In regard of methanol - water framework, a well solubleframework, distillation method for higher purity achievement is preferred.The Process and Equipment course is an incorporated cours
Introduction to materials
Methanol
Methanol, known as methyl alcohol or wood alcohol, is the simplest organic compound in the alcohol family, featuring a methyl group (CH3) bonded to a hydroxy group (OH).
Methanol is a clear, colorless liquid that boils at 65°C and freezes at -93.9°C It can create explosive mixtures with air and burns with a flame that lacks luminosity Completely miscible in water, methanol has an odor similar to that of ethyl alcohol; however, it is highly toxic and has been responsible for numerous cases of blindness and fatalities due to ingestion of contaminated mixtures.
Methanol production has evolved from its historical method of destructive distillation of wood to a more efficient process that involves the direct combination of carbon monoxide and hydrogen with a catalyst Today, the use of syngas—a blend of hydrogen and carbon monoxide sourced from biomass—is becoming increasingly popular for producing methanol.
Pure methanol is a crucial material in chemical synthesis, serving as a precursor for various derivatives that contribute to the production of synthetic dyestuffs, resins, pharmaceuticals, and perfumes Additionally, it plays a significant role in automotive antifreezes, rocket fuels, and as a versatile solvent As a high-octane, clean-burning fuel, methanol presents a promising alternative to gasoline for automotive applications Furthermore, methanol sourced from wood is primarily utilized to make industrial ethyl alcohol undrinkable.
Water
Water (H2O) is a polar inorganic compound that appears as a tasteless and odorless liquid at room temperature, with a slight blue tint Known as the "universal solvent," it is the most extensively studied chemical compound and the most abundant substance on Earth Unique among common substances, water can exist in solid, liquid, and gas forms on the Earth's surface.
Water exhibits high viscosity, surface tension, heat of vaporization, and entropy of vaporization due to its low molar mass and the extensive hydrogen bonding interactions among its molecules.
Methanol-water mixture
Table 1 Vapor-liquid equilibrium data of methanol-water mixture
Figure 1 Binary phase diagram of methanol-water mixure
Methanol production
Methanol is primarily produced on an industrial scale using natural gas as the main feedstock; however, it can also be derived from various sources such as coal, biomass, municipal solid waste, biogas, waste CO2, and renewable electricity This versatility in production methods positions methanol as a sustainable option for future fuels and chemicals.
The production process can be divided into four main stages:
Feed purification is essential for the two primary feedstocks, natural gas and water, before their utilization Natural gas is subjected to a desulphurization process, ensuring that the sulfur content is lowered to below one part per million Additionally, impurities in the water are minimized to undetectable levels, ensuring high-quality feedstock for further applications.
Liquid or parts per billion levels before being converted to steam and added to the process
Reforming is a crucial process that converts methane (CH4) and steam (H2O) into key intermediate reactants, including hydrogen (H2), carbon dioxide (CO2), and carbon monoxide (CO) During this stage, carbon dioxide is introduced into the feed gas stream to create an optimal mixture for efficient methanol production This transformation occurs in a reformer furnace, which is heated using natural gas as fuel.
Methanol synthesis involves compressing the reformed gas after excess heat removal, followed by its introduction into the synthesis reactor In this process, the reactants are transformed into methanol, resulting in a crude product consisting of approximately 68% methanol and 31% water The crude methanol is then condensed and directed to the purification stage for further processing.
Methanol purification involves refining a 68% methanol solution to achieve a high purity level of 99% methanol The purification process is rigorously tested at multiple stages, ensuring quality and safety Once completed, the refined methanol is securely stored in a large tank area before it is delivered to customers.
Figure 2 Methanol production process flow diagram
Within this project, we only concern about the very final purification step of the whole production, which is the distillation process to obtain pure methanol from methanol-water mixture.
Introduction to distillation process
Definition
Distillation is a separation technique that involves the selective boiling and condensation of components in a liquid mixture This process allows for the concentration of specific components or the extraction of nearly pure substances by leveraging the differences in boiling points among the mixture's constituents.
At the boiling point of a liquid mixture, all volatile components vaporize, but the concentration of each constituent in the vapor depends on its contribution to the overall vapor pressure Consequently, compounds with higher partial pressures tend to be more concentrated in the vapor phase, while those with lower partial pressures remain more prevalent in the liquid phase.
Distillation cannot yield a completely pure sample of a component from a mixture due to the impossibility of achieving zero partial pressure for any component However, high-purity samples can be obtained when one component's partial pressure approaches zero.
Types of distillation
Simple distillation is a process that heats a liquid mixture to its boiling point and condenses the vapors produced This technique is effective only when the boiling points of the liquids differ significantly, typically by at least 25℃ The purity of the resulting distillate is determined by Raoult’s law.
