Process Description
The Simtronics Ethanol Distillation simulator enables comprehensive training for fuel ethanol distillation unit operators. The simulated process is representative of the basic two-stage distillation process used to conventionally concentrate the ethanol in a beer feed to approximately 95 liquid volume %. To make a fuel-grade ethanol product, the distillation section is followed by a drying unit to eliminate most of the remaining water in the product ethanol produced by distillation of the feed beer. The simulated process includes the following main equipment:
– Beer Column (performs primary separation of ethanol from the beer)
– Rectification Column (eliminates most of the water from the ethanol)
– Stillage Flash Drum (removes ethanol from produced stillage)
– Ethanol Drying Unit (eliminates water from the ethanol)
– Fusel Oil Decanter Unit (removes byproduct fusel oils)
The conventional two-column distillation process allows operators to develop a good understanding of ethanol distillation principles and operations that would be applicable to commercial ethanol production facilities. This includes developing an understanding of the thermodynamic limit of ethanol concentration using conventional distillation.
A full range of operations can be learned and practiced on the Ethanol Distillation simulator. These include normal, startup, shutdown, and emergency shutdown procedures.
Ethanol Distillation Process Overview
The Ethanol Distillation process distills a dilute solution of ethanol in the feed to a vapor ethanol stream of about 95 volume percent concentration with the balance being water vapor. This distilled vapor stream is sent to a drying unit to remove water and produce a liquid, fuel-grade ethanol product, suitable for blending with gasoline. The main processing equipment are:
T-501 Beer Column
T-502 Rectification Column
D-501 Stillage Flash Drum
D-502 Reflux Drum
X-501 Ethanol Drying Unit
X-502 Fusel Oil Decanter
E-501 Beer Preheater No. 2
E-502 Beer Preheater No. 1
E-503 Condenser
EJ-501 Stillage Ejector
The feed to the unit is called beer and contains approximately 3.24 weight percent ethanol. The beer also contains non-volatile compounds such as unconverted sugars and fine solids from the fermentation process used to produce the beer. These compounds amount to about 16.8 weight percent of the feed. The balance of the beer consists of water and a small concentration of fusel oils generated in the fermentation process that produces the beer.
The beer is preheated in two heat exchangers (E-501 and E-502) using warm process streams prior to entering the Beer Column T-501. Vapor, rich in ethanol, flashes from the warm beer near the top of T-501. The balance of the ethanol in the beer is stripped out using direct steam injection into the bottom of the Beer Column.
Water, dissolved sugars, and solids (known as stillage) are removed from the bottom of the Beer Column and sent to the Stillage Flash Drum D-501 to remove any ethanol that was not stripped out in the Beer Column. Stillage Flash Ejector EJ-501 pulls vacuum on the stillage in the Stillage Flash Drum. The vacuum conditions in D-501 ensure very little ethanol is present in the final stillage, which is sent from D501 to storage. Heat is recovered from the stillage by preheating beer in Beer Preheater No. 2 E-501.
Ethanol concentrates to about 32.5 weight percent in the overhead vapor from the Beer Column. The balance of the overhead vapor is water vapor and a small amount of fusel oils from the beer feed. This vapor is sent to the base of the Rectification Column T-502.
The Rectification Column concentrates the vapor from the Beer Column to the maximum practicable concentration for a mixture of ethanol and water by refluxing overhead condensate back to the Rectification Column. The overhead vapor from T-502 is first cooled by preheating beer feed in Beer Preheater No. 1 E-502 and then is partially condensed using cooling water in Condenser E-503.
Ethanol Distillation Process Overview
Vapor from E-503 contains about 88.6 weight percent ethanol and is sent off to the Ethanol Drying Unit X-501 where more water is removed. The resulting product ethanol is essentially water free and is taken off to storage. A recycle stream containing a low concentration of ethanol is produced from X-501 because a portion of the dried ethanol is used to regenerate the dryer beds in X-501 and picks up water in the process.
