unit operation performance testing of cascade distillation

44th International Conference on Environmental Systems ICES-2014-0014 13-17 July 2014, Tucson, Arizona

Unit Operation Performance Testing of Cascade Distillation Subsystem

David Loeffelholz, Ben Baginski, Vipul Patel, and Allen MacKnight Honeywell, International, Torrance, CA 90504

and

Sarah Schull, Miriam Sargusingh, and Michael Callahan Johnson Space Center, Houston, TX

The Cascade Distillation System (CDS) is a waste water recovery technology being developed under NASA’s Advanced Exploration System (AES) water recovery project. The Cascade Distiller (CD) is the principal component of the CDS. The CDS prototype unit was extensively tested at NASA Johnson Space Center (JSC) during 2008 and 2009. In 2012 the need for additional CD testing was identified to determine thermodynamic, hydraulic, and distillation performance through experiment at operating conditions of interest. This paper discusses this operational testing performed at Honeywell in 2012 on the prototype Cascade Distiller. The thermodynamic performance demonstrated an average specific energy for recovered water of 92 watt-hr/kg. The limiting process recovery of distilled water was 95% of the feed on a weight basis. The distillation performance was tested using a chemical analysis method that used a water soluble red dye. This new method allows qualitative and quantitative measures of the concentrations of salt in the CD and the distillation efficiency.

Nomenclature AES = advanced exploration system CDS = cascade distillation system CD = cascade distiller GEN2 = generation 2 cascade distiller ISS = international space station LEO = low earth orbit RTD = rapid technology program THP = thermoelectric heat pump WRP = water recovery project

I. Introduction he Cascade Distillation system (CDS) technology has been in development for a number of years[1-12]. Beginning in early 2000, Honeywell (Torrance, CA) working in conjunction with Thermodistillation Company

(Kiev, Ukraine), sponsored and led the development of the cascade distiller (CD), a five-stage vacuum distillation mechanism. The CD distills wastewater at reduced pressure to efficiently produce purified water condensate and concentrated brine waste. In 2004, Honeywell received funding through NASA as part of a Rapid Technology Development (RTD) program to deliver a prototype cascade distillation subsystem. The objective of the RTD was to promote the CDS.

The Advanced Exploration Systems (AES) Water Recovery Project (WRP) was initialed by NASA to develop advanced water recovery systems that enable NASA human exploration missions beyond low Earth orbit (LEO)[13]. As part of the WRP Honeywell was contracted to design an improved Generation 2 Cascade Distiller (GEN2) in 2012. The GEN2 project is being managed by the Johnson Space Center Crew and Thermal Systems Division. Previous testing of the CD demonstrated its capability for water recovery but more precise test results were needed to support the GEN2 design activity. This paper discusses unit operation style tests completed on the CD in late 2012. Of principal interest was the relationship between rotational speed and CD performance.

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A. Unit Operations Test Objectives

Unit operations are used in chemical engineering to aid in the design of large scale systems with many functional operations. The unit operation defines the physical, thermal, hydraulic and chemical properties that in the operation. These tests have as their main objective to define how the CD operates. Previous tests have concentrated on the thermal and hydraulic effects in the CD but did not provide detail on the changes in mineral (salt) content of the solutions. The unit operations for the CD must provide quantitative measure of the principle function of the distiller which is to remove salt from the inlet feed stream. The quantitative measurement of salt concentrations is therefore a prime objective of these tests.

