Innovation in Iran - part two

Shiraz petrochemical complex

Shiraz Petrochemical Co. (S.P.C) was founded in 1959 with an initial investment of approximately US$400 million, in 500 ha. of land, out of which 72 ha. was dedicated to infrastructure. The company’s initial aim was to meet Iran’s need for fertilizer. It now produces 3.7 million tpy of different nitrogen compounds as well as methanol, and is counted as one of the major petrochemical companies in Iran and the most diversified in terms of nitrogen products. The major feedstock for the operating plants is natural gas, received from the main cross-country pipelines. Water is routed from a nearby dam.

S.P.C first commenced operation in 1963, producing ammonia, urea, nitric acid and ammonium nitrate. The soda-ash, sodium bicarbonate and sodium tripolyphosphate (STPP) plants annexed to these units formed Zone I. The inbuilt utility plant provided all the utilities for the process plants. Since 1963, two major expansion projects were implemented that have increased the current total capacity of the complex to 3.7 million tpy. The operation of older plants in Zone I has been discontinued due to out-of-date technologies and environmental considerations. The process plants in operation today are divided into two major zones.

Zone II

The first expansion project was executed in 1984 with the construction of ammonia, urea, ammonium nitrate and nitric acid plants with more than ten times the capacity of the old units, as well as a dedicated utility plant. The project also included the installation of bulk handling facilities for produced materials. The argon and hydrogen recovery unit was annexed to the ammonia plant in 1994. Zone II is self-sufficient in terms of utilities. Five steam turbine driven generators provide the required electricity. In addition, an 8.8 MW gas turbine driven generator has been installed for emergency cases. The total electrical power generation capacity is 68.8 MW. The nominal production capacity in the plants in Zone II is 438 000 tpy of ammonia, 547 500 tpy of urea (peril grade), 377 400 tpy of nitric acid (58%), 237 250 tpy of ammonium nitrate and 5475 tpy of argon.

Zone III

The second major expansion project, which doubled the total capacity of the complex, started in 2006 with the engineering and constructing of new ammonia and urea plants as well as another dedicated utility plant and a bulk handling section. Three gas turbine driven generators provide the required electricity with a total capacity of 75 MW, equipped with heat recovery stream generator (HRSG) units to recover the heat from the flue gas and produce the required steam for process plants. The normal production capacity of the plants in Zone III is 748 250 tpy of ammonia and 1.18 million tpy of urea (granular grade). The plants started operation in March 2016 and gave the Shiraz petrochemical complex the ability to produce three types of nitrogen compounds as well as prilled urea, granular urea and prilled ammonium nitrate at the same time.

S.P.C’s methanol plant, the first in Iran, started production in 1990 with 93 075 tpy of nominal capacity. The methanol process technology is based on the Lurgi concept. Methanol is produced by steam reforming of natural gas and applied in the synthetic fibres, plastics and glues industries as well as methyl tertiary butyl ether (MTBE) production.

Cold box leakage

After 27 years of continuous operation of the argon/nitrogen (Ar/N₂) plant with a capacity of 5500 tpy, process conditions showed a small internal leakage in the cold box 18 months ago; however, operation was continued until 15 April 2020 before the unit was halted for overhaul. The first decision made after the reporting of the leakage was to unload the perlite insulation. Perlite insulation is a type of very low density material that is used for very low temperature service (i.e. less than -200°C) for protection of equipment in the cold box of an Ar/N₂ plant, the space around the equipment and the wall of the cold box. The composition and mechanical properties of perlite are shown in Table 1. Low density and the small particle diameter of perlite insulation make it very difficult to unload from a cold box because the particles float in air by very slow air movement or air streamlining around devices. The grain size consideration is shown in Table 2.

Nowadays vacuum cleaners are used for unloading perlite. The vacuum cleaner is equipped with a bag filter device, which has a limited volume. When the bag filter is filled with perlite, the user needs to open the baghouse and clean the filters, which takes time. The vacuum cleaner also needs an electrical motor and fans. These problems forced S.P.C to design and construct a device to unload perlite, without using an electric motor or fans, at a low cost. The system consists of a small ejector, main cyclone filter, secondary cyclone filter, and pipes and fittings.

Figure 1. Main and secondary cyclone filters.

Figure 1 shows the cyclone filters and Figure 2 shows details of a 3D model of the system. The ejector creates a vacuum to remove the perlite from the drain nozzle of the cold box, and the main cyclone separates 86% of particles between 100 µm to 800 µm in length. The secondary cyclone separates 80% of particles that are less than 100 µm in length. The efficiency of the system depends on the relative humidity, plenum chamber temperature and the pressure of the motive gas used by the ejector (motive gases are air or N₂). Figure 3 shows efficiency vs motive pressure. The positive pressure of the cold box is affected but by less than 5%.

Figure 2. 3D schematic of system.

In the bottom of the cold box is a 6 in. drain nozzle and valve. Installing a small ejector on the first 90° long radius elbow produced -14 mm H₂O to -40 mm H₂O vacuum pressure on the cold box though a ½ in. motive air/N₂ pipeline with a backpressure of 2 barg to 13 barg. In the initial calculation, backpressures of 1 barg and 2 barg were used but this rose to 4.5 barg during testing and 13 barg during operation. Rainfall (leading to humidity and low plenum temperature) meant that the conditions during operation partially differed from those of the testing and design phase. The total efficiency of the system was 86 – 95% in dry air and less than 40% in relative humidity.

Figure 3. System efficiency and environmental effect.

The 320 m³ cold box had emptied after 8 hours and approximately 208 m³ of wet perlite (nearly 10 t) had drained. As leakage occurred at the cold box, the perlite was humid and became damaged because of leakage of chemical process fluid and water. The humidity had an effect on gas residence time and particle drift velocity in cyclones. The efficiency of cyclones depends on backpressure, environmental relative humidity and temperature. The main cyclone has 95% efficiency for particles of between 125 – 800 µm in length and the secondary cyclone is efficient for particles that are less than 100 µm.

This experimental and analytical investigation has helped the industry improve the process of reloading perlite from cold boxes during the maintenance period. The device was modelled by the Aspen HYSYS programme. The calculations and operating results endorsed the programme.

Author: Shahram Hashemi, Shiraz Petrochemical Complex, Iran

Read the article online at: https://www.worldfertilizer.com/special-reports/27042021/innovation-in-iran--part-two/