Enhanced for Ethanol

FROM THE OCTOBER ISSUE: A one-step clean-in-place solution designed for ethanol plants decreases chemical usage while cleaning both inorganic and organic compounds.
By Michael Raab | September 16, 2019

In the dry grind corn-to-ethanol process, operators are continually seeking ways to improve process efficiency and lower operating costs to help drive the overall profitability of their plants. Multi-step, clean-in-place (CIP) procedures are employed in many process steps to chemically and physically remove deposits that have fouled the surfaces of equipment, including tanks, piping and heat-transfer surfaces.

Plant design, water quality, pH, enzyme mix, phytase addition and corn oil extraction are just a few of the dozens of factors that contribute to the fouling of the evaporator chain, as well as many other areas in the process stream. Understanding the nature of the fouling is key to effectively removing these deposits.

Two Types
Deposits can be divided into two types: organic and inorganic. Organic deposits generally consist of proteins, fiber, starches and fats. Proteins adhere rapidly to heat exchanger surfaces, and protein/carbohydrate interactions (such as the Maillard reaction) can occur, as well.

Inorganic deposits are mineral in nature and are driven by species solubility based on concentration and temperature. These inorganic species demonstrate inverse solubility and, at high temperature, precipitate out of solution. This change is typically irreversible, so a change in concentration or temperature will not resolubilize these deposits. Typical inorganic deposits in this type of system are magnesium phosphate, calcium phosphate and calcium oxalate.

Most deposits contain a matrix of the two types, but the ratios at which they can be present vary drastically. Analysis of the deposits from evaporators are critical to understanding how to best approach instituting an effective and robust cleaning process.
Efficient Approach
Historically, the approach to effective CIP has been a combination of acid cleaning, alkaline cleaning and hydroblasting, which is a physical, not chemical, approach. Plants often undergo an extended acid cleaning, followed by an extended alkaline cleaning, and then evaporator tubes are hydroblasted to remove any remaining deposits.

This multistep approach leads to significant downtime and chemical costs. Each of the cleaning steps can range from six to eight hours or more. Additionally, these chemistries will end up in the distillers  dried grains with solubles (DDGS), so they must be approved for animal feed. This restriction puts severe limitations on which chemistries are allowed in the process. It is possible to employ a single-step alkaline cleaning approach that can address the broad range of deposit samples encountered and meet the animal feed regulations.

By addressing the removal of both organic and inorganic deposits in the same cleaning step, a decrease in the overall downtime, as well as a decrease in overall chemical usage, can be achieved. In some cases, the physical hydroblasting step can be eliminated, as well.

It can be done with a solution additive to the caustic (sodium hydroxide) CIP already used throughout the plant. The additive would ideally work over a wide range of alkaline pH, as CIP make-up and life vary greatly from plant to plant, and even from week to week within the same plant. This approach incorporates additives to help the caustic do its job better, with improved wetting and solids suspension. Most critically, additional additives can be included that would directly target inorganic deposits in an alkaline cleaning environment, such as in the evaporator chain and other heat-exchange surfaces.

A Cleaning Case Study
Suez conducted a case study on the CIP approach at an ICM-designed plant on evaporator No. 6. The evaporator was fouled with a matrix of organic and inorganic deposits. The CIP was conducted over a period of 24 hours, with an average temperature of 120 degrees Fahrenheit and a CIP caustic concentration target of 5 percent.

The one-step CIP removed both types of deposits, and operators noted a much larger amount of organic material sloughing off into solution than they had in past CIPs. (See Figure 1.) Borescope analysis confirmed the removal of deposits down to bare metal along the length of the tubes. Laboratory analysis of the CIP solutions confirmed the presence of a majority of magnesium phosphate, with some calcium phosphate and oxalate, as well.

Samples were collected at periodic intervals throughout the cleaning and sent to the laboratory for analysis. This analysis confirmed the removal of magnesium and calcium deposits, as evidenced by the rapid increase of both materials (as well as phosphate) in solution. When compared to caustic-only CIP, the enhanced one-step CIP was able to remove roughly 10 times the amount of magnesium, and six times the amount of calcium.

By addressing both types of cleaning in one step, a significant downtime reduction can be expected, from one-half to possibly one-third of the time typically allotted for evaporator CIP. In this case study, cleaning was effective enough that hydroblasting was not necessary. It eliminated the use of sulfamic acid and improved heat transfer efficacy, generating an estimated savings of $20,000.

Utilizing a single-step, enhanced alkaline cleaning solution is a viable and attractive alternative to the historical approaches for CIP cleaning. This approach is effective over a broad range of operating conditions and deposit type combinations.

Author: Michael Raab
Lead Technologist,
SUEZ – Water Technologies & Solutions