risks, organisations of many sizes usually outsource and commission a process plant consultant also known as engineering, procurement and construction management (EPCM) to design the process plant. The process engineers can be part of either the EPCM’s organisation or the owner's engineering design team. Process engineers in the EPCM’s organisation are part of the project team executing the design and performing the deliverables in the project. The process engineers that are part of the owner's design and engineering team are the engineering consultants responsible for reviewing and approving the deliverables from the contracted EPCM organisation to ensure that the deliverables comply with the design specifications and requirements.
Whichever organisation you work for, as a process engineer, it is important to understand your role and know that being the main driver in the design executions requires a good engineering foundation to perform the plant design. A good understanding of your activities and deliverables and your collaboration with other engineering disciplines contribute to the successful execution of the project.
CASE STUDY - BIODIESEL PROCESS DEVELOPMENT
Process Chemistry
In this pilot plant, it is intended that waste cooking oil (WCO) from various sources around Singapore, for example, Hawker Centres, is used as the feedstock. It is the primary intention of this pilot plant to convert WCO by reacting it with a suitable alcohol in the presence of a catalyst to yield ethyl esters, also known as biodiesel. The figure below shows a balanced transesterification chemical reaction equation that will take place in this pilot plant. In this reversible reaction, WCO reacts with CH3OH in the presence of a suitable catalyst to yield biodiesel and glycerol as a by-product. Our experienced process engineers established the process chemistry prior to embarking on process engineering tasks.
The catalyst is prepared by mixing CH3OH and a strong base such as NaOH or KOH. During the preparation, the NaOH breaks into ions of Na+ and OH-. The OH- hydroxyl radicals will combine with the hydrogen H+ ions from CH3OH to form water whilst leaving the CH3O- radicals available for reaction. Water will increase the possibility of a side reaction with free fatty acids (FFA) to form soap, an unwanted reaction, thus it is preferable to have lower fatty acids in WCO. From the chemical equation, one mole of triglyceride will react with 3 moles of CH3OH, so excess CH3OH has to be used in the reaction to ensure complete reaction. The three attached carbons with hydrogen react with OH- ions and form glycerol, while the CH3 group reacts with the FFA to form methyl ester. The table below shows the preferred WCO properties for this process.
Process Engineering
Stage 1 – Pre-treatment of WCO
Prior to the feeding of WCO to the esterification process, it has to go through a pre-treatment process. First, WCO has to be filtered to get rid of any external and remains of food. This can easily be achieved using filter mesh of 100 nm, this pre-treatment process helps to improve the efficiency of the reaction. During the first step, the WCO will be preheated by passing through a shell-and-tube heat exchanger before being charged into the reactor. In the reactor, the WCO will be heated to about 50-60 deg C. This heating process will help in the removal of moisture content in oil, saponification, as well as the FFAs concentration.
Stage 2 – Esterification Process
One of the challenges of the process chemistry is that the WCO feedstock may contain particulate matter, high free fatty acids (FFA) and water. Unlike other feedstocks, the quality of the incoming WCO cannot be pre-determined or controlled. Thus, esterification process steps must be undertaken prior to undergoing the transesterification process. Such process steps include filtering the particulate matter, removing the FFA and removing the presence of water. Referring to the process flow diagram (PFD) below which our process engineers developed, WCO feedstock is preheated to 60 deg C by heat exchangers. CH3OH and H2SO4 or HCl catalysts are homogenously mixed in a batch mixer being charged into the esterification reactor. This esterification reaction will be conducted at 90 deg C and at atmospheric pressure. Products from the esterification reactor are cooled to 45 deg C and the catalysts are removed or neutralised. These products are then pumped into a gravity settling tank to separate the CH3OH /water mixture from the esters. From the top of the gravity settling tank, CH3OH /water mixture is removed and fed into a distillation column 1 to separate the CH3OH from water. This separated CH3OH will be reused in the process. The esters sitting at the bottom of the gravity settling tank is then fed into the transesterification reactor.