The general objective of the unit operations test was to determine the steady state operation of the distiller as a

function of rotor speed and process duration. More specific goals were to: Quantify the fluid flow rates and temperatures as function of the rotor speed and process duration Determine water recovery rates and water purity as a function rotor speed and process duration Determine hydraulic and process power as a function of rotor speed and process duration Quantify the salt concentration in the product and brine as a function of speed and process duration using

accurate chemical analysis methods Determine distiller thermal efficiency as a function of rotor speed and process duration

B. Cascade Distillation System

The cascade distillation system is shown in simplified form in Figure 1. The system consists of two main components: the multistage vacuum rotary distiller and a thermoelectric heat pump. The feed liquid, such as preserved urine, is fed to cascade distiller (CD) where evaporation and condensation of water takes place. The multiple stages operate in parallel to provide a high rate of water production. The energy for the process comes from the thermoelectric heat pump (THP), where the distillate water is cooled and the process liquid is heated. Both streams are pumped by the CD in loops to the heat pump and return to the CD. The temperatures of the process are 40 to 50C for the hot loop and 20 to 25C for the cold loop.

The other components of the system are used for the storage and control of the liquids used in the process. The feed and withdrawal of liquids are controlled by differential pressure regulators that do not require a digital controller. The feed liquid is held in a run tank at ambient pressure and is delivered to the hot loop through a differential pressure regulator. The system operates at a vacuum and when the volume of the hot loop decreases due to distillation, its pumped pressure decreases and more feed is driven into the CD. Condensate is delivered to a product tank through a pressure-controlled valve operating in the reverse direction. In this case the product tank is also held at the system vacuum pressure. When the cold loop volume increases due to distillation, its pumped pressure increases and the valve will open to deliver product.

The process is operated as batch process to obtain the maximum recovery of water from the feed liquid. The CD distills product water from the hot loop depleting the volume of the hot loop. Feed liquid is added to the hot loop to maintain a constant volume in the hot loop. This process continues until the hot loop becomes concentrated brine and the distillation temperature increases. At this point the heat pump is turned off and pressure is restored to ambient. This usually occurs when over

Figure 1. Cascade Subsystem Functional Diagram


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90% of the water in the feed is distilled and collected in a product tank. The brine is then pumped out of the system to the brine tank and the CD is turned off. A typical batch consists of processing 10 liters of feed, producing nine liters of product water and one liter of brine.

C. Test Facility

The unit operation tests were conducted at the Honeywell grey water test lab in Torrance, California. The lab has been operational since 1999 and was modified to accommodate the needs of the unit operations tests. The general test setup is described in detail in a previous paper [9]. A picture of the test facility is shown in Figure 2. The round object is the CD. It is surrounded by tubing, valves, sensors, and white plastic sample ports used to take samples of the fluids during the tests. Product water and brine samples were taken every 5 minutes during a test and were used to determine brine concentrations.

Figure 2. Partial view of experimental setup. Partial view with CD ports show near top center

II. Test Plan The approach to the unit operations tests of the CD was to conduct distillations of batches of sodium chloride

feed solutions. During the distillation run 27 values indicated in Table 1 were measured at 5 minute intervals. A complete test run typically required 19 measurement intervals, generating about 513 data points per run for further analysis.

The objective of the tests was to determine the CD performance as a function of rotor speed. Distillation runs were taken for rotor speeds of 1000, 1100, 1200, 1300, 1400, and 1495 rpm. The maximum speed for the CD was 1495 rpm which was limited by the motor power. The full project has a data bank of 3078 data points.

A. Concentration Measurements

One of the objectives of the unit operations testing was to quantify the salt concentrations in product water and brine as the distiller operated. Prior CDS system tests had used conductivity probes as an indicator of salt concentration. This measurement method was inadequate for our purposes since the conductivity measurement was subject to many systematic errors and was very limited in its capability to measure the range of concentrations present in the CD. The systematic errors are due to its sensitivity to noise variables of temperature and ion concentration. During operation, the range of salt concentrations developed in the CD span 4 orders of magnitude, from salt concentrations from 0.0058 g/liter in the product water to 70.0 g/liter in the brine.


