MXPA99007612A - Inhibition of calcium oxalate scale in aqueous based solutions - Google Patents

Abstract

A method of inhibiting the precipitation and deposition of calcium oxalate scale in an aqueous system is disclosed, in which an effective amount of a combination of a high molecular weight lignosulfonate compound and a phosphate compound is added to the system.

Description

INHIBITION OF CALCIUM OXALATE SCALES IN AQUEOUS SOLUTIONS BACKGROUND OF THE INVENTION Most commercial water contains alkaline earth metal cations such as calcium, magnesium, etc. and anions such as carbonate and oxalate. When combinations of these anions and cations are present in concentrations that exceed the solubility of their reaction products, they form precipitates until their concentrations of product solubility are no longer excessive. For example, when the concentrations of the calcium ion and the oxalate ion exceed the solubility of the calcium oxalate reaction product, a solid phase of calcium oxalate will be formed as a precipitate. The concentrations of the solubility product exceed for various reasons, such as the evaporation of the aqueous phase, the change in pH, pressure or temperature, and the introduction of additional ions can form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on the surfaces of a water carrier system, they form flakes. Scales or incrustation prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes and are sources for bacteria. Scale formation is a costly problem in many industrial water systems, resulting in delays and interruptions for cleaning and disposal. Calcium oxalate is a common substituent in the formation of scales on metallic surfaces of devices used for the thermal treatment of aqueous solutions and suspensions. It is known that the scale of calcium oxalate can be removed by washing pipes, tubes or other metal surfaces in which deposits of calcium oxalate have been formed, with diluted aqueous acid solutions, such as solutions of hydrochloric acid or nitric acid . However, due to the limited solubility of calcium oxalate in these acids, repeated washings of prolonged durations are necessary. In this way, acid washing is a costly and time consuming operation. Acid washes also deteriorate the equipment. The ethanol obtained from aiz is a mainly aqueous stream (80-90% water) which is usually produced by continuous fermentation in which the glucose is converted by yeast or other microorganisms into ethanol. The fermentation feed streams (which include starch or glucose, light maceration water, dilution water and fermentation countercurrent) contain different concentrations of calcium and oxalate ions. It is the combination of these currents and the conditions of temperature and pH in the fermentation that give rise to the precipitation of calcium oxalate in the process. These crystals present coalescence and form deposits in the equipment, which reduces the efficiency and capacity of the production. Prolonged interruptions are usually necessary to physically remove these deposits.

DETAILED DESCRIPTION OF THE INVENTION We have found that the calcium oxalate scale can be adequately controlled by adding an effective amount of a high molecular weight lignosulfonate compound and a phosphate compound to the desired aqueous system. It has been found that the high molecular weight lignosulfonates from the sulfite process (Ufoxano 2, molecular weight 55,800, hereinafter referred to as lignosulfonate A and Ultrazine Na, of molecular weight 63,600, hereinafter referred to as lignosulfonate B, both available from Lignotech) they are especially effective for the purposes of the present invention. In a preferred embodiment of the present invention, a combination of from about 0.1 to about ICC ppm of the lignosulfonate compound and a hexametaphosphate, based on one million parts of the aqueous system, is added to the system in need of treatment.

High molecular weight lignosulfonate is a molecular weight of at least 50, 000 A range of molecular weight from about 55,000-65,000 is preferred. The following range of concentrations can be used: Lignosulfonate 0.1-100 ppm, preferably 50 ppm. Phosphate 0.1-100 ppm, preferably 50 ppm. It was also found that the aforementioned lignosulfonate compound effectively disperses the calcium oxalate crystals that precipitate in an aqueous solution of ethanol in the maize to ethanol production process, as described below.

EXAMPLES In order to identify the calcium oxalate dispersants, two tests of hindered sedimentation were carried out, and then the effect of the pH on these dispersants was evaluated. The slurry synthesized for the tests contained 10% w / w calcium oxalate, 12% w / w ethanol and 2000 ppm phosphate. The high level of phosphate was necessary to equalize the conditions in the process streams in the field (1800-2000 ppm). Note that despite this high concentration, field mineral deposits are composed entirely of calcium oxalate since, at a pH of 3.75, all phosphate exists as phosphoric acid and precipitation of the phosphate salt does not occur. However, these levels of phosphate can affect the activity of the dispersants by adsorption on the surface of the solids and modify the surface charge or compete with the dispersant for the absorption sites. In both tests, the pH of the slurry was adjusted to 3.75 and 50 ml aliquots of the slurry were placed in graduated cylinders with a stopper of 50 ml. The treatment was added at a rate of 250 ppm of the active, the cylinders were covered, inverted and sedimentation of the contents was allowed. The height of the solids / liquid interface was recorded at regular intervals. Additional steps for the second test (pH dependence) included measuring the pH of the slurries after the test was completed to determine if it was between 3.8 and 4.4. If the pH was outside this range, the pH of the treatment solution was adjusted to approximately 4.0 and the test was repeated. The experimental results are found in Tables I and II.

Table I Testing of blocked sedimentation in calcium oxalate batch "Conditions: 250 ppm of active treatment sedimentation time 20 minutes CaC20 at 10% p / p T = environment PH = 3.75 2000 ppm phosphate Treatment Interface height (mi) t = 2 min t = lOmin t = 20 min Control (average of 9 45.5 31.2 20.9 processes) Lignosulfonates: Polyphon © H "50 45 37 Lignosulfonate A 50 43 37 Lignosulfonate B 50 42 34 1 the pH was not adjusted again after the addition of the treatment - Kraft lignin, commercially available from Westvaco, molecular weight 7000.