Fractional distillation is a technique used to separate liquid mixtures with similar boiling points through multiple vaporization-condensation cycles in a fractioning column During the process, the heated liquid converts into vapors that ascend the column, where they are cooled and condensed on the condenser walls This results in new vapors being generated from the heated condensed liquid, enhancing the purity of the distillate with each cycle.
Steam distillation is a key technique for separating heat-sensitive components in mixtures by passing steam through them, which vaporizes certain elements at lower temperatures This method allows for efficient heat transfer and is commonly employed to extract essential oils and herbal distillates from aromatic flowers and herbs.
Vacuum distillation is an effective technique for separating liquid mixtures with high boiling points, making it preferable when high-temperature heating is inefficient By reducing the surrounding pressure, the boiling point of the components is lowered, allowing them to vaporize These vapors are subsequently condensed and collected as distillate Additionally, vacuum distillation is utilized to obtain high-purity samples of compounds that decompose at elevated temperatures.
Distillation equipment
In industrial distillation processes, the efficiency of mass transfer relies on equipment that provides a large area for phase contact Tray columns are among the most commonly used hardware for this purpose, facilitating effective mass transfer in various operations, including distillation.
The choice and design of trays significantly influence the efficiency of distillation, absorption, or stripping systems Each tray must be engineered to optimize vapor-liquid contact while maintaining economic feasibility.
Sieve trays are flat, perforated plates designed to allow vapor to rise through small holes in the tray floor, creating bubbles in the liquid below in a uniform manner They offer a capacity that is comparable to valve trays, making them an effective choice for various separation processes.
Valve trays consist of perforated sheet metal decks featuring round, liftable valves, strategically placed parallel to the outlet weir These trays facilitate vapor flow through the valves, offering a combination of high capacity and exceptional efficiency across a broad operating range.
Bubble cap trays facilitate the upward movement of vapor through risers or uptakes, allowing it to escape through slots as bubbles into the surrounding liquid on the tray These trays are primarily utilized in specialized applications.
Table 2 Comparison between different types of tray
Sieve trays Valve trays Bubble cap trays
Most flexible (high or low vapor/liquid rates)
Less-flexible to varying loads than other two types.
Less efficient mixing as compared to bubble cap trays.
Difficult to operate and clean.
High fouling tendency High cost High pressure drop
Within this project, we use simple distillation method, equipped by the bubble cap tray column for the distillation process of methanol-water mixture.
Selection for project design
Design a bubble cap tray distillation system for methanol recovery in aqueous (CH3OH – H2O mixture) as following specifications.
Mass flowrate of feed: GF = 7000 (kg/h)
● Feed: x F = 0.7 (kg methanol/kg solution)
● Distillate: x D = 0.97 (kg methanol/kg solution)
● Bottom: x B = 0.005 (kg methanol/kg solution)
Process flow diagram
The process begins with a ground-level storage tank that utilizes two pumps to continuously transport the feed mixture to an overhead reservoir From there, the mixture flows to a preheater, where it is heated by superheated steam to achieve a saturated liquid state before entering the distillation column.
The distillation column is divided into two main sections: the rectifying part and the stripping part In this process, the liquid phase flows downward while the vapor phase rises upward Vapor from the lower tray passes through bubble caps, interacting with the liquid on the upper tray, which leads to partial condensation and an increasing concentration of volatile constituents in the liquid as the height of the column rises Consequently, as the concentration of these volatile components in the liquid increases, the concentration in the rising vapor also elevates, resulting in a decrease in boiling temperature.
In the distillation process, vapor is condensed into liquid using a condenser, where a portion of the liquid is refluxed back into the tower to enhance separation efficiency The remaining liquid is then cooled with water and collected as the final product.
At the base of the tower, the liquid product is directed into a reboiler where it is vaporized using superheated steam, allowing the vapor to be refluxed back into the tower for redistillation A weir within the reboiler maintains the liquid level, ensuring consistent contact with the heating surface If the liquid exceeds the weir's height, the excess bottom product is discharged from the reboiler and utilized to preheat the initial feed mixture before it enters the storage tank.
(The detailed process flow diagram can be found in the attachment.)
MASS BALANCE 9 1 Overall mass balance
Reflux ratio
Based on methanol-water VLE diagram, x F =0.568⇒y F ¿ =0.825
The empirical correlation between the practical and minimum reflux ration is:
We choose the formula: R=1.3R min +0.3=0.922
Operation lines
Properties of liquid and vapor phase
Average liquid phase composition in the column:
2 =0.285 Average vapor phase composition in the column:
Based on liquid-vapor equilibrium diagram of methanol-water, we obtain:
Average molar mass and density of vapor phase:
Average density of liquid phase:
● Rectifying section: x R = x R ,ave × M M x R, ave × M M +( 1− x R, ave ) × M W
● Stripping section: x S = x S ,ave × M M x S ,ave × M M +( 1− x S,ave ) × M W
ENERGY BALANCE 12 1 Overhead condenser
Reboiler
Based on liquid-vapor equilibrium diagram of methanol-water, x B =0.00282⇒T B 0℃
Assume that heat loss is equal to 5% of total useful heat.