Stage 3 – Transesterification Process
NaOH or KOH catalysts and CH3OH are mixed homogeneously in a batch mixer and fed into the transesterification reactor for them to react with the esters. This transesterification reactor process conditions include maintaining its temperature at 60 deg C, at atmospheric pressure and 1:6 molar ratio of oil to CH3OH. The reacted products from the transesterification reactor are pumped into gravity settling tank 2 and from this unit, the biodiesel/ CH3OH product stream is pumped into distillation column 2 to separate them. The separated CH3OH from the distillation column is recycled and reused. The biodiesel obtained is then purified using a liquid-liquid extraction column before being pumped to a storage tank. A heating element will vaporise remaining water moisture in the biodiesel storage tank and this moisture is fed into a counter-flow packed scrubber. Water will be used to strip remaining CH3OH from the moisture before discharging the air into the atmosphere. Water with a slight CH3OH from the scrubber is reused by feeding it into the liquid-liquid extraction unit. On the other hand, glycerol/ CH3OH mixture sitting at the bottom of gravity settling tank 2 is pumped to distillation column 3 where CH3OH from the top of this distillation column is reused whilst glycerol from this column is stored as a by-product. The deliverables from chemical process engineering are then employed to plant engineering design.
Plant Process Variables
The yield of a biodiesel in transesterification process depends on several important variables, namely:
(1) reaction temperature
(2) reaction time
(3) CH3OH to oil molar ratio
(4) type and quantity of catalyst
(5) reactor mixing effectiveness
Reaction Temperature
The yield of biodiesel is affected by the reaction temperature. Higher temperature increases the reaction rate and reduces the reaction time as the viscosity of oils is decreased. However, if the reaction temperature is increased beyond the optimal temperature, it will lead to a decrease of the biodiesel yield, as higher reaction temperature speeds up the saponification of triglycerides and facilitates the vaporisation of CH3OH resulting in reduced yield. Therefore, the transesterification reaction temperature should be kept below the boiling point of alcohol to prevent the evaporation of alcohol. The optimal reaction temperature may vary from 50 deg C to 60 deg C, depending upon the WCO and should be near the boiling point of alcohol for faster conversion. Methyl esterification of the FFA can be performed at ambient temperature up to 78% conversion after 60mins.
Reaction Time
An increase in reaction time has been found to increase fatty acid esters conversion. At the commencement of the reaction, the reaction rate is slow due to mixing and dispersal of alcohol and oil. But once the reactants are mixed, the reaction proceeds very fast and the maximum ester conversion has been determined to be attained within 90mins. Further increase in the reaction time does not cause any increase in the yield of biodiesel. In fact, if the reaction time is longer than 90mins, the yield of biodiesel is decreased because transesterification is a reversible reaction. This may lead to the loss of esters and instead, lead to the formation of more soap.
CH3OH to Oil Molar Ratio
The transesterification reaction is reversible so in order to shift the reaction to the right and increase the yield of biodiesel, either the use of excess alcohol or the removal of one of the products from the reaction mixture is recommended. The removal of one of the products from the reaction mixture is generally preferred for the reaction to proceed to completion. The reaction rate has been determined to be at the highest when 100% excess CH3OH is used. Other alcohols such C2H5OH, C3H7OH or C4H9OH can be used but CH3OH is preferred due to its relatively low cost. The effect of volumetric ratio of CH3OH to oil has been studied and it was observed that the highest biodiesel yield was nearly 99.5% at 1:10 oil to CH3OH ratio.
