Distillation: apparatus – Apparatus – Systems
Reexamination Certificate
2000-07-18
2004-02-24
Manoharan, Virginia (Department: 1764)
Distillation: apparatus
Apparatus
Systems
C159S006100, C159S013200, C159S024100, C159S026100, C159S027300, C159SDIG001, C202S202000, C202S236000, C202S237000, C202S238000, C202S187000
Reexamination Certificate
active
06695951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a saline and sewage water reclamation system and process using an extremely efficient vapor compression/vacuum distillation cycle. Though the system and process has its greatest use with saltwater or contaminated water, it also can be used to reclaim other fluids or to remove toxic wastes from fluids.
2. State of the Art
Distillation is a common method known to remove unwanted substances from a contaminated water supply or to remove salt from brine. The process occurs by a selective phase change between the differing vapor pressures of the contaminants and the water vapor. Phase change by evaporating water is the process by which rainwater has been recycled continuously since water first appeared on earth. The earth's water bodies are open systems. Consequently, the balance of differing vapor pressures between the water body and the atmosphere and the heat flux from solar radiation acting on the water body affects the amount of evaporation.
Distillation is slow at atmospheric pressures unless the heat flux is raised to the boiling point of water, 212° F. (100° C.) at sea level. (Metric conversions are approximations.) Therefore, to distill water at atmospheric pressures, heat energy must raise the temperature from ambient to 212° F. (100° C.). At that temperature, water boils and vaporizes, changing from liquid to gas. Once the water vaporizes, a cold source must be present to condense liquid water from the water vapor. One must use additional energy to remove heat from a cold trap and create a continuous cold source to condense the fluid.
Boiling contaminated liquid at atmospheric pressures usually has not been economically viable. For desalination or waste management, the temperature change is from about 70° F. to 212° F. (21° C. to 100° C.), a 142° F. (61° C.) temperature difference (&Dgr;T). In colder climates, the temperature difference often is greater. The necessary energy required to heat water to boiling and to maintain condensers cool enough to condense the vapor makes traditional water distillation systems prohibitively expensive to operate without using costly, “multi-effect” boiling chambers. They have found their niche in specialized applications. For example, in desert regions near an ocean or soda lake, for ships and in space applications, one may trade off the high energy cost for the need for potable water. One also may accept the high energy costs where the contaminants are so toxic that they must be removed from the water.
Not only is higher temperature distillation expensive, it can cause an additional problem. When processing contaminated liquids that contain minerals or organic molecules, higher temperature can cause chemical reactions between the molecules. Some reactions can form high molecular weight molecules that can obstruct boiler walls and make cleaning difficult. High temperatures also break down the walls of organic cells within the contaminates, which can release toxic materials into the liquid. High temperature boiling can cause some lower molecular-weight contaminates to vaporize and migrate with the water vapor toward the condenser.
Reverse osmosis (R.O.) systems also are common for saline water reclamation. They also are costly and are not used for large applications.
Despite problems with ambient-pressure distillation and R.O., desalination capicity in the United States has increased. According to the Office of Technology Assessment, in 1955, for example, the United States had almost no capacity, and less than 30 million gallons per day (Mgal/d) (113.5 million liters per day) could be produced in 1970. By 1985, capacity exceeded 200 Mgal/d (757 million liters per day). Still, that amount is quite small compared to the annual water use in the United States. For example, the United States Geological Service reports that overall fresh water withdrawals in the United States in 1995 was 341,000 Mgal/d (1.29×10
12
liters per day).
Conventional distillation systems use conventional boilers. Boilers are an advanced art whose efficiencies have been studied and documented. See, e.g., McAdams, W. H.,
Heat Transmission
2d Ed., McGraw-Hill 1942, pp.133-137.
Boiler and Condenser Phase Change Processes: The energy required to produce a liquid-to-gas phase change is defined by the heat of vaporization equation given by:
Q=w&Dgr;h
v
. (1)
Where:
Q
=Heat energy; (BTU) (1a)
w
=Weight of fluid to be vaporized; (lbs) (1b)
&Dgr;
h
v
=Heat of Vaporization of the fluid (BTU/lb). (1c)
During a continuous feed flow process, the required energy per unit time is simply the time derivative of equation (1) and defined by:
{dot over (Q)}={dot over (w)}&Dgr;h
v
(2)
where,
{dot over (Q)}
=Heat energy flow; (BTU/hr) (2a)
{dot over (w)}
=“Mass” (weight) flow of fluid vaporized (lbs/hr). (2b)
The heat transfer rate flowing between the boiler and condenser is by a combination of both convective and conductive processes and is given by Newton's Law of Cooling defined by:
{dot over (Q)}=UA&Dgr;T,
(3)
where,
U
=Overall heat transfer coefficient; (BTU/(ft
2
hr ° F.)) (3a)
A
=Overall boiler & condenser area; (ft
2
) (3b)
&Dgr;
T=T
C
−T
B
=Temperature difference between boiler & condenser (° F.). (3c)
One computes the overall heat transfer coefficient by the standard parallel addition of local, individual heat transfers, which yields:
1
U
=
1
h
C
+
1
h
B
+
1
(
k
wall
t
wall
)
.
(
4
)
See, e.g., McAdams, W. H.,
Heat Transmission
, 2d Ed., McGraw-Hill 1942, pp. 133-137. This expression has the following definitions:
h
C
=Condenser local heat transfer coefficient; (BTU/(ft
2
hr ° F.)) (4a)
h
B
=Boiler local heat transfer coefficient; (BTU/(ft
2
hr ° F.)) (4b)
k
wall
,k
fluid
=Thermal conductivities of wall and fluid; (BTU/(ft hr ° F.)) (4c)
t
wall
,t
fluid
=Thickness of the common wall (ft). (4d)
The mass flow, {dot over (w)}, in pounds per hour of purified fluid from the distillation unit can be computed by combining equations (2) and (3) yielding:
w
.
=
UA
Δ
T
Δ
h
v
.
(
5
)
In units of gallons per day, the mass flow, {dot over (W)}
G
equals:
w
.
G
=
C
G
UA
Δ
T
Δ
h
v
.
(
6
)
where,
{dot over (W)}
G
=Mass flow rate; (Gal/day) (6a)
C
G
=Constant conversion to gal/day=(24/8.3454). (6b)
Quantities in each preceding equation are temperature and pressure dependent. Consequently, the optimum thermodynamic cycle for contaminated water or any other fluid depends on the fluid and contaminants. Most fluids have known properties, however. Accordingly, one can account for the particular fluid. Further, a computer microprocessor feedback and control system can adjust for any specified requirements.
Heat Transfer Performance: Equations (5) and (6) show that a linear increase in the overall heat transfer coefficient U increases the system output flow rate linearly. Increasing the temperature difference requires added energy consumption. The ambient input temperature, which is not controlled, determines the working temperature. Therefore, maximizing the heat transfer coefficient without increasing the working temperature T or the temperature difference &Dgr;T is advantageous.
Thin boiler wall thickness: It is important to utilize a very thin boiler/condenser wall surface thickness t
wall
, with metals that have high heat conductivity k
wall
. Typically, the wall thickness is between 0.010 inches to 0.015 inches (0.25 mm-0.38 mm). The heat conductivity for steel, a typical boiler wall surface, is about 25 BTU/(ft hr ° F.) (0.43 watt/cm-° C.), which yields a boiler wall conductivity heat transfer rate of between 20,000 and 30,000 BT
Bitterly Jack G.
Bitterly Steven E.
Kleinberg & Lerner ,LLP
Manoharan Virginia
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