131- Technical design discussion on the alkali wash coalescer and water wash coalescer dedicated for propane in the depropanization tower of the isooctane plant

In the isooctane plant, propane is mainly an added material. It works in synergy with isobutane to remove heat via flashing from the reactor and thereby regulate the reaction temperature. To prevent the accumulation of propane in the system, it is necessary to remove some of the propane from the system using a depropanation tower. Alkali wash coagulators and water wash coagulators are specifically designed for treating the propane obtained from the depropanization tower.

Since the alkylation unit in the isooctane project uses sulfuric acid as a catalyst, the reaction products contain acidic gases of sulfur oxides. In the propane stream obtained from the depropanization tower, it is necessary to use caustic alkali to alkalinize the propane and remove the sulfur oxide acidic gases carried by it; this process inevitably introduces caustic alkali as well as reaction by-product salts into the propane stream. The alkali wash coalescer is specifically used to remove the dispersed caustic alkali and by-product salt solution from propane.

In the Lummus process package, the propane caustic coalescer is referred to in English as Propane/Caustic Coalescer. The relevant operating parameters are as follows: 1 – Composition of the hydrocarbon feed (wt%): sulfur oxides 0.12, ethane 2.4, propane 96.91, isobutane 0.49, n-butane 0.08； 2-Composition of the alkaline phase feed (wt%), caustic alkali 2.02, salts 5.98, water 92 ； 3-Hydrocarbon phase flow rate: 2400 m^3/h ； 4-Hydrocarbon phase density: 470 kg/m^3; 5-Hydrocarbon phase viscosity: 0.08 cp ； 6-Base phase flow rate: 16230m^3/h ； 7- Alkali phase density: 1100 kg/m^3; 8- Alkali phase viscosity: 1.3 cp ； 9-Operating temperature: 40℃ ； 10-Operating pressure: 2.2 MPaG.

Requirements imposed by the Lummus process package on the propane alkaline scrubbing coalescer: 1- The water separation precision for the hydrocarbon phase at the outlet of the alkaline scrubbing coalescer must be above 5 microns, with a removal rate of 99% for the dispersed phase ； 2-Operating maximum pressure difference: 0.035 MPa ； 3-Internal component material: SS316L ； 4-Internal component replacement cycle: not less than 60 months ； 5-Shell material: CS; 6-Design pressure: 3.05 MPaG ； 7-Design temperature: 65℃ ； However, it needs to be purged with steam at 1.0 MPaG@190°C ； 8-Operational flexibility: 60%-110% ； 9- Design standard: ASME; 10- Residence time in alkaline water shall be no less than 5 minutes.

The propane stream exiting the top of the propane alkaline washing coalescer still contains a small amount of alkali solution in its dispersion phase, failing to meet the specifications for propane product. Water washing is also required to elute the remaining trace amounts of salts and alkalis. However, since water washing is still used to remove salts and alkalis, the propane stream contains a significant amount of dispersed water, and a water-washing coalescer must be employed to remove it in order to meet the requirements for the propane product.

The propane/water coalescer involved in the Lummus process package is referred to in English as the Propane/Water Coalescer. The relevant operating parameters for this device are as follows: 1 – Composition of the hydrocarbon feed (wt%): 0.02 for caustic alkali, 0.06 for salts, 0.92 for water, 2.38 for ethane, 96.06 for propane, 0.47 for isobutane, and 0.09 for n-butane； 2-Composition of the basic phase feed (wt%), caustic alkali 0.13, salts 0.38, water 99.5 ； 3-Hydrocarbon phase flow rate: 2400 m^3/h ； 4-Hydrocarbon phase density: 470 kg/m^3; 5-Hydrocarbon phase viscosity: 0.08 cp ； 6- Alkali phase flow rate: 16000 m^3/h ； 7- Alkali phase density: 1020 kg/m^3; 8- Alkali phase viscosity: 0.8 cp ； 9-Operating temperature: 40℃ ； 10-Operating pressure: 2.0 MPaG.

Requirements of the Lummus process package for the propane washing coalescer: 1- The precision required for separating water from the hydrocarbon phase at the outlet of the washing coalescer is above 5 microns, with a removal rate of 99% for the dispersed phase ； 2-Operating maximum pressure difference: 0.035 MPa ； 3-Internal component material: SS316L ； 4-Internal component replacement cycle: not less than 60 months ； 5-Shell material: CS; 6-Design pressure: 3.05 MPaG ； 7-Design temperature: 65℃ ； However, it needs to be purged with steam at 1.0 MPaG@190°C ； 8-Operational flexibility: 60%-110% ； 9- Design standard: ASME; 10- Residence time in alkaline water shall be no less than 5 minutes.

This post was last edited by luoli519 on 2019-7-2 13:50. Although propane-based washing coagulators and water-based washing coagulators serve different purposes, they are similar in design. Here, a propane alkaline washing coalescer is taken as an example for design explanation and discussion. Our design recommendation for the propane alkaline scrubbing coalescer in the aforementioned Lummus process package is as follows: 1–Given the high requirements for the precision of coalescing separation, a two-stage separation approach that combines pre-coalescing with vane-type coalescing elements is necessary to ensure satisfactory separation results ； 2-For the reasons stated in the previous technical article on olefin feed coalescers, horizontal designs are preferred for the alkali wash coalescers and water wash coalescers discussed in this article ； 3- Once the size of the separator is determined, distribution pipes and flow-stabilizing internals must be installed at the inlet and outlet ports for each phase; moreover, a back-type distribution pipe corresponding to the structure of the horizontal coalescer should be selected, with professional companies specializing in dynamic separation technology carrying out precise dynamic calculations and configuration design ； 4- It should be further noted that the aforementioned coalescer requires not only accurate dynamic calculations and a proper structural design for the distribution pipes, but also maximization of the two-phase travel distance, in order to provide a longer residence time that allows the smaller-sized dispersed phase to have sufficient time to complete sedimentation and separation ； 5- It should be emphasized that individual barrier-type coalescing internals, such as filter elements and wire meshes, do indeed exert a coalescing effect; however, the small and medium-sized dispersed phases resulting from this coalescing process cannot be effectively separated within the residence time. Therefore, it is necessary to use a set of vaned coalescing internals in order to enable the effective separation of these small and medium-sized dispersed phases during the residence time. For the reasons, please refer to my previously published article on liquid-liquid dynamics separation technology.

This post was last edited by luoli519 on 2019-7-2 at 13:54. The diagram below shows the structure of the propane alkali scrubbing coalescer, which was developed by our team using NOVEL’s precise dynamics separation calculation and configuration system platform; it is provided for your reference

It should be added that the new type of composite weir-chamber coalescer is our proprietary technology. For more technical information on separation technologies and equipment, please visit the Nuowei Separation Technology dedicated website.