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Table 1 Test Run Data Collection

Product

Out Ambient CD Chiller THPIn Out In Out In Out Feed Product Brine

Pressure X X X X X X X

Temperature X X X X X X XVolumetric Flow Rate X X XTank Weights X X X

Rotational Speed X

Power X X

Concentration X X X X

Hot Loop Cold Loop Feed Tanks

Measurements were taken with instruments as shown in Table 2

Figure 3. Red dye #40 calibration curve for standard solutions. In an effort to generate a measurement that was less sensitive to feed concentration and temperature, an indirect

spectrophotometric measurement was chosen. While direct measurement of ion concentration would have been preferable for a given feed, unknown future feed compositions favored the development of an indirect method.

Table 2 Instrumentation for Data Collection

Measurement Meter Type Manufacturer Range Accuracy Mass Flow Coriolis Meter Micromotion 0-5 gpm 0.5 % Temperature Surface Mount Watlow 100 ohm +/- 1 °F Platinum RTD Generic 100 ohm +/- 1 °F Type T

Thermocouple Generic -328 to 662 °F +/- 2 °F

Pressure Transducer GE - Druck 0 to 20 psia +/- 1 % Rotation Motor Controller +/- 1 RPM Mass Platform Scale Kistler 0 to 50 kg +/- 0.01 kg Concentration UV/Vis

Spectrometer HP 8453 300 – 700 nm See below


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Tracer dye (Red Dye #40, Allura Red) was introduced into the feed at a known NaCl concentration. The measured concentration of the dye at any time was then proportional to the concentration of non-volatiles remaining in the sample. The dye was selected for non-toxicity and a strong signature visible spectrum. Use of the dye had the important added benefit of providing immediate qualitative feedback on the unit’s operation. Measurement of concentration was accomplished with an HP-8453 photometer at 499 nm.

Solution calibration standards established a transfer function for NaCl solution concentration as a function of

absorbance at 499 nm in the range of 1.01E-3 M to 1.01E-7 M. For all but the highest end of the range, a log-log straight line correlation was found. For concentrations greater than 6.07E-5 M, a polynomial fit is used. Figure 3 illustrates the calibration relationships used.

B. Procedure

The CD tests were run with a feed solution of approximately 1% NaCl (exactly : 7.198 g/liter, 0.1232 moles/liter, 0.7183 wt%) with a small amount of tracer dye added. The feed tank was charged with 5-8 liters of feed solution and the duration of each run was determined by the initial amount in the feed tank. That is, the CD was operated until all available feed was consumed. During this time, a pseudo steady-state was achieved and samples were taken from the brine and distillated recirculation loops every five minutes. Energy balances were determined from data at five-minute intervals. After the run, a mass balance was completed to account for feed, brine, product, samples, vacuum trapped volatiles, and DI water used for startup. Recovery was computed on a system-wide basis. An abbreviated outline of the operating procedure is as follows:

1. Start the lab facilities 2. Record Tare weights, assure system is as clean and dry as possible 3. Using established procedure, pull vacuum to < 3 kPa 4. Start the CD motor; adjust set point to achieve the desired speed. 5. Idle the motor to avoid startup power consumption bias noted in prior tests. 6. Prepare the feed solution 7. Fill the Cold Loop using established procedures. Fill rate should be approx. 10 ml/sec 8. Start the THP and immediately begin the feed to the hot loop. 9. When the first distillate appears, begin sampling the hot and cold loops. 10. Start the auxiliary chiller to maintain a thermal balance across the system. 11. Continue to record data and sample every 5 minutes until the feed stops. 12. Close feed valves, disable THP and chiller. 13. Drain the hot loop to the brine tank. 14. Release the vacuum on the system. 15. Drain the distillate and cold side recycle. 16. Record weights of all drained fluids. Account for all fluids in the system including samples and any

condensed volatiles caught in the vacuum cold trap. 17. Rinse with DI water and prepare for the next test.

III. Mass and Hydraulic Test Results Review of the data determined that most of the parameters of the individual tests remained nearly constant

during an single test so average values could be determined. Variation of results were generally less than 1%. A graphing approach was use to analyze with dependent results.

