As discussed throughout this volume, the main objective of purification stage (section 4) is to produce high purity methanol at minimum cost of production through minimisation of energy consumption. For this reason, it is highly desirable to achieve energy saving through heat integration and feasible engineering designs. In overall, the critical review in section 4 is mainly focus on distillation column and heat exchangers.
5.2	Design Methodology
Extensive research revealed that 2 stage distillation columns is the widely used separation operation in methanol production industries due to low capital cost investment (Sina, 2012). Although the distillation process is a well-established separation process, it has disadvantages such as high energy consumption and low efficiency (Christopher, 2012). For this reason, relevant data to the column scheme and mitigation methods have been thoroughly researched to improve the overall efficiency of distillation column. Based on its significantly reduced capital expenses, simpler control structure and manageable safety and environmental concerns, a two-column distillation scheme has been deemed to be the most appropriate in the design.
5.3	HYSYS Modelling
When the initial process selection was made, a reality check was performed on the obtained industry data through simulation model. A reliable and realistic model is essentially required to determine the feasibility of our proposed design and also to facilitate optimal design. Hence, a commercial simulation software package, HYSYS is used to construct a comprehensive model of high purity methanol distillation column which has allowed us to optimise the conceptual design and operations.
5.3.1	Higher Alcohol Modelling
To improve the practicability of our model, the higher alcohol, ethanol and also dimethyl-ether are modelled in the simulation. Due to the higher alcohol modelling, the simulation provides a better estimation of equilibrium constant and relative volatility, allowing a more accurate distillation column design. If the higher alcohol and dimethyl-ether are not modelled, this would result an inaccurate simulation model which provides an underestimated data.
5.3.2	Fluid Package
On the other hand, the property package used in both distillation columns is Uniquac activity model and incorporated with Peng Robinson fluid package which provides a better prediction in fluid properties for methanol and water separation. The correct use of fluid package has enabled a better accuracy in predicting methanol and water separation which subsequently improve the equipment design and economic estimation.
5.3.3	Distillation Column Modelling
As mentioned before, distillation is the most widely used separation operation but associated with high energy consumption. With increasing energy cost and growing environmental concerns, an efficient distillation design and operation is therefore needed to save the overall plant energy consumption and minimise environmental impact. Practically speaking, the economic optimization of a distillation column involves the correct selection of the number of trays and construction materials, as well as the operating conditions (Udugama, 2013). In effort to achieve energy efficiency, the distillation column has to operate at optimal pressure, temperature, feed location and reflux ratio (Judson, 1980).
To optimise the distillation process, the operating pressures for the topping column and refining column are specified at 60kPa and 101.3kPa respectively since it is the ideal pressure for methanol water separation  (Kung, 1994). In addition, a pressure drop of 35kPa has been specified in the column to increase the reliability of the distillation process. Furthermore, a combination of purity and recovery specifications in distillation column has been chosen as the operating parameters since energy consumption can be minimised (Sinnott, 2005).
 A partial condenser is modelled for topping column to remove non-condensable gases such as hydrogen and carbon monoxide as it requires substantial amount of cooling duty to condense them. Direct disposal of the gases into atmosphere will not only contribute to climate change but also reduce the air quality, jeopardising the life of operators. To mitigate this, the gas stream is utilised as fuel for steam generation in fire boiler. Through this implementation, the resource efficiency is achieved while the cost of production is also reduced.  With detailed designs, the actual number of stages and ideal feed point location are determined. However, limitations of HYSYS are realised in detailed calculations which will thoroughly discussed in section 5.3.6. Nonetheless, through better modelling, the constructed model has a better prediction of distillation process which in turn helps to improve the feasibility and practicability of our design. Besides, the effective model has reduced the methanol content in wastewater stream produced from refining column, therefore ease the wastewater treatment system.
5.3.4	Optimization of Distillation Feed
The optimal thermal condition of the feed contributes significantly in increasing the thermodynamic efficiency of distillation column while also minimising the duty of reboiler (Lee, 2011). Besides, For this reason, a feed preheater is included in design to raise the feed temperature to dew point of mixture which is 60°C. Through implementing this, the heating duty requirement by reboiler duty is reduced by around 15%.
5.3.5	Limitations of Modelling
As discussed in section 5.3.3, one of the limitations of HYSYS has been realised during detailed design of distillation column. The tray efficiencies in HYSYS are specified at 100% which basically implies that distillation is performed in perfect efficiency. However, in actual practice, the trays are not perfect as there are deviations from ideal conditions (Lockett, 2009). For this reason, HYSYS has simulated the model based on ideal operating condition which subsequently can render the practicability and reliability of the design.
