James C. Watters

Model based control for reverse osmosis desalination processes

Abstract Computer simulations were performed using the Dynamic Matrix Control (DMC) algorithm for the control of a reverse osmosis (RO) desalination pilot plant. The plant dynamics were modeled by the use of empirical transfer functions developed at the Kuwait Institute for Scientific Research. The two output variables considered were: production flow rate and product water electrical conductivity as a measure of purity. With the selection of proper tuning constants the DMC approach allows for substantial improvement over proportional-integral (PI) control based on the integral square error (ISE) performance criterion. A setpoint change in production flow rate was considered in the simulations since this type of change is the one most likely to occur in an RO plant. For the same magnitude setpoint change in flowrate, the ISE results for the flowrate were comparable between the two controllers; however, the ISE results for conductivity were substantially better using DMC control. The ISE for conductivity using the PI controller is 1688.92 while for the DMC controller, with the proper tuning parameters the ISE may be much less than 1.0. These results indicate that much more flexibility in the operation of an RO plant is available with DMC control.

Advanced control of a reverse osmosis desalination unit

Abstract An experimental investigation of constrained model predictive control (CMPC) for a reverse osmosis (RO) desalination unit has been conducted. For comparison purposes, results with traditional PID-type control have also been obtained. The experimental unit consists of a series of four cellulose acetate membranes. A 486-PC is used as the data acquisition and control computer. It is interfaced to the experimental unit via analogue-to-digital and digital-to-analogue converter boards. The models required for CMPC and PID-type controls are obtained by step testing. The RO system has four outputs and two inputs. The outputs are (1) permeate flow rate, (2) permeate conductivity, indicative of the salt content in the product, (3) trans-membrane pressure, and (4) inlet pH. The inputs are (1) flow rate of reject water and (2) inlet acid flow rate. The production objectives are to produce the specified flow rate of permeate, having the desired salt content, subject to the constraints that the inlet pH and the trans-membrane pressure are within specified bounds. It is shown that CMPC can achieve these goals. It is also demonstrated that CMPC can maximize the throughput subject to the constraints on the other three outputs. A comparison of the results with CMPC and PI control reveals the excellent capability of CMPC for RO desalination plant operations.

Ultrafiltration of a Textile Plant Effluent

Abstract This preliminary study was initiated to determine the feasibility of using ultrafiltration to remove dyes and other contaminants from industrial textile plant waste streams. Various runs were conducted on samples of the waste stream by using a lab-scale UF unit fitted with a polysulfone XM50 hollow fiber membrane. The effects of temperature and pressure on permeate flow rate and rejection coefficient were investigated. Spectrophotometric analysis was used to determine the rejection coefficients. The average rejection coefficients ranged from 30 to 90%. The permeate-to-feed ratios ranged from 1.4 to 15.2%. Increasing the pressure increased the permeate flow rate, but also decreased the rejection coefficient. The effect of temperature was inconclusive. Fouling varied with the waste solutions, but could be enough to clog the whole unit. The pH remained at the same value of 10 for the permeate, retentate, and feed in all the runs.

Advanced process control of a B-9 Permasep® permeator desalination pilot plant

An experimental application of advanced control and optimization on a hollow-fiber membrane module (B-9 Permasep® permeator by DuPont) is presented. The objective of the study was to compare the performance of standard proportional-integral (PI) control with the performance of a constrained model predictive control (CMPC). A proper control strategy, whether PI or CMPC, should allow for the manipulation (servo control) of the product flow rate while maintaining product quality. In doing so, a plant can adjust the production of water to meet demand. Several PI control experiments involving set point changes in product flow rate and conductivity (a measure of quality) were conducted. It was found that PI control was unable to properly control the quality of the product by means of manipulating the pH of the feed. The PI controller over-compensated for offset in product conductivity. In contrast, CMPC displayed superior performance in the control of the pilot plant by holding the process outputs within specified bounds; especially the feed pH which prevented the conductivity PI control loop from becoming unstable. Furthermore, CMPC was able to maximize the product flow rate by 13.6% while improving the conductivity (quality) by 1.1%.

