Evaporation

Reprinted from The Alcohol Textbook 5th Edition 2009 with permission from Lallemand Ethanol Technology and Nottingham University Press

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Evaporation technology uses convective heat transfer to concentrate nonvolatile substances in solution or suspension, producing higher-solids-concentration products. The energy-driving evaporation process uses steam or other process streams. In evaporation, energy is applied to a liquid at constant pressure, raising the temperature to saturation - the point where it holds as much energy as possible without boiling. As additional energy is applied, the vapour pressure of the liquid reaches the vapour pressure of the surrounding environment, and the liquid begins to vaporise. The heat of vaporisation is the amount of energy required for the liquid to change state to a vapour without a change in temperature. The resulting vapour separates from the residual liquid, increasing the concentration of the nonvolatile fraction.

The heat-transfer process is defined by Fourier's law (Figure 9), where

Figure 9. Fourier's law equation defining the heat-transfer process.

Thin stillage is predominantly an aqueous suspension of soluble and insoluble grain solids and nondistillable fermentation end products. During stillage evaporation, thin stillage, which contains from 5 to 10% total solids, is concentrated to produce a nominal 30 to 50% total solids. Figure 10 illustrates the temperature, enthalpy and state relationship of water, showing the quantity of energy required and available as water changes from a liquid to a vapour during the evaporation process.

Figure 10. The relationship of water temperature, enthalpy and the state of water.

Evaporator systems

A simple industrial evaporator system (Figure 11) will contain the following:

• Calandria, or heat exchanger, which transfers energy from the source stream to the solids-containing fluid, raising the fluid's temperature to the boiling point
• Circulation, or feed, pump, which supplies feed to the evaporator's heat exchanger
• Distributor, which distributes feed or circulating fluid evenly across the faces of the tube sheets of the tubular evaporator calandrias, ensuring that the surfaces of the gravity-fed tubes are thoroughly wetted
• Transfer pump, which moves enriched-solids-containing fluid from the evaporator calandria
• Vapour separator, which separates the water vapour from the enriched-solids-containing fluid
• Condenser, which removes energy from the evaporator via heat transfer with another fluid
• Vacuum source, which removes noncondensable components in the vapour

The design of a simple evaporator is shown in Figure 11, where approximately one unit of steam is condensed on the shell of the calandria, transferring the heat of condensation to feed located in the tubes, evaporating one unit of water. When the feed temperature is below saturation, additional energy will be required to raise the liquid to the boiling point at the system pressure. The vapour produced in the evaporator flows through the separator, removing entrained liquid before condensing and transferring the energy to cooling water, with the concentrated product pumped to storage. The uncontaminated steam condensate is returned to the boiler for reuse. Together, the components are referred to as an evaporator 'effect'.

For energy transfer to occur, a temperature differential must exist across the heat-transfer surface area. Because steam and stillage are both predominately water, a temperature differential must be accompanied by a corresponding pressure differential, as illustrated in Figure 8.

Figure 11. Components of a simple evaporator.

Figure 12. Single-effect evaporator.

The efficiency of the simple evaporator system in Figure 12 results in about one unit of steam removing one unit of water with a near-equal quantity of energy transferred to the cooling water. Figures 13 through 15 illustrate options for further improvement in system efficiency. In these designs, the vapour directed to the condenser (shown in the simple evaporator illustrated in Figure 11) is routed to succeeding evaporation stages, or effects. So, the first evaporator effect condenses the incoming steam, producing a near-equal amount of vapour that condenses in the second effect.

Figure 13. Two-effect evaporator

Figure 14. Three-effect evaporator

Figure 15. Four-effect evaporator

As seen in these figures, additional evaporator effects remove more water per unit of steam supplied, and system efficiency improves. From this, it appears that evaporation systems could be infinitely efficient through the addition of effects, but design and operating parameters dictate otherwise. Critical issues include

• the minimal practical condensing temperature of the final vapour, which is a function of the cooling water temperature,
• the maximum practical product side temperature in the first effect, which is a function of the thermal stability and fouling potential of the feed,
• the practical differential temperature across the individual effects, considering operating parameters such as fouling of heat-transfer surface area and boiling-point elevation of the product and
• cleaning frequency.

A multi-effect stillage evaporation system, with a first-effect product side temperature of 210°F and a final condensate temperature of 130°F, that operates with a 15°F differential temperature across each effect could be designed with approximately five effects.

Thermocompression evaporators

Alternatives exist to improve evaporator efficiency without the continued, capital-intensive addition of effects. Such evaporator systems have been used in dryhouse designs for over 25 years. These systems increase efficiency by recycling vapour from later effects to preceding effects in the evaporator. To accomplish this, the pressure of the vapour must be increased by thermocompression (Figure 16) to offset the design-basis pressure drop of the system.

Figure 16. Thermocompression evaporator

Figure 17. Steam ejector (Croll Reynolds Company, Inc.)

