September 2019

Special Focus: Refining Technology

Horizontal distillation column technology

Gas-liquid contact operations are required for the separation of mixtures based on differences in the chemical properties (volatility, solubility, etc.) of various components.

Katyal, A., Independent Researcher/Inventor

Gas-liquid contact operations are required for the separation of mixtures based on differences in the chemical properties (volatility, solubility, etc.) of various components. Gas-liquid contact operations form the backbone of numerous hydrocarbon-based industries, such as the refining, petrochemicals, natural gas liquids (NGL) and alcohol industries.

FIG. 1. The design of a horizontal gas-liquid contact process.
FIG. 1. The design of a horizontal gas-liquid contact process.

Conventional gas-liquid contact operations are traditionally conducted in vertical tray columns or packed columns. Due to economic considerations, vertical tray columns are used in all cases where column diameters are > 0.6 m or where numerous stages are required. Vertical tray columns are also used when working under stressed conditions, such as large variations in temperatures and pressures, as packings may become damaged under such conditions. Vertical tray columns are preferred over packed columns due to their ease of cleaning and are generally used more frequently compared to packed columns, unless special conditions of low pressure drop (i. e., vacuum distillation), corrosive chemical handling and highly foaming liquid are encountered.

Vertical tray columns are used widely in refineries and other chemical plants to separate hydrocarbons and other compounds by gas-liquid contact operations (distillation, gas absorption and gas stripping) and sometimes for humidification and dehumidification operations. Vertical tray columns with a height of 30 ft–40 ft are common in chemical plants; sometimes they can reach as high as 100 ft. The transportation of these columns is costly, and installation, operation, maintenance and troubleshooting require difficult and unsafe aerial operations.

Vertical tray columns have been used for more than 100 yr, and numerous attempts have been made by various researchers to provide a horizontal solution to vertical tray columns to make the transportation and installation of infrastructure, and the operation, maintenance and troubleshooting of gas-liquid contact operations—such as distillation, gas absorption and gas stripping—easier, safer and less expensive. Until now, a workable horizontal solution to vertical tray columns has been elusive.

A new solution

A new horizontal solution to vertical tray columns that assists in carrying out gas-liquid contact operations is detailed here. The design of the horizontal gas-liquid contact process consists of small, interconnected vessels placed horizontally. Trays with the same structure and the same or similar size as an equivalent vertical tray column are placed near the bottom of each vessel. The tray spacing is equivalent to a vertical tray column and is replicated with the height of the vessel. Vessels have the same or reduced diameter as the diameter of an equivalent vertical tray column. Each vessel acts consistently to a stage of an equivalent vertical tray column. The shell bottom of each vessel is connected to a downcomer at the shell top of the next vessel in the liquid flow, replicating the liquid flow in an equivalent vertical tray column.

FIG. 2. The design of a horizontal gas-liquid contact process.
FIG. 2. The design of a horizontal gas-liquid contact process.

The crossflow of liquid and vapor-like vertical tray columns are maintained by connecting the vapor space of one vessel to the bottom of the next vessel in the vapor flow path, and by transferring a controlled amount of liquid from the bottom of one vessel to the top of the next vessel in the liquid flow path, using a pump and valves arrangement.

Depending on the gas-liquid contact operation, a suitable reboiler and condenser and feed vessels are placed in the horizontal array of vessels, replicating the reboiler, condenser and feed stages of an equivalent vertical tray process. A centrifugal pump of suitable pressure rating and flowrate is placed in the liquid flow line connecting two successive vessels. A flow control valve (FCV) in the pump outlet controls the liquid flow from one vessel to the next. The excess liquid from the pump outlet is recycled back to the vessel on the pump inlet side through a liquid recycle line to avoid excessive backpressure on the pump.

A liquid recycle non-return valve (NRV) is placed in the recycle line to avoid backflow of liquid/vapor. A level transmitter is placed in each vessel to transmit the liquid level to the FCV in the next connecting pipe. The opening of the FCV is controlled by the level in the preceding vessel. At high liquid levels, the FCV opens more, resulting in less recycle of liquid; it closes completely at a low liquid level, resulting in full recycle of liquid. This maintains the liquid level on the tray at the most optimum value, resulting in the distillation system running near the designed liquid holdup.

In an open-ended, gas-liquid contact operation, a compressor is placed in the vapor flow line to ensure cross-current vapor flow. NRVs are placed in all liquid and vapor connecting pipes to stabilize the process and increase its operational flexibility. Demisters are placed in vessels, as required, to avoid liquid entrainment in the process. The design of a horizontal gas-liquid contact process is shown in FIGS. 1 and 2.