The primary process flow streams are of feed and the product streams. The average flow rates for these streams were determined from tank weights as shown in Figure 4. Weight measurements have proven to be accurate and reliable. Data is started at 10 minutes to avoid startup transients. Product weight corrected for tare. Linear regressions of both sets of data are highly correlated with a deviationof 0.12% . Weight and time intercepts are also accurate; 8.265 correlation vs. 8.27 experiment for feed intercept and 11.0 correlation vs. 10.0 experiment. The slopes of the linear correlations are the primary process flow rates.

Feed Flow Rate 83.6g/min 5.016 kg/hr Product Flow Rate 79.5 g/min 4.770 kg/hr


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A. Process Capability The water recovery process capability is the maximum water recovery possible from the test CD. It is the ratio

of the product flow rate divided by the feed flow rate. This is also called the derivative or instantaenous recovery rate. For the CD at 1300 rpm the process capability is 95.1%.

In actual practice the amount of water recovered is controlled by several factors:

Process capability Fill and drain holdup losses Water vapor losses Amount of feed processed Final brine concentration

If the CD was able to process very large amounts of feed the water recovery would approach the water recovery process capability.

B. Mass Results Using the starting and final weights of the tanks the amount feed

delivered and product recieved can be found. The amount of salt added to the CD can be found from the product of the feed salt weight percentage and the total amount of feed delivered. The wieght of brine is found by substration and the final brine concentration can be found. The water recovery factor is the ratio of the product weight divided by the feed weight. The average feed and product flow rates can be determined by dividing the total weights by the process time. These results for the 1300 rpm test are shown in Table 3. As seen in the table the actual recovery rates are less than the process capability as expected from the previous discussion.

C. Primary Process Flow Rates The primary process flow rates are the feed and product flow rates.

These flow rates were determined for a number of CD rotor speeds using the same analysis methods as the expample of 1300 rpm test. Figure 5. shows the results of all these tests. The flow rates for both streams are nearly equal as expected from the efficient water recovery of the CD. Both flow streams rates track similar behavior with rotor speed. They reduce with increasing speed until 1200 rpm and then remain constant.

Figure 4. Feed and product tank weights for a 1300-rpm distillation run.

Table 3. Mass Results for 1300 RPM Test

Time 93 min

Feed delivered 7.84 kg

Salt concentraion 0.747%

Salt delivered 58.6 g

Water delivered 7.78 kg

Product received 6.57 kg

Brine 1.27 kg

Brine conc 4.61%

Recovery 83.8%* 84.4%** Average Consumption 84.3 g/min

Average Production 79.2 g/min

* Based on feed weight ** Based on available water


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The water recovery process capabilty as a fuction of the rotor speed is shown in Figure 6. The process capability is the maximum recovery possible for the CD as described above. The process capability is nearly constant for all speeds tested. the highest value at 1200 rpm is 97%. All of the other speeds are above 95%.

D. Process Loops Flow, Pressure, &Temperature Results

During the operation of the CD cold product water and hot brine are pumped by pitot pumps within the CD through the THP to maintain their temperature differences which provides the thermal power for the distillation. These flow loops are termed the process flow loops. The performace of the distiller depends on how these loops function. Results of the unit operations tests for the process loops are shown in Figure 7 for the flow rates, loop pressures and loop temperatures.

Figure 6. Process water recovery capability.

Figure 5. Primary process flow rates.


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The flow rates of the hot and cold liquid loops showed consistent results when viewed as a function of rotary

speed. The observed increase in flow rates is consistent with higher pitot pressures at higher rotary speeds. The nomenclature is for the flows going to into the CD and leaving out of the CD. The cold and hot loop “out”

pressures are as expected from the pitot pumps. The pressure increases with speed for the “out” ports again as expected for pitot pumps driven by the rotational speed of the CD. The return “in” flows are reduced due to the pressure loss in the individual circuits through the THP and the miscellaneous pressure losses encountered during the fluid’s travel through the recirculating loops.