The liquid from Condenser E-503 is collected in Reflux Drum D-502 and is completely pumped back to the Rectification Column. The concentration of ethanol in the reflux is about 85.9 weight percent.
Reflux to the Rectification Column results in concentration of ethanol in the vapor phase which results in a corresponding concentration of water in the liquid phase at the bottom of T-502. The liquid phase at the bottom of the Rectification Column contains only about 4.3 weight percent ethanol. This liquid is collected and pumped back (refluxed) to the Beer Column.
There are two side-stream draw lines from the Rectification Column to the Fusel Oil Decanter X-502. This system separates an immiscible layer of fusel oil in the draw streams from T-502. Fusel oil is drawn off, washed with fresh water to remove any dissolved ethanol and sent to storage. The aqueous streams from X-502 are combined and returned to T-502 under gravity flow.
Overview of Ethanol Distillation Unit Operating Conditions
Beer normally contains 3.42 weight percent ethanol and is fed in at a rate of 1,630 GPM. Direct low pressure (LP) steam injection to T-501 is normally 40,000 lb/h and motive steam for the Stillage Flash Ejector is 50,000 lb/h. The produced rate of stillage from the Stillage Flash Drum is 1,755 GPM. It exceeds the beer feed rate because the steam injected into the Beer Column is condensed and refluxed via the Rectification Column, so it ends up in the stillage. The stillage product has very low ethanol content. All the sugars and solids in the beer feed end up in the stillage. The overhead vapor from T-501 contains about 32.5 weight percent ethanol with the balance being water and a small concentration of fusel oils.
The ethanol concentration of the vapor leaving the top of Rectification Column T-502 is 86.4 weight percent. After partial condensation in E-502 and E-503, the net vapor produced from distillation has an ethanol concentration of 88.6 weight percent.
The net vapor from distillation is dried and condensed in X-501. The recovery ratio of ethanol in X-501 is assumed to be 95%. The balance is taken off as a recycle stream to the fermentation section at battery limits. The Ethanol Drying Unit produces 63.6 GPM of fuel grade ethanol (assumed to contain no water) and 9.4 GPM of recycle ethanol/water mixture.
The Ethanol Distillation Unit overhead runs at atmospheric pressure (14.7 PSIA). The two distillation columns run at slightly higher pressures owing to the pressure drop across the columns and heat exchangers in the unit. The Stillage Flash Drum D-501 normally runs at 10.1 PSIA.
T-501 Beer Column
The beer feed is pumped from battery limits to the Ethanol Distillation Unit and is preheated in two heat exchangers (E-501 and E-502) using T-502 overhead vapor and stillage to storage, respectively. The warm beer enters the Beer Column T-501 two trays from the top tray of T-501. The Beer Column consists of 30 special, high-capacity distillation trays designed to handle solids contained in the liquid phase (baffle tray design). This type of tray helps minimize solids in the beer from accumulating within the distillation column. Ethanol and some water vapor flash from the warm beer as it enters T-501. The feed flash vapor combines with ethanol and water vapors rising from the bottom of T-501. Reflux from the bottom of T-502 is pumped to the top tray of T-501. The top two trays of T-501 serve as a wash section to keep any entrained beer from reaching the outlet vapor line from the top of T-501. The top vapor from T-501 is routed to the bottom of Rectification Column T-502.
The liquid beer remaining from the feed flash combines with reflux falling from the top of T-501 and falls down through the baffle trays of T-501. The liquid mixture contacts rising vapors generated by steam injection to the bottom of the Beer Column. The steam strips out the ethanol from the falling beer. There are two sources of steam: direct low pressure (LP) live steam injection and exhaust steam from the Stillage Flash Ejector EJ-501. The ejector also compresses flash vapor from Stillage Flash Drum D-501 (mainly water vapor and a small amount of ethanol) which is routed into the bottom of T-501 along with the motive steam for EJ-501. The liquid that accumulates in the bottom of T-501 is taken off to D-501 by a combination of gravity and differential pressure developed by EJ-501.
