Heating options for thermal separation plants
Traditionally, an evaporator or crystallizer is heated by live steam, but waste heat (e.g. recompressed vapor, drier vapor, hot water or thermal oil) can be used as energy source as well, as long as the amount of energy required for the thermal separation process is given.
Steam-heated (Single / Multiple-effect)
A Product / B Concentrate / C Condensate / D Heating steam / E Vapor
For the evaporation of water in a single effect evaporator, about 1 t/h of live steam will produce about 1 t/h of vapor since the specific evaporation heat values on the heating and product sides are about the same.
If the product vapors from an evaporation effect are used to heat another evaporation effect operated at lower pressure, the steam consumption of the overall system will be reduced accordingly.
If the amount of vapor produced by primary energy is used as heating steam in a second effect, the energy consumption of the overall system is reduced by about 50 %. This principle can be continued over further effects to save even more energy.
| Live steam [t/h] | Vapor [t/h] | Specific steam consumption | |
| 1-effect plant | 1 | 1 | 100% |
| 2-effect plant | 1 | 2 | 50% |
| 3-effect plant | 1 | 3 | 33% |
| 4-effect plant | 1 | 4 | 25% |
The maximum heating temperature of the first effect in combination with the lowest boiling temperature of the final effect, define an overall temperature difference which can be divided among the individual effects.
This means, in case the number of effects increases, the temperature difference per effect decreases accordingly.
For this reason, the heating surfaces of the individual effects need to be dimensioned accordingly larger to achieve the required evaporation rate at a lower mean temperature difference (∆ Tm). A first approximation shows that the total heating surface of all effects increases proportionally to the number of effects. As a result, the investment costs rise considerably while the amount of saved energy becomes proportionally lower.
Thermal vapor recompression (TVR)
A Product / B Concentrate / C Condensate / D Heating steam / E Vapor
During vapor recompression, vapor from the separator is recompressed to the higher pressure of a heating side of the tube bundle.
Approximately half of the vapors produced by the evaporation process can be reused for heating, the other half flows to the next effect to drive the process there. A certain steam quantity, the so-called “motive steam”, is required for the operation of a thermal vapor recompressor.
For thermal vapor recompression (TVR) steam jet compressors are used. They operate according to the steam jet pump principle. They have no moving parts and therefore no wearing parts, this ensures maximum operational reliability. Thermal vapor recompressors are designed in-house.
GEA has over a hundred years of experience supplying steam jet pumps and compressors.
Mechanical vapor recompression (MVR)
A Product / B Concentrate / C Condensate / D Heating steam / E Vapor / El Electrical energy
Whereas steam jet compressors only compress part of the vapor that leaves the evaporator, mechanical vapor recompressors (MVR) are capable of recycling all of it.
The vapor is recompressed to the pressure of the corresponding heating steam temperature of the evaporator, using a small amount of electrical energy compared to the enthalpy recovered in the vapor. The energy of the vapor condensate is frequently utilized for the preheating of the product feed.
Depending on the operating conditions of the plant, a small quantity of additional steam or the condensation of a small quantity of excess vapor may be required to maintain the overall evaporator heat balance and to ensure stable operating conditions, especially during start-up.
Due to their simplicity and maintenance-friendly design, single stage centrifugal fans (supplied as high-pressure fans) are used in evaporation plants. They operate at high flow velocities and are therefore suited for large flow rates at vapor compression ratios of 1:1.2 to 1:2. Rotational speeds typically ranges from 3,000 up to 12,000 rpm. For high pressure increases, multiple fans can be used.
Steam vs. Electricity
Depending on local conditions and project-specific utility costs the most ecological and cost-efficient heating option can be evaluated based on typical consumption figures.
The possibility to use electricity from renewable sources instead of steam from fossil fuels is one of the biggest advantages that the electrical heating offers.
Specific utility consumption per ton of evaporation
| Steam [t] | Electricity [kWh] | Cooling water [m³ (∆T = 10K)] | |
| Live steam | 1 | minor | 60 |
| 2-effect | 0.5 | minor | 30 |
| 3-effect | 0.33 | minor | 20 |
| 4-effect | 0.25 | minor | 15 |
| TVR | ~0.5 | minor | 30 |
| MVR | minor | ~30-50 | minor |
Not less important and of special relevance for arid regions, the use of electrical heating reduces drastically the need for cooling water. All in all and with circular economy and the new tendencies to reduce energy consumption taking an ever more preponderant role, MVR is surging as the option to go with.
Several crystallization processes are operated at low temperatures due to the constraints of the phase system or temperature sensitive products. Therefore, efficient cooling is required which could be induced either by Vacuum Cooling or by Surface Cooling.
Dryer vapor heated (Evaporation plants)
The main criteria for the use of dryer vapor to heat evaporation plants is the dew point of the water steam-air mixture given by the presence of inert gases and/or air in it. The higher the dew point, the higher the water steam content and thus the usable energy content.
If the dryer vapors contain dust and grease vapors, these could land in the calandria, disrupting or even preventing the heat transfer from the heating to the boiling chamber. This can be avoided by cleaning the dryer vapor in a GEA vapor scrubber. Usually, a jet scrubber, a self-priming liquid jet fan, is perfectly suited for this application.
The vapor scrubber can be operated with the vapor condensate from the evaporation plant. Thus, no additional process water is necessary -this way, the energy was reused for heating the evaporator and simultaneously the dryer exhaust was cleaned.
Vacuum Cooling (Crystallization plants)
Vacuum cooling is the preferred method as the cooling is only generated by adiabatic expansion of the solvent and no active cooling surface is required. Such an active cooling surface would need to be relatively large due to limited heat transfer coefficients and present the risk of scaling due to decreasing solubilities at lower temperatures.
The major parameter as well as the limitation for vacuum cooling is the pressure of vapors generated during the process. Depending on the required pressure, the most cost-efficient mix shall be chosen from between the following available options:
- Surface condensation or direct condensation
- Cooling water or chilled water
- With steam ejector or without steam ejector
Surface Cooling (Crystallization plants)
Surface cooling is applied when the required temperature cannot be reached by vacuum cooling.
This method uses an active cooling surface (tube bundle heat exchangers) cooled by any available cooling media suitable for the required process temperatures.
Those systems present the risk of showing scaling tendencies at heat exchanger tubes due to high local supersaturations at cold surfaces.
To maximize the operating cycle of such units, its design needs to be highly sophisticated with regards to cooling profile, design heat transfer coefficients, solid densities and tube velocities as well as quality of the tubing. Further to that, efficient cleaning procedures are available minimizing the downtime to an acceptable level.
Steam Ejector vs. Chiller (Crystallization plants)
Either a Steam Ejector or a Chiller shall be used for cooling purposes.
While Steam Ejectors are compressing the vapors to a temperature level, enabling the condensation against cooling water, Chillers use electrical energy to generate a cold media enabling the condensation of low-pressure vapors.
Depending on local site conditions as well as utility availability and pricing, the decision is individually taken to offer the most cost-efficient and energy-savvy solution to every customer.
With circular economy and the new tendencies to reduce energy consumption taking an ever more preponderant role, the Chiller is surging as the option to go with. Already existing plants have been revamped to change their Steam Ejector for a Chiller.
