BP spins the vinyl

AS BP Chemicals pressed the button in 1996 to start up its state-of-the-art vinyl acetate production plant in Korea, engineers at BP’s Research and Technology Centre in Hull were planning how to make the technology behind the 150,000-tonnes-a-year plant redundant. The result is that BP Chemicals will start work next year in Hull to build […]

AS BP Chemicals pressed the button in 1996 to start up its state-of-the-art vinyl acetate production plant in Korea, engineers at BP’s Research and Technology Centre in Hull were planning how to make the technology behind the 150,000-tonnes-a-year plant redundant.

The result is that BP Chemicals will start work next year in Hull to build the world’s first fluidised-bed vinyl acetate plant. Its patented technology has allowed BP to design a single reactor with an annual capacity of 250,000 tonnes far bigger than anything the industry has been able to construct so far.

It will also save 30% on construction costs and slash operating bills. The plant will come on stream at the end of 2000, cutting the lead time from lab to commercial production by three years.

BP claims it is the most significant development in 30 years for producing vinyl acetate monomer (VAM), used in paints, adhesives and paper coatings. At its heart is its patented Leap process, which involves a jump in process engineering technology and in the way chemical plants are scaled up from small laboratory test rigs to full commercial operation.

The company has ditched its traditional process technology of a fixed-bed catalytic reactor in favour of a new fluid-bed process and catalyst design. It has used powerful computer modelling systems to enable it to go straight from its original 3-inch diameter desk-top reactor in the Hull laboratory to the design and construction of the plant.

Today, VAM is made by passing the reactants acetic acid and oxygen pre-mixed with ethylene under pressure down through thousands of narrow steel tubes packed with 5mm silica balls coated with a catalyst (generally palladium), aided by a promoter such as potassium, sodium or calcium.

Some 7 8,000 tubes 25 40mm in diameter are packed into a stainless steel reactor vessel 4 5m diameter and 6 8m long. The tubes are supported by a ‘tube sheet’ and surrounded by water to remove heat from the reaction.

Although BP had invested heavily in VAM technology for 30 years, fundamental problems remained. Passing reactants down narrow tubes leads to widely changing reaction and catalyst conditions along the length of the pipes. And the state of the promoter cannot be assessed until the plant is shut down.

Also, as reactors get bigger, the tube sheets have to get thicker to support the reaction tubes. Finding engineering companies able to produce a one-piece 4m diameter stainless steel plate perhaps six inches thick with 8,000 holes in it was hard. This limited reactor size to around 150,000 tonnes capacity, forcing BP to increase capacity in large, costly steps.

It wanted to attain the same conditions throughout the reactor and make it easier to maintain. In 1996 the Hull researchers visited the US to look at work at BP’s Warrensville Research Centre in Cleveland on fluid-bed technology for ethylene and acrylonitrile production.

Fluid-bed technology offers a number of advantages. First, there are no tubes or tube sheet. Instead, the catalyst is introduced as a fine talcum-like powder sitting on a perforated grid at the bottom of the reactor vessel. As ethylene is blown up through the grid, the powder ‘flows’ like a liquid, the turbulence providing intimate mixing between the catalyst and the tiny bubbles of ethylene, acetic acid and oxygen. Heat is removed by simple heat exchanger coils inside the vessel.

The reaction is more uniform and heat transfer is improved so reaction can be controlled to within a degree or so of optimum temperature.

Oxygen is injected directly into the fluid bed to provide consistent maximum oxygen levels throughout the vessel. This improves yield, producing more VAM per pass and less unused reactants to be recycled back into the reactor. Samples of catalysts can now easily be removed to check the state of the promoter.

The engineering is simpler and the size of recycling plant is reduced, so construction costs are lower. Also, with no engineering size limitations, reactors of up to 500,000 tonnes capacity are possible, says BP.

But perhaps the biggest achievement at Hull has been the virtual elimination of scale-up trials. Engineers have got rid of the costly and time consuming need to build and test pilot plants of increasing size until they are confident full-scale commercial plants will run.

Instead they used BP’s unique X-ray rig at its research site in Sunbury, London. Here, X-ray videos were taken of what was happening inside the lab reactor, particularly gas bubble size and movement. Using powerful computers, they were able to mathematically model the fundamental processes. Once the model had been proved by lab trials, it was a simple step to scale up the process to design the new Hull plant. BP reckons it saved £10 15m by not building pilot scale plants for the Hull project. It also eliminated the three years it takes to build and run pilot plants.