Use superheated steam at T 0℃, P=0.2MPa and 5% moisture content to fuel the preheater.
Feed preheater
Based on liquid-vapor equilibrium diagram of methanol-water, x F =0.568⇒T F 0℃
Use superheated steam at T 0℃, P=0.2MPaand 5% moisture content to fuel the preheater.
Overhead product cooler
The outlet temperature of distillate flow is chosen at 35℃.
2 ≈51℃⇒{C p, M 894( kmol K kJ ) C p,W u.249 ( kmol K kJ ) ⇒ C p, D = 86.289( kmol K kJ )
3600 ×86.289×(66−35)9.845(kW) The inlet and outlet temperature of cooling water are 25 o C and 40 o C respectively F water = Q COOL1
75.39( kmol K kJ ) ×( 40− 25) × 997 ( m kg 3 ) × 18 ( 1 kg kmol )
Bottom product cooler
The outlet temperature of bottom product flow is chosen at 35℃.
2 ≈68℃⇒{C p, M 34( kmol K kJ ) C p,W u.40 ( kmol K kJ ) ⇒C p, B u.445( kmol K kJ )
75.39( kmol K kJ ) ×( 40− 25) × 997 ( m kg 3 ) × 18 ( 1 kg kmol )
The total amount of cooling water used:
The total amount of steam used:
Theoretical number of trays
According to the diagram, the theoretical number of trays for this distillation process is 9 trays, in which:
● 1 feed tray (the feed flow enters the column at the fifth tray)
Practical number of trays
The efficiency coefficient is the function of volatility and viscosity:
T R, ave i℃⇒ {μ M =0.0003311(Pa s)=0.3311(cP)μ W =0.0004147(Pa s)=0.4147(cP) log logμ R =x R,ave ×log logμ M +( 1− x R ,ave ) × log log μ W =0.758×log log 0.3311+(1−0.758)×log log 0.4147
T S , ave s℃⇒{μ M =0.0003192(Pa s)=0.3192(cP)μ W =0.0003916(Pa s)=0.3916(cP) log logμ S =x S , ave ×log logμ M +( 1− x S,ave ) × log log μ W =0.285×log log0.3192+(1−0.285)×log log 0.3916
0.39.26≈11trays Based on the calculation, the column has a total of 20 practical trays
(including 1 feed tray) and the material is fed into the 9th tray.
Diameter of trays
Average vapor flow through rectifying section can be calculate by: g R,ave =Go+g 1
The amount of overhead vapor: Go = D×(R+1) = 309.99 (kmol/h)
The amount of vapor entering the first stage of rectifying section is determined by the below system of balance equations (p.181,[2]):
In the rectifying section, the latent heat of the mixture entering the first stage is denoted as r1, while r0 represents the latent heat of the overhead mixture Additionally, g1 indicates the quantity of vapor entering the initial stage of the rectifying section.
G 1 : the amount of liquid in the first stage of rectifying section x 1 =x F =0.568, y 0 =x D =0.948
● Velocity of vapor flow in rectifying section
In which, ρ R , x and ρ R , y : average density of vapor phase and liquid phase in rectifying section h tray : distance between 2 trays (m) φ(σ): surface tension coefficient
Figure 4 The relation between tray’s diameter and tray’s distance data
(p.184, [2]) Table 3 Trial results for tray’s diameter and tray’s distance h tray (m) (ρ R, v × ω R ) ave ( kg m 2 s¿¿ D (m)
Choose h tray =0.45(m), the diameter for rectifying section calculated from this height is 1.83 (m) ⇒ Choose standard diameter: D=2(m)
Re-calculate the velocity through rectifying section ω R, ave =0.0188 2 × g R ,ave
Average vapor flow through stripping section can be calculated by: g S ,ave =g n ' +g 1 '
Because the amount of vapor escaping the stripping section is equal to the amount of vapor entering the rectifying section ⇒ g n ' =g 1. g S ,ave =g 1+g 1 '
The amount of vapor entering the first stage of strippiNG section is determined by the below system of balance equations (p.182,[2]):
In which, r 1 : latent heat of the mixture entering the first stage of rectifying section r 1 ' : latent heat of the mixture entering stripping section g 1 ' : the amount of vapor entering stripping section
G 1 : the amount of liquid entering stripping section x B =0.00282, y 1 ' =y B =0.0063, y 1 =0.773
● Velocity of vapor flow in stripping section
Choose h tray =0.45(m) (in order to be uniform with the rectifying section)
The result for diameter is quite appropriate with the chosen height.
