Advanced process control helps global chemical maker achieve greater productivity
By Ladislav Jurenka and Don Morrison
There came a point when benzol and crude coking tar maker Deza knew it needed to improve the performance of its phthalic anhydride (PA) plant at Valasske Mezirici in the Czech Republic.
A specialist in processing benzol and crude coking tar, byproducts of coke production from coal, Deza needed to do what everyone else was doing and that is to keep increasing its productivity. With its benzol processing capacity of 150,000 metric tons per year (MT/a) and 400,000 MT/a of crude coal tar, Deza is a major player in the global market.
Deza’s move to improve their process performance began back in 1997, and it has been evolving ever since.
In an age where manufacturers are looking for competitive advantages for their plants, even the smallest capacity boosts are valuable, especially when they do not require a major investment.
To work with their legacy systems, more plants these days are considering advanced process control (APC) technologies to improve profitability through enhanced process stability and operating flexibility. The results include increased throughput and yield, decreased operating costs, and a better-quality product. All of that adds up to better productivity and increased performance.
Deza knew APC could be a useful tool when it came to improving process performance and increasing capacity.
Deza down low
Deza’s business includes production of basic aromatic commodities, organic intermediates, aromatic specialties, and phenol homologues, as well as the purchase and sale of chemical products and R&D and analytical activities in organic chemistry.
The Deza PA process consists of seven functional processes and two support processes: air blower and preheater; ortho-xylene feed system; naphtalene feed system; oxidizing reactor; desublimation section; waste gas incinerator; and crude distillation section, supported feed preparation, and the PA storage system.
The turbo-blower sucks and compresses the amount of oxidizing air necessary for the reactor via the air filter. The air then heats up in the steam-heated air pre-heater. After it heats in the pre-heater, a minor portion of air aspirates into the auxiliary turbo-blower, which then conveys as carrier gas through naphthalene evaporators. That is when naphthalene saturates the gas. The required concentration of naphthalene is set according to the temperature of air at the outlet, which regulates to the required temperature by means of the heating steam.
Meanwhile, the oxidizing air undergoes enrichment with ortho-xylene injected under pressure via spray nozzle. The naphthalene vapors, ortho-xylene vapors, and air then mix and enter the reactor. The reactor contains 14,000, 3.7 meter long vertical pipes connected in parallel, which fill with a four-layer catalyst. An eutectic mixture of potassium nitrate and sodium nitrite surround the tubes. The mixture continuously re-circulates via pump. The mixture of naphthalene and ortho-xylene vapors entering the rector heat up by the reaction with molten salt. At a temperature of 360-390°C, naphthalene and ortho-xylene partially catalytically oxidize by the atmospheric oxygen, yielding mainly phthalic anhydride.
A smaller portion of naphthalene is at the same time converted to 1,4 maleic anhydride or it becomes completely oxidized. If the reaction is too cool, it will produce a greater amount of 1,4 naphtaquinone. Conversely, if the temperature is too high, the proportion of maleic anhydride increases, and the majority of naphthalene completely oxidizes. A part of ortho-xylene also converts to maleic anhydride or completely oxidizes. Partial ortho-xylene oxidation produces phthalide as a by-product.
The reactions taking place on the catalyst are very exothermic. The temperature in the reactor remains uniform by built in structures and by re-circulating the salt bath. The salt bath cools by evaporating condensate in an evaporator, which produces a mixture of steam and water. This, in turn, separates into saturated steam and condensate in a high-pressure steam drum.
A control valve with an increased pressure rating than the rest of the plant controls the cooling system pressure. The salt bath temperature can vary ±0.25°C in a stable operation. Increasing the salt bath temperature increases the catalyst temperature and vice versa.
For cooling, the hot reaction gas from the lower part of reactor conveys into the common housing of a two-stage cooler, where it can generate steam. For further cooling, it channels into a desublimator. This process deposits phthalic anhydride on the fins of the tubes in the form of rod-like crystals, with an efficiency of up to 99.5%. The gas goes to the catalytic incinerator for final purification.
The gases from desublimators contain residues of organic matter, which have not been isolated, such as phthalic anhydride and maleic anhydride, as well as carbon monoxide and carbon dioxide. They need to be catalytically incinerated to form carbon dioxide and water before they can be emitted to the atmosphere. In the incinerator, the waste gas first heats up in a steam pre-heater, followed by a second pre-heater using a counter-current of clean waste gas, until it is hot enough for catalytic combustion using a two-level platinum catalyst.
After a detailed survey taking into account all PA reaction sections, research found the air compressor, which would pose a bottleneck during peak production, feed preparation, and the reaction sections would benefit most from APC.
Deza officials also developed several objectives to improve performance at the plant:
Maintain the reactant concentration in the oxidizing reactor below its upper limit.
Ensure the reactant concentration in the feed section did not reach explosive limits.
Keep the turbo blower throughput below or at the operation specified upper limit.
Maintain the oxidation reactor catalyst temperatures within safe limits.
Make sure controller outputs stay inside specified limits.
Maximize crude PA production.
The solution ended up being multivariable control and optimization software, covering the air compressor, feed preparation section, and oxidizing reactor. In addition, the application validates the oxidizing reactor temperature and continuously updates them.
Additionally, the new process historian database reports and issues trend forecasts for key data, helping improve process plant analysis. The performance monitoring package provides useful statistics on the multivariable controller’s performance in terms of online time, standard deviation from controlled variables (CVs), constraint activity, and more.
This particular solution has seen use in the refining and petrochemical industry. It centers on a range control algorithm, which controls all controlled variables within their ranges and allows the controller to explicitly constrain all of the controlled variables dynamically into the future. The range control algorithm keeps the multivariable controller online in situations where others become unstable.
Another part of the solution is the product value optimization, which is more dynamic rather than steady-state, which lets the multivariable controller optimize the entire unit faster and potentially offers a more precise and more profitable final solution. That part finds not only the steady-state solution, but also the optimal dynamic path leading there. The product value optimization is part of closed-loop control and unifies into the range control algorithm. As a result, optimization speed can tune independently of the speed of CV error correction.
APC has yielded significant economic and operational benefits for Deza. The survey found the increase in crude PA production would lead to a nine-month payback for the project. A subsequent performance guarantee audit found the project payback was less than six months, and the controller service factor has been greater than 99% since commissioning in October 2006. Comparing the performance of the PA unit using APC with its performance without it shows that not only has the production rate increased, every ton of PA produced now consumes less utilities and raw materials than before.
Key to the project’s success was operator acceptance throughout the project schedule and strong management support allowing control engineers to fully participate and take responsibility for all project phases including controller design, basic controller tuning, plant step testing, implementation, and commissioning of the multivariable predictive controllers. Rigorous attention and resolution of regulatory control problems before implementation of the advanced controls was a major factor in achieving the high level of control.
ABOUT THE AUTHORS
Ladislav Jurenka (email@example.com) is the phthalic anhydride process engineer for Deza at Valasske Mezirici, Czech Republic. He has been actively involved in the design, implementation and commissioning of the process control strategy. Don Morrison (Donald.firstname.lastname@example.org) , based in Cincinnati, Ohio, is a senior principal engineer responsible for advanced applications in the areas of advanced process control, optimization and abnormal situation management. Morrison holds a BS in chemical engineering from Purdue University.
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