Distillation process with reduced fouling

Details Distillation process with reduced fouling Distillation process with reduced fouling

1998-01-14

2002-03-12

Manoharan, Virginia (Department: 1764)

Distillation: processes, separatory

With measuring, testing or inspecting

C203S002000, C203S003000, C203S010000, C203S024000, C203S026000, C203S023000, C203S027000, C203S071000, C203S098000, C159S024100, C159S044000, C159S047300, C159S901000, C210S774000

Reexamination Certificate

active

06355145

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 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*
(
T
1
−
T
2
)  (2)
Q
(latent heat)
=m*L
  (3)
where
Q=quantity of heat transferred (BTU hr
−1
)
U=overall heat transfer coefficient or ability of system to transfer heat (BTU hr
−1
ft
−2
F
−1
)
A=heat transfer surface area (ft
2
)
LMTD=log mean temperature difference or the thermal drive of the system (F)
m=mass flow of fluid in liquid or vapor state (Ib hr
−1
)
Cp=fluid specific heat (BTU hr
−1
F
−1
)
T
1
,T
2
=temperature of fluid entering or exiting system (F)
L=latent heat of vaporization or condensation (BTU Ib
−1
)
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 is predicated upon the marriage of 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 employing a conventional vapor recompression circuit 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 and maintaining the desired forced convection circuit non-conductive to scaling exchangers, which is typically encountered by practicing standard distillation methods.
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 the present invention is to provide a method of removing a contaminant from a water feed stream containing the contaminant, comprising the steps of:
a) providing a feed stream;
b) heating the feed stream in a first step to at least partially remove some of said contaminants from the feed stream and recover energy from a concentrate and distillate;
c) heating the feed stream in a second heating step in a heated separator to generate a vapor fraction and a concentrate liquid contaminant fraction;
d) compressing the vapor fraction to generate a temperature differential in reboiler exchanger;
e) passing the vapor fraction into contact with the reboiler exchanger to provide a condensed distillate from the reboiler;
f) circulating at least a portion of the concentrate through the reboiler exchanger and the heated separator to maintain a ratio of circulating mass to vapor mass of about 300 to about near 2; and
g) collecting the condensed distillate 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 2 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 cold heat exchange surfaces remain wetted, at a temperature near that of the circulating 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 an additional localized concentration factor of less than 1.1, avoiding localized precipitation of scaling compounds;
4. As the vapor fraction increases and the concentration factor increases while passing through the reboiler, the stream velocities increase significantly 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;
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 compression energy very low; and
7. By adjusting the heat flux, the temperature of the wet surfaces for condensing and evaporating are maintained near that of the saturated steam condition. The type of boiling experienced will range from primarily forced convection to stable nucleate boiling of the wetted surfaces.
One object of the present invention is to provide a method for removing contaminants from a feed stream containing contaminants by employing a heated separator and a heat exchanger and preventing the fouling of and formation of scale on said separator a

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