Apr-2014

Zeolite based dryers in ethylene plants

A review of the role and nature of adsorbent dryers in the production of ethylene by steam cracking. Steam cracking represents more than 95% of ethylene production. This article focuses on product stream drying duties in ethylene production based on steam cracking of paraffinic hydrocarbons.

VASSILIOS ZAFIRAKIS and HANS HOEFER
Grace Materials Technologies

Article Summary

Steam cracker feeds can be separated into two categories:
• Natural gas liquids (NGLs): ethane, propane and butane
• Heavier hydrocarbons such as naphtha and gasoils.
North America (>5%) and the Middle East (>80%) use gaseous feedstock to produce ethylene, whereas in Europe (>80%), in Asia-Pacific (>81%) and in Latin America (>65%) naphtha and gasoil feeds dominate. On the world scale, ethylene feedstocks are as follows:
• Naphtha   52%
• Gasoil      6%
• Ethane     28%
• Propane   8%
• Butane     3%
• Other        3%

Process sections of an ethylene plant
In general, ethylene production has four major sections:
1. Cracking heater section including transfer line heat exchangers (TLE)
2. Gasoline fractionator and quench water tower
3. Compression section with both acid gas removal and a main drying unit
4. Chilling train and separation section for cracked gas separation into the desired products.

A variety of commercial process routes is available. Whereas cracking heaters, quench towers and compression sections are similarly configured (sections 1-3 above), for separation (section 4) three main process routes are established. They are characterised by the first separation tower and the position of the unit for hydrogenation of C2 acetylenes in the cracked gas. These three process sequences are:
1. Demethaniser first with tail-end hydrogenation
2. Deethaniser first with front end hydrogenation
3. Depropaniser first with front end hydrogenation.
   
Cracking heater section and TLEs
The ethylene industry is served by a limited number of licensors and contractors with their own ethylene process technologies. They have designed and built most of the world’s ethylene plants. In the process route shown in Figures 1, 2, 3 and 4 hydrocarbon feedstock (ethane, propane, butane, naphtha and gasoil) is fed into a pyrolysis heater (furnace) and preheated to temperatures of 500-650°C. Steam enters the heater coil and the hydrocarbon/steam mixture is further heated to temperatures of 750-890°C. Within this range, saturated hydrocarbon molecules crack to form olefins such as ethylene, propylene and butene, and diolefins, as well as other hydrocarbon molecules such as methane, benzene, toluene, and hydrogen and other products. After this pyrolysis reaction the produced gas from the heater coils is rapidly cooled in tubular heat exchangers (transfer line exchangers, TLE) against boiler feed water, producing high pressure steam (HPS). In the case of gaseous feeds, the gas leaves the TLE at a temperature of 300-400°C. For liquid feeds such as light to heavy naphtha, temperatures are 420-450°C, and for gasoil 550-650°C. The lowest temperature refers to the start of the cracking cycle (SOR), while the highest temperature represents the maximum temperature, where the coil and the TLE need to be cleaned (EOR) and the heater has to be shut down.

Gasoline fractionator and quench water tower
One of the major differences between liquid and gas crackers is the use of a gasoline fractionator. Liquid crackers require gasoline or primary fractionators. This is not necessary for crackers using ethane or propane as feedstock (see Figure 2). 

Furthermore:
•  For liquid feedstock downstream of the TLEs, the required cooling to temperatures of 200-230°C cannot be performed in standard shell and tube heat exchangers; condensation of a significant portion of liquids in the cracked gas leads to excessive fouling by carbon deposits. Therefore, the cracked gas is further cooled by injection of oil (oil quench) by means of devices called quench fittings. Oil quenching is performed downstream of the TLEs after each heater, or in the combined cracked gas line (transfer line) from all heaters. The mixed two-phase flow of cracked gas and quench oil is separated in the bottom section of the primary fractionator. In this tower the cracked gas is separated from the fuel oil, which leaves the tower from the bottom. The overhead is directed to the quench water tower
•  For gaseous feedstock, cracked gas from the TLEs is further cooled to about 200°C against feed or boiler feed water (BFW) in the so-called secondary TLE, before entering the quench water tower where the cracked gas is further cooled.
In a liquids cracking plant, this tower essentially works as a partial condenser to the upstream oil fractionators, which condense both the steam and a significant part of the gasoline components. Part of the gasoline is used as reflux for the upstream oil fractionator. The remaining gasoline leaves the unit after stripping in a side tower for stabilisation.

The cracked gas leaves the top of the quench tower at a temperature of 35-40°C, slightly above atmospheric pressure and enters the compression section of the plant.

Compressor section, acid gas removal and drying of cracked gas
The cracked gas is compressed in four to five stages to pressures of 35-37 bar. Usually, compression of the charge gas from gaseous feedstock cracking takes place in centrifugal compressors employing four stages, while five compressor stages are required for compression of cracked gas from the cracking of liquid feedstocks such as naphtha or gasoil. Consequently, in plants processing gaseous feedstock a higher compression ratio is employed, yielding a higher discharge temperature compared to liquid based plants. The number of stages depends primarily on gas composition and the highest temperature allowed for the discharge of the different compression stages. Such a temperature is typically below 100°C in order to avoid excessive equipment fouling through polymerisation of diolefins and other precursors in the cracked gas. As the pressure increases at the different compressor stages, water and hydrocarbons are condensed in the downstream coolers and separated from the gas in the interstage separators.

Water is returned to the quench water tower while the hydrocarbons from the first three stages are recycled to the gasoline stripper in the upstream hot section of the plant. The acid gas removal system is usually positioned after the third or fourth compression stage. Cracked gas leaving the acid gas removal unit has a remaining acid gas content of about 1 ppmv. Depending on the technology selected downstream of the acid gas removal system, one or two additional compression stages may be employed. Additional C2- and C3 rich condensate is produced in these compression stages. This is fed into the stripper, where the C2 and lighter fractions are forced through the tower top back to a compression section operating at lower pressure. The bottoms of this tower are sent to the downstream depropaniser. If the operating temperature in the depropaniser is lower than 15°C, the bottom stream has to be dried to avoid the formation of hydrates.

The cracked gas is saturated with water before compression as well as after each intercooler stage and down-stream of the acid gas removal system. The condensate is also saturated with water, while excess water separation depends on the design of the separator equipment. Therefore the condensate always contains traces of free water. This needs to be considered in the design of downstream condensate dryers. Complete removal of water is mandatory for the charged gas or liquid streams to be cooled to temperatures below 15°C in the downstream chilling train and the separation section, to avoid the plugging of lines and equipment due to formation of hydrates and ice. For the removal of moisture from these streams, the most current technology uses solid alumino-silicates (zeolitic molecular sieves) which will be discussed later. Previously, dual systems (glycol followed by alumina) were employed.