The temperatures for the process flow loops (see Figure 8) remain nearly constant as a function of rotor speed as

expected since the temperatures are driven by the THP input power which was held nearly constant for the tests. The warmest and coldest temperatures are the “in” flows as they come directly from the THP. The temperature difference between the "in' and "out" flows are the temperature changes that occur within the distiller due to distillation.

Figure 8. Process loops temperatures.

Figure 7. Process flow rates.


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IV. Thermodynamic Test Results

A. Process Power

Electrical power is supplied to the CD and the THP independently. The CD power drives the motor which controls the rotor speed. The THP power provides heating and cooling to the process flow loops. The process power supplied during the tests in shown in Figure 9.

Process power consumption remained mostly constant by test design, as the main driver for power consumption

was the THP and that power was held constant throughout the testing. CD power increased with higher rotary speeds as more power was required to maintain higher flow rates with higher friction losses.

B. Mass and Power Balance Closure Errors

As noted in the brief description of the test procedure, a complete inventory of all fluid used during each distillation run was completed. Liquid material was collected from the product tank, the feed system, the cold and hot loops, samples, material in the vaccum trap, and all drainable lines. For each distillation trial the total relative error in this mass accounting was less than 1.0% and the total mass collected was approximately 8.9 kg.

From temperature, flow, and electrical power readings, a first law power balance was completed with the

assumption that there are some parasitic heat gains and losses to the laboratory environment. These losses were modeled with a convective cooling rate law applied to measured temperature differences through tubing, valve bodies, uninsulated pieces of the CD housing, etc. For all distillation runs, the maximum error in the total power for each case was less than 8.0%. The total power input to the system was fairly constant, being dominated by the constant THP load.

The maximum relative errors in both mass and power balances are shown in Figure 10.

C. System Thermal Efficiency

The thermal efficiency of the cascade distiller is measured by the amount of energy required to distill one kg of water with the process, or its specific energy consumption. A better way of presenting this data is to compare the CD’s specific energy consumption to the heat of vaporization of water. Figure 11 shows the cascade distiller specific energy consumption as a fraction of water’s latent heat of evaporation.

At low speed, the distiller produced water at less than 10% of the energy required for a normal flash distillation.

The features that provide for this remarkable performance are: Five stages of repeated distillation recovering heat at each stage High thermal performance of the Thermal-Electric Heat Pump Cascade Distiller has exceptional thermal performance

Figure 9. Process power.


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Figure 10. Mass and power balance closure errors.

Distillation Test Results The purpose of the distillation test was to characterize the ability of the CD to produce water distillate from an

aqueous ionic solution. The unit operations tests were conducted over a range of rotational speeds at constant input power to the CD to understand the effects on system operation and product quality.

In general higher product quality was obtained at higher rotational speeds. 1200 RPM is the lowest speed at which good product quality was achieved. In addition, at the highest speeds the CD operation became very sensitive to back pressure in the product line/cold recirculation loop. It seemed that at the highest speed, the CD would quickly accumulate brine solution and additional rotational inertia. If the motor controller could not maintain speed, the CD could not recover dynamic stability and as speed continued to decrease due to fluid loading, product quality significantly decreased.

Although the purpose of this test was not to determine a set of optimum operating conditions, we clearly observed that a trade between robust operation, product quality, and power consumption clearly exists.

A. Product Quality Test Product water quality measurements were obtained on all the unit operations tests. These consisted of

liquid samples taken every 5 minutes from four process loops. The loops were: Port 3 Product out to tank Port 4 Cold loop "out" Port 1 Hot loop "in" Port 5 Hot loop "out" Tracer dye (Red Dye #40) was introduced into the feed at a known NaCl concentration. The samples were

stacked in chornological order during the test and photos taken to document the resulting colors. Later the samples

Figure 11. System thermal efficiency.


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were measured with a colorimeter to obtain quantitative NaCl concentration results. The photo of the stack of samples for the 1300 rpm test is shown in Figure 12.

Figure 12. Distillation samples for 1300-rpm test run.