Evaporator vapour is generally boosted by taking a portion of the vapour from one of the effects and directing it to a steam ejector (Figure 17). This device produces a Venturi effect, where ejector fluid under high pressure is converted into a high-velocity jet at the throat of the nozzle, which creates a low pressure at that point. The low pressure draws the suction fluid into the nozzle, where it mixes with the motive fluid, resulting in an intermediate pressure-vapour mixture. The quantity of vapour recycled is a function of the design of the ejector, the motive steam pressure and the pressure of the evaporator vapour. A disadvantage of steam ejector systems is that the motive steam is often contaminated with impurities present in the evaporator vapour. In stillage evaporators, the condensate contains measurable concentrations of ethanol and organic acids and cannot be reused as boiler feed makeup water.

Mechanical compression evaporators

Figure 18. Mechanical vapour recompression (MVR) evaporator

Another variant of vapour-compression technology uses electrical or steam-turbine-driven devices such as fans, blowers or compressors to boost the pressure and recycle the evaporator vapour. In mechanical vapour recompression (MVR) evaporation (Figure 18), the vapour from the separator, free of entrained liquid, is compressed, elevating the condensing temperature. The vapour directed to the shell of the evaporator body condenses, transferring energy back to the circulating fluid. A small amount of additional energy is required to balance the system's enthalpy, 'replacing' energy that is required to raise the temperature of the incoming feed to the operating conditions of the evaporator.

Design-basis considerations for MVR systems are a compromise between

• compressor power (increasing the pressure differential increases power demand, resulting in lower system reliability) and
• exchanger surface area (higher boost pressures increase vapour temperatures, providing greater differential temperatures across evaporator bodies, reducing heat-transfer surface areas).

In stillage evaporation, compressing large vapour flows translates to high power demand. MVR systems are best suited for applications where

• high-pressure steam is available for an exhaust,
• an extractive turbine-drive is applied and
• low-cost power allows for an electrical drive.

Fluid properties and evaporator system design

Fluid physical properties must be taken into consideration during the design of evaporator systems. Stillage is a complex mixture of inorganic salts, organic acids, soluble and insoluble proteins, peptides and amino acids, carbohydrates, sugar alcohols such as glycerol, lipids and fibre fines.

A key property is fluid boiling-point elevation. When a solute is added to a solvent, the vapour pressure of the solvent (above the resulting solution) is less than the vapour pressure above the pure solvent. The resulting boiling point of the solution will be greater than the boiling point of the pure solvent. This is because the solution (which has a lower vapour pressure) must be heated to a higher temperature in order for the vapour pressure to become equal to the external pressure. During evaporation, as the solids concentration of the stillage increases, boiling-point elevation reduces the effective differential temperature and increases the required heat-transfer surface area.

Another physical property having a significant impact on evaporator design and performance is fluid viscosity. As dilute feed streams concentrate, fluid viscosities increase, exhibiting both Newtonian and non-Newtonian properties:

• A Newtonian fluid is a fluid whose viscosity does not change with the rate of flow or shear stress.
• A non-Newtonian fluid is a fluid whose viscosity changes with the rate of flow or shear stress.

Figure 19 illustrates the shear-temperature relationship in thick-stillage streams that contain high concentrations of suspended solids.

Figure 19. Concentrated stillage viscosity profile over a range of temperature and revolutions per minute during testing

Elevated fluid viscosities interfere with film formation in falling-film evaporation systems, resulting in uneven wetting of heat-transfer surfaces. In high-velocity forced circulation evaporation systems such as finishing evaporators, increasing viscosities generate laminar flow, reduced Reynolds numbers and corresponding lower heat-transfer coefficients. Increasing fluid velocity in an effort to improve heat transfer has the adverse effect of increasing system pressure drop and power demand. In all instances, increased fluid viscosities accelerate fouling of heat-transfer surfaces, with fouling being most severe in high solids effects.

Evaporator clean-in-place (CIP) systems

The design-basis criteria of a stillage evaporation system must consider fouling rate and the associated cleaning process. Most evaporation systems are equipped with CIP systems that chemically clean heat exchangers, separators and associated product-side piping without opening the equipment. Heat-transfer-surface fouling rate and the associated CIP frequency and CIP cycle time detract from the onstream time of the evaporator system. In heavily fouled stillage evaporators, one finds complex, mixed organic and inorganic deposits on the heat-transfer surface. Cleaning stillage evaporators requires the following steps, which can require up to twenty hours to complete:

• Initial rinse. Hot water flushes stillage from the system and rinses away easily removable deposits, reducing chemical consumption during subsequent CIP steps.
• Caustic wash. Dilute caustic solution attacks and partially solubilises the deposit's organic matrix.
• Intermediate rinse. Hot water rinses dilute caustic from the system in advance of the ensuing acid wash.
• Acid wash. A dilution acid solution, such as sulfamic acid, attacks and partially solubilises the calcium oxalate and calcium-sulphate-rich inorganic matrix of deposits.
• Final rinse. Hot water flushes residual acid and dislodged deposits from the system.

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