D-501 Stillage Flash Drum
The Stillage Flash Drum D-501 receives stillage (water, dissolved sugars and solids) along with a small concentration of ethanol from the bottom of the Beer Column T-501. The pressure in the D-501 is lower than the pressure in the Beer Column T-501 because of the compression of stillage flash vapors by Stillage Flash Ejector EJ-501. This lower pressure ensures that very little ethanol will be present in the flashed stillage liquid.
Stillage Flash Ejector EJ-501 pulls vacuum on the stillage in D-501 using medium pressure (MP) steam as the motive fluid for the ejector. The flash vapors combine with the ejector motive steam and are sent to the bottom of the Beer Column.
Stillage collected in D-501 is pumped by Stillage Pumps P-501A/B which are rotary lobe types of pumps so they can handle the solids present in the stillage. The Stillage Pumps are driven by electric motor. Normally only one pump is in operation.
The stillage from P-501A/B is routed to Beer Preheater No. 2 E-501 to cool down the stillage and heat the beer feed prior to sending the stillage to storage facilities at battery limits.
T-502 Rectification Column
The Rectification Column T-502 concentrates the vapor from the Beer Column using all the overhead condensate from the Reflux Drum D-502 as reflux. In lieu of a reboiler as would be found on a conventional distillation column, the Rectification Column relies on the heat content of the vapor from Beer Column to drive the separation of water and ethanol. T-502 consists of 35 valve trays. Reflux is received on the topmost tray. Net liquid from the bottom tray is collected in the bottom of T-502.
Overhead vapor from T-502 is routed to Beer Preheater No. 1 E-502. Some of the ethanol and water is condensed in E-502 while it heats up the beer feed from battery limits. The warm beer feed continues on to Beer Preheater No. 2 E-501. The partially condensed mixture from E-502 flows into Condenser E-503.
Liquid collected in the bottom of T-502 is pumped to the top of the Beer Column T501 by Rectification Column Bottom Pumps P-502A/B. These are motor-driven centrifugal pumps. Normally, only one pump is in operation.
In order to prevent accumulation of fusel oils in T-502, two side draw lines are provided to continuously purge the oils from the column by routing them to the Fusel Oil Decanter X-502. Fusel oils are produced in the fermentation process that produces the beer feed. Because the boiling points of these oils are often close to that of the temperatures in the Rectification Column, they can end up getting trapped in the column, resulting in a buildup of liquid in the middle of the column which can lead to erratic operation of the Rectification Column. Normally, the lower purge line is in service. Either line draws a fraction of the liquid entering the tray they serve.
The Fusel Oil Decanter X-502 is simulated as a black box in the simulator. It is assumed that all the feed oil to the decanter system is separated and that all the ethanol is recovered by wash water in the secondary separator. The following equipment is assumed to be employed in X-502:
- Primary separator that provides enough residence time to allow the oils to separate from the ethanol/water phase.
- Wash water mixer – oils drawn off the primary separator are mixed and washed with fresh water to remove any ethanol that may have dissolved in the fusel oils.
- Secondary separator that separates the oils and the wash water. The fusel oil byproduct is taken off to storage at battery limits. The water phase from the secondary separator is combined with the ethanol/water phase from the primary separator and returned to a lower tray in the Rectification Column by gravity.
Ethanol/Water Azetrope
NOTE
Because of the non-ideal relationship of vapor pressure versus ethanol concentration for ethanol/water mixtures, it is impossible to achieve an overhead ethanol concentration exceeding 95.6 weight percent using conventional distillation at roughly atmospheric pressure. Unlike more ideally behaving mixtures, the vapor pressure of solutions of ethanol and water reaches a maximum at this ethanol concentration. Such a mixture is called an azeotrope, which derives from Greek meaning “no change in boiling”. Pure ethanol is more volatile than water, so that mixtures containing lesser concentrations of ethanol will more readily boil off ethanol than water. However, as the concentration of ethanol in the liquid increases and approaches the azeotropic composition, the relative volatility of ethanol and water become the same, owing to non-ideal interactions between the two compounds. Thus, boiling off more vapor when the azeotropic point is reached will result in no more concentration of ethanol in the vapor. In fact, the vapor produced by additional boiling of the mixture will have the same exact composition as the liquid. At atmospheric pressure, an azeotropic mixture of ethanol and water will boil at 173.1 DEG F.