The samples are arranged in four groups left to right: samples from Port 3, two blank sample, samples from Port 4, divider, samples from Port 1, two blank samples and, samples from Port 5. Within each group the sample sequence follows a left to right, top to bottom pattern with samples 1-4 in the first row, samples 5-8 in the second row, samples 13-16 in the penultimate row, and samples 17 and 18 in bottom row. This represents the full 90 minute test run samples taken.

The color pattern evident in the sample images in Figure 12 illustrates a general pattern seen in all runs. The first samples in the hot loop (ports 1 and 5) reflect concentrations of the feed solution. The first samples in the cold loop (ports 3 and 4) reflect concentrations of some initial condition within the CD. With time (and increasing sample number), the hot loop color intensifies showing that concentrations steadily increase while the cold loop samples become colorless showing that in this case, cold loop concentration become very low. Nearly pure water was produced.

The effect of rotor speed can be seen in Figure 13, which is a photo of all the water quality samples taken for the unit operations tests.

Figure 13. Water quality samples for all of the unit operation tests.


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As seen in this picture, higher speeds produce both higher quality product and more highly concentrated brine solution. The CD has mist separators in each of the stages as the steam moves to the subsequent condensing stage. The mist separators are designed to eliminate contamination of the steam (product) by the brine containing mist present in the evaporation stage. The mist separators depend on centripetal forces for their operation. Increasing the rotational speed increases the centripedal forces by the square of the rotational speed. This effect explains the improvement in water quality at the higher rotor speeds.

The product water cases also show an inconsistent amount of color in the first few samples of a run. This is attributed to residule brine form the previous test. If extra care was taken in flushing the CD there is no startup color. For example see the test for 1400 rpm.

In runs 1200 RPM through max RPM, the most concentrated of the samples begin to show evidence of dye precipitation. Graphs of the concentration trajectory for these runs show a break in the steadily increasing concentration curve toward the end of the run time.

B. Water concentration measurements The results of NaCl concentrations for runs of 1200, 1300 and 1400 are shown in Figures 14-16. These data

used the photometric method described previously. The figures identify the data sets accoding to port number. Ports 1 and 5 are hot loop concentrations and ports 3 and 4 are cold loop concentrations as indicated by the arrows. The hot loop concentrations are 100 times the cold loop concentrations.

Figure 14. Concentration measurments at 1200 rpm.

Figure 15. Concentration measurments at 1300 rpm.


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Figure 16. Concentration measurments at 1400 rpm. These figures show quantitatively the effects of rotor speed seen in the previous photos of the water samples. The cold loop concentrations are at the detection limit for speed of 1300 rpm and 1400rpm. While at 1200rpm the cold loop water takes the entire test run to reach that point. The higher speeds result in better mist separation as discussed previously. The hot loop (brine) concentrations increase throughout the run for all speeds shown. The rate of concentration change for the hot loop becomes greater as the concentration is higher. This is a result of less water present in the brine as it become more concentrated.

V. Summary and Conclusion The CD produced purified water that exceeds all current regulations and guidelines for sodium in drinking water

when challenged with a feed concentration of 7198 mg/liter. After 5 minutes of startup the 1300 rpm test delivered water that averaged at 10 mg/liter for the entire 80 minutes of the test.

Typical guidelines require 75-50 mg/liter for drinking water Milk has 500mg/liter, Club Soda has 211 mg/liter, and Wine has 48 mg/liter

That outstanding distillation performance is supported by equal thermal and mechanical performance of the distiller. This report shows that the cascade distiller has outstanding thermal performance. The specific energy has an average of 92 watt-hr/kg. This means that the cascade distiller uses 87% less power than a standard flash distillation. Unit operation tests have defined the hydraulic, thermal and distillation performance of the prototype cascade distiller.

Acknowledgements Gratitude to Dave Cardenas, Clark Lukens and Cesar Maldonado for their support during testing.

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unit operation performance testing of cascade distillation

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