Type and Quantity of Catalyst
In transesterification reactions, alkali catalysts, such as NaOH or KOH, are preferred to acidic catalysts such as H2SO4 or HCL, because they have a higher reaction rate, require low alcohol, such as CH3OH and results in a higher yield at ambient temperature. Despite the advantages of using alkali catalysts, the presence of FFA and water in WCO will render these alkali catalysts ineffective. The presence of high FFA in WCO will react with the alkali catalysts and will lead to incomplete transesterification reaction. Due to this reason, the level of FFA and water must be reduced to < 1% prior to the transesterification process taking place. To reduce the high FFA, acidic catalysts can be used to convert the FFA into monoesters. The presence of water in WCO will speed up the hydrolysis reaction, lead to saponification reaction, decreases the catalyst activity and produces more soap instead of biodiesel. Water can be removed through evaporation from the WCO by boiling the feedstock at 100-110 deg C. To obtain 90% yield of biodiesel, the water content shall be < 0.5%. Now that the particulate matter has been removed, high FFA and water have been reduced, the WCO is ready for transesterification. The yield of biodiesel usually increase with increasing quantity of catalyst since there are more available active sites. But using excessive catalyst is uneconomical due to the cost of the catalyst. Thus, it is necessary to determine the optimum quantity of catalyst required in the reaction.
Reactor Mixing Effectiveness
Mixing is very important in the transesterification process since oils and alcohols are not completely miscible. Since the reaction can only take place at the interface between these reactants, transesterification reaction is therefore a slow process. This means effective mixing between the reactants is very important. We have a long history in the design and optimisation of reactor and mixing tanks. Our simulation team uses computational fluid dynamics (CFD) at the design stage to verify and ensure the reactor will work effectively even before fabrication starts. Only when the reactor’s performance has been verified by CFD results do the fabrication begin.
Plant Process Simulation
To ensure that the biodiesel plant that we design will work as intended and will perform optimally, process plant simulation was performed on the pilot plant using Aspen Plus. Aspen Plus is the leading chemical process simulator in the chemical engineering world. This software allows our process engineers to build a process model and then simulate it using complex calculations (models, equations, math calculations, regressions, etc).
The simulation solution covers the entire pilot plant and its use will:
(1) check to ensure all engineering calculations are correct and the design is optimised
(2) predict with high level of accuracy the performance of the biodiesel pilot plant prior to procurement and construction
CHEMICAL PROCESS ENGINEERING
Our chemical process engineering consultants provide conceptual design, front-end engineering design and detailed design within the chemicals, renewable fuels, petrochemicals and mineral processing. Below are the typical process flow diagram (PFD) and piping & instrumentation diagram (P&ID) that our process engineers had delivered. In the design of a chemical plant, our process engineering designers will execute the following tasks:
Process Engineering Design
- Develop or perform:
> process design basis
> process conceptual design
> process flow diagram (PFD)
> mass & energy balance
> verification of process via process simulation
> process engineering calculations
> piping and instrumentation diagrams (P&IDs)
> functional specification
> hydraulic calculations
- Selection and sizing of:
> equipment, such as, mixing tank
> valves, such as, globe and ball
> pumps, such as, centrifugal and gear
> instrument, such as, level indicator
> piping and fittings
- Preparing list for:
> equipment
> valves
> pumps
> instrument
> piping
- Hydraulics study report
- Utilities summary list
- Preparing datasheets for instrument such as:
> flow indicator and transmitters
> level indicator and transmitters
> pressure indicator and transmitters
> temperature indicator and transmitters
> pH indicator and transmitters
> conductivity indicator and transmitters
- Selection and/or design of process equipment
- Sizing of process equipment
- Preparing datasheets for equipment such as:
> mixing tanks and static mixers
> CSTR and plug flow reactors
> control valves
> various pumps
> heat exchangers
> chiller unit
> heating unit
- Preparing specification for vendor package
- HAZOP study and HAZOP closeout report
- Developing SOPs for:
> start up
> normal operations
> shutdown
> emergency
- Commissioning procedure
In addition, our process engineers also perform sensitivity analysis on crucial variables to determine which are the most important variables that result in the most significant changes.
Our chemical process engineers will develop accurate mathematical models that can be used to predict process changes as a consequence of changing the value of certain variables. Using Microsoft Excel, these mathematical models are then transformed into a form that is user-friendly for the client.
AUSTRALIA SINGAPORE