Application of Antifoulants in POX-Units with Waste Heat Boilers

Introduction

In a POX (partial oxidation) unit CO and H₂ are generated from pure oxygen, high pressure steam and hydrocarbons. In downstream plants ammonia and methanol are produced of these products. Co-products are CO₂, H₂S und carbon black pellets.

Hydrocarbon gases and heavy oils are used as feed for these plants. Especially heavy oils as main feed cause considerable fouling in the waste heat boilers. The boilers are located directly behind the reactors (see graph 1). This fouling leads to insufficient down-cooling of the product gases with the effect that a plant shutdown becomes necessary. The reason for the shutdown is that the material of the downstream parts are not designed for high temperatures.

Shutdown means reduced product quantities and strain of the plant material due to enormous reduction of the temperature in the reactors (from 1,350 °C to 250 °C).


Graph 1: Schematic design of a POX plant

Application of KURITA AP-2505

Kurita developed a special antifoulant to solve the fouling problems in POX units. This antifouling product KURITA AP-2505 is already successfully applied in several plants. It is dosed undilutedly into the heavy oil feed stream to the reactor. The application of KURITA AP-2505 completely stops the fouling in the waste heat boilers. With a certain dosage rate of KURITA AP-2505 a cleaning effect in the waste heat boilers can be achieved. With its special formulation the antifoulant prevents growth of deposits.

The antifoulant´s effectivity has been tested in several plants. Below you will find examples for testruns in two plants (plant A + B). In plant A the critical parameter is the outlet temperature of the raw gas behind the waste heat boiler. As a check of efficiency the dosing of KURITA AP-2505 has been stopped and the change of the temperatures were recorded. Three tests showed congruent results: the outlet temperature increased clearly without dosing KURITA AP-2505. Upon resumption of the dosage the temperature increase stopped immediately.


Graph 2: Outlet temperatures of waste heat boilers, plant A

In plant B the critical parameter is the pressure loss over the waste heat boiler. Graph 3 shows the progression of the pressure loss. During the operation period April/June 2003 KURITA AP-2505 has not been dosed. The dosage started directly before the period end of July/August 2003. The end of operation period was determined by a fixed maximum pressure loss of 2.0 bar. This limit should prevent material damages.


Graph 3: Pressure loss over waste heat boiler, plant B

Results

In both partial oxidation plants (heavy oil gasification) the application of KURITA AP-2505 led to congruent results. By using the antifoulant the deposits in the waste heat boilers have been reduced in a way so that a plant shutdown due to fouling is not necessary anymore.

For this reason enormous cleaning expenses have been saved. But much more important is the fact that there is no production loss due to fouling or deposits in the waste heat boilers and boiler feedwater exchangers anymore.

Since the continuous dosage of KURITA AP-2505 both plants operate without losses caused by shutdown measurements based on fouling.

---