Nonlinear Chemical Dose Controller

Overview

Figure 1: Draft nonlinear chemical dose controller design.

Abstract

Accurate alum dosing is vital for plant operation as it has a great impact on the effectiveness of flocculation and sedimentation. The nonlinear chemical dose controller (CDC) is designed to handle turbulent flow chemical dosing used in conjunction with the newly designed Rapid Mix Tube. In contrast, the linear CDC requires that the chemical flow in the dosing tube is laminar.

The linear CDC uses the relationship between laminar flow and major losses in the doser tube to maintain a constant chemical dose with varying plant flow rates. However, when the flow in the dosing tube is turbulent, the linear relationship no longer exists. In this case, a nonlinear CDC, one that uses minor losses to control flow rates, can be used to maintain a constant chemical dose with the varying plant flow rates. By using an orifice to control chemical flow, the CDC will have the same nonlinear response to increasing flow as the plant flow rate which is controlled by an orifice. A dual scale system on the lever arm will increase accuracy in dosing at smaller doses.

Figure 1, above, shows a conceptual design of the dosing system. A float in the entrance tank is connected to one end of the lever arm and allows for the arm to move up or down based on varying plant flow rates. As plant flow rates increase, the float rises, and the dosing orifice falls making more head available to power chemical flow. As the flow rate decreases so does the available head and the chemical flow rate slows down.

There are two dosing tubes coming out of the constant head tank each with a different size orifice fitting attached to the end. The two different sized orifices allow for the plant operator to dose alum at two different scales, a high and low scale. Based off the raw waters turbidity, the operator will choose which orifice to use based off whether he needs a higher or lower flow rate of alum. If, for example, a high flow rate of alum is desired then the larger orifice will be attached to the sliding scale, and the proper scale on the dosing arm will be chosen. The two scales on the lever arm, as seen in Figure 1, correspond to one of the dosing tubes; for instance, the higher scale (20-100 mg/L) will be used when the larger orifice size is needed. The dosing tube which is not in use is merely lifted to a higher elevation than the constant head tank level and is clipped onto a hook to prevent the flow of alum through this unused orifice.

After the alum exits the dosing orifice it flows down through a rigid tube which then injects the alum over a rapid mix conduit which connects the entrance tank and the flocculator. The rapid mix tube has two orifices which create macro and micro eddies to ensure the uniform distribution of coagulant into the raw water.

Introduction and Objectives

As the AguaClara project grows, larger plants are being designed. These larger plants have a greater flow rate and thus there is a need for the linear dose controller to be redesigned for accurate dosing for these higher flow rates. The Non-linear Dose Controller Team is working on redesigning the dose controller, including the flow controller, the doser, and the flow measurement device. We are also working on designing the rapid mix tube for large-scale and small-scale mixing.

After the new design is completed, the non-linear dose controller will be constructed. Experiments will be done to verify the accuracy of the dose controller compared to theoretical results. This new design will be presented at the EPA's P3 competition in Spring 2010 for the Phase II prize of $75,000. The design will also be implemented at the new Agalteca plant in Honduras.

"Non-Linear Chemical Dose Controller Fall 2009" Goals and Meeting Minutes

Theory

The nonlinear doser uses a dosing orifice (minor losses) instead of a dosing tube (major losses) to control the relationship between changing plant flow rates and chemical dose. The flow rate through the CDC is related to the available head by the equation:

where

• Unknown macro: {latex}

  $$Q_

  Unknown macro: {Cdc}

  $$

  is the chemical flow rate
• Unknown macro: {latex}

  $$ K_

  Unknown macro: {orifice}

  $$

  is the orifice coefficient
• h is the available head

The chemical dose to the plant can be determined by a simple mass balance:

where

• C _c is the chemical stock concentration
• C _p is the chemical dose

The water leaves the entrance tank through the Rapid Mix Tube and the alum is dosed directly into the tube. An orifice is located in the tube to generate small-scale mixing.
The relationship between flow rate and head loss is governed by the orifice equation:

where

• Unknown macro: {latex}

  $$ Q_

  Unknown macro: {Plant}

  $$

  is the plant flow rate
• Unknown macro: {latex}

  $$ h_

  Unknown macro: {EtOrifice}

  $$

  is the height of water above the entrance tank orifice

The CDC uses a lever arm with a float and counterweight to relate the dosing to the changes in the entrance tank water level, which is a function of the flow rate.

to relate head above the centerline of the rectangular plant entrance orifice to head in the dosing orifice. This means that the available head for the dosing orifice is the same as the head controlling the plant flow rate. The increase in head links the chemical flow rate to the plant flow rate and the chemical dose will be constant as plant flow varies as long as the exponent of the head is the same for both the plant flow and the chemical flow.

The dosing tube must be designed to minimize major losses so that the major losses that deviate from the

relationship do not cause excessive errors. The deviation from

is especially significant when the flow through the dosing tube becomes laminar. The dosing tube must be flexible to accommodate the lever arm motion and dose adjustment. There will be two dosing tubes from the constant head tank to the level arm (one for each scale and one tube from the lever arm to the rapid mix tube.

Methods

Designing the Orifice Sizes and Dual-Scale for the Lever Arm

The dosing orifices and dual scale were designed to produce the difference in head loss between the maximum CDC head loss and the actual head loss in the flexible dosing tube:

where

• h _l the difference in head loss between the maximum CDC head loss and the actual head loss in the flexible dosing tube
• K _DoseOrifice is the required minor loss coefficient through the orifice
• V _DoseTube is the velocity in the dosing tube

Designing the Float

A float on the lever arm was designed to help the dosing system react to changes in water height in the entrance tank. The float size is a parameter that can be easily adjusted to calibrate the dosing system in a specific plant.

Designing a Prototype Doser Frame

AutoCad was used to design a prototype doser frame. 80/20 plug-ins were used to facilitate modeling and eventual full-scale construction of the dosing system.

Designing the Rapid Mix Tube

A rapid mix tube was designed for large-scale and small-scale mixing of alum with water before flocculation.

Constructing a Small-Scale Model of the Plant

A model of the plant will be constructed using acrylic plastic for use in educational and promotional presentations.