Introduction

Monoethylene glycol (MEG) is an important raw material for industrial applications. End uses for MEG range from clothing and other textiles, through packaging to kitchenware, engine coolants and antifreeze. Polyester and fleece fabrics, upholstery, carpets and pillows, as well as light and sturdy polyethylene terephthalate drink and food containers originate from ethylene glycol. The humectant (water attracting) properties of MEG products also make them ideal for use in fibers treatment, paper, adhesives, printing inks, leather and cellophane.

Mono Ethylene Glycol Process

Ethylene, Oxygen and Water are the main raw materials used in the process of production of Mono Ethylene Glycol. The process is segregated on the high level as four major sections namely Ethylene Oxide Reaction section, Ethylene Oxide separation section, Ethylene Glycol Reaction section and Ethylene Glycol Separation section. The raw materials Ethylene and Oxygen are mixed and reacted in a Catalyst bed reactor resulting the production of Ethylene Oxide and other undesirable products like CO2, unreacted Ethylene and other light ends. The Ethylene Oxide produced from the reaction section is separated from the undesirable products by process of Absorption and Stripping to separate Ethylene Oxide alone for the production of Ethylene Glycol.

The Ethylene Oxide is then reacted with Water in the Tubular reactors to produce Ethylene Glycol in the Ethylene Glycol Reaction section which contains water and other impurities along with the Ethylene Glycol which is processed in the Separation section to produce the desired Mono Ethylene Glycol (MEG), Di Ethylene Glycol (DEG) and Tri Ethylene Glycol (TEG).

Operation/Economic Issues

The cost of production of Mono Ethylene Glycol is dependent on the cost of Ethylene and cost of Steam as these components play a significant factor on the production cost. Mono Ethylene Glycol process unit was studied for operation and required improvisations before Advanced Process Control was implemented and the Steam consumption in the unit most of the time was more than what was required and the operation of the separation columns were not tightly controlled, resulting in desirable Tri Ethylene Glycol product loss in the column bottom stream. The operation of the Ethylene Glycol Reaction section is usually manual operation with the Aqueous Ethylene Oxide was adjusted frequently by the board operator based on the level in the Ethylene Oxide buffer vessel without any level of automation.

APC in Mono Ethylene Glycol Unit

Typically model based multi-variable controls used in Chemical plants are designed to improve the Yield of operation by controlling the reaction parameters within the desired operation limits whereas the strategy for Model based multi variable control using Yokogawa’s Advanced Process Control package was designed in such a manner to optimize the Steam consumption in the plant and recover the valuable products that are lost in the unrecoverable section of the plant.

This Advanced Process Control application is implemented on the Ethylene Oxide Absorber and Ethylene Oxide Stripper, CO2 Removal section, Light Ends Removal by Absorption (LERA) column, Ethylene Glycol Reaction section, Ethylene Glycol Concentrators, Ethylene Glycol Dehydrator and Separation Columns with the objective being to minimize steam consumption in all areas of the plant and maximize desirable product TEG production by pushing TTEG into the TEG stream. The Advanced Process Control application is always operated in Optimization mode to meet the objectives with the operation parameters controlled within the specified operation limits. The impact of the steam consumption reduction in the Ethylene Oxide buffer vessel is accurately predicted and the Aqueous EO feed is continuously adjusted by the controller to control both the buffer vessel level and the EG reaction kinetics as desired.

Steam Optimization

Steam is one of the major component used in the Mono Ethylene Glycol unit both on the Ethylene Oxide section and Ethylene Glycol section. The major consumptions of steam are in the Ethylene Oxide Stripper column, CO2 Stripper column, Ethylene Glycol Concentrators 1/2/3, Ethylene Glycol Dehydrator, Glycol Bleed Flasher, Mono Ethylene Glycol Column, Mono Ethylene Glycol Recovery Column, Di Ethylene Glycol Column and Tri Ethylene Glycol Column. Before implementation of the Advanced Process Control the operation of the units and the control of steam to each of the columns were manually adjusted by operator based on the operation needs infrequently and when any abnormality observed. This manual operation of the plant resulted in variation of the plant parameters resulting in loss of valuable products or over consumption of utilities. With Advanced Process Control the cost factor was incorporated for each of the critical variables in the plant there by a cost minimization objective was achieved by continuous control thereby a direct measure of Steam consumption reduction was observed.   

Conclusion

The Advanced Process Control application implemented on the Mono Ethylene Glycol unit helped control the plant with less variation during load changes and with less operator interactions during plant operation. This resulted in maximizing the Tri Ethylene Glycol production and reducing overall steam consumption.

A post-implementation study estimated that the overall Mono Ethylene Glycol Unit APC controller returns a savings about 18% greater than the originally estimated benefits. Although both the steam consumption reduction and Tri Ethylene Glycol production increase were greater than originally estimated, the prices for steam and Tri Ethylene Glycol used in the post-implementation study were reduced by 10% and 45%, respectively, from their original prices.

Read other articles on APC here and other articles from Saravanan here.