As a consequence, the maximum achievable concentration of ethanol in the overhead vapor from the Rectification column is the azeotropic composition. In theory, it would take a large number of trays (and cost) in a distillation column to reach the azeotropic composition. In practice, commercial ethanol distillation columns are designed to produce ethanol concentrations of about 89 weight percent ethanol (corresponding to 95 liquid volume percent or 190 proof). Downstream drying units (as in this simulator) or specially designed distillation processes that use a circulating solvent are commercially employed to remove the water which cannot
be separated using conventional distillation. To qualify as fuel-grade ethanol the water content typically must be lower than 1.0 volume percent (about 98 weight percent ethanol). It is commercially preferable that the water content be even lower than this specification.
Condenser & Drying Section
The partially condensed mixture from E-502 flows into Condenser E-503 where additional condensation of ethanol and water occurs by heat exchange with cooling water. Not all the vapor is condensed in E-503 because the concentrated ethanol stream will be taken off to the Ethanol Drying Unit X-501 which uses molecular sieves operating in the vapor phase to remove the residual water. The amount of condensate and vapor produced in E-503 will depend on how much heat is removed by the cooling water.
Reflux Drum D-502 separates the vapor and liquid from E-503 and collects the liquid condensate. Normally all the vapor is taken off to X-501. In case of an upset to X501, the vapor from D-502 can be vented to atmosphere to maintain a safe pressure in the process equipment.
Collected liquid in D-502 is pumped by Reflux Pumps P-503A/B to the top of Rectification Column T-502. P-503A/B are motor-driven centrifugal pumps. Normally, only one pump is in operation. In case of upsets which might cause overfilling of D502, it is possible to send some or all of the reflux (wet ethanol) to storage.
Ethanol Drying Unit X-501 is simulated as a black box because drying units are highly automated and normally require very little operator attention. The product stream flow rates from the dryer are calculated assuming a constant recovery ratio of ethanol in the vapor feed to the unit (95%). The recycle stream flow rate is computed by material balance.
The drying unit is assumed to employ the following equipment:
- Blower/compressor to move the vapor from the Reflux Drum to the dryer beds
- Dryer beds (molecular sieves)
- Valves and lines for automatic bed switching and regeneration
- Product ethanol condenser, receiver, and product pump
- Regeneration gas heater
- Regeneration gas condenser, receiver and recycle pump
Instrumentation
Beer Column Instruments & Controls
The beer feed to the unit from battery limits is controlled by FIC-501. The beer ethanol concentration is indicated on AI-501 and the temperature is indicated on TI-501. After passing through E-502 and E-501, the preheated beer temperature is indicated on TI-503 prior to entering the Beer Column T-501.
The top pressure of T-501 is indicated on PI-501 and the overhead vapor temperature is indicated TI-504. Tray 20 temperature is indicated on TI-505. The differential pressure across the trays of the Beer Column is indicated on PDI-503. The bottom level of stillage in T-501 is controlled by LIC-501 which adjusts the flow of stillage to the Stillage Flash Drum D-501. The stillage temperature from T-501 is indicated on TI-506. Direct Low Pressure (LP) steam flow to the bottom of T-501 is controlled by FIC-503. The supply pressure of LP steam is indicated on PI-503.
The level of stillage in the Stillage Flash Drum D-501 is controlled by LIC-502 which adjusts the flow of stillage to storage at battery limits. The flow rate of Medium Pressure (MP) steam to the Stillage Flash Ejector EJ-501 is controlled by FIC-504. The supply pressure of MP steam is indicated on PI-504. The pressure of the Stillage Flash Drum is indicated on PI-502. This pressure is affected by the net vapor compression capacity of EJ-501 which is, in turn, affected by the MP steam flow to the ejector. Therefore, adjustments to the MP steam flow will affect the pressure in D-501. Keep in mind that any MP steam flow adjustments will also affect the performance of the distillation columns. More steam will eventually increase the ethanol concentration in the vapor to the drying unit and will increase the reflux in the two columns.
