Natural Gas Evaporator

[Desmond Hutchins]

Naturalcirculation evaporators are highly efficient for managing wastewater with low levels of pollutants, which contain components that can be easily evaporated with low boiling points, such as salts, certain organic waste, or chemical products.

In terms of specific applications, this equpment offers excellent results and good profitability in industries such as food processing, chemicals and pharmaceuticals. Natural circulation evaporators also offer a good performance in the production of pure water and the recovery of valuable products or raw materials from aqueous solutions.

Natural circulation evaporators work by applying heat to the industrial effluent, which causes the evaporation of the water and the concentration of the pollutants in the form of a solid residue. Natural circulation refers to the way in which the wastewater circulates inside the evaporator, since the movement of the effluent is produced by the difference in density between the denser solution at the bottom of the evaporator and the lightest solution at upper part.

The production of clean water that can be achieved with this line of evaporators ranges from 10 to 120 liters/hour. These systems are an excellent investment due to their combination of distillate quality, waste concentration capacity and robustness.

Vacuum evaporators can concentrate a residual effluent as many times as it is required, reaching zero liquid discharge if necessary. Thanks to this technology, industrial companies have an efficient solution to manage complex wastewater that can not be treated with more conventional techniques due to its lack of effectiveness or viability.

In these systems, the process starts with the external energy supply to the evaporator to start the evaporation process. The operation of the mechanical vapor compression vacuum evaporators is based on the recovery of the condensation heat from the distillate as a source of heat to evaporate the feed. The steam produced is extracted and compressed by a volumetric compressor with the intention of increasing its temperature. This superheated steam is then returned to the evaporator as a heating fluid. Once the cycle has begun, no more external heat input is required, as the mechanical compression of the steam provides enough heat to maintain the evaporation of the liquid. Upon passing through the exchanger of the evaporator itself, this compressed, and therefore superheated, vapor has two effects: (1) it heats the liquid to be evaporated and (2) it condenses, thereby reducing the need for a refrigeration fluid.

The main advantage of mechanical vapor compression vacuum evaporators lies in the fact that the energy consumption of the volumetric compressor is lower than the cost of producing steam for multi-effect evaporation systems. However, the volume to be evaporated must be high enough so that the saving generated compensates for the investment in the volumetric compressor.

Natural circulation mechanical vapor compression vacuum compressors are an excellent investment due to their combination of distillate quality, high technology and robustness. The distillate production capacity of this product line ranges from 10 to 120 L/h.

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Well suited for working with products insensitive to high temperatures that require large evaporation ratios as well as with products with a high tendency to foul or with Non-Newtonian products, where the apparent viscosity may be reduced by the high velocities.

Where the boiling chamber of the circulation evaporator is divided into several separate chambers, each one equipped with its own liquid circulation system, the heating surface required for high final concentrations can be considerably reduced compared to an undivided system.

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The evaporation rate E is governed by the surface energy balance between net radiation and heat losses due to turbulent convection and evaporation (Fig. 1d). Combining this energy balance with equations of heat and mass transfer can predict E over a saturated water surface from meteorological data (i.e., net solar radiation, relative humidity, air temperature, and wind speed)16. This model has been adapted to understand changes in E over varying surfaces, such as plants17 and soil18, 19.

Because the evaporation rate depends on the vapor pressure deficit between the engine surface and the atmosphere, an increase in w causes a reduction in evaporation rate. Second, the total energy required to evaporate water and extract energy from an evaporation-driven engine is the sum of the latent heat L and the work energy w. We define the ratio of this total energy to the unperturbed case as β:

Here, L is the molar latent heat of vaporization of water in J/mol. Thus, β represents the energy penalty for evaporating water through an evaporation-driven engine versus the case with no engine. Consequently, w affects the energy balance between net radiation and heat loss due to convection and evaporation, because some portion of the energy from net radiation is now removed from the system as work.

Using parameters α and β, it is possible to derive a model that predicts the evaporation rate and power generated from it. Note that w can be dynamically adjusted during operation by varying the resistance of the load so that the water responsive material in the engine must exert a larger force on the load. Thus, it is possible to control α and β. At steady-state, the net radiation leaves the engine surface via convection, evaporation (i.e., latent heat), and power generation. The convective heat flux is proportional to the temperature difference between the engine surface and the atmosphere, whereas the latent heat flux is proportional to the difference in vapor pressures between the engine surface and the atmosphere. The magnitudes of these two energy fluxes also depend on the transport characteristics of the air, which is primarily determined by turbulence and wind speed. Using these relationships, we derived an equation that relates the latent heat flux, F, to α and β (Methods):

Our estimates of steady state evaporation rates and power do not currently consider potential changes in atmospheric conditions due to the reduction in evaporation rates. This can be viewed as a feedback interaction between the engine and the atmosphere. Such feedback mechanisms can be critical to distributed renewable energy systems. For example, atmospheric feedback imposes limits to the maximum power generation of wind turbines28, 29. Therefore, it is important to consider potential feedback effects in our model.

One potential feedback pathway is caused by the changes on the atmosphere due to covering lakes and reservoirs with evaporation-driven engines. The evaporation-driven engine reduces the evaporation rate while increasing the rate of convective heat loss (due to higher surface temperatures). This shift of energy from evaporation to convection mimics the conditions seen when moist soils become dry, where higher convective heat fluxes warm the air due to reduced water availability for evaporation. Previous studies30,31,32,33,34,35,36 show that as previously moist soil become drier, the atmosphere becomes more arid, consistently shifting toward higher air temperatures and lower relative humidities37, 38. These changes contribute toward a reduction in cloud cover39, 40 (i.e., an increase in net radiation). Individually, these changes would increase the potential for evaporation that could result in power densities greater than those for fixed weather conditions, as seen in eq. (3).

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