Table II Testing of blocked sedimentation in calcium oxalate batch * Conditions: 250 ppm active treatment sedimentation time 20 minutes CaC204 at 10% w / w T = environment PH = 3.75 2000 ppm phosphate Treatment Interface height (mi) t = 2 min t = lOmin t = 20 min Control (average of 17 45.5 31.2 processes)) Lignosulfonates: Polyphon © H: 47 36 27 Lignosulfonate A 50 50 Lignosulfonate B 50 50 50 1 the pH was not adjusted again after the addition of the treatment - Kraft lignin, commercially available from Westvaco, molecular weight 7000.

A treatment that produced higher interface heights than the control experiments was considered a calcium oxalate dispersant. The above tests indicate that the high molecular weight lignosulfates effectively disperse the calcium oxalate crystals independently of the pH, while a low molecular weight lignosulfonate Kraft only exhibited calcium oxalate dispersing activity when the pH of the system was allowed to rise.

Heat exchanger simulations The chemical changes associated with the fermentation of glucose to ethanol release heat energy in significant quantities per pound of alcohol produced. In order to reject this heat and control process temperatures, each fermentation vessel is usually fitted with a refrigerant pipe for recirculation. As calcium oxalate is embedded in the surfaces of the equipment, the heat exchangers in these pipes lose their ability to cool the process stream through partial blockage of process flow and degradation of heat transfer efficiency . In order to simulate the incrustation in the refrigerant, a one-way heat exchanger apparatus was designed. This apparatus consisted of a tube-in-tube heat exchanger with the process fluid passing inside the inner tube and the cooling water passing through the outer tube.

Two separate ethanol solutions were fed to the heat exchanger simultaneously, one of which contained 200 ppm of the oxalate ion, the other contained 40 ppm of calcium ion (the concentrations are approximately twice those found in the actual corn process). ethanol). The exchanger was placed for service in downward flow to avoid premature seals in the intake joint at the point of supply of the solution, which avoid the accumulation of deposits in the exchanger and mask the results of the anti-scale formation tests . The key parameter that identifies the effect of the treatment is that of online hours until obstruction. It was found that the calcium oxalate that formed as the two feed solutions passed through the apparatus would form plaques inside the inner tube, particularly around the discharge end of the inner tube. Finally, the accumulation at the discharge end of the tube totally blocks the flow of fluids. The tests showed that certain treatments had a pronounced effect on the time that the apparatus remained operating until the interchanger clogged. The results of these tests can be found in Table III Table III Calcium oxalate inhibition studies Heat exchanger experiments Treatment Time to obstruction, hours (average) 25 ppm SHMP 44 10 ppm SHMP 57 5 ppm SHMP 18 10 ppm Lignosulfate A 7 5 ppm SHMP + 5 ppm Lignosulfate A 23 5 ppm SHMP + 5 ppm Lignosulphate B 17 10 ppm SHMP + 5 ppm Lignosulphate B 39 10 ppm SHMP + 10 ppm lignosulfate A 86 SHMP: sodium hexametaphosphate The previous tests show the previous results obtained by combining the individual components. A particularly preferred embodiment of the present invention (100 ppm SHMP + 10 ppm lignosulfonate A) was found especially effective. Although this invention has been described with respect to the particular embodiments thereof, it is evident that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention should generally be considered to cover all of these obvious forms and modifications that are within the spirit and actual scope of the present invention.

Claims (13)

1. A method for inhibiting the precipitation and deposition of calcium oxalate scale in an aqueous system is to add to the system an effective amount of a high molecular weight lignosulfonate compound and phosphate compound.

The method as recited in claim 1, wherein the lignosulfonate compound is added in amounts ranging from about 0.1 to about 100 parts per million parts of the aqueous system.

3. The method as recited in claim 1, wherein the molecular weight of the lignosulfonate compound is at least about 50,000.

4. The method as recited in claim 3, wherein the molecular weight of the lignosulfonat compound is from about 55,000-65,000.

The method as recited in claim 1, wherein the phosphate compound is added in amounts ranging from about 0.1 to about ICO parts per million parts of the aqueous system.

6. The method as recited in claim 1, wherein the phosphate compound is sodium hexametaphosphate.

The method as recited in claim 1, wherein the aqueous system consists of an alcoholic fermentation stream.

8. In a system for the production of ethanol from maize, a method to reduce the deposit of calcium oxalate on the surfaces of the processing equipment in contact with an alcoholic fermentation stream in the system, the method is to add to the alcoholic fermentation stream an effective amount of a high molecular weight lignosulfonate compound and a phosphate compound.

The method as recited in claim 8, wherein the lignosulfonate compound is added in amounts in the range from about 0.1 to about 100 parts per million parts of the aqueous system.

The method as recited in claim 8, wherein the molecular weight of the lignosulfonate compound is at least about 50,000.

The method as recited in claim 10, wherein the molecular weight of the lignosulfonate compound is from about 55,000-65,000.

The method as recited in claim 8, wherein the phosphate compound is added in amounts ranging from about 0.1 to about 100 parts per million parts of the aqueous system.

13. The method as recited in claim 8, wherein the phosphate compound is sodium hexametaphosphate.