Horizontal system design

The new system’s horizontal design makes it inexpensive, as well as easy and safe to transport, install, operate, maintain and troubleshoot, compared with an equivalent vertical tray column. Due to its vertical structure, a vertical tray distillation column must be designed considering dead loads, seismic loads and wind loads. A costly foundation must be laid, ladders must be added and intermediate supports are required. These components can be avoided for the horizontal system, resulting in reduced capital cost. The new system can be manufactured in modular form in the factory rather than onsite, saving time and money. All regulatory permissions and compliances required for vertical tray columns can be avoided. The added energy consumption resulting from pump-driven flow rather than gravity flow, along with the increased footprint of the horizontal system, can be partly offset with some design/layout variations. These variations can be achieved through segmented operation rather than the single-stage, linear and snake-like spatial arrangements described in later sections.

Gas-liquid contact applications

The new system can be used universally to perform numerous distillation operations, such as continuous distillation, batch distillation, semi-batch distillation, etc., at lower cost and with better operational flexibility than conventional vertical tray distillation columns. The configurations and arrangements proposed in the new system can be used in other gas-liquid contact operations (gas absorption, gas stripping, humidification, dehumidification) that are conventionally carried out in vertical tray columns.

Design and operating procedures

The new system is based on the same theoretical principals as a conventional vertical tray process. It enables the same calculation procedures (i.e., McCabe-Thiele and Ponchon-Savarit) used for binary distillation in a conventional vertical tray distillation process to be used for the proposed process. Additionally, the same calculation methods used for various gas-liquid contact operations can be used for the proposed process. Similar internals are used as in the conventional vertical tray process, requiring similar design calculations and standard procedures. Startup, shutdown and normal operating procedures for this system are like those for equivalent vertical tray columns, in theory, requiring suitable variations during practical implementation due to the design differences of the two systems. This avoids any required development of different calculation methods and operating procedures for the system, guaranteeing the efficient design and operation of the new system based on tried and tested principals, calculation methods and operating procedures.

FIG. 3. The operating area for the proposed process on a vapor flowrate vs. liquid flowrate curve, as compared to the operating area for an equivalent vertical process.
FIG. 3. The operating area for the proposed process on a vapor flowrate vs. liquid flowrate curve, as compared to the operating area for an equivalent vertical process.

Swelling of operating region

Due to the modular design of the system, it is possible to install NRVs in liquid and vapor flow lines connecting various modules. Demisters can also be installed in desired modules with better maintainability compared to an equivalent vertical design. This results in relaxed weeping and flooding conditions, causing the operating area for the proposed process on the vapor flowrate vs. liquid flowrate curve to swell, as compared to the operating area for an equivalent vertical process (FIG. 3). The new system is operationally more flexible and, therefore, less prone to shutdowns, resulting in uninterrupted service and monetary savings.

More efficient design parameters

The system can operate in a swelled operating area on a vapor flowrate vs. liquid flowrate curve compared to an equivalent vertical tray column. Due to more flexible operating conditions, system design parameters like the downcomer area, perforated area, hole diameter, hole pitch, weir height, etc., may result in improved operational efficiency.

FIG. 4. The snake configuration is a compact spatial arrangement where vessels are arranged in a zig-zag configuration.
FIG. 4. The snake configuration is a compact spatial arrangement where vessels are arranged in a zig-zag configuration.

The system’s separate vessels act as different stages of an equivalent vertical process. If required, each vessel and its internals can be designed independently of other vessels. Such design flexibility is not practically possible in conventional vertical tray columns.

Choice of spatial configurations

The system can be arranged in two spatial configurations: a linear configuration and a snake configuration. The snake configuration, shown in FIG. 4, is a more compact spatial arrangement where vessels are arranged in zig-zag configuration. Both configurations are operationally the same with the same inputs providing the same outputs, so any of these configurations can be chosen depending on convenience and need.

Segmented gas-liquid contact operations

The segmented gas-liquid contact process, which is an extension of the horizontal gas-liquid process, divides a vertical tray column-based, gas-liquid contact process into various vertical segments, with each vertical segment comprising more than one stage. The flow of liquid within each vertical segment takes place by gravity. The flow of liquid from one vertical segment to the other takes place through a pump, NRV and FCV arrangement like the horizontal gas-liquid contact process. Vapor NRVs are placed between each of the two successive segments in the vapor flow path. The segmented distillation process is shown in FIG. 5.

FIG. 5. The segmented distillation process.
FIG. 5. The segmented distillation process.

The segmented gas-liquid contact process is operationally less flexible than the horizontal gas-liquid contact process due to the absence of liquid and vapor NRVs between stages within a vertical segment. However, the segmented gas-liquid contact process is operationally more flexible due to the presence of liquid and vapor NRVs between two successive segments. The energy requirement of the segmented gas-liquid contact process is less than the horizontal gas-liquid contact process due to the lesser number of pumps, while the energy requirement of the segmented gas-liquid contact process exceeds the vertical gas-liquid contact process, where liquid flow takes place entirely due to gravity rather than a pump.

Other advantages of the segmentedgas-liquid contact process include ease and safety of transportation, installation, operation, troubleshooting and maintenance, as well as capital cost reduction due to fewer dead loads, seismic loads and wind loads. HP

NOTE

The design for the horizontal distillation technology is patented under US patent number 9855515 B2. An Indian and a UK patent are pending.

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