Polyelectrolyte-Enhanced Ultrafiltration of Copper from a Waste Stream

Abstract The concentration of copper in a simulated wastewater was successfully reduced from 100 ppm to less than 1 ppm by polyelectrolyte-enhanced ultrafiltration. In polyelectrolyte-enhanced ultrafiltration, copper is ionically bound to the polymeric chain of an anionic polyelectrolyte. Ultrafiltration then filters out this large copperpolyelectrolyte complex. At the same polyelectrolyte-to-copper ratio in the 1:1 to 3:1 (weight basis) range and a feed temperature in the 15 to 40°C range, sodium carboxymethyl cellulose was more efficient for copper removal and exhibited higher flux values than sodium polystyrene sulfonate. The experiments showed that transmembrane pressure has a minimal effect on removal of copper whereas the polyelectrolyte-to-copper ratio has considerable influence. It was also seen that the permeate copper concentration decreased with a decrease in feed temperature. Decreasing the feed temperature also caused a reduction in flux because of an increase in the viscosity of the feed sol...

Reduction of the Lactose Content of Skim Milk by Continuous Countercurrent Cascade Ultrafiltration

Abstract Lactose was continuously removed from skim milk using two ultrafilters in series, with intermediate recycle. Reduced recycle flow rates at constant lactose stream (permeate product) flow rate resulted in slightly better removal of lactose from the milk feed, although this mode of operation increased the protein loss from the final milk retentate product. Increased permeate flow rate at constant recycle rate removed more lactose from the feed but also resulted in more loss of protein. However, at its maximum, the loss of protein reprented only about 6% of the nutritional value of the milk. Under the experimental conditions studied, 58% of the lactose in the original skim milk could be continuously removed for the best combination of recycle and permeate flow rates. The experimental values of flow rates and compositions were reasonably well predicted by a mathematical model of the process.

Pneumatic conveying characteristics of kentucky coal-limestone mixtures

Abstract The feasibility of pneumatically conveying limestone and 6:1 Kentucky coal-limestone mixtures has been demonstrated. Attrition of the particles was about 4% when conveyed through a 150-foot line while a decrease in average particle size of about 30% was observed when the material was conveyed through a 450-foot line. It is deduced that in a commercial fluidized bed combustor the feed conveying lines be not more than about 200 feet long and that the coal feed bunker be placed downstream of the limestone bunker as the coal was observed to break down much more than the limestone. In an atmospheric fluidized bed combustor where the freeboard or de-entrainment height is designed based on the terminal velocity of the average-sized particle, it is predicted that material which has been carried through 450 feet of feeder pipeline will yield about 33% more carryover of fines than would be observed with the unconveyed material. This excess carryover is reduced to 4% with material which has been coveyed only 150 feet.

The hydrogenation behavior of various Kentucky coals

Abstract Kentucky coals numbers 6, 9 and 11 and Illinois coal number 6 were hydrogenated with and without a hydrodesulphurization catalyst in a 0.3-liter autoclave. Reaction conditions simulated the syncrude mode of the H-Coal process (pressure of 14 MPa, temperature of 725 K). Conversion of all coals to oil plus asphaltenes was in the 80–85% range, irrespective of seam, mine, use of catalyst or time. Reaction to preasphaltenes was minimal in runs which used a catalyst, except for KY no. 6 coal. Without catalyst about 4% preasphaltenes were isolated in the product. Sulphur balances indicated that the catalyst, Cyanamid HDS-2A, was effective at keeping sulphur in the solid residue. Between 75 and 95% of the sulphur stayed in the residue for catalyzed reaction as opposed to 25 to 45% for the uncatalyzed cases. Actual conversion values were consistently lower than those predicted based on petrographic data. This is postulated as being due to the reaction conditions.

A PNEUMATIC FEEDER-SPLITTER SYSTEM FOR FLUIDIZED BED COMBUSTORS

Abstract One of the problems associated with the commercialization of Atmospheric Fluidized Bed Combustors (AFBC) is that of feeding coal-limestone mixtures into the bed without breakdown of the particles. Breakdown of the particles should be avoided in order to prevent the emission of inordinately large quantities of ash fines and the resulting heavy duty to the flue-gas cyclones. A system has been constructed which has successfully split individual streams of crushed walnut shells. limestone or coal from a 100 mm (4 inch) feedline to four 50 mm (2 inch) lines with minimal (10%) attrition and acceptable uniformity. The system consists of a horizontal section 10 meters long, followed by a vertical section 5 meters high at the top of which is affixed a four-way splitter cap. Samples are knocked out by cyclones in each of the four arms and analysed both for total weight and size distribution. Based on these and previous analyses some parameters have been set for the length of pneumatic conveying lines for f...