On the other hand, it is impossible to perform heat integration among distillation columns due to limitation of HYSYS. For this reason, the consumption of saturated steam is therefore substantially high which consequently led to unfeasible heating duty required by reboiler for refining column. resulting in the need for ridiculously sized heat exchangers for heat transfer. Therefore, the purchased cost estimated for reboiler is unrealistically high due to the ridiculously sized kettle reboiler. Hence, two reboilers are installed in place to reduce the boiling duty, therefore providing a much more accurate prediction of CAPEX.
In addition to limitation of heat integration, another major limitation of HYSYS is the modelling of side stream. In the plant initial design, a fusel oil stream which is basically a side stream that used to draw out stream with high content of ethanol was in place in HYSYS modelling. The main purpose of the side stream is to reduce the heating duty of reboiler as boiling up higher alcohol requires additional energy. However, the modelling of side stream in HYSYS had proven to be overly complex and difficult to control. Hence, the side stream was neglected and the ethanol was removed through the bottom stream of refining column. For this reason, the heating duty of reboiler is appeared to be too high if compared to real industry values.
5.4	 Distillation Column and Heat Exchanger Design
5.4.1	Distillation Column Equipment Design Assumptions
One of the most critical assumptions made distillation column designs is related to heat effects and heat losses. Throughout the design, the heat effects and heat losses are assumed negligible and invariant with time. However, the heat loss from the distillation column is the prime factor in the efficiency loss of distillation column which can affect the heat added and removed at the reboiler and condensers respectively.  When there is heat loss along the distillation column, this would result dramatic impact on separation efficiency as higher heating duty is required in reboiler (Christopher, 2012).
In addition to heat effects, the assumption of constant relative volatility in distillation column design is not always the case as the latent heat of vaporization of the different components is generally not equal (Sørensen, 2006). Hence, the assumption of constant relative volatility might underestimate values which subsequently affect the design of distillation column. Besides, the assumption of equilibrium with respect to mass transfer is not often valid. This is primary attributed to insufficient time of contact and insufficient degree of mixing between liquid and vapour product during distillation process (Seader & Henley, 2006).
5.4.2	Equipment Design
Through reality check with real plant, the obtained design for both distillation columns is deem reasonable and realistic as the critical factors such as length/ diameter ratio and maximum height limit are not violated. As a rule of thumb, the length/diameter ratio of distillation column has to be within the range of 20-30 while the maximum permissible height is 175ft (Seader & Henley, 2006). Besides, the determined shell diameter for topping column is proved to be reasonable while the diameter of refining column is slightly oversized. Therefore, the engineering design of refining column has to be further evaluated to determine a more realistic design. This will be further discussed in section 5.5.
In terms of heat exchangers, the size of reboiler for refining column appears to be unrealistic due to the tremendous heating duty required as mentioned in section 5.3.5. To mitigate this, a serial of two reboilers connected in series is installed to share the boiling duty, leading to a higher energy efficiency and lower consumption of saturated steam.
5.5	Suggestions
With the ever increasing energy cost and environmental concerns, it is therefore essential to minimise energy consumption where practicable to maintain sustainability and to ensure safe operations for 30 years. A case study has been performed to determine the economic feasibility of the proposed design. The case study revealed that the annual operating cost of methanol purification section alone has influential impact on the plant’s total economic and it is predicted that operating cost might outweigh the benefit of lower CAPEX offered by two distillation column scheme in future.
As recommendations, a 3 distillation column configuration appears to be the feasible solution as it allows for greater production rates as well as reduced utilities consumption through improved heat integration between columns (lurgi, 2010). Through implementation of heat integrated three-column distillation schemes, the Low Pressure steam consumption of the distillation section can be substantially reduced by 30-40% (Filippi & Ostuni, 2013). In addition, the lower energy consumption can help to minimize the environmental impact while also maximizing the profit of the company.
In addition, the distillation efficiency can be greatly enhanced with improved control since the variability in the controlled variable is reduced, allowing the operating target to be moved closer to its limit. In other words, a better control allows the distillation column to operate at optimal conditions, thus separation efficiency can be greatly enhanced. Nonetheless, the improved control can improve product quality controls, and lower energy consumption in distillation process (Douglas, 2012).
On the other hand, there is a high pressure drop of 50 bar across the valve between methanol separator and feed preheater. It is therefore highly recommended to install a pressure exchanger to recover and utilize the high pressure of the liquid stream to provide load for steam generation in furnace boiler. Through installing the pressure exchanger, the operating costs can be minimized significantly.
Another suggestion for methanol purification section is the revised engineering design for condenser and reboiler of refining column due to their significant consumption in energy. Most of the shell and tube orientation of the designed heat exchangers are based on 1 shell pass and 1 tube pass which limits the heat transfer between the liquid, therefore a higher coolant or steam flow rate is required.
Throughout the design, pragmatic material selection is the only mitigation to corrosion issues. However, it is highly recommended to consider other mitigation strategies to ensure a safe and reliable operation. Among the methods, cathodic protection appears to be a feasible mitigation strategy for the plant (International, 2000).