The motors for Stillage Pumps P-501A/B are operated by switches HS-501A/B, respectively. The warm stillage temperature leaving P-501A/B is indicated on TI-507. The temperature of stillage after cooling in E-501 is indicated on TI-508. The flow rate of stillage to storage at battery limits is indicated on FI-502.
Rectification Column Instruments & Controls
The ethanol concentration of the vapor feed to the Rectification Column T-502 from the Beer Column T-501 is indicated on AI-502. The level of the bottom of T-501 is controlled by LIC-511 which adjusts the setpoint of the column bottoms liquid flow controller FIC-511. Rectification Column bottoms is reflux for the Beer Column T-501. The T-502 bottoms liquid temperature is indicated on TI-515. The ethanol composition of the bottoms liquid is indicated on AI-511.
The motors for Rectification Column Bottoms Pumps P-502A/B are operated by switches HS-502A/B, respectively. The flow of column bottoms from P-502A/B to Beer Column T-501 is controlled by FIC-511.
Rectification Column Instruments & Controls
The differential pressure across the trays of T-502 is indicated on PDI-512. The flow of the upper side draw to the Fusel Oil Decanter X-502 is controlled by HIC-511. The upper draw tray temperature is indicated on TI-513. The flow of the lower side draw to the Fusel Oil Decanter X-502 is controlled by HIC-512. The lower draw tray temperature is indicated on TI-514. The wash water flow to X-502 is controlled by FIC-512. The net produced fusel oil product flow to battery limits is indicated on FI-513.
The temperature of tray 28 is controlled by TIC-512 which adjusts the cooling water flow through Condenser E-503. Increasing the cooling water flow will produce more condensate from E-503 which, in turn, will cause the reflux flow to the top of T-502 to increase. As the reflux flow increases, the temperature of tray 28 will decrease because the tray’s ethanol concentration will increase at the expense of the water concentration.
The pressure at the top of T-502 is indicated on PI-511. The overhead vapor temperature entering Beer Preheater No. 1 E-502 is indicated on TI-511. The temperature of the partially condensed vapor leaving E-502 is indicated on TI-516. The beer feed temperature leaving E-502 is indicated on TI-502.
Condenser & Drying Section Instruments & Controls
The process side outlet temperature of Rectification Column Condenser E-503 is indicated on TI-521. The cooling water flow through E-503 is controlled by tray 28 temperature controller TIC-512. Note that TV-512 has a minimum stop of 10% to prevent complete stoppage of cooling water flow through E-503.
The pressure of Reflux Drum D-502 is normally controlled by PIC-521B which controls the flow of concentrated ethanol vapor to the Ethanol Drying Unit X-501. The flow of vapor to X-501 is indicated on FI-523 and the ethanol content of the vapor is indicated on AI-521. PIC-521A will automatically vent the ethanol vapor from D-502 if the pressure gets too high. Its setpoint is 20 PSIA.
The flow of product ethanol from X-502 is indicated on FI-524. The flow of recycle ethanol is indicated on FI-525.
The level of D-502 is normally controlled by LIC-521A which adjusts the setpoint of the Rectification Column reflux flow controller FIC-521. In case the level in D-502 gets too high, LIC-521B will divert wet ethanol from D-502 to storage at battery limits. The setpoint of LIC-521B is 90%.
The motors for Reflux Pumps P-503A/B are operated by switches HS-503A/B, respectively. The flow of reflux (wet ethanol) P-503A/B to Rectification Column T-502 is controlled by FIC-521. The concentration of ethanol in the reflux is indicated on AI-522 and the temperature of the reflux is indicated on TI-522. The flow rate of wet ethanol to storage at battery limits is controlled by FIC-522.