Re-calculate the velocity through stripping section ω S, ave =0.0188 2 × g S, ave
Bubble cap’s parameters
The number of bubble caps per tray: n=0.1× D 2 d 2 h =0.1× 2 2
Bubble cap’s thickness normally varies from 2 to 3 (mm), here we choose: δ cap =0.0025(m) According to equation IX.213 (p.236, [2]), the upper gap between riser and bubble cap is: h 2 =0.25× d h =0.25×0.1=0.025(m)
Height of bubble caps:h cap =h h +h 2=0.12+0.025=0.145(m)
According to equation IX.214 (p.238, [2]), bubble cap’s diameter is: d cap =√❑
The distance between slots varies from 0.003 to 0.004 (m), choose: c=0.003(m)
The gap between trays and edge of caps varies from 0¿0.025 (m), choose:
Height of slots varies from 0.01¿0.05 (m), choose:b=0.025(m)
Number of slots in each bubble cap: i=π c× ( d cap − 4 d ×b h 2 ) = 0.003 π × ( 0.145 − 4 × 0.1 0.025 2 ) = 47.12 ⇒ Choose 48 slots
48 −c=0.0065(m)Minimum distance between bubble caps: l 2.5+0.25× d cap I(mm)
Tray’s thickness
The thickness chosen for the trays must resist the deflection, which causes dimple to the tray’s surface and eventually breakage (especially with those having large diameter)
The material used for trays is steel 304 (SA-240 plate) Assuming the weight is distributed evenly on the tray, deflection can be calculated by: w= p× R 4
In which, p:uniform pressure ontheupper side of the tray¿
D f :flexural rigidity(Pa m 3 ) φ: correction factor, choose φ=0.5 since perforated plates sag more easily than solid trays.
In section V.5 of this report, the mass of gadgets on tray is calculated as follows: m weirs 84.88(kg), m risers H5.8(kg), m caps 21.85(kg) m w =( ∆+h c ) × π D 4 2 × ρ liquid (kg)
{Rectifying section:m w 7.14(kg)Stripping section:m w #8.19(kg)⇒Takem w #8.19(kg)∈calculation
E: Young’s modulus of material, E3×10 9 (Pa) t: tray’s thickness (m¿ ν: Poisson’s ratio of material, ν=0.27
Deflection should not exceed half thethickness:w=1.124×10 −9 × 1 t 3 h min #300×ρ y ρ x × ( n× π × d F × ω y cap ) 2 (m)
In which, h tray : distance between trays (m) h min : minimum distance between trays (m) ω y : vapor velocity (m/s)
Mechanical design
The distillation column is designed accordingly to the ASME standard.
The working condition of distillation column is at atmospheric pressure
The design temperature is chosen at T highest 0℃73K!2℉, in order to preclude the risk of overheating, overloading operation or the heat transfer between all parts of the column.
The vessel is constructed of 304 stainless steel (grade SA-240, nominal composition 18Cr-8Ni) with spot welding seam
At the design temperature, the maximum allowable stress is S = 16000 (psi).
The design pressure is calculated as follows:
∑ ❑ ❑ ∆ P: total column pressure drop ρ S, x : average liquid density of stripping section
The vessel minimum thickness is calculated as follows:
C A : additional thickness for chemical corrosion, mm Assume that methanol’s corrosion rate is 0.05 mm/year and the usage length is 20 years ⇒C A =1(mm).
C B : additional thickness for mechanical corrosion Assume that C B =0.
C C : additional thickness for manufacturing error Assume that C C =1(mm).
✔ Maximum allowable working pressure (MAWP)
According to the standard document QCVN 02-2009/BXD, the average wind pressure at Ho Chi Minh city is recorded as W o =0.83( kN m 2 ) 34 ( lbf ft 2 )
Assume the height of skirt support from bottom seam to the ground is h T =1.5(m)=4.92(ft).
Moment at the base: M=W o D H2 2 =V H2 (ft lbf)
D: inner vessel diameter (ft), D=2(m)=6.562(ft)
H: total height (ft), H=H body +h T 7.73(ft)
Moment at bottomseam:M T =M−h T ( V −W o D h 2 T ) ( ft lbf )
The required thickness in order to tolerate wind pressure is calculated by: t wind = M T
S: maximum allowable stress (lbf ft 2 ¿; S000(psi)#04000(lbf ft 2 )
Longitudinal stress and wind load are exerted onto the bottom seam, therefore, the thickness should be the sum of required thickness for both.
⇒t bottom seam =t wind +t longitudinal =0.28+1=1.28mm