Borzienkova S., Ladieva L.

The National Technical University of Ukraine “Kyiv Polytechnic Institute”

The evaporation control in the thermal cracking process

Thermal cracking or dry distillation under pressure was designed for processing heavy fuel-oil residues and distillate materials. This method contributes to a significant increase in selection depth of light oil-products from oil.
Dry processing based on decomposition of hydrocarbon molecules and is the most common way of chemical processing of oil-products. The destructive distillation process of fuel oil and tars was invented to increase gas-oil fractions resources – the cracking plants feedstocks for hydrocarbons mixture with different structure (distillate yield increased by almost 35% compared with conventional distillation). The peculiarity of the process is a combination of a raw distillation with a tarry residue thermal decomposition in the evaporator.

Fig.1. Technological scheme of installation for destructive distillation of fuel oil and tars

Installation with single continuous passing of material is showed in Fig. 1. It consists of the high-temperature section (oven, evaporator), the fractionation (stripping and rectification columns, etc.) and the cooling (refrigerators, etc.) sections.

Gas-oil fractions principally formed on the evaporator. It`s a part of the high-temperature section, which works at medium (420 ... 425 °C) temperatures and low overpressure (0.2 ... 0.3 MPa). The formation takes place under the live steam influence: a mixture of fuel oil with a small amount of water vapor and cracking gases enters the bottom of the evaporator. There the mixture begins to be purged by heating steam and decomposes into heavy resinous residue and volatile mixture of cracked gases.

Fluid level and overpressure are the main parameters of the evaporator operational condition.
Next control system (Fig. 2) was created to ensure the management of the evaporator. It includes feedforward and error-closing control.
Pressure control is carried out by superheated steam with using a cascading control system. The internal control loop consists of flowmeter of water vapor (6-1), normalizing converter (6-2), the regulator (6-3) and the actuator (6-4). The external loop includes a pressure meter (4-1), the regulator (4-2) and summator (4-3), which forms internal tasks for regulator. The signal from the disturbances compensation loop is applied to summator. Compensation loop consists of flowmeter for cracking gases (5-1), normalizing converter (5-2) and the dynamic equalizer (5-3).
Fluid level control is carried out by feeding liquid fuel oil. It used a similar system to control. The internal control loop includes flowmeter (3-1), normalizing converter (3-2), the regulator (3-3) and the actuator (3-4). External circuit: level gage (1-1), the regulator (1-2), summator (1-3), which receives the signal from disturbances compensation loop. It consists of measuring the flowmeter for severe cracking residue (2-1), normalizing converter (2-2) and the dynamic equalizer (2-3).
Additionally, the solvent supply control loop was provided to reduce the impact of speed removal of heavy residue. It consists of the viscometer (7-1), the regulator (7-2) and the actuator (7-3).

Besides the temperature control loops of all incoming and outgoing flows were introduced monitoring of the evaporation process (8-11 contours).

Fig.2. Automation scheme of evaporators work

The relevance of fuel-oil dry distillation technology is recycling and using products of primary oil processing.

Also, the designed scheme of vaporizer automation control enables to increase the accuracy of measurement. Therefore the process control is going to be better. But the main advantage of this scheme is reducing the support resources. And it is economically profitable.