Cost effective APC helps Czech plant to step up production

Valasske Mezirici, Czech Republic - Deza, a Czech processor of benzol and crude coking tar has reported significant performance gains from the application of advanced process control (APC) on its phthalic anhydride (PA) plant at Valasske Mezirici. Since it was commissioned in late 2006,  the system has delivered a project payback of less than six months, while that the controller service factor has been greater than 99% .

The project was based on the findings of an APC benefits study, which was carried out by Honeywell. After surveying all PA reaction sections including the air compressor, the feed preparation section, the reactor, the desublimation, and the waste gas incineration section, Honeywell concluded that the air compressor, which would pose a bottleneck during peak production, feed preparation and the reaction sections would benefit most from APC.

Deza's process

The 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. A turbo-blower sucks and compresses the amount of oxidizing air necessary for the reactor via air filter.

The air heated up in the steam-heated air pre-heater. A minor portion of air is aspirated by auxiliary turbo-blower and then conveyed as carrier gas through naphthalene evaporators where it is saturated with naphthalene. The required concentration of naphthalene is set according to the temperature of air at the outlet, which is regulated to the required temperature by means of the heating steam.

Meanwhile, the oxidizing air is enriched with ortho-xylene injected under pressure by spray nozzle. Oxidation in standard operation, the naphthalene and ortho-xylene vapours and air within the explosion limits are mixed under the upper reactor cover, in the mixers and, in the starting operation, for a limited period of time, also in the naphthalene evaporators.

The naphthalene and ortho-xylene mixture enters the reactor from above. The reactor contains 14 000, 3.7m long vertical pipes connected in parallel, which are filled with a highly efficient four-layer catalyst. The tubes are surrounded by a salt bath - an eutectic mixture of potassium nitrate and sodium nitrite, which is continuously re-circulated by pump.

The mixture of naphthalene and ortho-xylene vapors entering the rector at is first heated up to the heat of reaction by the molten salt. At a temperature of 360-390°C, naphthalene and ortho-xylene are partially catalytically oxidized 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 is completely oxidized. If the reaction is too cool, a greater amount of 1,4 naphtaquinone is produced, if the temperature is too high, the proportion of maleic anhydride increases and the majority of naphthalene is completely oxidized. A part of ortho-xylene is also converted to maleic anhydride or is oxidised completely. 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 is distributed uniformly through suitable in-built structures and by re-circulating the salt bath. The salt bath is cooled by evaporating condensate in an evaporator, which produces a mixture of steam and water. This in turn is separated into saturated steam and condensate in a high-pressure steam drum. .

A controlling valve with an increased pressure rating than the rest of the plan controls the cooling system pressure. The salt bath temperature can vary ±0.25°C in stable operation. Increasing the salt bath temperature increases the catalyst temperature and vice versa.

The hot reaction gas from the lower part of reactor is conveyed into the common housing of two-stage cooler, where it is used to generate steam. For further cooling, it is channeled 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 is conveyed to catalytic incinerator for final purification.

The gases from de-sublimators 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 is first heated up in a 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.

APC targets 

The APC project targetted maintaining the reactant concentration in the oxidizing reactor below its upper limit and ensurign that the reactant concentration in the feed section does not reach explosive limits. Other key aims included keeping the turbo blower throughput below or at the operation specified upper limit, keeping the oxidation reactor catalyst temperatures within its safe limits and maximising the crude PA production

Honeywell’s solution was to install its multivariable control and optimisation Profit Controller software, covering the air compressor, feed preparation section and oxidising reactor. In addition, the company’s Profit Toolkit application validates the oxidizing reactor temperature and continuously updates them in Profit Controller; the Uniformance PHD (process historian database) reports and issues trend forecasts for key data, helping Honeywell with improved process plant analysis; and the company’s performance monitoring package provides useful statistics on the controller’s performance in terms of on line time, standard deviation from controlled variables (CVs).

Profit Controller is a multivariable predictive controller technology that has been widely used in the refining and petrochemical industry. It centres on the patented range control algorithm (RCA), which controls all CVs within their ranges and allows the Profit Controller to constrain all of the CVs dynamically into the future. The number of CVs can be greater or fewer than the number of manipulated variables (MVs), which are generated by the controller to keep the CVs within their limits.

The initial study predicted that the increase in crude PA production would lead to a nine-month payback for the project. This has since been revised to six months. Table1 shows the performance of the APC controller over a 24 hour period. 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 success, said Honeywell, was operator acceptance throughout the project schedule and strong management support allowing control engineers to 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 robust control, the vendor added.