MTR's VapSep System

   Membrane Technology & Research, Inc.

 Due to the high cost of raw materials, efficient use of chemical feedstocks is a major concern in polyolefin manufacturing. For example, monomer losses from polyolefin plant vent streams typically range from 1 to 2 percent of the total plant feedstock. With 300 polyolefin plants worldwide, and total capacity in excess of 60 million tons per year, worldwide monomer losses are costing the industry nearly one-half billion dollars per year.

Membrane Technology and Research, Inc. (MTR) has developed a membrane-based process to separate and recover hydrocarbons, including propylene and ethylene, from nitrogen and light gases in polyolefin plant vents. The process, called VaporSep®, uses a membrane that selectively permeates hydrocarbon vapors compared to nitrogen and other light gases.

Since the first set of commercially membrane system run successfully in 1988, over 70 sets of VaporSep systems have built in the world and widely applied in PE, PP, PVC, EO, VAM, flare recovery, LPG recovery, natural gas conditioning ,etc. Then 1,000,000 tons organic vapor can be recovered annually. As MTR contributes on the PE and PP monomer recovery, MTR won the Kirkpatrick Award in 1997.

   VaporSep Membranes

The membrane is a high flux, thin film composite type that is 10 to 100 times more permeable to hydrocarbons than to nitrogen. The membrane shown in Figure next, consists of three layers; a non-woven fabric which serves as the membrane substrate, a solvent-resistant microporous support layer for mechanical strength, and a defect-free selective layer which performs the separation.

Membrane structure

MTR can achieve efficient separations by exploiting differences in permeability through MTR’s robust, high-flux polymeric membrane.

Solubility selective membrane

For use in VaporSep processes, MTR incorporates membrane into spiral-wound membrane modules. These modules contain membrane envelopes wound around a central collection pipe. Mesh spacer materials create channels through which the feed gas and permeate vapors travel. As a feed gas stream containing organic vapor passes across the membrane surface, the more permeable fraction passes preferentially through the membrane and enters the permeate channel. The permeate fraction spirals inward through the permeate channel to the central collection pipe.

   The Advantages of the VaporSep system

  Ø         Simple process and easy operation；

Ø         Guaranteed performance；

Ø         High recovery of valuable raw materials；

Ø         Short payback time on investment；

Ø         Long membrane Life；

Ø         Few or no moving parts

Ø         High on-stream Time；

Ø         Reduced emissions

   Application of VaporSep in Chemical and Petrochemial Industry

    Application of VaporSep in Ployolefin plants

   Application of VaporSep in the offgas of Ployolefin plants

There are three sections in the polyolefin production process where monomers are typically lost. These three sections are raw material purification, chemical reaction, and product purification and finishing. All three sections provide significant opportunities for more efficient monomer use. A schematic of the process is shown in the following figure.

 Polyolefin production process

   Application of VaporSep in the Raw Material Purification Section

Purification of raw material is the first step in the polyolefin process. This step is very common because raw materials are not always available at required purities. In addition, they have the capability to use low quality feedstocks which provide greater operating flexibility and lower operating costs. Ethylene supply and purity are very important aspects of polyethylene production. At polymer plants where an olefin plant is on-site, ethylene purification is accomplished in an ethylene-ethane column within the olefin process train.

However, many polyethylene plants are stand-alone and purchase feedstocks from refineries or other sources by pipeline. For stand-alone plants, purification of feedstocks is achieved in separate C2 splitter column. When nitrogen and other light gases (hydrogen and methane) are present in the feed, they build up in the overhead of the column and must be vented. This vent stream also contains a significant amount of ethylene, which in a typical plant is valued at more than $500,000 per year.

The VaporSep system is shown in the following figure. The objective of the unit is to remove a fixed amount of the lights and to recycle the enriched ethylene back into the column. Since the stream is at 300 Psig, no feed compressor is required. The vent from the overhead condenser of the column is feed to the membrane. The membrane separates the feed into an ethylene enriched permeate and an ethylene depleted residue. The permeate is sent to an existing compressor and then recycled back to column while the residue is sent to the flare or fuel.

Column overhead recovery

Opportunities for raw material purification are also found in PP plants as well.

   Application of VaporSep in the Reaction Section

In a large number of reaction processes, only a fraction of the feed reacts during a single pass through the reactor to form the desired product. After separation from the product, the unreacted feedstock is recycled back to reactor. In this recycle process, contaminates build-up to unacceptable levels over time and must be purged. Unfortunately, in addition to the contaminants, unreacted monomer is also lost in the purge. By applying Vaporsep in the reaction section of the process, 90% of the reactant lost in the purge can be recovered.

