Multiple effect distillation process with reduced fouling

Details Multiple effect distillation process with reduced fouling Multiple effect distillation process with reduced fouling

1999-12-22

2003-04-22

Manoharan, Virginia (Department: 1764)

Distillation: processes, separatory

With measuring, testing or inspecting

C023S29500G, C159S024100, C159S045000, C159S047300, C159S901000, C203S010000, C203S024000, C203S026000, C203S027000, C203S071000, C203S098000, C203S048000, C203SDIG008, C210S774000

Reexamination Certificate

active

06551466

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a highly efficient water distillation process and an apparatus thereof and more particularly, the present invention is directed to a highly efficient water distillation process which minimizes fouling and scaling of operating equipment over long periods of operation.
BACKGROUND OF THE INVENTION
Generally speaking, water distillation is a highly effective method of vaporizing a pure water distillate and recovering a concentrated liquid containing a large quantity of non-volatile components. This process method can be an effective means to recover clean pure water from contaminated sources. However, water distillation processes typically have several problems not the least of which can be fouling or scaling of the apparatus With minerals or other components from the fluid being distilled. Common scaling compounds consist of calcium, magnesium and silicon. Fouling, or to a greater extent, scaling of the heat transfer surfaces have a detrimental effect on the capacity of the heat transfer components, causing conventional distillation processes to become inoperable.
Another common problem with typical water distillation processes is that of the high energy input requirements. Without a means to effectively recover the input energy, the energy required is equivalent to the latent heat of vaporization of water at a given pressure/temperature. Water distillation, under this condition is not commercially viable for water remediation applications.
Several variables must be considered to overcome the problems with conventional distillation methods. The following three equations describe the basic heat transfer relationships within a water distillation system:
Q
(
total
)
=
U
*
A
*
LMTD
(
1
)
Q
(
sensible
⁢
 
⁢
heat
)
=
m
*
CP
*
(
T1
-
T2
)
(
2
)
Q
(
latent
⁢
 
⁢
heat
)
=
m
*
L
(
3
)
In order to have an efficient distillation system, the quantity of heat exchanged and recovered, Q, expressed by the above stated equations, must be maximized, while at the same time obeying the practical limits for the remaining variables and preventing scaling and fouling. For a given fluid and fluid dynamics within a given heat exchange apparatus, the variables, U, Cp and L are relatively non-variable. Therefore, careful consideration must be given to the variables A, Q/A, LMTD, m, and T
1
& T
2
to overcome the problems associated with distillation of contaminated water.
To fully overcome the problems related to distilling contaminated water and eliminate scaling, other essential factors must be considered beyond the basic equations stated above:
the rate by which the heat is transferred within the distillation system, known as heat flux or QA
−1
(Btu hr
−1
ft
−2
)
the level of contaminates in the concentrate;
the final boiling point of the concentrate relative to the saturation temperature of the vapor stream;
the degree of supersaturation and level of precipitation of the concentrate; and
level of vaporization of the evaporating stream.
Until the advent of the present invention, maximizing the quantity of heat transferred and recovered with a water distillation process, without the tendency of fouling or scaling, could not be realized over a long term continuous period.
A process has been developed which is both energy efficient and eliminates the problems of scaling previously encountered in the distillation of contaminated water, contaminated with organics, inorganics, metals, inter alia.
SUMMARY OF THE INVENTION
The invention further advances the concepts established in the initial application. These former concepts linked two distinct concepts, both of which have been previously identified singularly in the prior art, but which have not been uniquely configured with the synergistic effect that results with the present invention. It has been found by combining a conventional vapor recompression circuit and excess low grade heat energy source together with a uniquely configured forced convection heat recovery and transfer circuit, that very desirable results can be obtained in terms of maximizing heat transfer, minimizing compression power requirements and maintaining the desired forced convection circuit non-conductive to scaling exchangers, which is typically encountered by practicing standard distillation methods.
It has now been found that the use of the waste stream energy can be recovered in the heat transfer circuit and this source of low grade energy, most commonly discharged as unrecoverable energy, is employed to reduce the quantity of requisite compression to treat waste water.
By this methodology, the excess vapors passing from the evaporator that are created by the recovered heat energy, do not pass into contact with the compressor, but rather are externally condensed. The result of this is the accumulation of energy, which acts as a driving force for a subsequent crystallization step, leading to the precipitation of solids for disposal.
One object of the present invention is to provide an improved efficient process for distilling water containing organic, inorganic, metals or other contaminant compounds with the result being a purified water fraction devoid of the contaminants which additionally does not involve any scaling of the distillation apparatus.
A further object of one embodiment of the present invention is to provide a method of removing contaminants from a water feed stream containing contaminants, comprising the steps of:
a) providing a water feed stream;
b) providing a source of waste energy for vaporizing the water stream;
c) providing a fluid circulation circuit including a heated separator and a reboiler exchanger in fluid communication;
d) passing the water feed stream into the heated separator;
e) passing the waste energy into the reboiler for recovery of heat energy;
f) vaporizing the water stream with the waste energy in the reboiler exchanger to generate a vapor fraction and a concentrate liquid contaminant fraction;
g) circulating at least a portion of the concentrate liquid fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of 300 to near 2 to result in a vapor fraction of near 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler; and
h) condensing the vapor fraction with an external condensing means;
i) collecting condensed vapor fraction substantially devoid of contaminants.
It has been found that by precisely controlling the ratio of circulating mass in a range of less than 300 to near two times that of the vapor fraction being compressed, several desirable advantages can be realized:
1. The circulating concentrate through the evaporating side of the reboiler will contain a precisely controlled vapor fraction near 1% to 50% of the mass of the circulating concentrate;
2. By precisely controlling this vapor fraction, the temperature rise of the circulating concentrate remains very low (about 1F) and reboiler heat exchange surfaces remain wetted, at a temperature near that of the circulating concentrated fluid. This reduces the risk of fouling of these surfaces;
3. With this controlled low vapor fraction, the concentrated fluid within the exchanger is subjected to a greatly reduced localized concentration factor of less than 1.1, avoiding localized precipitation of scaling compounds on the exchanger surfaces;
4. As the vapor mass is formed toward the exit of the reboiler, the stream velocities within the exchange passages increase significantly promoting good mixing and thus reducing the risk of fouling;
5. By allowing a controlled vapor fraction in the evaporating fluid, significant heat transfer can be realized through the means of latent heat, without scaling and causing a temperature cross within the heat exchanger;
6. Because the temperature rise of the evaporating side of the reboiler is kept very low, the LMTD of the reboiler is maintained, thereby keeping the input energy requirement very low;
7. By adjusting the heat fl

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