In the production of Linear Low Density Polyethylene（LLDPE）or High Density Polyethylene(HDPE), nitrogen is added to the reactor to control the partial pressure of ethylene. The nitrogen is later purged from the reactor, taking a substantial quantity of ethylene. This stream is normally sent to flare, resulting in monomer losses in excess of $500,000 per year.

A simple flow diagram of this application is shown in the following figure. The objective of the VaporSep unit is to remove a fixed amount of nitrogen and to recycle the enriched hydrocarbons back to reactor. Since the feed stream is at 300 Psig, no feed compressor is required. The vent from the reactor is initially fed to a heat exchanger to cool the stream and no condense any high boiling hydrocarbons. After the exchanger, the membrane separates the feed into a hydrocarbon enriched permeate stream and a hydrocarbon depleted residue stream. The permeate is sent to an existing compressor for recycle back into the reactor while the residue is sent to flare.

Reactor Purge Recovery

   Application of VaporSep in the Product and Finishing Section

After the polyolefin products are produced, they must be further purified before they are ready to be shipped to the customer. Raw Polyolefin product, which is produced as a powder, contains significant amounts of unreacted hydrocarbons. Before the powder can be extruded, these unreacted hydrocarbons must be removed. The hydrocarbons are removed from the powder by using hot nitrogen in a stripper column, also known as the purge bin. In many plants, the vent gas from the purge bin is sent to the flare and both hydrocarbons and nitrogen content are lost. In a typical polyolefin plant, the value of the monomers in this stream is in excess of $1 million per year.

In the manufacture of High Density Polyethylene (HDPE), the resin degassing vent stream contains a substantial quantity of Iso-butane. The process design combines compression-condensation with membranes as shown in Figure 6.The feed stream is compressed to 200psig and then partially condensed with cooling water in a shell and tube heat exchanger. The vent from the condenser, which still contains a significant fraction of hydrocarbons, passes across the surface of the hydrocarbon selective membrane. The membrane separates the gas into two streams: the permeate enriched in hydrocarbon vapor, and a residue stream depleted in hydrocarbons. The permeate stream is recycled back to the inlet of the compressor. The residue stream, which contains a small amount of carbon dioxide and oxygen (as well as nitrogen), is sent to flare. The condensate is the recovered hydrocarbon stream.

Hydrocarbon Recovery from HDPE

   LPG Recovery

Recovery of heavy hydrocarbons (C3+) or liquefied petroleum gas (LPG) from refinery purge and fuel gas streams is more profitable than using these high-value components as fuel. LPG components are produced in many refinery operations such as reforming, isomerization, hydrocracking, and hydrotreating. Traditionally, absorption and cryogenic systems have been used for the recovery of LPG. These systems require numerous moving parts and/or external chemicals and have high capital and operating costs.
VaporSep offers a simple alternative for recovering LPG from refinery gases. In a typical VaporSep system, the feed gas enters the membrane and the LPG components preferentially permeate, producing an LPG-enriched permeate stream. The permeate is compressed and LPG is recovered as a liquid in the condenser. The residue stream from the membrane depleted in LPG and enriched in hydrogen and lighter hydrocarbon gases (methane, ethane), remains at pressure and can be sent directly to the fuel headers or for further hydrogen purification. In some cases, the hydrogen purity is high enough to allow direct recycle to other refinery processes.
Pressure swing adsorption (PSA) systems are often used to purify hydrogen from refinery streams. About 80-85% of the hydrogen is recovered at high purity. The remaining hydrogen and LPG are rejected as a low-pressure tail gas upon regeneration of the PSA bed. The tail gas is typically passed to the fuel gas header, and the hydrogen and LPG components are reduced to fuel value.
The VaporSep process separates and recovers these valuable components. In the VaporSep process, the tail gas is compressed to the feed pressure of the gas entering the PSA unit. The compressed gas is cooled and LPG is condensed. The gas leaving the condenser still contains significant amounts of LPG and enters the membrane. The LPG preferentially permeates the membrane and is recycled to the suction of the tail gas compressor. The residue stream from the membrane is enriched in hydrogen and other light ends. A fraction of this stream is sent to the fuel header to purge the methane and light ends, while the remaining fraction is recycled to the PSA for further hydrogen recovery. The VaporSep system not only achieves high LPG recovery, it also increases the total hydrogen recovery by 5 to 10%.

The VaporSep system for PSA tail gases